CA1290819C

CA1290819C - Distributed station welding system

Info

Publication number: CA1290819C
Application number: CA000521140A
Authority: CA
Inventors: Malcolm T. Gilliland
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-10-25
Filing date: 1986-10-22
Publication date: 1991-10-15
Anticipated expiration: 2008-10-15
Also published as: US4870248A; US4896018A; US4716274A

Abstract

Abstract of the Invention An improved welding system is shown. A
single high capacity power supply (10) provides operating power for several weld selector stations (16). Each weld selector station (16) operates independently, is adjustable, and allows a welder to obtain the voltage and current characteristics desired for his particular welding operation. Each weld selector station (16) operates over a wide range of input voltages so that long runs of low-voltage interconnecting cable (14,15) can be used without adversely affecting the quality of the weld.
Furthermore, the weld selector station (16) is small, lightweight and easily transportable. The result is a more efficient, less expensive, versatile welding system.

Description

~9~

~DISTRIB~T~D STATION W~LDI~G SYST~W

Technical Field The present invention relates to welding power supplies and in particular to improved distributed welding power supply systems having a single, relatively high voltage power source, and a plurality of physically distributed power regulator~
and DC-to-DC converters for use by a plurality of individual welders.

Backqround of the InYenti~on The present invention is an improvement to welding power supplies particularly suited for environment~ in which a relatively large number of welder~ are working in the area. The present invention is particularly u~eful in environments in which welding must be done in relatively small cramped areas. In particular, the environment of a shipyard is one in which the present invention is particularly useful.
~ s is known to tho~e skilled in the art, welding jobs in shipyards normally include a relatively large number of welders welding ... .

O~g simultaneously. Additionally, in the construction or repair of a ship, it is often necessary for a welder to work in relatively cramped quarters within the interior of a ship.
s P ior art arrangements have normally included a relatively large number of power supplies, one for each welder. Such an arrangement i~ time consuming and expensive since it requires multiple drops o~ three-phase high volta~e AC line~ to supply the various power supplies. As i9 known to those s~illed in the art, accidental cutting of high voltage AC power lines leads to much ~ore dangerou~
situations than similar accidentR involving the lower voltage DC lines.
Furthermore, lack of reasonable acces3 to the AC power lines and/or the impracticality of or undesirability of moving the prior art power supply sometimes creates a situation wherein the prior art power supply is substantially removed from the welder and a long run of welding cable i~ required. Since the low voltage high current outputs of such welding power supplies require large gauge cables, pulling long runs of these cableR around a shipyard or similar environment is cumbersome, results in incxeased I2R losses as the cable length increa~es, and causes variations in the quality of the weld because of the distributed inductance, capacitance and resistance of long runs of cable.
Additionally, the use of multiple individual welding power ~upplLe~ necessarily mean3 duplication of one of the most expensive components of a power supply, a large, high current three-phase transformer.
Thus, as explained in greater detail hereinbelow, the present invention provides a ~2~0~3~9 distributed welding power ~upply system which overcomes ~ome of these disadvantage~. The ~ystem of the pre~ent invention generally consist oE a single high power regulated power supply and a plurality of distributed weld ~elector ~tations. Within each weld selector station there are a number of element~
es~entially ~imilar to a ~maller welding power supply.
Therefore, it is appropriate to consider other aspects of the background of the art of solid state welding power ~upplies in connection with the weld ~elector stations used in the present ~ystem.
Again con~idering the environment of a shipyard as a typical environment for welding, it i~
highly de~irable to provide a power ~upply which i~
usable in a relatively large number of welding processesO In particular, there are a number of variations in the electrical output characteristics of a welding power ~upply which affect its utility in particular welding processes. Among these are the output impedance and the turn-on and turn-off time or the dV/dt (dI/dt) characteri~tics of the voltage output (current output) of the supply. For example, in tungsten inert gas (TIG) and stick welding, it i5 generally desira~le to have a power supply with a high output impedance so that it~ characteristics approximate a constant current ~ource. In metal inert gas (MIG) and pulse arc welding, it is often desirable to have a power supply with a low output impedance which approximate~ a con~tant voltage source. There are, of course, other type~ of welding which require a compromise between these characteristics.
Another aspect of a weldlng environment well known to those skilled in the art i9 the fact that large transients are present in the main current ~''3~ 9 _ carrying cable~ within a welding ~yste~. The use of pulse width modulator~ to control solid state switching device~ to adjust output voltage has been known in the art for some period of time. Generally, such pulse width modulators are constructed u~ing relatively low voltage solid state integrated circuits. Such circuits are provided with well regulated and well by-passed low voltage power supplies so that they will operate properly.
One problem with prior art power ~upplie8 of thi~ type occur~ from tranQients which occur on the DC input to a welding power supply ~witching regulator operated by the pulse switch modulator.
When an arc is struck, there i9 normally a largs current surge initially drawn from the output of the power supply which normally lowers the voltage at the input of the regula~or. ~ problem which arose in designing a system of the present invention wa~ the fact that multiple welders are operating weld selector stations off of a single power ~upply output.
As variou~ welders using the system strike arc~, the overall output voltage of the main ~upply, and thu~
the input voltage to the various weld selector station~, will drop. Thus, it i~ desirable to provide a feedback system for pulse width modulators used in a distributed welding power ~upply sy~tem which will respond to changes in the input voltage to the weld selector ~tation.
Additionally, in some welding environments there are a large number o~ remote welding units which include wire feed motor~ and ~hielding gas valves, for example, of the type shown in applicant's U.S. Patent 4,119,830, issued october 10, 197~. The voltage supply to the wire feed motor normally fluctuate~ with the voltage supplied by the power ~3~ L9 supply output. Also, in some types of MIG weldinq, the output voltage i5 lower than that required to drive the wire feed motor. Thu~, there is a need to as~ure that the voltage to the wire feed motor will not become exce~sive so a~ to damage the motor or inadequate 30 as to ~ause stalling or erratic operation.
Furthermore, in many prior art syqtems, a high power resiRtance box i~ used to obtain the 10 de~ired output voltage, current and impedance. These boxes can con~ume ~ub~tantial amounts of power which must be dissipated a3 waste heat, lower~ the ener~y and co~t efficiency of the system, and requires additional air f low or cooling .
LaRtly, there i~ a problem in tbe ~tate of the prior art, not limited to the environment of welding power supplie~, which has been overcome by the present invention. As is well known to tho~e skilled in the art, the design of reliable pusb-pull 20 amplif iers, whether they be linear amplifier~ or pu~h-pull devices u3ed in switching power ~upply regulators, ha~e always been con~idered to require clo~ely matched pairs of output devices. Thi~ goes back to the days of push-pull vacuum tube audio amplifiers. In the de~ign of push-pull solid state amplifier~, this problem ha~ become more critical ~i~ce excessive gain in one tran~i~tor of a pu~h-pull amplifier stage normally leads to greater power dissipation and thus a higher operating temperature for that particular tran~istor. ~s the operating temperature increase~, the beta of the tran~istor tends to increase, and this ultimately lead~ to a condition known A4 thermal run-away in which the higher beta translqtor will be de3troyed.
As is al~o known to those skilled in the ~29~819 art, matched pairs of transistors of a particular type tend to be considerably more expensive than individual transistors of the same type. This cost increase becomes even greater when relatively high current devices, such as those used in relatively large switching power supplies, are used. Thus, there is a need in the art to provide a practical and reliable arrangement for designing push-pull amplifier output stages (which may be used in switching power supplies, linear amplifiers, and other applications for push-pull topology) which can eliminate the requirement for transistors having closely matched betas and thermal characteristics.
In switching power supplies, particularly those where push-pull operation is employed, the stress placed upon the transistor when switching from the on state to the off state frequently causes premature failure of the transistors. To some degree, these failures were alleviated by including an internal diode in the transistor, oriented so that the diode became forward biased if circumstances tended to cause the transistor to operate in the inverted mode. However, this internal diode did not fully alleviate the failure problem. In some cases, premature failure of the switching transistors could only be alleviated by using switching transistors which had higher voltage and current ratings.
However, this solution increases the cost of the switching power supply.
Therefore, there is a need for an apparatus whlch will protect switching transistors from the vo]tages and currents experienced in normal operation.
More particularly, -there is a need for an apparatus which will prevent the application of excess reverse voltages to the transistor. Also, there is a need for an apparatus which will prevent excessive currents from flowing through the internal diode in the switching transistor.

6a Summary of the Invention The present invention overcomes the drawbacks of the prior art recited above and provides some less expensive advantageous arrangments for accomplishing some end results known in the prior art.
The invention in one claimed aspect pertains to an improved welding system for conducting a plurality of independent welding operations from a single welding power source, comprising central power supply with sufficient capacity to power a plurality of welding operations and a plurality of weld parameter selection means. Each weld parameter selecti.on means provides one of the welding operations with electri.cal power having selectably variable characteristics for the welding operation, the weld parameter selection means being powered by the central power supply. A plurality of connecting means connects the central power supply means to weld parameter selection means.
An improvement to the weld parameter selection means for reducing adverse effects between the welding operations comprise a rectifier having a first terminal connected to a predetermined conducting means and a second terminal connected to the power input terminal of the weld parameter selection means, the rectifier being for providing less noisy power to -the power input terminal of the weld parameter selection means by reducing electrical transients placed upon the conducting means by other the weld parameter selection means.
An improvement to the weld parameter selection means for reducing adverse effects between the welding operations, also contemplates an energy storage device connected to a predetermined conducting means and the power inpu-t terminal of the weld parameter selection means, the energy storage means being for providing less 38~ ~
6b noisy power to the power input terminal of the weld parameter selection means by reducing electrical transients placed upon the conducting means by other weld : parameter selection means.
A still further aspect of the invention pertains to an improved welding power supply for providing welding power which is compensated for variations in input voltage, compr.ising power supply means for providing a source of DC power, weld parameter selection means for providing the welding power with a selectable set of predetermined electrical parameters desirable for a predetermined welding operation, and conducting means for connecting the power supply means to the weld parameter selection means. The weld parameter selection means comprises a power input terminal, voltage sensing means connected to the power input terminal and responsive to the input voltage at the power input terminal for providing an input voltage signal, and parameter selection means responsive to the input voltage signal for varying at least one of the predetermined electrical parameters of the welding power.
Further the invention comprehends an improved pulsed welding power supply, comprising pulse generating means for providing a pulsed output signal with predetermined electrical parameters, a power source, a power supply output terminal, and at least one semiconductor device responsive to the pulsed output signal for selectively connecting and disconnecting the power source and the power supply output terminal. The semiconductor device has a first terminal connected to the power source, a second term.inal connected to the power supply output terminal, and a third termi.na] for selectively varying the conductance between the first terminal and the second terminal, the third terminal being connected to the pulse generating means.

.. . . ...

A still -further aspect oE the invention provides an electronic welder, comprising a power supply for providing operating voltage and operating current, a voltage divider responsive to the operating voltage for providing a first signal, control means for providing a second signal, a pulsewidth modulator for providing a third signal having a pulsewidth responsive to the first signal and the second signal, and output means responsive to the third signal for providing welding power to a welding operation.
Althouyh the above statements refer to broad aspects of the invention claimed, the invention and various other aspects disclosed are summarized herein.
More particularly, the system disclosed is a distributed welding power supply system in which one large relatively high voltage high power supply (80 volts at 1500 amps continuous in the preferred embodiment) is used to power a plurality of relatively small weld selector station units which are essentially high current ~C to DC converters. Each weld selector station may be operated in a constant current mode, a constant voltage mode, or a variable combination of the two.
Additionally, each weld selector station uses a pulse width modulated switching to control the output power from the weld selector station. The pulse width modulator controlling the output of each weld selector station is arranged to respond to drops in the input voltage rather than ~Ise of conventional arc voltage feedback in order to maintain a nominally constant output.

~g~9 Because each weld selector station unit is supplied from a higher voltage source than most conventional welding equipment, the weld selector station may be connected the main power supply via smaller gauge cabling than that normally u~ed to connect a weld torch to a conventional 4t)-volt supply.
The weld selector stations of the preferred embodiment are such that they can be readily implemented in a small cabinet, weighing approximately eighty pounds, which may be easily transported into confined spaces by two people.
; ~mong the advantageou~ features of the weld selector station of the present invention are a switch for automatically selecting a source of input voltage for a wire feed motor to be used with a welding torch connected to the output of the weld selector station. For application3 in which the nominal output voltage from the weld selector station is adequate to drive the wire feed motor, the wire feed motor circuitry i5 connected directly to the output of the weld selector station. When a lower output voltage is selected, for example in a number of metal inert gas (MIG) processes, the system automatically switches the source of the wire feed motor voltage to one of several user selectable voltage dividers, one of which is selected to pick off a predetermined portion of the input voltage to the weld ~elector station.
Another advantageous aspect of the weld selector station of the present qystem, which applies to welding power supplies in general, i~ the drive circuitry for the main switching transistors.
The base of the switching transistors are driven by a pair of metal oxide semiconductor field eEfect 908~3 transi5tors (MOSFETs), one of which is a P-channel type and one of which is an N-channel type. The FETs have their source terminals tied together to the base of the main switching transistor. The gates o each s of the FETs have independently adjustable impedance~
connected between them and the source of a pulse width modulated control signal for controlling the output of the weld selector station. By adopting this arrangement, the independently adju~table gate impedances varies the rise and fall times of the output of the main switching tran~istor~ As is known to those skilled in the art, various welding proce~ses respond differently to the tran~ient characteristics of the output from the s~itching device from a switching power supply.
Additionally, the P-channel FET i~
selectably switchable in and out of the base drive circuit. When it is in the circuit, it causes the base of the main switching transistor to be driven to a voltage lower than the emitter, thus depleting charge carriers in the base region, thereby causing the main transistor to turn off very rapidly. When more gradual turn-off the switching transi~tor~ is desired, the drive to the P-channel FET may be reduced or it may be removed from the ba~e drive circuit.
Additionally, the present invention includes a novel arrangement for biasing a pair of transistors used in a push-pull configuration. In the preferred embodiment of the present invention, the push-pull stage in que~tion is driving the primary of a power transformer which is used in generating well regulated lower voltage power supplies used to power a number of the lower voltage integrated circuit components in the preferred ~29~)8~9 embodiment. However, the principle of this aspect of the present invention has utility in a large number of other applications in which power transistors are interconnected in a fa~hion in which they need to conduct ~ubstantially equal amount~ of average current in order to avoid the circum~tance~ of one transistor going into thermal run-away. For example, push-pull power output stages in audio amplifiers may benefit from this aspect of the present invention.
Also, parallel power transist~r conflguration~ may be rendered less expen~ive by adoption o thi~ aspect of the present invention.
Generally stated, thi~ aspect of the present invention can be used to provide considerable cost savings to the manufacturer of device~ which normally require matched pairs of set of transistors.
As is known to those skilled in the art, matched pairs of transistors are ones (often obtained from the same die during fabrication) which have closely matched characteristic~. Of particular concern is the forward current gain or beta, of transistors having similar or common base drives. ~g noted above, in thepreferred embodiment, the arrangement meeting this requirement is that of a push-pull switching amplifier driving an inductive load.
As is known to those 3killed in the art, if one of the transistors in such an arrangement has a somewhat hlgher beta than the other. It will draw more current than the other transi~tor. This leads to a ~ituation in which the junction temperature of the higher beta transistor becomes higher than that of the other transistor. As junction temperature increases, beta increases and thus the situation is exacerbated by a form of positive feedback.
As is known to tho~e skilled in the art, ~ 9 eventually this situation lead~ to thermal run-away and de~truction of the higher beta transistor. This is why such amplifier arrangements are conventionally constructed u~ing matched pairs of devices. In the S situation of parallel drive transistors, unle~q power wasting emitter resi~tor~ are used, one tran~istor tends to ~hog" more current than the other~. Under the~e circumstances, the collector-emitter saturation voltage i8 also decreased as the transi~tor heats up, and destruction of one o the devices often result~, frequently followed by a chain reaction de~tructlon of the other transi~tors as they try to accommodate the increase in current caused by the destruction of the first transi~tor.
The main drawback to matche~ tran~istor~, particularly relatively high power device~, i3 that they are much more expensive than buying a plurality of transistors of the same type which do not necessarily have their characteristics closely matched, other than the extent to which they are matched by virtue of being the same type device. ~he present invention provides a mechanism or using non-matched pairs of transistorq in circumstances in which the conventional wisdom of the prior art dictates that matched pairs be used.
Broadly stated, this aspect o~ the present invention provides thermally sensitive base drive to each of the transistors in questlon. Each of the thermally sensitive base drive elements is thermally coupled to a heat sink connected to the other transistor of the pair. In the preferred embodiment, each transistor includes a base emitter resistor having a positive temperature coefficient which is mounted on the heat sink or case of the other transistQr. As A ~econd transistor becomes hot~er 3~

than a first transistor, the base emitter resistor of the first transistor heats up, thus increasing its value.
This has the effect of shunting less of the input current from the base of the first transistor, thus increasing the first transistor's base drive. Therefore, the first transistor begins conducting more current and the pair moves toward an equilibrium condition.
In the preferred embodiment, a potentiometer is connected as a variable resistor between the common connection between the emitters of the drive transistors and a switch. The switch is configured to selectively connect the other end of the variable resistor to the base of either of the drive transistors. This arrangement is used to initially calibrate the transistor lS pair. The switch is moved back and forth between the two bases as the variable resistance is adjusted until an initial condition of equality between the currents conducted by the two transistors is established. Once the device is subsequently operated, the above-referenced thermal feedback connection is used to maintain this initially established equilibrium.
Therefore, the present invention seeks to provide an improved system for providing welding power in an environment in which a large number of welders are working simultaneously.
Further the present invention seeks to provide a distributed welding power supply system employing one master power supply and a plurality of lightweight weld selector stations which is less expensive and more energy efficient than provision of an equivalent amount of power through the use of individual supplies.
Other aspects of the invention will be apparent from the detailed description of the preferred embodiment below.

