US 3651753 A
An electric cooker for the direct heating of food by the passage of alternating current therethrough has a two-way electronic valve, triggered at the beginning of each half-cycle of its power supply, connected in series therewith. A control voltage derived from a series resistor, varying with the magnitude of the load current, blocks the trigger circuit of the valve during half-cycles of a given polarity whenever the current exceeds a predetermined threshold, the trigger circuit being unswitchable during half-cycles of the opposite polarity so that only complete cycles are suppressed in the case of overload.
Description (OCR text may contain errors)
[ 51 Mar. 28, 1972 ELECTRIC COOKER WITH AUTOMATIC CURRENT CONTROL  Inventor: Walter Schmidt, Lindau-Bodolz, Germany  Assignee: lntertrade Warenverkehrsgesellschalt Hild & (19;! i ml  Filed: Sept. 17, 1970  Appl.No.: 73,030
 Foreign Application Priority Data Sept. 17, 1969 Austria ..A8824/69 June 15, 1970 Austria ..A5394/70  US. Cl.. ..99/358, 99/337, 219/501, 219/509, 323/4, 323/9  Int. Cl. ..H05b 7/00  Field of Search ..99/358, 337; 219/284, 327, 219/334, 379, 412, 492, 501, 509; 323/1, 4, 9, 225 C, 24, 38
 References Cited UNITED STATES PATENTS 3,319,152 5/1967 Pinckaers ..L ..323/225 Zero- Detector 2,642,794 6/1953 Spiess et al. ..99/358 3,197,691 7/1965 Gilbert 323/225 C 3,356,784 12/1967 Bertioli et al. ....323/24 X 3,486,042 12/1969 Watrous ..323/225 C 3,514,580 5/1970 Brockway ..219/501 X Primary Examiner-A. D. Pellinen Attorney-Karl F. Ross [5 7] ABSTRACT An electric cooker for the direct heating of food by the passage of alternating current therethrough has a two-way electronic valve, triggered at the beginning of each half-cycle of its power supply, connected in series therewith A control voltage derived from a series resistor, varying with the magnitude of the load current, blocks the trigger circuit of the valve during half-cycles of a given polarity whenever the current exceeds a predetermined threshold, the trigger circuit being unswitchable during half-cycles of the opposite polarity so that only complete cycles are suppressed in the case of overload.
8 Claims, 4 Drawing Figures ELECTRIC COOKER WITH AUTOMATIC CURRENT CONTROL can be cooked with the aid of an alternating current applied thereto for a certain length of time, e.g., by direct contact with two electrodes or by way of an electrolytic solution. The use of direct current for this purpose is impractical since it might lead to electrolytic decomposition of salt and other ingredients.
The rate of heating, which determines the necessary cooking time, is a function of the electrical resistance of the goods to be treated and is thus subject to considerable variations.
With the supply voltage substantially constant, as is normally .the case, the current is directly proportional to the conductivity of the goods, as is the resulting heating effect. This represents an inconvenience since it prevents the selection of specific treatment times as a measure for the intensity of the cooking, a technique readily applicable to other cooking methods utilizing a substantially constant source of heat.
In order to minimize this inconvenience, and at the same time to provide safeguards against overcooking by an excessive current flow, it has already been proposed to insert a relatively high resistance in series with the load, i.e., with the cooking utensil proper. A disadvantage of such a protective resistance, however, is the expenditure of large amounts of energy and the problem of dissipating the resulting waste heat.
The general object of my present invention, therefore, is to provide a self-regulating power supply which eliminates these drawbacks and which substantially stabilizes the current flow through a load of unknown resistance, especially a cooker, with protection against overload and automatic compensation for changes in load resistance and/or supply voltage.
A more particular object is to provide, in a system in which the presence of direct-current components would be detrimental, means for effectively suppressing such components in controlling the flow of an alternating current.
In accordance with my presentinvention, the alternating current supplied by a suitable source, such as the usual utility mains (operating at 50 or 60 cycles per second), is passed through the load by way of a normally open electronic circuit breaker in series with the load, this circuit breaker being periodically closable in the rhythm of load current by trigger pulses generated by a network connected to the current source. A current-responsive sensor in the load circuit disables the trigger-pulse generator whenever the load current exceeds a predetennined threshold, thereby maintaining the average current flow substantially constant.