Brie~ cr~El_on of the Dra~inq~
Fig. 1 i~ a block diagram of the preferred embodiment of the present invention.
Fig. 2 is a block diagram of the weld selector station of the preferred embod$ment.
Fig. 3 is ~ schematic diagram of the switching and shutdown driver of t:he prefexred embodiment.
Fig. 4 i~ a ~che~atic diagram of the current detection circuits of the preferred embodiment.
Fig. 5 is a ~chematic diagram of the STICK
& TrG control circuit of the preferred embodiment.
Fig. 6 is a s~hematic diagram o~ the low frequency pu lsewidth m~dulator of the preferred embodiment.
Fig. 7 is a schematic diagram of the high frequency pulsewidth modulator of the preferred embodiment.
Fig. 8 is a schematic diagram of the high/low feeder voltage switchover control circuit of the preferred embodiment.
Fig. 9 is a schematic diagram of the power supply circuitR of the prefarred em~odiment.
Detailed Descxiption Turning now to the drawings, in which like numerals r'eference like elements throughout the ~everal drawings, the preferred embodiment of the present invention will be described. As shown in Fig. 1, the preferred form of the pre~ent invention i9 a central welding power ~upply 10 feeding a number of remote weld selector 3tation~ 16a 16nO Central welding power supply 10 contain~ a single AC-to-DC
converter 11 for providing power to the weld ~elector stations 16a-n.

~9~9 Converter 11 i8 connected to a source of three-pha~e AC power ~not shown), normally of 460 or 230 volts, by conductors 5, 6 and 7. The positive output of AC-to-DC converter 11 is connected by conductor 12 to a number of po~itive output stud~
8a-8nO The negative, or return, output of converter 11 i~ connected by conductor 13 to a number of grounded output studs 9a-9n. It will be appreciated that the stud~ 8 and 9 are conventional in nature and provide the connection point at the weld selector station for the welder.
Convertar 11 provides an output of 80 volt~
DC ( nominal ) at a current ~ufficient to power the desired number of remote weld Yelector ~tations 16a-16n. In the preferred embodiment, conver er il provides 1500 amp~ (continuo~s) current. Method~ of construction of central power supply 10 are well known to those skilled in the art. Remote weld selector stations 16a-16n are connected by conductor L4a-14n and 15a-15n to studs 8a-8n and 9a-9n, respectively.
Each remote weld selector station 16 can accept an input voltage of 30 to 150 volts (80 volt~
nominal) and has its own controls for varying voltage and current characteristic~O ~his allow~ a welder using a remote weld selector station 16a to ad just the voltage and current output characteristics to match the type of welding that welder is performing without affecting the voltage and current characteristics of remote weld selector station~
16b-16n.
It will be appreciated the present invention requires only a ~ingle central power supply 10 and a single AC power co~nection instead of the multiple power supplies and AC power connections of )8~

conventional systems. Furthermore, it will be appreciated that since the remote weld ~elector station 16 weighs only 80 pound~ it can be readily moved f rom place to place as required.
S It will also be appreciat~d that, in conventional welding systems, becau~le of the high current~ involved, the length of the electrical cable between power supply and welder has a ~ignificant effect on the quality of the welding performedO ~ach remote weld selector station 16 therefore acts as a buffer between the central power supply 10 and the welder. It will therefore be appreciated that the present invention effectively reduces the length of electrical cable to the short distance between the remote weld selector station 16 and the welder. ~he present invention therefore reduces the co~t of a welding system by minimizing the amount of expsn~ive control cable and gas hose u~ed. The pre ent invention also reduces the cost of a welding system 20 by consolidating several smaller power supplies into one central power supply 10 and greatly reducing the number of AC power connection~ that must be installed.
Weld selector statlon 16 may be briefly described as a high efficiency, overload protected, ~electable constant current or constant voltage switching power supply with automatic compensation for input voltage fluctuation, and ~electable low frequency pulse, high frequency pulse, and chopped low frequency pulse outputs.
Turn now to Fig. 2, which i~ a block diagram of a remote weld selector station 16. High efficiency is obtained by the use of switching transistors 56a-56f, inductor 62, and a free wheeling diode 61. Transi~tors 56a-56f are rapidly switched V~3~9 _ on and off to provide a desired current or voltage at output studs 17 and 18 instead of the conventional manner of using an array of high power di~sipation resistors.
The positive 80 volt DC conductor 14 is connected to the anode of a diode 20. Diode 20 protects weld selector 16 in the event that a negative voltage is appl~ed to conductor 14. The cathode of diode 20 is connected by conductor 21, tbe internal po~itive 80 volt DC line, to a resistance bar 25; the STICR & TIG control circult 30, the low frequency pulsewidth modulator 32, the high frequency pulsewidth modulator 34, the circuit power supplie~
38, the high/lo~ voltage switchover circuit 86, the po~itive terminal of 10,000 microfarad ~toraye capacitor 22, and to one end of the coil of relay 43.
Storage capacitor 22 ~tore~ power from and ab~orbs voltage surges on VIN conductor 14 and provides a low impedance source which can provide large welding currents for a short period. Diode 20 also prevents capacitor 22 from discharging into VIN
conductor 14 in the event that the voltage on VIN
conductor 14 should fall below the voltage on conductor 21. Diode 20 and capacitor 22 therefore i801ate the output of weld selector station 16 from transients caused by other welding operations and rom the distributed inductance/capacitance/
resistance characteristics of conductors 14 and 15.
The ground connector 15 i8 connected to one end of a fuse 24 and to the ground output stud 19.
The other end of fuse 24 i~ connected by conductor 23 to the negative terminal oE capacitor 22, the circuit power supplies 38, the anode of diode 61, one end of ON/OFF power ~witch 46, and the high/low voltage ~IL2~ 9 switchover circuit 86. The other end of switch 46 is -connected by conductor 47 to the other end of the coil of relay 43. Although only a ~ingle ON/OFF
switch 46 is shown, it will be appreciated khat it i2 representative of multiple, interlocking ~witches ~o that relay 43 may be energized/de-energized from a local or remote location, ~uch a~ an ON/OFF ~witch on the welding torch.
The other end of re~istance bar 25 i9 connected by conductor 42 to one contact of a normally open pair of contact~ in relay 43. The other contact of relay 43 is connected by conductor 44 to four snubber circuits 50a-SOd, six power transistor circuit~ 5Sa-55f, and arc-~ustaining resistor 45. The other end of the snubber circuit~
50a-50d, of the power transistor circuits 55a-55:E~
and of arc-sustaining resistor 45 arc connected by conductor 41 to the cathode o diode 61, one end of inductor 62, and the return output of shutdo~n driver ; 20 37. The other end of inductor 62 is connected by conductor 63 to one end of resistor 64 and on,e input of overcurrent detector circuit 73. The other end of resistor 64 is connected by conductor 65 to the other input of overcurrent detector circuit 73, one end of re~istor 66, and the MIG & PUISE stud 18. The other end of resistor 66 is connected by conductor 67 to one end of inductor 70. The other end of inductor 70 is connected by conductor 71 to the STICR & TIG ~tud 17, one input of the high/low voltage switchover 30 circuit 86, one end of 44 ohm resistor 93 and the input of short circuit detector circuit 72. Inductor 62 and resistor 64 provide regulation and smoothing of the output for MIG & PULSE welding. Inductor 70 and resistor 66 provide the additional output smoothing required for STICR & TIG welding. The 308~L9 other end of re~istor 93 i~ connected to 80v return conductor 23 which is the ground for power supply 11 ~Fig. 1). ~esistor 93 ha~ two tap points which are connected by conductors 91 and 94 to two of the iaputs of high/low voltage switchover circuit 86.
Re~i~tor 93 provides a minimum, stabilizing load for transi~tors 56a-56f when the weld selector station 16 is on but the welder ha~ not yet struck an arc.
Re~istance bar 25 has several tap points.
The first and second tap points are connected by conductor~ 26 and 27 to the positive and negative inputs, respectively, of STICR & TIG control c~rcuit 30. The ~hird and fourth tap points are connected by conductor 76 and 77 to the po~itive and negative inputs, respeçtively, of peak current detector circuit 80.
The output of STICR & TIG control circuit 30 is connected by conductor 31 to one input of high frequency pulsewidth modulator 34. The output of low frequency pulsewidth modulator 34 i9 connected by conductor 33 to the other input of high frequency pulsewidth modulator 34. The po itive and negative outputs of high frequency pul~ewidth modulator 34 are connected by conductor~ 35 and 36 to a first po~itive and a fir~t negative input, respectively, of switching, shutdown and driver circuit 37.
The outputs of overcurrent detector circuit 73 and 3hort circuit detector 72 are connected by conductor 74 to one input of peak current detector 80.
The output of peak current detector 80 i~ connected by conductor 81 to a second positive input of switching, shutdown and driver circuit 37. The common reference of overcurrent detector 73, short circuit detector 72, and peak current detector 30 are connected by conductor 75 to the second negative ~L~9~1~19 input of ~witching, shutdown and driver circuit 37.
-The driver output and the reference outputoE switching, shutdown and driver circuit 37 are connected by conductors 40 and 41 to th~ driver input and reference input of power transistor circuit~
55a-55f, respectively.
The circuit power supplies 38 provide the following output voltage to other circuit~ of the weld selector station: + 14, +7, +15, +66.7, ~1.0~
Arc-sustaining resiRtor 45 ha~ one tap point whlch is connected by conductor 82 to one input of high/low voltage switchover circuit 86.
One end and the wiper of potentiometer 84 are connected by conductor 83 to a ~econd input of high/low voltage switchover circuit 86. ~he oth~r end of potentiometer 84 is connected by conductor 85 to a third input oE switchover circuit 86. The speed control output~ of switchover circuit 86 are connected by conductors 95 and 96 to the speed control inputs of feeder control 99. The feeder power outputs of switchover circuit 86 are connected by conductors 95 and 96 to the feeder power inputs of feeder control 99.
Power transi~tor circuits 55a-55f are identical and each contain a transistor 56 and an emitter re3istor 57. The collector of transiqtor 56 i9 connected to conductor 44. The emitter of transistor 56a is connected to conductor 41 through resistor S7a. The base of transistor 56a is connected to conductor 40.
Power transistors 56a-56f are each rated at 350 volt~ and 100 amps continuous. Although ~ix NPN
transistors 56a-56~ are shown, lt will be appreciated that the number of transistors 56 required will be dependent upon the maximum weld current desired and the rating~ of the individual transi~tor. It will -also be appreciated that PNP tran~i~tor~, field effect tran~istor~ (FET), or other semiconductor devices can al30 be used. At present, bipolar transi~tor~ are preferred becau e of their higher ratings and lower C09t.
Each power tran~istor 56 has an emitter resi~tor 57 valued at 0.55 ohm~. It will be appreciated that the purpose of re~istor 57 is to prevent current-hogglng and destructive thermal runaway by any of the parallel transistors S6. It will also be appreciated that F~T's have a positive resistance-temperature coefficient and do not suffer from the ~ame type of current hogging. Therefore, emitter resistors 57 woul~ not be required if transistor~ 57 were an FET. However, the performance of paralleled FET's can suffer somewhat due to slightly differing gate characteriqtics. Therefore, if FET'~ are u~ed for transiqtor 57 a resistor should be placed in ~eries with the gate of each FET. The value of the re~istor will be dependent upon the gate characteristics of the particular F~T used.
The four snubber circuits 50a-SOd are identical. Snubber 50a contains an 8 microfarad capacitor 51a with one end connected by conductor 52a to the anode of a fast recovery diode 53a and one end of a 0.25 ohm resistor 54a. The other end of capacitor 51a is connected to conductor 44. The cathode of diode 53a and the other end of resistor 54a are connected to conductor 41. Snubber~ 50a-SOd protect power transistors 56a-56f from ~witching transients. It ~hould be noted that the value~ for capacitor 51aand resistor 54a axe not the values that would be obtained by the uRe of standard snubber circuit de~ign equation~ and tables becau~e of the ~;~9~)8~9 pre~ence of arc~sustaining resistor 45.
A~ will be explained in detail below, power transistors 56a-56f are switched on and off at a rapid rate~ It will therefore be appreciated that 5 when tran3istor~ 56a~56f are off, there will be no current to sustain the weLding arc and the arc will be quenched. lrhis undesirable condition i9 preYented by arc-sustaining resiqtor 45 connected between conductor~ 44 and 41. Resistor 45 has a value of 2.4 10 ohms, which will pass sufficient current to keep the welding arc alive for a short period while transistors 56a-56f are off. When transistors 56a-56f are on, the voltage acroqs resistor 45 will be reduced to a low value and the current supplied by 15 resistor 45 will be negligible compared to the current conducted through transistors 56a-56f.
Arc sustaining resistor 45 also provide~ an additional benefit. As~ume for a moment that arc sustaining resistor 45 is not present, and that 20 transistor 56a develops a short. When driver 37 turns off tranqistors 56b-56f, the full load current wiLl flow through transistor 56a which will, iQ most cases, cause it to burn out and permanently become an open circuit. When driver 37 turns transistors 25 56b-56f on again, they will have to also pas~ the current that shou ld have been passed by transistor 56a. This extra current may cauRe transistor 56b to fail shorted, then opened, and so on in a chain reaction untll all six transistors have failed in the 30 open mode~
Now insert resi~tor 45. If transistor 56a develop~ a short, then when driver 37 turns off transistors 56b-55f, the load current will be partially absorbed by resistor 45. This prevents 35 transistor 56a from opening and therefore prevents ~29~ .9 the chain reaction from occurring. Resistor 45 thuq causes shorted transistor 56a to stay shorted, instead of opening, and preserves tran~istor~
56b-56f.
5Of course, if transiYtor 56a shorts, the weld selector ~tation 16 will not operate properly, but the cost of repair will be reduced.
Asqume that transistors 56a-56f are on, that the welding equipment i9 connected to the STICR
& TIG stud 17, and that welding is in progre.4~ so that current i3 flowing through inductor~ 62 and 70 and resistors 64 and 66. It will be appreciated that inductors 62 and 70 will oppose any rapid change in current.
15Free wheeling diode 61, conductor 23, and fu~e 24 therefore provide a path so that, when transistors 56a-56f turn off, the current through inductors 62 and 70 will decrease gradual}y instead of instantaneously, thereby preventing a large negative voltage from appearing on the emitter of transistors 56a-56f. The decay rate of the current will be determined primarily by the values of inductors 62 and 70, reslstors 64 and 65, and the inherent resistance, inductance, and capacitance of the welding equipment and the arc.
Free wheeling diode 61 also protects the weld ~elector 16 from the application of a negative voltage to ~tuds 17 and 18. If a negative voltage is applied to studs 17 or 18, a large current will flow 30through re~istor 64, inductor 6~, diode 61, conductor 23, and fuse 24. Fuse 24 is rated at 150 amp~. The large current flow will blow fuse 24, thereby isolating and protectiny the circuitry in weld selector 16.
35Weld selector station 16 has three current ~ X~ l9 limiting circuits: peak current detector 80, overcurrent detector 73, and short circuit detector 72. These current limiting circuit~ are explained in detail below. The peak current detector 80 cau~e~
switching, shutdown and dri~er circuit 37 to momentarily turn off transi~tor~ 56a-56f if the in~tantaneou~ current exceed3 2000 ampY.
Overcurrent detector 73 cause~ driver circuit 37 to turn off transistors 56a-56f if the average current exceeds 300 amp~ for more than one second. Short circuit detector 72 cause~ driver circuit 37 to turn off transistor~ 56a-56f if the average voltage on stud~ 17 or 18 drop~ below approximately 4 volts. It will be appreciated that overcurrent detector 73 and short circuit detector 72 interact. Assume that a low value re~istance is placed across either stud 17 or 18 and stud 19 so that an average current greater than 300 amps flo~
out of stud 17 or 18. The overcurrent detector 73 will cause transistor~ 56a-56f to be turned off. The current will then drop and, the voltage between stud 17 or 18 and stud 19 will drop to a low value. Short circuit detector 72 will then keep transistors 56a-56f in the off ~tate until the low value resi~tance is removed.
The circuit power supplies 38 provide~ + 14 VDC, ~ 7 VDC, and ~15 VDC, each with an isolatecl return, and also provide~ +66.7 VDC and ~1.0 VDC, both having conductor 23 as a common return. Power supplies 38 i~ described in detail below.
Switching, shutdown and driver circuit 37 is controlled by current detectors 72, 73 and 80, pulsewidth modulator~ 32 and 34, and sTrcR ~ TIG
control circuit 30. Driver circuit 37 turns transistors 56a-56f on and off, as required. The _ operation of driver circuit 37 is de~cribed in detail below.
It will be appreciated that different type~
of welding require different voltage/current characteristics. The STICR ~ TIG mode uf welding i~
best performed with a con~tant current ~ource.
STICR fi TIG control circui~ 30 sense~ and compensates for the current flowing through resi~tance bar 25 90 that, if the STICR ~ TIG mode is ~elected, weld selector ~tation 16 will ~pproximate a constant current -qource.
If the STICK h TIG control circuit 30 1~
not ~elected, weld selector 16 and stud 18 approximate a constant voltage 30urce, which i~
preferred for MIG & PULS~ type ~elding~
As previously described, weld ~elector station 16 is a switching power supply, ~ith a selectable switching frequency of 10 to 1500 ~z. It will be appreciated that the type of welding and the 20 welding gas and material~ used require different switching frequencies, waveforNs, and ~oltage-current characteristics for best performance. Pulsewidth modulators 32 and 34 allow the welder to select the~e parameters. Both modulators 32 and 34, contain circuitry which increases the output pulsewidth if the input voltage, VIN, on conductor 14 decrease~, so that the energy supplied to the weld remains approximately con~tant over a large ~30 volt~ to 150 volts) range o input voltage VIN.
Low frequency modulator 32 provides an output signal with a range of 10 to 400 Hz. 8igh frequency modulator 34 provides an output signal with a range of 400 to 1500 Hz. The output~ of modulator~
32 and 34 can also be combined to provide a low frequency (5 to 400 Rz) output which is chopped at a 308~9 high frequency ~400 to 1500 Hz). This chopping action prevent~ inductor~ 62 and 70 from saturating, thereby improving the output regulation provided by them. The output of either modulator 32 or 34 can be combined with th~ output of STICR & TIG control circuit 30 to provide a pul~ed con~tant current ~ource. In the preferred embodiment, only one or two of the circuits 30, 32 and 34 can be activated at any one time~
10The feeder control circuit 99 controls the speed at which the welding material i~ fed (inched) into the weld. Two commonly u~ed feeder control circuits are the arc voltage feeder, which typically requires approximately 20 to 40 volts, and the DC
controlled feeder, which typically requires approximately 30 to 50 volts. Hish~low voltage switchover circuit 86 automatically ~elects the voltage on one of conductor~ 71, 82, 91 and 94 to ~aintain the proper operating voltage for feeder control circuit 99.
A typical feeder control circuit 99 will also have a potentiometer for varying the rate at which the weldinq material i8 inched into the weld.
However, it will be appreciated that the welder will desire a much lower inch feed speed when he is adjusting the extended length of the welding material than the inch feed speed desired during welding.
Righ/low voltage switchover circuit 86 automatically switches the inch feed speed from the higher rate during welding to a lower rate when welding is not actually being performed.
Turn now to Fig. 3, which is a ~chematic diagram of the switching, shutdown and driver circuit 37. Driver circuit 37 receives ~7 volts on conductor 35100 and -7 volts on conductor 102, both referenced to ~ 29~:)81~3 a 7v return conductor 101, and receive~ +14 volts on conductor 104 and -14 volt~ on conductor 106, both referenced to a 14v return conductor lOS.
A first positive input of driver 37 is connected to conductor 35, and the fir~t negative input to a first signal return conductor 36. The ~econd po itive input of driver ~7 i9 connected to conductor 81, and the second negative input to a second signal return conductor 7S. The output of driver 37 is connected to conductor 40, referenced to output return conductor 41. Driver 37 may be thought of, in a grossly over-simplified manner, as an electrically isolated two-input ~ND gate with the ~econd input, conductor ~1, being inverted.
- 15 Therefore, if a logic 0 voltage i5 present on conductor 35 and/or a logic 1 i~ present on conductor 81, the output voltage on conductor 40 will be a logic 0, selectable as 0 volts or -6 volt~.
Conversely, if a logic 1 voltage is pre~ent on conductor 35 and a logic 0 i~ also precent on conductor 81, the output voltage on conductor 40 will be a logic 1, approximately +6 volts.
Referring briefly to Fig. 2, it will be seen that conductor 40 i9 connected to the bases of transistors S6a-56f, and conductor 41 i5 connected, through re~istors 57a-57f, to the emitters of transistor~ 56a-56f. Therefore, a logic 1 (+6 volts) on conductor 40 will turn on transistors 56a-56f, and a logic 0 will turn off transistors 56a-56f. It wlll be appreciated that the ~tored charge in the base3 of transi~tors 56a-56f will have a substantial effect on their turnoff time. Some types of welding require that the turnoff be rapid, whereas other types of welding require a longer turnoff time. As will be shown below, driver 37 has a selectable logic 0 ~ 29(~819 output of 0 volts, which provide~ a longer turnoff ._ time, or -6 volt~, which rapidly sweeps away the base charge and provides a rapid turnof f time .
Going back to Fig. 3, input 5ignal S conductor 81 i5 connected through curxent limiting re3istor 110 and conductor 111 to the anode ~ PIN 1 ) of the LED of i~oLator 112. Signal return conductor-75 is connected to the cathode ~PIN 2) of the L~D of isolator 112. Isolator 112 is a schmitt-trigger, inverting, open collector optoisolator ~uch as the MOC-5008, manufactured by Motorola, Inc., Phoenix, Arizona. Isolator 112 contains a light emittlng diode ~LED) ~ a photodetector, a schmitt-trigger, and an open collector output tran~istor. The ground terminal tPIN 5) of isolator 112 is connected to the negative supply conductor 114. The VCC terminal ~PIN
6) of isolator 112 is connected to the output of a regulator 130 and the positive terminal of a filter capacitor 132l by conductor 113. The output of regulator 130 is nominally +5 volt~ with respect to conductor 114 and provides the 5 volt power required for operation of isolator 112. The negative input of regulator 130 and the negative terminal of filter capacitor 132 are connected to the negative supply conductor 1140 The positive input of regulator 130 i~ connected to the 14V return conductor 105. The operation of regulator 130 is well known to those skilled in the art.
The output ~PIN 4) of isolator 112 is connected through the ~erles combination of conductor 115, blocking diode 116, and conductor 117 to the cathode of a 3.6 volt zener diode 120, one end of a resistor 121, and to the enable input (PIN 2) of a half-bridge driver 133. A logic 1 on conductor 81 causes isolator 112 to place a logic 0 on conductor g~38i~.~3 117 .
The anode of zener diode 120 i connected to the negative ~upply conductor 114. The purpo~3e of zener diode 120 is to assure that the voltage on conductor 117 does not exceed the maximum rated input voltage of half-bridge driver 133.
In the inventor ' s pro'cotype of the pre~ent invention, driver circuit 37 wa~ in clo~e proximity to the power ~upply circuits 38. ït wa~ determined that transients from the power ~upply were being coupled into isolator 112, thereby affecting it~
function. Diode 116 ~erves to prevent theE3e tran~ients from affecting isolator 112. Diode 116 is a Schottky diode 90 that the logic 0 voltage provided by i~olator 112 on conductor 117 will be within the input voltage specification for half bridge driv~er 133 .
Conductor 35 is connected to one end of a loading reC~istor 122 and to the anode ( PIN 1~ of the LED of isolator 123. Signal return conductor 36 i~
connected to the other end of re~istor 12 2 and to the cathode ( PIN 2 ) of the LED of isolator 123 . Isolator 123 is an optoisolator with two out-of-phase output transistors such a~ the M57215L, manuactured by Mit~ubishi Electric Company, Tokyo, Japan. The ground terminal (PIN 4) of isolator 123 is connect~d to the negative supply conductor 114 . The f irst VCC
terminal ~ PIN 3 ) i8 connected to the +5 volt output of regulator 130 by conductor 113. The ~econd VCC
terminal ~PIN 8) i~ connected to the 14v return conductor 105.
The collector of the f irst output transistor ~PIN 7) i~ connected to the 14v return conductor 105 through current limiting resi~tor 124.
The emitter of the first output transi~tor (PXN 6) ~9~819 and the collector of the ~econd output transi~tor (PIN 5) are connected together, to the other end of resistor 121, and to one end of pulldown resistor 127 by conductor 126. The other end of re~i~tor 127 is connected to the negative upply conductor 114. A
logic 0 on conductor 35 turn~ off the fir~t output transistor and turns on the second output tran3istor of isolator 123, thereby placing a logic 0 on conductor 126.
Since i301ator 112 has an open collector output it can only place either a logic 0 or an op~n circuit onto conductor 117. However, i~olator 123 can place a logic O or a logic 1 onto conductor 126.
It will therefore be appreciated that if the voltage on conductor 35 correspond~ to a logic 0 and/or the voltage on conductor 81 corresponds to a logic 1 the voltage on conductor 117 will correspond to a logic 0. Likewise, if the voltage on conductor 35 corresponds to a logic 1 and the voltage on conductor 81 corresponds to a logic 0, the voltage on conductor 117 will correspond to 3 logic 1.
As previously stated, conductor 117 is connected to the enable input (PIN 2) of a : half-bridge driver 133 ~uch aq the SG3635A
manufactured by Silicon General, Garden -Grove, California. The pulse input (PIN 5) and the ground terminal (PIN 3) of driver 133 are connected to negative supply conductor 114~ The VCC input (PIN 1) of driver 133 is connected to positive supply conductor 136. Driver 133, as configured, functions a~ a voltage-shifting inverter.
The output (PIN 4) of driver 133 is connected by conductor 134 to one end of a 3.3 kilohm load re~istor 135r the cathode of 15 volt zener diode 145, the cathode of 20 volt zener diode 163, the ~g~8~