In order to prevent an unbalanced interruption of the current flow which would give rise to objectionable DC components, particularly when the system is used for direct cooking as outlined above, I prefer to use as a trigger-pulse generator a network which is unswitchable during half-cycles of a given polarity so that, in the presence of an overload, the interruption and subsequent restoration of the current flow can take place only during half-cycles of the opposite polarity, preferably at the beginning of such half-cycles, whereby only entire cycles are suppressed. Depending on the extent of the momentary overload, the suppression may apply to a single cycle at a time or to several cycles in a row. With proper timing, the average current will remain approximately constant even if the instantaneous load current during individual cycles varies by as much as 100 percent.
A convenient way of preserving the same time position for the switching-on and switching-off points of the trigger-pulse generator in different half-cycles, with the further advantage of minimizing transients interfering with radio reception in the vicinity, is to utilize a zero-voltage detector of a type known per se to make the trigger pulses substantially coincide with the crossover of the supply voltage from one polarity to the other. Such detectors are used, for example, in a conventional integrated-circuit module marketed by General Electric under the designation PA424.
A component of this nature or its equivalent may be used to trigger a gate-controlled two-way electronic valve of the type known as TRIAC serving as the normally open circuit breaker. Other electronic gates of this description include thyristors or solid-state controlled rectifiers.
The above and other features of my invention will be described in detail hereinafter with reference to the accompanying drawing in which:
FIG. 1 is a block diagram of a power supply for an electric cooker in accordance with the invention;
FIG. 2 is a similar block diagram for a modification of the system of FIG. 1;
FIG. 3 is a more detailed circuit diagram of an embodiment similar to that shown in FIG. 2; and
FIG. 4 is a circuit diagram for another embodiment.
The system shown in FIG. 1 comprises a source of alternating current, represented by a pair of terminals A & B, defining between them a load circuit 4 including a cooker l for the direct heating of foods by the current flowing in that circuit, a circuit breaker 2 in series with the load 1 and a sensor 5 for determining the magnitude of this current flow. A branch line 7 terminates at a controller 3 which periodically closes the circuit breaker 2 in the rhythm of the supply voltage either once per cycle or during every half-cycle to energize the load I. In the normal operation, the open period of the circuit breaker 2 may be so short as to result in virtually continuous energization of the load.
Sensor 5 works into a processor 6 which acts as a rectifier and storage circuit delivering to the controller 3 a signal voltage to be compared with a predetermined but preferably adjustable reference voltage. If the signal voltage exceeds the threshold thus established, controller 3 is inhibited from closing the circuit breaker 2. With the the load circuit now open, the sensor output disappears so that the overload signal from processor 6 decays at a rate depending upon the storage capacity of this circuit. After one or more cycles, therefore, controller 3 resumes its normal operation of periodically closing and opening the circuit breaker 2.
In FIG. 2 the elements already discussed in connection with FIG. 1 have been given the same designations with the addition of a prime mark. Unit 2' is here shown as a TRIAC, with unit 5' essentially consisting of a low-ohmic resistor in series with the TRIAC and the load 1'. Processor 6', having its input connected across the resistor 5', feeds the controller 3 which, however, is here shown to act upon the circuit breaker not directly but through a trigger circuit 8 associated with TRIAC 2! Circuit 8 normally delivers to TRIAC 2' a series of trigger pulses P which, as more fully described hereinafter with reference to FIG. 3, can be turned on and off by switching signals S in the output 9 of controller 3 only if these trigger pulses occur during half-cycles of a given polarity. During half-cycles of the opposite polarity, trigger circuit 8 is unswitchable; the corresponding trigger pulses are invariably generated if the TRIAC 2' conducts during the immediately preceding half-cycles. Thus, the load current traversing the circuit 4' is interrupted only for entire cycles.
Trigger circuit 8 may be a module of the type PA424 referred to above and described in detail hereinbelow.
FIG. 3 shows a system generally similar to that of FIG. 2, including a load in the form of a cooker 1 connected in series with a TRIAC 10, a trigger circuit 11 of the type PA424, a low-ohmic series resistor 12 forming part of a sensing circuit to determine the magnitude of the load current, a processor 50 receiving the output voltage of sensing resistor 12 by way of a transformer 51, and a control circuit generally designated 60 for switching the trigger circuit 11.