anode of 20 volt zener diode 152, the source of tran~istors 149 and 160, one end of a 25 ohm load resistor 164, and output signal conductor 40. The 14v return conductor lOS is connected to the other s end of resistor 135, to one end of switch 153, to the cathode of 15 volt zener diode 144 and to the wiper and one end of 400 ohm potentiometer 146. The anode of zener diode 144 is connected to the anode of zener diode 14S. Zener diodes 144 and 145 assure that the 10 voltage on conductor 134 does not exceed the gate-to-source voltage of transistors 149 and 160.
The other end of potentiometer 146 i~ connected to the gate of transistor 149 by conductor 148 through a damping ferrite bead 147. The other end of switch 153 is connected by conductor 154 to the wiper and one end of 400 ohm potentiometer 155. The other end of potentiometer 155 is connected to the gate of transistor 160 by conductor 157 through damping ferrite bead 156.
The drain of transistor 149 is connected to one end of a 0.425 ohm potentiometer lSl by conductor L50. The wiper and other end of potentiometer 151 and the cathode of zener diode 152 are connected to the +7 volt conductor 100. The drain of transistor 160 is connected to one end of a 0.05 ohm potentiometer 162 by conductor 161. The wiper and other end of potentiometer 162 and the cathode of zener diode 163 are connected to the -7 volt conductor 102. The 7v return conductor 101 is connected to the other end of resistor 164 and the output signal return conductor 41. Diodes 152 and 163 limit the drain to-source voltage on transistors 149 and 160, respectively.
Tran~istor 149 is an N-channel enhancement mode power MOSPET quch as the IRF250 manufactured by ~ ~C~8~.9 ~1 International Rectifier Corporation, Los ~ngeles, California. Transistor 160 is a P~channel enhancement mode power MOSFET ~uch as the IRF9130, also manufactured by International Rectifier S Corporation. As configured, transistor~ 149 and 160 function as a power inverter.
~ ssume that the voltage at the enable input of half-bridge driver 133 corresponds to a logic 1;
driver 133 will place approximately -14 volts on conductor 134 with respect to conductor 105. Thi3 turns on transistor 149 and turns off tran~i~tor 16Q, thereby placing approximately ~6 volt~ on conductor 41 with respect to conductor 41. It will be appreciated that thi turns on transistors 56a-56f of - 15 Fig. 2.
Assume now that the voltage at the enable input of driver 133 corresponds to a logic 0; driver 133 will place approximately +14 volt~ onto conductor 134 with respect to conductor 105. Assume also that switch 153 is closed. This turn~ off transi3tor 149 and turns on tran istor 160, thereby placing approximately -6 volts on conductor 40 with re~pect to conductor 41. It will be appreciated that this rapidly turns off transistors 56a-56f of Fig. 2 by rapidly sweeping away any stored base charge.
However, if switch 153 is open, then both transistors 149 and 160 will be turned off, thereby placing 0 voltq an conductor 40 with respect to conductor 41.
It will be appreciated that this will turn off 3" transistors 56a-56f of Fig. 2, but at a slower rate ~ince the stored base charge decays at a ~lower rate~
The negative supply conductor 114 is connected to the negative end of capacitor 141 and ' o one end of potentiometer 143. The wiper and the other end of potentiometer 143 are connected to the ~9(~1819 -14 volt supply conductor 106. The positive supply conductor 136 i5 connected to the positive end of capacitor 140 and to one end of potentiometer 142.
~he wiper and other end of potentiometer 142 are connected to the +14 volt supply conductor 104. The other end~ of capacitors 140 and 141 are connected to the 14v return conductor lOS.
It will be appreciated that potentiometers 142 and 1~3 vary the maximum positive and negative voltage, re9pectively, that driver 133 place~ on conductor 134 and therefore vary the on-resistance of transistors 160 and 149, respectively.
It will be noted that transi~tor~ 1~0 and 149 have a gate-to-source input capacitance of approximately 2000 picofarads~ It will be appreciated that potentiometer~ 155 and 146 vary the rate at which this capacitance can be charged or discharged and therefore vary the rate at transistor~
160 and 149, respectively, turn on and off.
. Potentiometers 151 and 162l which are in series with the drain of transistors 149 ~nd 160, respectively, vary the maximum current available to turn transistors 56a-56f of Fig. 2 on and off, respectively.
It will therefore be appreciated that potentiometers 142, 143, 146, 155, 151 and 162, and switch 153 allow the user to vary the switching characteristics of tran~i~tors 56a-56f to obtain the arc characteristics desired for the type of welding being done.
Returning to Fig. 2, ~witching, shutdown and driver circuit 37 is therefore controlled by the outputs on current detectors 72, 73 and 80 on conductors 81 and 75, and by the output of pulse modulator 34 on conductors 35 and 36.

1;~9 [)8~.~

Resistor 164 provides additional loading for tran~istors 149 and 160 and circuit power supplies 38 so that the output voltage on conductor 40, with respect to conductor 41, doe~ not exceed the allowable base-e~itter voltage of tran3i~tor~ 56a-56f of Fig. 2.
Turn now to Fig, 4, which i~ a schematic diagram of peak current detector 80, overcurrent detector 73, and ~hort circuit detector 72.
10 Referring briefly to Fig. 2, peak current detector 80 causes driver circuit 37 to turn off transistor~
56a-56f whenever the current through resi~tance bar 25 exceeds the maximum rated peak curre~t for paralleled transistor 56a-56f~ In the preferred lS embodiment, the allowable peak current i~ 2000 amp~.
Regulator 170 ~tep~ the +15 volt~ present on conductor 440 down to +5 voltq. The ground of regulator 170 i~ connected to the 15v return conductor 441. The output of regulator 170 i5 20 connected to a filter capacitor 172, the VCC terminal (PIN 8) of current ~ense latch 176 and the driver circuit input ~FIG. 2) by conductor 81.
Current sense latch 176 i~ a latching comparator with a low, 0.1 volt, thre~hold, ~uch as } 25 the SG1549 manufactured by Sllicon General, Garden Grove, California. The "HI CM" input~ (PINS 1 and 2) are not used and are connected to each other by conductor 177. Current signal input conductor 76 is connected to one end of 47 ohm re~istor 173, one end 30 of capacitor 174, and to the "LO CM ~" lnput ~PIN 3) of latch 176~ The current signal return conductor 77 is connected to one end of a S00 ohm potentiometer 175. The wiper and the other end of potentiometer 175 are connected by the lSv return conductor 441 to 35 the other end of resistor 173, the other end of 0.02 microfarad capacitor 174, and the "LO CM -n/ground ~PIN 4~ of latch 176. Resi3tor 17~ and potentiometer 175 form a variable voltage divider for calibrat:ing the peak current trip level. Capacitor 174 f ilter~
out high frequency noise.
The open collector output ~PIN 5) of latch 176 is connected to conductox 75. ~herefore, when the input voltage (PIN 3) exceeds approximately 0.1 volt with respect to conductor 441, conductor 75 will be shorted to conductor 441. Since conductor 81 is connected to the +5 volt output of regulator 170, conductor 81 will be at ~5 volt~ ta logic 1) with respect to conductor 75. A~ previou~ly discussed, a logic 1 on conductor 81 will cause driver 37 to turn off transistors 56a-56f ~Fig. ~).
Referring again to Fig. 2, the current through transistors 56a-56f al~o flows through re~istance bar 25 and develop~ a voltage drop acros~
resistance bar 25. Conductors 76 and 77 present a portion of this voltage to latch 176. The value of resistance bar 25 between conductors 76 and 77 i~
approximately 0.1 volt/2000 amps = 50 microohms.
When 2000 amp~ flows through transistors 56a-56f, the voltage between conductors 76 and 77 will be approximat21y 0.1 volt~, thus setting latch 176 (Fig.
4) and turning off transistors 56a-56f. It will be appreciated that a latching device was chosen for device 176 so that a well-defined shutdown of tran~istors 56a-56f occurs when the peak current rating is exceeded. If a latch was not used for device 176 then, when the peak current exceeded the safe value, transistors 56a-56f would be turned off;
this would cause the current to quickly drop below the safe value, which would cause tran~i~tors 56a-56f 3S to turn back on again~ cauqing the current to quickly ~.2~

ri~e above the safe value. The net re~ult would be a high fraquency o~cillation with an average current, instead of a peak current, of 2000 amps.
Turning back to Fig. 4, since device 176 i9 a latch there must be a mean~ to re~et it. Although a manual reset could be used, an automatic reset i3 preferable. The output of pul~e clock 181 i~
connected by conductor 180 to the reset input (PIN 7~
o~ latch 176. In the preferred embodiment of ~he present invention pul~e cloc~ 181 has a frequency of 60 Hz. The frequency of pulse clock 181 i~ not critical but should be fast enough to allow weldi~g to continue without a noticeable delay. The period of the pulse i8 not critical, but must be at lea~lt 150 microseconds for latch 176 ~pecified to be reset~
Referring briefly to Fig. 2, ~hort circuit detector 72 monitors the voltage on STICR & TIG stud 17 and, through resistor 66 and inductor 70, the ! voltage on ~IG & PULSE stud 18. If the voltage on stud 17 or 18 drops below a predetermined trip voltage because of a short circuit or because overcurrent detector 73 has tripped, ~hort circuit detector 72 causes switching, shutdown and driver circuit 37 to turn off transistors 56a-56f. Short circuit detector 72 will then keep tran~i~tors 56a-56f off until the short is removed and the voltage at ~tuds 17 and 18 rises above the trip voltage.
Since transistors 56a-56f are now off, they cannot rai~e the voltage on studs 17 and 18, even after ~he ~hort is removed. ~he re~tart voltage is provided by the 2.4 ohm arc ~u~taining resistor 45.
It will be appreciated that resistor 45 cannot pa~
sufficient current to raise the voltage at studs 17 and 18 until the ~hort circuit is removed. Once the ~Z~308~9 short circuit is removed, re~istor 45 will pull studs 17 and 18 above the trip voltage and short circuit detector 37 will allow driver circuit 37 to turn on transistor~ 56a-56f so that welding can start again.
s Although the input of short circuit detector 72 is shown connected to the STICR & TI~
stud 17, it will be appreciated that the input may be connected to any point in the circuit that will have a low voltage when stud 17 or 18 is shorted. Two of the most likely other points are MIG & PULSE tud 18, and conductor 67 between resistor 66 and inductor 70.
Turning back to Fig. 4, conductor 71 is connected to the anode of blocking diode 191. The cathode of blocking diode 101 iR connected by : 15 conductor 192 to one end of 500 ohm resistor 193 and to the positive terminal of 680 microfarad capacitor 194. Diode 191 allows capacitor 194 to charge, but not to discharge, through conductor 71. The other end of resistor 193 is connected by conductor 196 to one end of the 12 volt, 400 ohm coil of relay 197 and to the cathode of 15 volt zener diode 195. The other end of the coil of relay 197, the anode of zener : diode 195, and the negative terminal of capacitor 194 are connected to the 80v return conductor 23.
Since the coil of relay 197 is rated at 12 volt~ and the voltage on conductor 71 can be 80 volts or more, re~istor 193 and zener diode 195 serve to limit the voltage on the coil of relay 197~
During normal welding the voltage on conductor 71 will be sufficient to keep relay 197 energized. However, if stud 17 and/or stud 18 are shorted to stud 19, the voltage on conductor 71 will be inadequate to keep relay 197 energized. Capacitor 194 serves as a holding capacitor to keep relay 197 energized for a short period of time. Thi~ prevents ~2908~9 short circuit detector 72 from detecting brief shorts.
During a brief short, peak current detector 80 provides protection.
If a prolon~ed short occurs, capacitor 174 will discharge through resi~tor 193, diode 195, and the coil of relay 197, and relay 197 will be de-en0rgized.
Relay 197 has a pair of normally closed contacts. If a short is not present, the voltage on conductor 71 wil1 be adequate to keep relay 197 energized, and the contacts will be open. If a prolonged ~hort occurs, relay 197 will drop out and the contact~ of relay 197 will closeO one contact 1~
connected to lSv conductor 441 and the other contac~t is connected to conductor 75.
When a prolonged ~hort occurs, relay 197 will drop out, the contacts will close, and conductor 75 will be connected to conductor 441. Conductor Bl will therefore be at ~5 volts (a logic 13 with respect to conductor 75. As previously described, a logic 1 on conductor 81 causes driver circuit 37 (Fig. 2) to turn off transistors 56a-56f (Fig. 2).
This condition will remain until, as described above, the short i~ removed and the voltage on ~tud~ 17 and 18 rises above the trip voltage.
Returning briefly to Fig. 2, overcurrent detector 73 monitors the current flowing through transistors 56a-56f and turns them off when the average current exceeds the maximum rated average current of transistors 56a-56f. In the preferred embodiment of the present invention~ overcurrent detector 73 i5 set to trip at 250 to 300 amps.
Assuming that the current flow is 300 amps, then there will be approximately 0.03 ohms x 300 amps = 9 volts developed across resistor 64. This voltage is ~x~ 9 pre~ented to overcurrent detector 73 by conductor~ S3 and 65.
Turning now to FigO 4, conductor 63 i8 connected to one end of 25 ohm potentiometer 182.
The wiper of poten~iometer 182 i~ connected by conductor 183 to the positive end of 4700 microfarad capacitor 186 and to one end of the 400 ohm coil of relay 190. Conductor 65 is connected to the other end of potantiometer 182 and, through the series combination o diode 184, 47 ohm resistor 185, and conductor 187, to the other end of capacitor 186 and the other end of the coil of relay 198.
The voltage placed between conductors 63 and 65 will, when the current exceed~ the trip value, cause relay 190 -to pull in. Potentiometer 182 allow3 the trip point to be ~et to match the average current rating~ of transistor 56a-56f ~Fig. 2). Re~i~tor 185 i~ a current limiting resi~tor and diode lB4 prevents capacitor 186 from discharging through potentiometer 182 and re~istor 64.
One contact of a pair of normally open contacts in relay 190 is connected to conductor 75.
The other contact in relay 190 i~ connected to the 15v return conductor 441. When the current flowing through transistors 56a-56f (Fig. 2) i le~ than the trip value relay 190 will not be energized and the contact~ will be open. However, when the current exceed~ the trip value, relay 190 will be energized and the contacts of relay 190 will be clo~ed, thereby connecting the 15v return conductor 441 to conductor 75~ ~his places ~5 volts, a logic 1, on conductor 81 with respect to conductor 75 and, as previously described, a logic 1 on conductor 81 cause~ driver circuit 37 ~Fig. 2) to turn off transiqtor~ 56a-56f ~Fig. 2). The current then falls below the trip ~9{~8~3 3g value~ Capacitor 186 and the resi~tance of the coil of relay 190 have a time constant of approximately two second~. This a~ures that tran~istor~ 56a-56f (Fig~ 2) are off for a ~ufficient time to causs the output current and voltage to fall to zero~
Returning to Fig. 2, it will be appreciated that if overcurrent detect 73 is tripped, the current flow will be interrupted for around two econd~.
This time perivd is more than adelquate for the voltage on stud~ 17 and 18 to fall below the trip voltage of short circuit detector 72. Therefore, even after the current fall~ and overcurrent detector 73 has reset, short circuit detector 72 will keep transi tors 56a 56f turned off until the short i5 removed and resistor 45 pull~ the voltage on studs 17 and 18 back above the trip voltage.
Continuing with Fig. 2, STICK/TIG control circuit 30 monitors the current out of studs 17 and 18 and causes weld selector 16 to function as a con~tant current 30urce. Current flowing out of ~tud~ 17 and 18 must flow through re~istance bar 25.
One of the taps of resi~tance bar 25 i~ connected to positive Rignal conductor 26. Another of the taps of re~istance bar 25 is connected to the negative signal conductor 27. Conductor~ 26 and 27 are connected to the inputs of control circuit 30.
The voltage output of control circuit 30 on conductor 31 increases a~ the current through resistance bar 25 decrea~es, This causes pulsewidth modulator 34 to increase the on-time of it~ output on conductors 35 and 36. This causes driver circuit 30 to increase the on-time of lts output on conductors 40 and 41, thereby increasing the on-time of transistor~ 56a-56f, which increases the output current.

~ 3L.~3~)8~

Turn now to Fig. 5, which i~ a ~chematic diagram of STIC~/TIG control circuit 30. Control circuit 30 i~ powered by the +13.3 volt difference between +80 volt conductor 21 and ~66. 7 ~olt conductor 430, which function~ a~ the circuit ground for control circuit 30. ~he po~iti~re signaL
conductor 26 is connected to one end of 100 ohm resistor 221. The other end of resistor 221 is connected by conductor 222 to one end of 100 ohm resi~tor 212, the po~itive terminal of 100 microfarad capacitor 213, and to one encl of 30 kilohm resi~tor 223. ~rhe other end of re~i~tor 223 i3 connected ~o one end of 2 kilohm potentiometer 224. The other end of potentiometer 224 is connected through 50 klloh:m resistor 225 to circuit ground 430. The wiper of potentiometer 224 is connected by conductor 22fi to the non-inverting input of operational amplifier ~op-amp) 217.
The negative signal conductor 27 is connected to one end of 100 ohm re~istor 210. The other end of resistor 210 is connected by conductor 211 to the okher end of resistor 212, the other end of capacitor 213, and to one end of 50 kilohm resia~tor 214. The other end of resistor 214 is connected by conductor 215 to one end of 500 kilohm feedback re~i~tor 216, the inverting input of op-amp 217, and to one end of 50 kilohm resistor 220. The other end of re~istor 216 is connected to the output ~PIN L) o~ op-amp 217 by conductor 227. The other end of resistor 220 i~ connected to circuit grouncl 430.
It will be appreciated that op-amp 217 i~
configured as a differential-input, negative-feedback gain stage with a gain control potentiometer 224.
Capacitor 213 i~ a ~moo'ching capacitor which 1~9(18~1.9 ~1 elim~nates high f requency transient~.
The output of op-amp 217 is also connected by conductor 227 to one end of 5.1 kilohm stabilizing resistor 232, and one end of 5.1 kilohm resi~tor 240.
The other end of resi~tor 240 i8 connected to circuit ground 430 through 5.1 kilohm resistor 241, and is connected by conductor 242 to the inverting input of op-amp 246 and one end of 30 kilohm potentiometer 243.
The ground input of op-amps 217 and 246 axe connected to circuit ground 430. Conductor 21 i8 connected to the VCC input of op-amps 217 and 246, and one end of 200 ohm re~i~tor 230. The other end of resistor 230 i5 connected by conductor 231 to the o her end of resistor 232, to the anode of temperature compensating diode 233, and one end of 5.1 kilohm re~i~tor 235. The cathode of diode 233 is connected to the cathode of 6.8 volt zener dic~e 234.
The anode of zener diode 234 is connected to circuit ground 430. The other end of resistor 235 i3 connected to the non-inverting input of op-amp 246 by conductor 237, and connected to circuit ground 430 through 5.1 kilohm re~istor 236.
Because of the temperature~ encountered in weld ~elector 16, re~istor~ 214, 216, 220, 223, 225, 232, 235, 236, 240 and 241 should be metal film resistors so that the output o STIC~/TIG control circuit 30 will not be overly temperature sensitive.
It will be appreciated that diode~ 233 and 234 form a temperature-compensated 7~5 volt reference and that resi~tors 235 and 236 p}ace one-half of this reference voltage onto the non-inverting input of op--amp 246.
The output of op-amp 246 i~ connected by conductor 247 to one end and the wiper of 6.8 kilohm ~L~9V~