Processor 50 comprises a rectifier bridge 14 working into an integrating network 84 which feeds a potentiometer 17 connected between a conductor 102 and a bus bar 107 originating at terminal A. Another bus bar 105, tied to terminal B through the intermediary of a resistor 52, is connected to conductor 102 through a diode 106 so poled that this conductor will be driven negative during negative half-cycles of the supply voltage on terminal B; for purposes of the following discussion, references to positive and negative half-cycles relate to the potential of that terminal relative to terminal A, assumed to be grounded.
The internal structure of module 11, known per se, includes a zero-crossing detector 100 connected between bus bars 105 and 107, a differential amplifier 101 with a blocking lead 13 terminating at the base of one of its transistors, and a Darlington-type output transistor 103 which is normally triggered into conductivity by detector 100 whenever the alternating voltage on bus bars 105 and 107 goes through zero; amplifier 103, when thus conducting, draws gate current via a lead 104 from TRIAC to trigger the same into a conductive condition which persists for the remainder of the half-cycle.
A condenser 53 is connected across bus bar 107 and conductor 102 so as to charge during negative half-cycles, this charge being available in the following positive half-cycles to supply the gate current for TRIAC 10 so as to trigger the latter into conductivity. Amplifier 103, however, can draw this gate current only as long as blocking lead 13 carries negative voltage with reference to conductor 102; when this blocking lead goes positive, the left-hand half of differential amplifier 101 conducts and inhibits the zero-crossing detector 100.
Lead 13 is connected to lead 102 through a voltage divider including resistors 61, 62 and 63, the junction of the two latter resistors being tied to the base of an NPN-transistor 21 fortning one stage of a bistable multivibrator or flip-flop whose other stage is a similar transistor 22. The base of transistor 22 -is normally biased positive by a circuit connected across bus bars 105 and 107, this circuit including a collector resistor 64 of transistor 21, a coupling resistor 65, a diode 66 and a condenser 23. The junction of this diode and condenser is connected to bus bar 107 through a resistor 67 and, in parallel therewith, through a voltage divider consisting of a pair of resistors 68, 69, the collector of transistor 22 being tied to a common terminal of resistors 61, 62, 68 and 69. Another diode 70 is inserted between conductor 102 and the base of transistor 22 with a polarity blocking the transmission of negative potential from the conductor to that base.
A monostable circuit of the Schmitt Trigger type, consisting of two transistors 18 (N PN) and 19 (PNP) conditions the flipflop 21, 22 for possible switchover. Transistor 18 has its collector connected via a diode 71 to the collector of transistor 22 and by way of a resistor 72 to the bus bar 107 as well as to the emitter of transistor 19 whose collector is joined to that of an NPN transistor 20 through a resistor 85. A further NPN- transistor 15 has its collector tied through a diode 74 to the base of transistor 19 which is grounded for high frequencies through a condenser 75. Transistor 15 has its base and its col lector connected to the collector of transistor 18 by way of two resistors 76 and 77, respectively, the latter resistor being in series with a further capacitor 16. The base of transistor 15 is also connected to conductor 102 through a resistor 78 forming a voltage divider with resistor 76; the corresponding emitter is connected via a resistor 79 to the variable tap of a potentiometer 17 which is shunted by condenser 53. The two junctions of network 17, 53 are connected to the emitter of transistor 18 through a resistor 80 and a condenser 81, respectivelyfFinally, the base and the emitter of transistor are bridged by a resistor 82, the base being connected through a resistor 83 to live bus bar 105.
Monoflop 18, 19, whose off-period is determined by the time constant of an RC network including the condenser 16 along with the resistance of transistor 15, is tripped whenever condenser 16 acquires a sufficiently negative charge to bias the transistor 19 into conduction. With transistor 20 cut off, stage 18 is turned on by the resulting increase in its base potential and drives the lead 13 negative to enable continued firing of TRIAC 10. Since the same negative bias is applied to the base of transistor 21, flip-flop 21, 22 remains in its aforedescribed condition. This flip-flop is unswitchable during positive half-cycles and can be switched during negative halfcycles by a cutoff pulse applied to the base of stage 22, via condenser 23 and diode 66, only if lead 13 is not negatively biased by monoflop stage 18.