current gain limit potentiometer 244, to circuit ground 430 through 6.3 kilohm load resistor 245, to one end of current limiting 10 ohm re~istor 250, and to one end of 100 kilohm re~istor 252. The other end of limit potentiometer 244 i~ connected to the wiper and the other end of current gain potentiometer 243.
The other end of resi~tor 250 iR connected to circuit ground 430 through 1000 microfarad capacitor 251.
The other end of resistor 252 i~ connected to the anode of blocking diode 253. The cathode of diode 253 is connected to output conductor 31 through switch 254.
Op-amp 246 i~ configured as a vari ble gain amplifier with a reference voltage ~3.7S volt~) connected to the non-inverting inpu~ and the output of op-amp 217 connected to the inverting input. Th~
output of op-amp 246 on conductor 247 i9 therefore proportional to the negative of the differential voltage between input conductors 26 and 27.
. Potentiometer~ 243 and 244 control the gain of op-amp 246. Potentiometer 243 may be ~et by the we~der to obtain the deqired arc characteristics.
Potentiometer 244 i5 generally not acces~ible to the welder and is used to set ~he minimum gain sf amplifier 246.
Capacitor 251 and re~istor 250 heavily load the output of op-amp 246 50 that the voltage on conductor 247 change~ at a relatively slow rate.
This assures that the output of control circuit 30 i8 re~ponsive to those variations in the welding current caused by welding conditions and not responsive to tho~e variations cau~ed by the normal operation of pulsewidth modulators 32 and 34.
Returning briefly to Fig. 2, modulator 32 35 i9 a low frequency pulsewidth modulator. In the preferred embodiment, the pul~e frequency is variable from approximately 10 to 400 ~z, and the duty cycle is variable from 0 to over 90 percent. These values are not critical but do allow the welder con~iderable latitude for obtaining the mo t de~irable arc characteristicq .
Modulator 32 i9 input voltage compensated.
The pulsewidth increases as the input voltage on conductor 21 decrea~es 90 that the average power 10 delivered to the weld i9 unaffected by input volt~ge variations.
Turn now to Fig. 6, which is a schemati~
diagram of low frequency pul~ewidth modulator 32.
Modulator 32 is powered by the 13.3 volt difference between ~80 v~lt conductor 21 and +66. 7 volt conduc'cor 430, which also serves as th~ circuit ground. Conducts~r 21 i5 connected to 0.1 microfarad capacitor 260 and to the VCCIN terminal of modulator 261. Modulator 261 is a regulating pulsewidth modulator such as the LM3524 manufactured by ~7ational Semiconductor Corporation, Santa Clara, California.
Modulats)r 261 contains a +5 volt regulator 262, an error tran~conductance amplifier 264, a current limit amplifier 265, an oscillator 266, a ~hutdown control 267, a comparator 271, an edge-triggered toggle 272, two 3-input NOR gate~ 275 and 276, and two NPN output transistor~ 277 and 280.
The output of transconductance amplifier 264, current limit amplifier 265, and shutdown control 267 are connected to the inverting input of compa~ator 271 by conductor 270. Current limit amplifier 265 and ~hutdown control 267 are each able to override the output of transconductance ampllfier 264. Current limit amplifier 26S is not used 50 its inverting input and non-inverting input are connected to circuit ground 430 by conductor 286 and conductor 287, respectively. Shutdown control 67 i9 al~o not used so its input i9 connected to circuit ground 430 by conductor 290, s Regulator 262 3tep9 the ~1303 volt~ pre~erlt on conductor 21 (with respect to circuit ground 430) down to +5 volt~ and places thi~ regulated +5 volt~
onto conductor 263. This +5 volts al30 powers some of the internal circuitry of modulator 261. The input of regulator 262 i~ connected to conductor 21.
The ground terminal of modulator 26L is connected by conductor 2B5 to circuit ground 430.
The output of o~cillator 266 is cor~ ctea by conductor 273 to the togqle input of toggle 272 and one input of ~OR gate~ 275 and 276. The Q output of toggle 272 i~ connected by conductor 281 to a second input of NOR gate 275. The negated Q output of toggle 272 i~ connected by conductor 282 to a second input of NOR gate 276. The outpu~ of comparator 271 is connected by conductor 274 to the third input of NOR gates 275 and 276. The outputs of NOR gates 275 and 276 are connected to the bases of transistor~ 277 and 2û0, respectively. The collector~ of tran istor3 277 and 280 are connected to +5 volt conductor 263. The emitters of transi~t~rs 277 and 280 are connected to output signal conductor 33.
The R input of o~cillator 266 i~ connected to circuit ground 430 through the series combination of conductor 297 and 500 kilohm potentiometer 301.
The C input of o~cillator 266 i8 connected by conductor 296 to one end of 0.1 microfarad capacitor 300, and the non-invertiny input of comparator 271.
The other end of capacitor 300 i9 connected to circuit ground 430. Capacitor 300 and potentiometer ~2g~L9 301 determine~ the frequency of oscillator 266.
The output of oscillatox 266 on conductor 273 is a pulsed waveform 269 which, during the pulse period, toggles the toggle 272 andt through NOR gate~
275 and 276, turn off transistor~ 277 and 280, respectively. O~cillator 266 al40 cau~es a sawtooth voltage 268 to be pre~ent on conductor 296. It will be appreciated that when th~ voltage on conductor 296 ri~e~ above the voltage on conductor 270, the output of comparator 271 will be a logic 1, thereby turning off transistors 277 and 280 through NOR gates 275 and 276, re~pectively.
The regulated +5 volt conductor 263 i~
connected to one end of 0.1 microfarad filter capacitor 291 and to one end of 5.1 kilohm re~istor 292. The other end of capacitors 260 and 291 are connected to circuit ground 430. The other end of resi~tor 292 is connected by condu~tor 283 to the inverting input of tran conductance amplifier 264 and one end of 10 ohm resictor 293. The other end of resi~tor 293 is connected by conductor 284 to the non-inverting input of tran~conductance amplifier 264, one end of 68 kilohm resistor 294, and one end of 178 kilohm resistor 295. The other end of re~istor 294 is connected to circuit ground 430. The other end of resistor 295 is connected to +1 volt conductor 431.
It will be appreciated that resistor~ 292, 293, 294 and 295 form a voltage divider string which provides the dif~erential voltage input to transconductance amplifier 2~4. As will be described below, +66.7 volt conductor 430 is regulated at -13.3 volt~ with respect to ~80 volt conductor 21. Neither +80 volt conductor 21 nor +66.7 volt conductor 430 are regulated with re~pect to ~1 volt conductor 431.

~9~)8 It will al~o be appreciated that conductor 2~3 will always be positive with respect to conductor 284, therefore transconductance amplifier will always be trying to sink, rather than source, current from conductor 270.
Transconductance amplifier 264 ha~ a maximum output o a few hundred microamp~. A~
previously stated, the output of transconductance amplifier 264 is al~o placed onto conductor 270.
Since transconductance amplifier 264 is conneated in an open-loop configuration, and since the inverting input is always positive with reqpect to the non-inverting input, it may be expected that amplifier 264 would drive conductor 270 down l:o circuit ground 430. However, the input to amplifier 264 is the small voltage ( millivolts ) devel.oped across 10 ohm resistor 293. Thi~ small input voltage, the small output current of amplifier 264, and the 100 kilohm load resi.~tor 315 therefore force amplifier 264 to operate as a linear transconductance amplif ier, even when connected in an open loop configurationO
It will be appreciated that the differential vol~age acro~s resistor 293 will increase as the voltage on conductors 21 and 430 increases with re~pect to +l volt conductor 431.
This causes amplifier 264 to sink more current and pulls the voltage on conductor 261 down ~o as to decrease the width of the output pulses. It will be appreciated that thi~, in turn, causes the average current through transistors 56a-56f ~Fig. 2) to decrease, thereby compensating for the increa~e in the input voltage on +80 volt conductor 21.
The output of tran~conductance amplifier 264 i5 also connected by conductor 270 to one end of ~29()8~l~

0.1 microfarad capacitor 316 and one end of 100 _.
kilohm re~i~tor 315. The other end of re~istor 315 is connected to the cathode of blocking diode 314.
The anode of diode 314 i~ connected by conductor 311 to one end of 12 ki lohm re i~tor 313, one end of 1.0 microfarad capacitor 312, and to one end of s~itch 310. The other end of capacitor~ 312 and 316 and the other end of resistor 313 are connected to circuit ground 430.
The other end of switch 310 is connected by conductor 307 to one end of paralleled potentiometers 305 ~250 kilohms) and 306 (100 kilohms). The wiper~
and other ends of potentiometer~ 305 and 306 are connected by conductor 304 to one end of 25 kiloh~
potentiometer 303. The wiper and other end of potentiometer 303 are connected to +80 volt conductor 21 through 22 kilohm resistor 302. It will be appreciated that, when ~witch 310 is closed, resistor~ 302 and 313 and potentiometers 303, 305, and 306 form an adjustable voltage divider with filtering capacitors 312 and 316 and an output on conductor 270.
Conductor 270 is connected to the inverting input of co~parator 271 and sets a reference level.
The ramp voltage 268 on conductor 296 starts below the reference level on conductor 270 and so the output of comparator 271 is a logic 0, thu~ turning on elther Sransistor 277 or 280, depending upon the ~tate of the Q and negated Q outputs of toggle 272.
After the voltage on conductor 296 rises above the reference level on conductor 270, the output of comparator 271 is a logic 1, thu~ turning off both tran~istors 277 and 280. The voltage on conductor 296 continues rising until it reaches the internal reference level of 03cillator 266, at which point ~LZ~ 9 oscillator 266 discharges capacitor 296 thus cau~ing the voltage on conductor 296 to fall to zero, and also places a pulse on conductor 273 which toggles the toggle 272 and also keeps tran~istor3 277 and 280 turned off. The voltage on conductor 296 begins rising again and the cycle repeats.
It will now be appreciated that the voltage on conductor 270 sets the width of the output pulse~
on conductor 33. Potentiometers 303 ancl 305 are used to ~et the maximum and minimum pul~ewidth point~.
Potentiometer 306 allowc the welder to vary the pulsewidth between the maximum and minimum point3 to obtain the de~ired arc characteri~tics.
If switch 30 is open, conductor 270 will be pulled to circuit ground 430 by amplifier 2~4, and the output pulsewidth of modulator 261 will drop to zero. Switch 310 therefore serves a the ON/OFF
switch or low frequency pui~ewidth modulator 32.
Turn now to Fig. 7, which i a schematic 20 diagxam of high-frequency pulsewidth modulator 34.
Wi~h a few exceptions, described below, the operation of high frequency pulsewidth modulator is the same as the operation of low frequency pulsewidth modulator 32 and components 320 through 376 are identical to components 260-316, respectively, of Fig. 6.
In the preferred embodiment the pulse frequency is variable from 400 to 1500 Hz, the duty cycle is variable from 0 to over 90 percent, and pulse frequency potentiometer 361 has a value of 50 kilohms. The frequency range of high frequency pul~ewidth modulator 34 i8 not critical and wa~
selected to provide for anticipated welding needs.
However, weld selector 3tation 16 has been successfully tested with modulator 34 frequencies above 50 kHz. One benefit of operation at frequencies substantially in exces~ of 1500 Hz is 9L29~ 9 that the inductance, and therefore the size~ of -inductor~ 62 and 70 may be reduced without adversely affecting the regulating ability of weld selector station 16.
Five volt conductor 323 i~ connected to circuit ground 430 through 5 kilohm potentiometer 384.
The wiper of potentiometer 384 is connected to tbe cathode of blocking diode 383. The anode of blocking diode 383 is connected to conductor 330. This sets an upper llmit on the output pul~ewidth and, therefore sets a minimum time during each pulse that the output of modulator 321 is a logic 0. This minimum time is often called the deadband time.
It will be appreciated that this deadband - 15 time only affects the maximum po~sible pulsewidth o~
the pulse~ generated by modulator 321 and has no ef fect upon the maximum pulsewidth of the pulses generated by modulator 261 (Fig. 6 ) or upon the period between high frequency and low frequency pulses.
The maximum pulsewidth set by potentiometers 363, 365 and 366 i9 typically below the maximum pu lsewidth set by potentiometer 384 .
Although potentiometer 384 can be used to override th~ maximum pulsewidth time set by potentiometers 363, 365 and 366, its primary purpo e, as deccribed below, i~ to override the ma~imum pulsewidth set by the output of STICK/TIG control circuit 30 and~or of low frequency pulsewidth modulator 32. It will be appreciated that the output of high frequency pulsewidth modulator 34 i~ affected by potentiometers 363, 365 and 366 only if switch 370 is closed.
It will be recalled that the output of STICX/TIG control circuit 30 i~ a DC voltage respon~ive to the current through resistance bar 25 (Fig. 2), can pull conductor 31 up, but not down ~.z~ 9 ~F ig . 5 ), and can be placed on t removed fxom) conductor 31 by closing ~opening) switch 254 (Fig. 5).
The output of STICK/TIG control circuit 30 i9 connected by conductor 31 to conductor 330 of high frequency pulsewidth modulator 34.
~s~ume that switch 254 (FigO 5) i~ clo~ed and switch 370 iY open. If the output current of weld selector station 16 increases, the current through resistance bar 25 willincrease, and the output of STICK/TIG control circuit 30 on conductor 31 wi 11 decreaYe . This lower~ the voltage on conductor 330, which cau~es modulator 321 to decrea~e the outpu'c pu lsewidth, cau~es tran~i~tor3 56a-~6f to have less on time, thereby reducing the current through resistance bar 25. The re~ult is a high-frequency, variable pul~ewidth ad~u~table constant current source.
Diodes 253 ( Pig . 5 ) and 375 form a wired-OR so, if switch 370 i~ closed, the output of 20 modulator 321 will be the larger of ta) the pul~ewidth determined hy STICK/TIG control circuit 30 or (b) the pu l~ewidth determined by potentiometer~
363, 365 and 366. If switch 370 is open, the output of modulator 321 will be the pul3ewidth determined by STICR/TIG control circuit 30. However, irl either case, the maximum output pulsewidth of modulator 321 is determined, as explained above, by the setting of deadband potentiometer 384.
The output of low frequency pul~ewidth modulator 32 ~Fig. 6) i~ connected by conductor 33 to the pole contact of SPDT 3witch 380. It will be recalled that the output of low frequency modulator 32 is a low frequency pulsed output. ~he first position of switch 380 connect~ conductor 33 to the anode of blocking diode 382 through 100 ki lohm re~istor 381. The cathode of diode 382 i~ connected ~9 to conductor 330.
-A~sume that switch 380 i~ in the firstpo~ition, that STICR/TIG switch 254 (Fig. 5) and HIFREQ switch 370 are open, and that LOFREQ switch 310 tFig. 6) is closed. During the period that the output of low frequency pulsewidth modulator 32 is a logic O the output of high frequency pulsewidth modulator 34 will also be a logic 0. During the period that the output of low ~requency pulsewldth modulator 32 i~ a logic 1 the output of high frequency pul~ewidth modulator 34 will be a high frequency series of pul es, with the pul~e~idth of each pulse being determined by the setting of deadband potentiometer 384. The result i9 that the output of hig~ frequency modulator 34 will be a repeated series of a bur~t of high frequency pul~es followed by a period of no output pul~es. Since both modulators 32 and 34 are input voltage compen ated, weld selector station 16 (Fig. 1) approximate a high-frequency, low burst rate, constant voltage ~ource.
A~sume now that STICK/TIG switch 254 (Fig.
5) is also closed. When the output of low frequency modulator 32 is a logic 0, the voltage on conductor 330, and therefore the output pulsewidth of high frequency modulator 34, will be determined by the output of S~ICIC/TIG control circuit 30, within the limit ~et by deadband potentiometer 384. When the output of low frequency modulator 32 i5 a logic 1, the voltage on conductor 330, and therefore the output pulsewidth of high frequency modulator 34, will be determined by the setting of deadband potentiometer 384. The result i8 that the output of high frequency ~odulator 34 will alternate between a first series of high ~requency pulses whose width i~
determined primarlly by STICR/TIG control circuit 30, ~29~ 9 and a ~econd serie~ of high frequency pulse who~e width i~ determined by deadband potentiometer 384.
The duration of each series will be determined by low frequency modulator 32. Weld selector station 16 will therefore alternate, at the low frequency rate, between approximating a high pulse frequency;
variable pulsewidth, constant current ~ource and a high pulse frequency, fixed pulsewidth, constant voltage source.
10Assume that ~witch 380 i~ in the second position and that switch 310 ~Fig. 6) is closed.
Conductor 33 will be connected by switch 380 to conductor 35~ It will be recalled that the output of modulator 361 and of modulator 321 can pull up, but 15not down. Therefore, modulators 261 and 321 are configured in a wired-OR configuration. Also a~ume that switch 370 and switch 254 (Fig. 5) are open.
When the output of low-frequency modulator 32 i~ a logic 1, the voltage on conductor 35 will be a logic 1. When the output of low frequency modulator 32 is a logic 0, the voltage on conductor 35 will also be a logic 0. The logic state of conductor 35 will therefore follow the logic state of the output of low frequency modulator 32. Weld selector station will therefore approximate an input voltage compen~ated, low frequency~ pulsed voltage source.
Now assume that switch 370 i5 closed, but switch 254 ~Fig. S) i9 open. When the output of low frequency modulator 32 i5 a logic 1, the output on conductor 35 will be a logic 1. When the output of low frequency modulator 32 is a logic 0, the output of modulator 321 dominates, which will be a high frequency pulse whose pulsewidth i~ determined by setting of potentiometer~ 363, 365 and 366. The ~ignal conductor 35 will therefore alternate, at the low frequency rate, between a logic 1 and a bur3t of ~296)8~9 high frequency pulses.
Now a~ume that switch 370 i~ open and switch 254 (Fig. S) i3 closed. During the period that the output of low frequency modulator 32 i9 a S logic 1, the output signal on conductor 35 will be a logic 1. During the period that the output of low frequency modulator i~ a logic 0, the output signal on conductor 35 will be a high frequency series of pulses provided by modulator 3210 It will be recalled that, qiven the above switch conditions, the width of the output pulse~ provided by modulator 32:L
is controlled by STICK/TIG control circuit 30.
The inventor knows of no welding application wherein it would be de~irable to have switches 254 (Fig. 5), 310 ~Fig. 6) and 370 (Fig~ 7) all closed. Therefore, in the preferred embodiment, the operating condition wherein switch 254 (Fig. 5), switch 310 (Fig. 6) and switch 370 (Fig. 7) are all closed i5 an invalid condition and turns off weld selector station 16. Methods of using mechanical and/or electrical interlock~ to prevent the simultaneous closing of all ~hree switches 254, 310, and 370 are well known to those skilled in the art and are not detailed herein. ~owever, some welding application unknown to the inventor may require the closing of all three switches 254, 310 and 370 ancl therefore it may be desirable to allow this operating condition.
Returning briefly to Fig. 2, high/low voltage swltchover circuit 86 perform~ two function~:
switching the speed control input of feeder control 99 from potentiometer 84 to a fixed resistor inside switchover circuit 86; and regulating the power supply voltage provided to feeder control 99.
The welder set~ potentiometer 84 to get the desired wire feed speed for the actual welding ~ 9 operation. This wire feed speed is generally higher -than the wire feed speed desired when the welder i adjusting the wire feed length prior to commencing the welding operation. Switchover circuit 86 monitors the voltage on ~tuds 17 and 18, which will be hlgh when not welding and low when welding, and connects an internal re~istor, or potentiometer 8~, re~pectively, to feeder control 99, thereby automatically providing the more desirable wire feed speed.
A typical feeder control 99 will conta~n 30me voltage regulating circuitry ~o that minor variations in the input voltage do not affect the wire feed speed. The input voltage for a feeder control 99 is also typically obtained by using a resistance voltage divider to drop the +8G volts o~
conductor 14 down to the specified operating voltage.
However, a typical resiqtive voltage divider may consume over 100 watts of power, which is dis~ipated as heat. Switchover circuit 86 eliminate~ thi~
resistive voltage divider, and therefore improves the efficiency of weld selector s~ation 16, by switching between several voltage ~ource~ already in weld selector station 16 to obtain the specified operating voltage.
In the preferred embodiment switchover circuit 86 can select from a tap point on arc sustaining reqistor 45, STICK & TIG stud 17, or two tap points on load resistor 93. It will be appreciated from prior statements that resistor 45 already exi~tq in weld selector statlon 16 to provide a sustaining current for the arc when transi~tor~
56a-56 are turned off, and that resistor 93 already exists in weld selector station 16 to provide a minimum load for tranqistors 56a-56f when they are on but an arc has not been struck.

~1)8~g STICK & TIG stud 17 is connected to a first input of ~witchover circuit 86 by conductor 71. Stud 17 will typically be at +80 volts before the arc i5 struck and +20 volts after the arc i9 struck. A
first tap pvint on re~i~tor 93 is connected to a second input of switchover circuit 86 by conductor 94~
The voltage on conductor 94 will follow, but will be less than, the voltage on ~tud 17. A second tap point on resi~tor 93 is connected to a third input of switchover circuit 86 by conductor 91. The volt~ge on conductor 91 will follow, but will be less thu~, the voltage on conductor 94 or stud 17. A tap point on resistor 45 is connected to ~ fourth input of switchover circuit 86 by conductor 82. The voltage on conductor R2 will typically b~ ~80 volt~ before the arc i9 struck and, after the arc i~ ~truck, will be between +80 volts and the volta~e un stud 17.
It will be appreciated that, although ~tud 17 is the reference point, the voltage on ~tud 17 closely follows the voltage on stud 18.
Turn now to Fig. 8, which is a schematic diagram of high/low voltage switchover circuit 86.
The +80 volt conductor 21 i8 connected to the input of a 15 volt regulator 390 and to one contact of a normally closed pair of contacts on relay 411. The 80 volt return conductor 23 i5 connected to the ground of regulator 390, the negative terminal of filter capacitor 392, one end of filter capacitor 393, the anode of 8.2 volt zener dlode 395, the ground input of op-amp 397, the cathode o~ blocking diode 410, one end of 0.1 m$crofarad smoothing capacitor 403, one end of 27 kilohm resistor 404, and one end of 250 kilohm potentiometer 405. The regulated +15 volt output of regulator 390 i~
connected by conductor 391 to the po~itiqe terminal of capacitor 392, the other end of capacitor 393, one . .

~ ~30~3.~

end of 608 kilohm re~istor 394, and to the VCC input of op-amp 397.
The other end of resistor 394 is connected by conductor 396 to the ca hode of zener diode 395 and the inverting input of op-amp 397. Re~istor 394 and zener diode 395 therefore form a voltage reference for op-amp 397.
The STICR/TIG stud 17 i~ connected by conductor 71 to the anode of blocking diode 400. The cathode of diode 400 i8 connected to conductor 402 through 82 kilohm resistor 401. Conductor 402 i3 connected to the non-inverting input of op-amp 397, the other end of capacitor 403, the other end of re istor 404, and to the wiper and other end of potentiometer 405. It will be appreciated that resi~tor 401 and 404 and potentiometer 405 form an adju~table voltage divider. Potentiometer 405 is ~et so that, when the voltage on conductor 71 ri~es above the desired voltage input level of feeder control 39, tha voltage on the non inverting input of op-amp 397 i~ greater than the voltage on its inverting input. This causes the output of op-amp 397 to rise to near the po~itive rail, conductor 391. Capacitor 403 smooth~ out the voltage on conductor 71 so that op-amp 397 doe~ not re~pond to the voltage transients causes by the switching action of transistors 56a-56f of Pig. 2.
~he output of op amp 397 is connected by conductor 406 to the cathode of surge protection diode 407 and to one end of the coil of relay 411.
The anode of diode 407 and the other end of the coil of relay 411 are connected by conductor 40a to the anode of blocking diode 410. The cathode of diode 410 is connected to the 80 volt ret~lrn conductor 23.
It is well known to place a ~urge protection diode ~07 in parallel with the coil of a r ... ..

gl)8~9 relay ~uch as relay 411 to absorb the voltage spike when relay 411 is de-energized. ~owever~ even with diode 407, conductor 406 may be momentarily pulled below conductor 23 by the forward voltage drop of diode 407. This momentary voltage pulse ha~ cau~ed erratic vperation of op-amp 397. The inventor ha~
not taken step~ to a3certain whether this erratic operation i~ cau~ed by a phantom diode or phantom tran~i~tor in op-amp 39~, or by ~ome other characteri~tic. However, the insertion of blocking diode 410 prevent~ thi~ momentary voltage pulse ~rom affecting op-amp 397 by isolating diode 407 and the coil of relay 411 from conductor 23~
The other contact of relay 411 i~ connect~ed by conductor 412 to one end of the coil of relay 420, and to one end of the coil of relay 421 through 3.3 : kilohm vol~age reduction re~istor 422. The other end of the coil of relay 420 and relay 421 are connected to 80 volt return conductor 23. Relays 420 and 421 each have a SPDT set of contacts.
The pole contact of relay 421 is connected by conductor 95 to the speed control input of feeder control 99. The normally clo~ed contact of relay 421 is connected by conductor 423 to one end of a 12 ~ilohm resistor 424. The other end of resi~tor 424 is connected to conductor 96. The normally open contact of relay 421 i~ connected by conductor 83 to one end of 11 kilohm potentiometer 84. The wiper and other end of potentiometer 84 are connected by conductor 85 to conductor 96. Conductor 96 i9 connected to the speed control return input of feeder control 99.
The normally clo~ed contact of relay 420 is connected by conductor 91 to the second tap point on re~istor 93 of Fig. 2. The normally open contact of relay 420 is connected to the pole of SP3T switch 425.

- 31 29~ 9 The first position of switch 425 connects the normally open contact o relay 420 to conductor 71, which is connected to STICR/TIG stud 17 of Fig. 2.
The second position of s~itch 425 connects the normally open contact of relay 420 to the fir~t tap point on resistor 93 of Fig. 2 through conductor 94.
The third po~ition of switch 425 connects the normally open contact of relay 420 to the tap point on arc su~taining resistor 45 of Fig. 2 through conductor 82.
The pole contact of relay 420 is connected by conductor 97 to the power voltage input of feeder control 99. The power voltage return of feeder control 99 ls connected by conductor 98 to the 80 volt return conductor 23.
Beore an arc is struck, the voltage on conductor 71 will be high, approximately 80 volts.
The vol~age on conductor 402 will ~herefore be greater than the trip voltage on conductor 396. The output of op-amp 397 will then be approximately 14 volts, thereby energi2ing relay 411. This de-energize~ relays 420 and 421. The second tap point on resistor 93 of Fig. 2 wiLl therefore be connected through conductor 91, the contact~ of relay 420, and conductor 97 to the power voltage input of feeder control 99. If feeder control 99 is specified for, say, a 40 volt input, then the second tap point of resistor 93 of Fig. 2 iq adjusted to give 40 volt~
on conductor 97 when the voltage on conductor 71 i~
80 volts.
After an arc is struck, the voltage on conductor 71 will drop to approximately 20 volts.
The voltage on conductor 402 will fall below the trip voltage on conductor 396 and relay 411 will be de-energized. Relays 420 and 421 will be connected through conductor 412 and the contacts of relay 411 to ~80 volt conductor 21 and therefore energized.
-The power voltage input of feeder control 99 will therefore be connected through conductor 97 and the normally open contacts of relay 420 to the pole of 5 switch 425. Switch 425 is placed in a selected position so that the voltage on conductor 97 is closest to the desired voltage, 40 volt~ in thi~
example. In the example given, the voltage on conductor 71 is only 20 voltq, 90 switch 425 positions 2 or 3 would be selected.
It will be appreciated that a differe~t feeder control 99, such a~ an arc feeder control, will require a different power input voltage, typically 20 volt~. In this case ~witch 425 would most probably be set to position 1, the voltage on - conductor 71. It will also be appreciated that, in some applications~ it may be desirable to provide another switch Yimilar to switch 425 to allow selection of different voltage points when relay 420 is energized. It will also be appreciated that it may be desirable to have switch 425 connected to potentiometer 405, or a switchable voltage divider network instead of potentiometer 405, so that a welder could select a position on switch 425 and automatically obtain the desired feeder voltage on conductor 97 and the corresponding desired re3istance o~ potentiometer 425.
In a similar manner, before an arc is struck, relay 421 will be de-energized, thereby connecting resistor 424 across the speed control inputs of feeder control 99 to yield a 910w, fixed wire feed speed to allow the welder to adjust the wire length.
Likewise, after an arc is struck, relay 421 will be energized, thereby connecting potentiometer 84 across the speed control inputs of feeder control ~2908 99 to yield the higher, welder-set, wire feed speed desired for the actual welding operationO It will be appreciated that potentiometer 84 may be contained within feeder control 99 so that the welder will hav~
convenient acces~ for adjuqting the wire feed speed.
Turn now to ~ig. 9, which i~ a schematic diagram of circuit power upplie3 3B. Input power is provided to power supplie~ 38 by +80 volt conductor 21 and 80 volt return conductor 23 which al~o serves as the circuit ground. Regulator 453 and filter capacitorq 455 and 456 provide regulated 14 volts t:o PIN 15 of modulator 457. Modulator 457 i~ a regulating pu}sewidth modulator such a~ the SG3524B
manufactured by Silicon General, Garden Grove, 15 California. The SG3524B, although almost identlcal in overall function to the SG3524/LM3524, is an improved device with additional protection feature~
and should be used, in circuit power supplies 38, instead of the SG3524 manufactured by Silicon Generalj or the LM3524 manufactured by National Semiconductor Corporation, Santa Clara, California.
Capacitor 462 provides additional filtering for the ~5 volt regulated output tPIN 16) of modulator 457.
Resi 9 tor 470 and capacitor 473 were selected to yield a frequency of approximately 15 kRz for the oscillator in modulator 457. This frequency is not critical but should be within the input frequency ~pecification of power tran~former 492.
Resistors 463 and 465 form a voltage divider which places approximately 2.5 volts on the non-inverting input ~PIN 2) of the error amplifier of modulator 457.
The inverting inpu~ (PIN 1) of thi~ error amplifier i8 connected to circuit ground 23 through an input current balancing resistor 467.
The compensation input ~PIN 9) of modulator 4S7 is connected to circuit ground 23 through one ~l ~9(~8~9 mlcrofarad capacitor 474. It will be appreciated that, as configured, and except as described b~low~
the error amplifier of modulator 457 will charge capacitor 474 to approximately 4 to 5 volt~, thereby yielding an output pul~ewidth of about 90 percent of the period of the frequency of the oscillator of modulator 457.
The shutdown input (PIN 10) of modulator 457 19 not used and is connected to circuit ground 23.
Overcurrent protection for transistor~ 477 and 484 i~ provided by the current limiter of modulator 457 . The CL- input (PIN 5 ) of modulator 457 i~ connected to circuit ground 23. The CL~ input (PIN 4) of modulator 457 is, a3 explained below, connected to sen~e the curren~ pas~ing through tran~i~tor~ 477 and 484.
If thi~ current exceed~ the Rafe value for - tran~istors 477 and 48~, the current limiter of modulator 457 begins discharging capacitor 474 and lowering the voltage on the compensation input of modulator 457. This reduces the pulsewidth, and therefore the current through transistors 477 and 484.
The collector (PINS 12 and 13) of the output txansi~tor~ of modulator 457 are connected to +14 volt conductor 454 through ll0 ohm current limiting re~istor 460. The emitter ~PIN 14) of the Q
output tran~istor of modulator 457 is connected by conductor 475 to the base of NPN Darlington power transistor 477, one end o 47 ohm base shunt re~istor 480, a~d one contact of SPDT switch 48l. The emitter (PIN 11) of the negated Q output transistor of modulator 457 is connected by conductor 476 to the base of NPN Darlington power transistor 484, one end o 47 ohm ba~e ~hunt re~i~tor 483, and the other ~Z9~319 6~
contact of switch 4Bl~
The emitter of tran~istor 477 i~ connected to the anode of blocking diode 4680 The collector of transi3tor 477 is connected by conductor 490 to the cathode of reverse voltage protection diode 469 and one end of the primary of power transformer 492.
The emitter of tran~ister 484 is connected to the anode of blocking diode 47~. The collector of transi3tor 484 is connected by conductor 491 to the cathode of reverse voltage protection diode 479 and to the other end of the primary of power tran~former 492. The center tap of the primary of power tran~former 492 is connected to +80 volt conductor 21, 15The pole of switch 481 is connected to one end of 250 ohm potentiometer 482. The wiper and other end of potentiometer 482, the other end of base shunt rPsistors 480 and 483, the cathode~ of blocking diodes 468 and 478, the anodes of reverse voltage 20protection diodes 469 and 479, and the CL+ input of modulator 457 are connected by conductor 487 to one end of 0.04 ohm curr~nt sensing resistor 488. The other end of resi~tor 488 i~ connected to 80v return . conductor 23.
25Ignoring, for a moment, switch 481, potentiometer 482, resistor~ 480 and 483, and diodes 468,469, 478 and 479, it will be appreciated that modulator 457, transistor~ 477 and 484, resistor 488, and tran~ormer 492 form a conventional inverter power supply. Current passing through transistors 477 and 484 passe~ throuyh resistor 488 and the resulting voltage developed acrocs res$stor 488 i5 applied to the CL+ input of modulator 457. ~he width of the output pul~es of modulator 457 is therefore reduced if the current exceed~ the desired value.
However, in a conventional inverter transistor3 477 and 484 must be a matched pair. If -the match i5 not good, then one of the tran~i~tors will draw significantly more current, heat up, draw even more current, and so on, with thermal runaway and.destruction of the transistor being the usual result, frequently followed by de~truction of the other transi~tor.
Switch 481 and potentiometer 482 allow the safe use of unmatched transistors for transistors 477 and 484. Assume that transistor 477 has the higher beta and draws more current than transi3tor 484.
Switch 481 i9 therefore placed in the fir~t po~ition so that the base of tran~istor 477 i~ ~hunted by potentiometer 4R2. Potentiometer 482 i~ then adjusted to absorb some of the output of modulator 457 so that tr~nsistor 477 receives less base drive current and therefore draw~ the same collector current as transistor 484. The transistor~ 477 and 484 are now "balanced. n If transistor 484 ha the higher beta, switch 481 would be placed in the second position and potentiometer 482 adju~ted to reduce the base and collector currents of transistor 484. It will be appreciated that resistor 460 limits the available output current of modulator 457 so that potentiometer 482 can draw base drive current away from transistor 477 or 484, as appropriate.
~ he value of potentiometer 482 is not critical but should be large enough that, when in the maximum resistance setting, negligible drive current is shunted away from the transistor base, and low enough so that the adjustment is not confined to a very small portion of the range of potentiometer 482.
It will be appreciated that a prior art method of balancing unmatched transistors is to connect a potentiometer between the bases of ~i X9~38~9 _ transi~tor~ 477 and 484 and connect the wiper of the potentiometer to conductor 487. However, this prior art method is unsati~factory because of two problem~.
The capacitance across the potentiometer couples the base drive ~ignal from the ba~e of tran~istor 477 to the base of transistor 484, and vice-versa, thus ~imultaneously turning on both tran~i~tors 477 and 484, an undesirable, ine~ficientf and often destructive re~ult. Al~o, there i~ a ~mall, but significant, resistance between the re~i~tance element and the wiper of the potentiometer~ Thi~
also allow~ cross-coupling of the ba~e drive signal~
and can lead to the ~ame unde~ired, inefficient, and destructive rexult. In the preferred embodiment, potentiometer 482 does not connect between the ba~e~
of tran3istors 477 and 484 and therefore there i3 no cross-coupling of the base drive signals.
A~su me no w t hat s wi tc h 48 1 and potentiometer 482 have been set so that transi~tors 477 and 484 are ~balance~." Assume now that, becau~e of different collector-to-case heat transfer characteristics, different case-to-heatsink heat transfer characteri~ticc~ or some other reason, tran~i~tor 477 becomes hotter than transi~tor 484.
Transi~tor 477 will then draw more collector current, which causes it to become even hotter, and so on, with thermal runaway and destruction again being the likely result.
Resistors 480 and 483 act to prevent this undesired event from occurring. It will be appreciated that re~istor 480 and 483 shunt base drive current away from transistors 477 and 484, re~pectively. Re~istor3 480 and 483 have a po~itive resistance-temperature coefficient. Resistor 480 i~
thermally connected 486 to the case of transistor 484.
Resistor 483 is thermally connected 495 to the case ~.2~

of tran~istor 477. If transistor 477 draws more collector current and becomes hotter than normal, it will heat up resistor 483 hotter than normal. This increase~ the resi~tance of resistor 483 so transi~tor 484 receive~ more ba~e drive current and draws more collector current so that tran3istor3 477 and 484 remain ~balanced.~ Al~o, the increa~e in current causes modulator 457 to decrease the output pul~e width, thereby limiting the total average current of tran~i~tors 477 and 484 to a safe value.
In the preferred embodiment, tran~i~tor~
477 and 484 are 350 volt, 20 amp, NPN Darlington power transistors, ~uch a3 the MJ10000, manufactured by Motorola, Inc., Phoenix, Arizona. The MJ10000 has lS an internal diode, with it~ cathode connected to the collector, and its anode connected to the emitt0r, ~o help prevent a negative collector voltage from destroying the transistor. ~owever, the inventor ha3 found that, in the preferred embodiment, the negative collector voltage caused by transformer 492 did de~troy transistors 477 and 484~ It is not known whet~er the failure~ occurred because the internal : diode of the MJ10000 wa~ unable to handle the current, the base-collector junction became forward biased and the MJ10000 operated in the inverted mode, or because of some other mechanism. The use of higher power, more expen~ive power transistors for 477 and 484 did solve the problem but increased the cost, Diodes 468, 469, 478 and 479 prevent the~e failure~ from occurring. Diodes 468 and 478 prevent transistors 477 and 484 from operating in the inverted mode and also prevent current from passing through their internal diodes. Diode~ 469 and 479 are then necessary to absorb the negative voltage spike~ produced by transformer 492. Dioda~ 469 and 479 should be fast recovery diodes. Thi9 allow~ the -u~e oE the lower power~ le~ expensive MJ10000'~ for tran8i~tor8 477 and 484.
Transformer 492 ha~ ~everal ~econdary winding3. One of the~e winding~ i~ connected to 14 volt rectifier & filter ~95. Rectifier ~ filter 495 provides ~14 volts on conductor 104 and -14 volt~ on conductor 106. The 14v return i9 conductor 10S.
~econd winding ig connected to 7 volt rectifier ~
filter 494O Rectifier & filter 494 provides +7 volts on conductor 100 and 7 volt3 on conductor 102. The 7v return i~ conductor 101. A third winding 1 connected to 15 volt rectifier & filter 493O
Rectifier & filter 493 provides a regulated ~lS volt~
on conductor 440. The 15v return is conductor Separate ~econdary winding4 and ~eparate retur~
conductors are used because the circuit~ that are powered operate at different circuit ~ground~
potential~. The de~ign of rectifier & filter 493, 494 and 495 i5 well known to thoRe ~killed in the art.
The +80 volt conductor 21 is connected to the positive terminal of filter capacitors 500 and 502, one ter~inal of high frequency filter capacitor 503, and to the "ground~ input of -13.3 volt voltage regulator 501. The -13.3 volt output of regulator 501 1~ connected to the other end of capacitor~ 502 and 503 and provides a regulated -13.3 volt3 on conductor 430~ The voltage on conductor 430 i3 a regulated -13.3 volt wlth re~pect to +90v conductor 21. Conductor 430 therefore ha~ an unregulated, nominal potential o +66.7 volt~ with re3pect to 80v return conductor 23.
The VE~ input of regulator 497 i3 connected to the anode of blocking diode 497. The cathode of diode 497 and the other end of capacitor 500 are connected by conductor 431 to one end of 10 ohm resistor 496. The other end of re~ or 496 is connected to 80v return conductor 23. Conductor 431 provide~ the unregulated +l volt signal which i~ u~ed S in pul~ewidth modulators 32 and 34 of Fig. 2. The total current through regulator 501 i~ appro~imately 100 milliamps and develop~ the 1 volt drop acro~
re~i~tor 496.
The preferred embodiment of the present 10 invention discloses a welding system with a centr~l welding power supply and numerous, high efficiency weld selector stations and also di3close~ a weld selector statîon with the capability of adju~ting arc characteristics, with automatic input voltage lS compensation, with the capabili~y of accommodating different types of welding, and with a power 3upply which utilizes inexpensive, unmatched, low power transistors. Although the preferred embodiment of the present invention has been de~cribed with particularity, it will be understood that numerous modifications and variation are possible.
Accordingly, the scope of the present invention is to be limited only by the claims below.

3~

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improved welding system for conducting a plurality of independent welding operations from a single welding power source, comprising:
central power supply, said central power supply having sufficient capacity to power a plurality of welding operations;
a plurality of weld parameter selection means, each said weld parameter selection means for providing one of said welding operations with electrical power having selectably variable characteristics for said welding operation, said weld parameter selection means being powered by said central power supply; and a plurality of connecting means for connecting said central power supply means to said weld parameter selection means.

2. An improved welding power supply, comprising:
power supply means for providing a source of DC power;
weld parameter selection means for providing welding power with a selectable set of predetermined electrical parameters desirable for a predetermined welding operation; and conducting means for connecting said power supply means to said weld parameter selection means, said conducting means having a predetermined set of electrical characteristics;
wherein said weld parameter selection means further comprises means for reducing particular effects of said electrical characteristic of said conducting means upon said predetermined electrical parameters of said welding power,

3. The improved welding power supply of Claim 2, wherein said predetermined electrical parameters of said welding power are selectively variable.

4. The improved welding power supply of Claim 2, wherein said predetermined electrical parameters of said welding power include:
pulse width and pulse rate.

5. The improved welding power supply of Claim 4, wherein said predetermined electrical parameters of said welding power further include pulse rise time.

6 The improved welding power supply of Claim 4, wherein said predetermined electrical parameters of said welding power further include pulse fall time.

7. The improved welding power supply of Claim 4, wherein said predetermined electrical parameters of said welding power further include average current.

8. The improved welding power supply of Claim 2 further comprising:
a plurality of said weld parameter selection means, each of said weld parameter selection means providing said welding power for one of a plurality of predetermined simultaneous welding operations; and a plurality of said conducting means.

9. The improved welding power supply of Claim 2 wherein said weld parameter selection means comprises:
power switching means responsive to a first control signal for selectably providing said welding power;
current sensing means connected in series with said power switching means for providing a second control signal responsive to the current through said power switching means; and pulsewidth modulator means connected to said current sensing means and responsive to said second control signal for providing said first control signal.

10. The improved welding power supply of Claim 9 wherein said pulsewidth modulator means is also responsive to an input voltage applied to said weld parameter selection means from said power supply means.

11. The improved welding power supply of Claim 10 wherein said pulsewidth modulator means is selectably responsive to said second control signal.

12. The improved welding power supply of Claim 9 and further comprising:
an inductance connected between said power switching means and the output of said weld parameter selection means.

13. An improved multi station welding system, comprising:
power supply means for providing a source of DC power;
a plurality of weld parameter selection means for providing welding power with a set of predetermined electrical parameters desirable for a predetermined one of a plurality of welding operations; and a plurality of conducting means for connecting said power supply means to said plurality of said weld parameter selection means, each of said conducting means having a set of electrical characteristics and connecting a particular one of said weld parameter selection means to said power supply means;
wherein each of said welding parameter selection means includes means for reducing particular effects of said electrical characteristics of said conducting means upon said predetermined electrical parameters of said welding power.

73 l4. An improved welding power supply for providing welding power which is compensated for variations in input voltage, comprising:
power supply means for providing a source of DC
power;
weld parameter selection means for providing said welding power with a selectable set of predetermined electrical parameters desirable for a predetermined welding operation; and conducting means for connecting said power supply means to said weld parameter selection means;
wherein said weld parameter selection means comprises:
a power input terminal;
voltage sensing means connected to said power input terminal and responsive to said input voltage at said power input terminal for providing an input voltage signal; and parameter selection means responsive to said input voltage signal for varying at least one of said predetermined electrical parameters of said welding power.

15. The improved welding power supply of Claim 14 wherein said weld parameter selection means further comprises:
rectifying means connected between said power supply means and said power input terminal; and energy storage means connected to said power input terminal;
wherein said rectifying means and said energy storage means provide for a less noisy input voltage to said power input terminal by reducing electrical transients from said power supply means.

16. In a welding system for conducting a plurality of independent welding operations, said welding system including a central power supply, said central power supply having sufficient capacity to power a plurality of said independent welding operation, a plurality of weld parameter selection means, each said weld parameter selection means having a power input terminal and being for providing a predetermined one of said welding operations with electrical power having selectably variable characteristics for said predetermined welding operation, each said weld parameter selection means being powered by said central power supply, and a plurality of conducting means for connecting said central power supply to said plurality of weld parameter selection means, an improvement to said weld parameter selection means for reducing adverse effects between said welding operations, said improvement comprising:
rectifying means having a first terminal connected to a predetermined said conducting means and a second terminal connected to said power input terminal of said weld parameter selection means; and energy storage means connected to said power input terminal;
said rectifying means and said energy storage means being for providing less noisy power to said power input terminal of said weld parameter selection means by reducing electrical transients placed upon said conducting means by other said weld parameter selection means.

17. In a welding system for conducting a plurality of independent welding operations, said welding system including a central power supply, said central power supply having sufficient capacity to power a plurality of said independent welding operations, a plurality of weld parameter selection means, each said weld parameter selection means having a power input terminal and being for providing a predetermined one of said welding operations with electrical power having selectably variable characteristics for said predetermined welding operation, each said weld parameter selection means being powered by said central power supply, and a plurality of conducting means for connecting said central power supply to said plurality of weld parameter selection means, an improvement to said weld parameter selection means for reducing adverse effects between said welding operations, said improvement comprising:
a rectifier having a first terminal connected to a predetermined said conducting means and a second terminal connected to said power input terminal of said weld parameter selection means, said rectifier being for providing less noisy power to said power input terminal of said weld parameter selection means by reducing electrical transients placed upon said conducting means by other said weld parameter selection means.

18. In a welding system for conducting a plurality of independent welding operations, said welding system including a central power supply, said central power supply having sufficient capacity to power a plurality of said independent welding operations, a plurality of weld parameter selection means, each said weld parameter selection means having a power input terminal and being for providing a predetermined one of said welding operations with electrical power having selectably variable characteristics for said predetermined welding operation, each said weld parameter selection means being powered by said central power supply, and a plurality of conducting means for connecting said central power supply to said plurality of weld parameter selection means, an improvement to said weld parameter selection means for reducing adverse effects between said welding operations, said improvement comprising:

an energy storage device connected to a predetermined said conducting means and said power input terminal of said weld parameter selection means, said energy storage means being for providing less noisy power to said power input terminal of said weld parameter selection means by reducing electrical transients placed upon said conducting means by other said weld parameter selection means.

19. An improved multi-station welding system for conducting a plurality of simultaneous welding operations, comprising:
power supply means for providing a source of DC
power; and a plurality of weld parameter selection means, each said weld parameter selection means being for providing welding power with a set of predetermined electrical parameter desirable for a predetermined one of a said plurality of welding operations; and each said weld parameter selection means comprising:
a power input terminal;
rectifying means connected between said power supply means and said power input terminal;
energy storage means connected to said power input terminal;
current limiting means;
first switching means responsive to a first drive signal for providing said welding power through said current limiting means to an output terminal, said welding power comprising an output voltage and an output current;
arc sustaining means connected in parallel with said first switching means for providing a predetermined minimum welding power, said predetermined minimum welding power being capable of sustaining an arc;
driving means responsive to a second drive signal and a shutdown signal for selectably providing said first drive signal having a first set of electrical parameters to said first switching means;
feedback current sensing means for providing a current feedback signal responsive to said output current;
first oscillation means for providing said second drive signal, said second drive signal having a second set of electrical parameters, at least one of said second set of electrical parameters being responsive to a selected one of the following signals: said current feedback signal, a first manually adjustable signal, and both said current feedback signal and said first manually adjustable signal;
average current sensing means for providing said shutdown signal in response to the average value of said output current exceeding a predetermined maximum average output current value;
peak current sensing means for providing said shutdown signal in response to the peak value of said output current exceeding a predetermined maximum peak output current value; and first output voltage sensing means for providing said shutdown signal in response to said output voltage being below a predetermined voltage, said predetermined minimum voltage being less than an arc sustaining voltage required to sustain an arc;
a plurality of conducting means for connecting said power supply means to said plurality of said weld parameter selection means, each of said conducting means having a set of electrical characteristics and connecting a particular one of said weld parameter selection means to said power supply means;
wherein each of said welding parameter selection means includes means for reducing particular effects of said electrical characteristics of said conducting means upon said predetermined electrical parameters of said welding power.

20. An improved multi-station welding system for conducting a plurality of simultaneous welding operations, comprising:
power supply means for providing a source of DC
power; and a plurality of weld parameter selection means, each said weld parameter selection means being for providing welding power with a set of predetermined electrical parameters desirable for a predetermined one of a said plurality of welding operations; and each said weld parameter selection means comprising:
a power input terminal;
rectifying means connected between said power supply means and said power input terminal;
current limiting means;
first switching means responsive to a first drive signal for providing said welding power through said current limiting means to an output terminal, said welding power comprising an output voltage and an output current;
arc sustaining means connected in parallel with said first switching means for providing a predetermined minimum welding power, said predetermined minimum welding power being capable of sustaining an arc;
driving means responsive to a second drive signal and a shutdown signal for selectably providing said first drive signal having a first set of electrical parameters to said first switching means;
feedback current sensing means for providing a current feedback signal responsive to said output current;
first oscillation means for providing said second drive signal, said second drive signal having a second set of electrical parameters at least one of said second set of electrical parameters being responsive to a selected one of the following signals: said current feedback signal, a first manually adjustable signal, and both said current feedback signal and said first manually adjustable signal;

average current sensing means for providing said shutdown signal in response to the average value of said output current exceeding a predetermined maximum average output current value;
peak current sensing means for providing said shutdown signal in response to the peak value of said output current exceeding a predetermined maximum peak output current value; and first output voltage sensing means for providing said shutdown signal in response to said output voltage being below a predetermined minimum voltage, said predetermined minimum voltage being less than an arc sustaining voltage required to sustain an arc;
a plurality of conducting means for connecting said power supply means to said plurality of said weld parameter selection means, each of said conducting means having a set of electrical characteristics and connecting a particular one of said weld parameter selection means to said power supply means' wherein each of said welding parameter selection means includes means for reducing particular effects of said electrical characteristics of said conducting means upon said predetermined electrical parameters of said welding power.

21. An improved multi-station welding system for conducting a plurality of simultaneous welding operations comprising:
power supply means for providing a source of DC
power; and a plurality of weld parameter selection means, each said weld parameter selection means being for providing welding power with a set of predetermined electrical parameters desirable for a predetermined one of a said plurality of welding operations; and each said weld parameter selection means comprising:
a power input terminal;
energy storage means connected to said power input terminal;
current limiting means;
first switching means responsive to a first drive signal for providing said welding power through said current limiting means to an output terminal, said welding power comprising an output voltage and an output current;
arc sustaining means connected in parallel with said first switching means for providing a predetermined minimum welding power, said predetermined minimum welding power being capable of sustaining an arc;
driving means responsive to a second drive signal and a shutdown signal for selectably providing said first drive signal having a first set of electrical parameters to said first switching means;
feedback current sensing means for providing a current feedback signal responsive to said output current;
first oscillation means for providing said second drive signal, said second drive signal having a second set of electrical parameters, at least one of said second set of electrical parameters being responsive to a selected one of the following signals: said current feedback signal, a first manually adjustable signal, and both said current feedback signal and said first manually adjustable signal;
average current sensing means fox providing said shutdown signal in response to the average value of said output current exceeding a predetermined maximum average output current value;
peak current sensing means for providing said shutdown signal in response to the peak value of said output current exceeding a predetermined maximum peak output current value; and first output voltage sensing means for providing said shutdown signal in response to said output voltage being below a predetermined minimum voltage, said predetermined minimum voltage being less than an arc sustaining voltage required to sustain an arc;
a plurality of conducting means for connecting said power supply means to said plurality of said weld parameter selection means, each of said conducting means having a set of electrical characteristics and connecting a particular one of said weld parameter selection means to said power supply means;
wherein each of said welding parameter selection means includes means for reducing particular effects of said electrical characteristics of said conducting means upon said predetermined electrical parameters of said welding power.

22. In a welding system for conducting a plurality of independent welding operations said welding system including a central power supply, said central power supply having sufficient capacity to power a plurality of said independent welding operations, a plurality of weld parameter selection means, each said weld parameter selection means having a power input terminal and being for providing a predetermined one of said welding operations with electrical power having selectably variable characteristics for said predetermined welding operation, each said weld parameter selection means being powered by said central power supply, and a plurality of conducting means for connecting said central power supply means to said plurality of weld parameter selection means, an improvement to said weld parameter selection means for reducing adverse effects between said welding operations, said improvement comprising:
rectifying means having a first terminal connected to a predetermined said conducting means and a second terminal connected to said power input terminal of said weld parameter selection means;
energy storage means connected to said power input terminal, said rectifying means and said energy storage means being for providing less noisy power to said power input terminal of said weld parameter selection means by reducing electrical transients placed upon said conducting means by other said weld parameter selection means; and input voltage sensing means responsive to a voltage at said power input terminal for providing an input voltage signal.;
wherein said weld parameter selection means is responsive to said input voltage signal for varying at least one of said selectably variable characteristics of said electrical power.

23. An improved welding power supply, comprising:
power supply means for providing a source of DC
power; and weld parameter selection means for providing welding power with electrical parameters desirable for a predetermined welding operation, said weld parameter selection means comprising:
a power input terminal;
input voltage sensing means responsive to an input voltage present at said power input terminal for providing an input voltage signal;
current limiting means;
first switching means responsive to a first drive signal for providing said welding power through said current limiting means to an output terminal, said welding power comprising an output voltage and an output current;
arc sustaining means connected in parallel with said first switching means for providing a predetermined minimum welding power, said predetermined minimum welding power being capable of sustaining an arc;
driving means responsive to a second drive signal and a shutdown signal for selectably providing said first drive signal having a first set of electrical parameters to said first switching means;
feedback current sensing means for providing a current feedback signal responsive to said output current;
first oscillation means for providing said second drive signal, said second drive signal having a second set of electrical parameters, at least one of said second set of electrical parameters being responsive to said input voltage signal and a selected one of the following signals; said current feedback signal, a first manually adjustable signal, and both said current feedback signal and said first manually adjustable signal;
average current sensing means for providing said shutdown signal in response to the average value of said output current exceeding a predetermined maximum average output current value;
peak current sensing means for providing said shutdown signal in response to the peak value of said output current exceeding a predetermined maximum peak output current value; and first output voltage sensing means for providing said shutdown signal in response to said output voltage being below a predetermined minimum voltage, said predetermined minimum voltage being less than an arc sustaining voltage required to sustain an arc.

24. The improved welding power supply of Claim 23, wherein said first oscillation means comprises:
an oscillator for providing said second drive signal, said second drive signal having a pulse rate and a pulse width, said second set of electrical parameters comprising said pulse rate and said pulse width;
pulse rate control means connected to said oscillator for controlling said pulse rate; and pulse width control means connected to said oscillator for controlling said pulse width;
wherein said second set of said electrical parameters of said welding power are selectively variable.

25. The improved welding power supply of claim 24, wherein said driving means comprises:
amplification means responsive to said second drive signal for providing said first drive signal, said first drive signal having a pulse rise time and a pulse fall time, said first set of said electrical parameters of said welding power comprising said pulse rise time and said pulse fall time;
pulse rise time control means connected to said amplification means for controlling said pulse rise time; and, pulse fall time control means connected to said amplification means for controlling said pulse fall time.

26. The improved welding power supply of Claim 23, wherein said driving means comprises:
amplification means responsive to said second drive signal for providing said first drive signal, said first drive signal having a pulse rise time and a pulse fall time, said first set of said electrical parameters of said welding power comprising said pulse rise time and said pulse fall time;
pulse rise time control means connected to said amplification means for controlling said pulse rise time; and pulse fall time control means connected to said amplification means for controlling said pulse fall time;
wherein said first set of said electrical parameters of said welding power are selectively variable.

27. The improved welding power supply of Claim 23 further comprising:
a plurality of said weld parameter selection means, each of said weld parameter selection means providing said welding power for one of a plurality of predetermined simultaneous welding operations;
wherein each said weld parameter selection means further comprises:
rectifying means connected between said power supply means and said power input terminal; and energy storage means connected to said power input terminal;
wherein said rectifying means and said energy storage means reduce adverse effects of other said welding operations on said welding operation supported by said weld parameter selection means by reducing electrical transients caused by said other said welding operations, and reduce adverse effects on said other said welding operations by reducing electrical transients caused by said welding operation supported by said weld parameter selection means.

28. The improved welding power supply of Claim 23 wherein said first oscillation means is also responsive to an input voltage applied to said weld parameter selection means from said power supply means for compensating for variations in said input voltage.

29. The improved welding power supply of Claim 23 wherein said current limiting means comprises an inductor.

30. The improved welding power supply of Claim 23 wherein said first switching means comprises a bipolar transistor.

31. The improved welding power supply of Claim 23 wherein said first switching means comprises a field-effect-transistor.

32. The improved welding power supply of Claim 23 and further comprising:
wire feed speed control means responsive to said output voltage for providing a first wire feed speed for arc striking operations and a second wire feed speed for welding operations, said second wire feed speed being greater than first wire feed speed.

33. The improved welding power supply of Claim 32 wherein said wire feed speed control means comprises:
a first resistance for setting said first wire feed speed;
a second resistance for setting said second wire feed speed;
speed control means responsive to an external resistance; and second output voltage sensing means responsive to said output voltage for connecting said first resistance to said speed control means when said output exceeds a predetermined value and, otherwise, connecting said second resistance to said speed control means.

34. The improved welding power supply of Claim 33 wherein said predetermined value is less than an arc striking voltage and greater than said arc sustaining voltage.

35. The improved welding power supply of Claim 23 and further comprising:
second oscillation means for providing a third drive signal, said third drive signal having a third set of electrical parameters, at least one of said third set of electrical parameters being responsive to a second manually adjustable signal; and second switching means for selectively providing said third drive signal to said first oscillation means for causing said first drive signal to be a chopped pulse signal, providing said third drive signal to said driving means for causing said first drive signal to be a pulse signal, and providing both said second drive signal and said third drive signal to said driving means for causing said first drive signal to alternate between a low frequency pulse signal and a high frequency pulse signal;
wherein said first oscillation means is selectively responsive to said third drive signal for varying a predetermined one of said second set of electrical parameters.

36. The improved welding power supply of Claim 35 wherein said second oscillation means comprises:
an oscillator for providing said third drive signal, said third drive signal having a pulse rate and a pulse width, said third set of electrical parameters comprising said pulse rate and said pulse width;
pulse rate control means connected to said oscillator for controlling said pulse rate; and pulse width control means connected to said oscillator for controlling said pulse width;
wherein said third set of said electrical parameters are selectively variable.

37. The improved welding power supply of Claim 35 wherein said second oscillation means is also responsive to an input voltage applied to said weld parameter selection means from said power supply means for compensating for variations in said input voltage.

38. An improved multi-station welding system for conducting a plurality of simultaneous welding operations, comprising:
power supply means for providing a source of DC
power; and a plurality of weld parameter selection means, each said weld parameter selection means being for providing welding power with electrical parameters desirable for a predetermined one of said plurality of welding operations, each said weld parameter selection means comprising:
a power input terminal;
rectifying means connected between said power supply means and said power input terminal;
energy storage means connected to said power input terminal;
input voltage sensing means responsive to an input voltage present at said power input terminal for providing an input voltage signal;
current limiting means;
first switching means responsive to a first drive signal for providing said welding power through said current limiting means to an output terminal, said welding power comprising an output voltage and an output current;
arc sustaining means connected in parallel with said first switching means for providing a predetermined minimum welding power, said predetermined minimum welding power being capable of sustaining an arc;
driving means responsive to a second drive signal and a shutdown signal for selectably providing said first drive signal having a first set of electrical parameters to said first switching means;
feedback current sensing means for providing a current feedback signal responsive to said output current;

first oscillation means for providing said second drive signal, said second drive signal having a second set of electrical parameters, at least one of said second set of electrical parameters being responsive to said input voltage signal and a selected one of the following signals; said current feedback signal, a first manually adjustable signal, and both said current feedback signal and said first manually adjustable signal;
average current sensing means for providing said shutdown signal in response to the average value of said output current exceeding a predetermined maximum average output current value;
peak current sensing means for providing said shutdown signal in response to the peak value of said output current exceeding a predetermined maximum peak output current value, and first output voltage sensing means for providing said shutdown signal in response to said output voltage being below a predetermined minimum voltage, said predetermined minimum voltage being less than an arc sustaining voltage required to sustain an arc.

39. An improved welding power supply for providing welding power which is compensated for variations in input voltage, comprising:
power supply means for providing a source of DC
power;
weld parameter selection means for providing said welding power with a selectable set of predetermined electrical parameters desirable for a predetermined welding operation; and conducting means for connecting said power supply means to said weld parameter selection means;
wherein said weld parameter selection means comprises:
a power input terminal;
voltage sensing means connected to said power input terminal and responsive to said input voltage at said power input terminal for providing an input voltage signal; and parameter selection means responsive to said input voltage signal for varying at least one of said predetermined electrical parameters of said welding power.

40. The improved welding power supply of Claim 39 wherein said weld parameter selection means further comprises:
rectifying means connected between said power supply means and said power input terminal, and energy storage means connected to said power input terminal;
wherein said rectifying means and said energy storage means provide for a less noisy input voltage to said power input terminal by reducing electrical transients from said power supply means.

41. An improved pulsed welding power supply, comprising:
pulse generating means for providing a pulsed output signal with predetermined electrical parameters;
a power source;
a power supply output terminal; and at least one semiconductor device responsive to said pulsed output signal for selectively connecting and disconnecting said power source and said power supply output terminal, said semiconductor device having a first terminal connected to said power source, a second terminal connected to said power supply output terminal, and a third terminal for selectively varying the conductance between said first terminal and said second terminal, said third terminal being connected to said pulse generating means.

42. The improved pulsed welding power supply of Claim 41 wherein said predetermined electrical parameters are selectively variable.

43. The improved pulsed welding power supply of Claim 41 wherein said predetermined electrical parameters comprise:
pulse rate and pulse frequency.

44. The improved pulsed welding power supply of Claim 43 wherein said predetermined electrical parameters further comprise:
rise time;
fall time; and current amplitude.

45. The improved pulsed welding power supply of Claim 42 wherein said predetermined electrical parameters comprise:
pulse rate and pulse frequency.

46. The improved pulsed welding power supply of Claim 42 wherein said predetermined electrical parameters further comprise:
rise time and fall time.

47. The improved pulsed welding power supply of Claim 41 and further comprising:
surge absorbing means connected between said first terminal and said second terminal of said semiconductor device for protecting said semiconductor device from voltage surges.

48. The improved pulsed welding power supply of Claim 41 further comprising:
power absorption means connected to said second terminal terminal of said semiconductor device for providing a predetermined minimum load to said semiconductor device.

49. The improved pulsed welding power supply of Claim 47 and further comprising:
power absorption means connected to said second terminal of said semiconductor device for protecting said semiconductor device from voltage surges.

50. The improved pulsed welding power supply of Claim 41 and further comprising:
a diode connected to said second terminal of said semiconductor device to protect said semiconductor device from the application of a reverse polarity voltage to said second terminal, said diode being electrically oriented to become conducting when said reverse polarity voltage is applied to said second terminal.

51. The improved pulsed welding power supply of Claim 41 and further comprising:
voltage sensing means connected to said power source for providing a first control signal responsive to the output voltage of said power source;
wherein said pulse generating means is connected to said voltage sensing means for varying at least one of said predetermined electrical parameters in response to said first control signal.

52. The improved pulsed welding power supply of Claim 51 wherein pulsewidth of said pulsed output signal is one of said predetermined electrical parameters and same is decreased in response to an increase in said output voltage.

53. The improved pulsed welding power supply of Claim 41 and further comprising:
current sensing means connected between said power source and said first terminal of said semiconductor device for providing a second control signal responsive to the current flowing between said power source and said first terminal of said semiconductor device;
wherein said pulse generating means is connected to said current sensing means for varying at least one of said electrical parameters in response to said second control signal.

54. The improved pulsed welding power supply of Claim 53 wherein pulsewidth of said pulsed output signal is decreased in response to an increase in said current.

55. The improved pulsed welding power supply of Claim 41 and further comprising:
current sensing means connected between said power source and said first terminal of said semiconductor device for providing a first control signal responsive to the current flowing between said power source and said first terminal of said semiconductor device; and voltage sensing means connected to said power source for providing a second control signal responsive to the output voltage of said power supply means;
wherein said pulse generating means is connected to said voltage sensing means and said current sensing means for varying at least one of said electrical parameters in response to said first control signal and said second control signal.

56. The improved pulsed welding power supply of Claim 55 wherein pulsewidth of said pulsed output signal is decreased in response to an increase in said output voltage and to an increase in said current.

57. The improved pulsed welding power supply of Claim 41 wherein said semi-conductor device is a bipolar transistor.

58. The improved pulsed welding power supply of Claim 41 wherein said semi-conductor device is a field-effect-transistor.

59. An electronic welder, comprising:
a power supply for providing operating voltage and operating current;
a voltage divider responsive to said operating voltage for providing a first signal;
control means for providing a second signal;
a pulsewidth modulator for providing a third signal having a pulsewidth responsive to said first signal and said second signal; and output means responsive to said third signal for providing welding power to a welding operation.

60. The electronic welder of Claim 59 wherein:
said voltage divider comprises a first resistor, a second resistor, and a third resistor, connected in series, said second resistor having a resistance significantly less than the resistance of said voltage divider, and said first signal is the voltage developed across said second resistor.

61. The electronic welder of Claim 60 wherein:
said pulsewidth modulator comprises an operational amplifier having a differential input; and said voltage across said second resistor is applied to said differential input.

62. The electronic welder of Claim 61 wherein said operational amplifier is a transconductance amplifier operating in an open loop configuration.

63. The electronic welder of Claim 62 and further comprising:
a load connected to the output of said transconductance amplifier, said load having a resistance such that said transconductance amplifier operates in a linear region when said voltage across said second resistor is applied to said differential input.

64. The electronic welder of Claim 59 wherein said pulsewidth modulator decreases said pulsewidth in response to an increase in said operating voltage.

65. The electronic welder of Claim 59 wherein said voltage divider is connected between a regulated voltage and an unregulated voltage.

66. The electronic welder of Claim 65 wherein said pulsewidth modulator comprises a voltage regulator for providing said regulated voltage.

67. An electronic welding station for use with an external power supply, comprising:
input terminals for receiving operating voltage and operating current from said external power supply;
a voltage divider responsive to said operating voltage for providing a first signal;
control means for providing a second signal;
a pulsewidth modulator for providing a third signal having a pulsewidth responsive to said first signal and said second signal; and output means responsive to said third signal for providing welding power to a welding operation.

68. The welding station of Claim 67 wherein:
said voltage divider comprises a first resistor, a second resistor, and a third resistor, connected in series, said second resistor having a resistance significantly less than the resistance of said voltage divider; and said first signal is the voltage developed across said second resistor.

69. The welding station of Claim 68 wherein:
said pulsewidth modulator comprises an operational amplifier having a differential input; and said voltage across said second resistor is applied to said differential input.

70. The welding station of Claim 69 wherein said operational amplifier is a transconductance amplifier operating in an open loop configuration.

71. The welding station of Claim 70 and further comprising:
a load connected to the output of said transconductance amplifier, said load having a resistance such that said transconductance amplifier operates in a linear region when said voltage across said second resistor is applied to said differential input.

72. The welding station of Claim 67 wherein said pulsewidth modulator decreases said pulsewidth in response to an increase in said operating voltage.

73. The welding station of Claim 67 wherein said voltage divider is connected between a regulated voltage and an unregulated voltage.

74. The welding station of Claim 73 wherein said pulsewidth modulator comprises a voltage regulator for providing said regulated voltage.

75. The welding station of Claim 74 wherein said unregulated voltage is derived from a predetermined one of said input terminals.

76. For use with an electronic welder which provides a pulsed output having a selectable pulsewidth, a method for improving the performance of said electronic welder, comprising the steps of:
monitoring an input voltage of said electronic welder, and changing said pulsewidth in response to changes in said input voltage.

77. The method of Claim 76 wherein said step of changing comprises decreasing said pulsewidth in response to an increase in said input voltage.

78. The method of Claim 77 wherein said step of monitoring comprises measuring the voltage across a predetermined resistor in a voltage divider connected to said input voltage, said predetermined resistor having a resistance which is significantly less than the resistance of said voltage divider,