Normally, i.e., as long as the monitoring voltage from rectifier bridge 14 does not override the positive bias applied to transistor 15 by potentiometer 17, this transistor charges the condenser 16 at a relatively rapid rate which terminates the unstable condition of the monoflop within a timing interval less than a cycle of the supply voltage (thus, shorter than 20 ms in the case of a 50-c.p.s. power supply). Transistor 20, which in its saturated state blocks the transistor 18, is cut off during negative half-cycles as soon as the voltage of terminal B reaches a certain value, e.g., of 7 V in a specific embodiment. Since the monoflop 18, 19 is switched within less than a cycle after the cutoff of transistor 20, flip-flop 21, 22 will remainclamped during the initial period of the next negative half-cycle, i.e., until transistor 20 is again turned off.
If, however, the signal supplied by processor 50 exceeds the selected threshold, the preselected constant current normally drawn by transistor 15 is throttled so that the charging period of condenser 16 is increased beyond the length of a cycle. Flip-flop 21, 22 is now switchable during a short interval between the zero-crossing of the negative-going bus bar and the blocking of transistor 20 a short time thereafter. Thus, the rising negative voltage on bus bar 105 biases the transistor 22 into its high-resistance state, thereby stopping the current flow through resistor 69 and driving lead 13 positive, with resulting conduction of transistor 21 and completion of the reversal of the flip-flop. The positive blocking potential on lead 13 inhibits the emission of further gating pulses from trigger circuit 11 to TRIAC 10 until the voltage stored in integrating network 84 has decayed sufficiently to restore the system to normal operation.
FIG. 4 shows an alternate embodiment wherein a TRIAC 27 is connected via leads 25, 26 across power supply A, B .in series with a cooker 24 and a sensing resistor 40 substantially in the manner described above. The gate 28 of TRIAC 27 is returned to lead 26 via a pair of series resistors 37, 38 and cascaded diodes 32, 33, the junction of these diodes being tied to lead 25 by way of two other cascaded diodes 34 and 35. A condenser 30 shunts the resistor 37, another condenser 29 being connected in series with a resistor 36 and diode 35 across the load 24.
The circuit so far described operates as follows: during negative half-cycles of terminal B (terminal A being again considered grounded), a firing current flows through gate 28 by way of resistors 37 and 38 as well as diodes 33 and 32. With the TRIAC conducting, condenser 29 is charged through diode 35 and resistor 36 so that, at the beginning of the following positive half-cycle (with the potential difference between terminals A and B approximately zero), the discharge of this condenser triggers another gating pulse through diodes 34 and 32. Thus, the TRIAC 27 conducts substantially continuously, firing with every half-cycle.
A gate-controlled rectifier 39 or thyristor is connected between the junction of resistor 38 with diode 33 and the common terminal of load 24 and resistor 40. The gate 41 of this thyristor is returned to its cathode through a resistor 44 shunted by a variable portion of a potentiometer 46, the latter being in parallel with a condenser 43. This condenser is connected across resistor 40 in series with a further resistor 45, a diode 42 and part of an autotransformer 48; the latter may also be adjustable to vary the effective magnitude of the monitoring voltage applied to the processing stage 42, 43, 45 by sensing resistor 40.
As long as the DC voltage tapped off the potentio-meter 46 is insufficient to fire the thyristor 39, the system operates continuously in the aforedescribed manner.
When, under overload conditions, the thyristor 39 breaks down, which can occur only during negative half-cycles, gate 28 is virtually short-circuited so that TRlAC 27 remains cut off. Condenser 29, accordingly, does not charge during this half-cycle and no further trigger pulse can be generated in the remaining half of the cycle. After a sufficient decay of the blocking voltage stored on condenser 43, i.e., after one or more full cycles, normal operation is resumed.
A shunt condenser 47, bridging supply terminals A and B, serves (as also in the system of FIG. 3) to short out transients and to help suppress radio-frequency noise. Since, however, the TRIAC 27 fires only during zero crossings, transients due to the aforedescribed switching operations will be minimized even without such additional protection. Naturally, the usual safety requirements (e.g., fusing) will have to be observed.
in a system of the aforedescribed type in which the overload level is at approximately 8 amps, a current exceeding this level may result in the suppression of a sufficient number of cycles to keep the mean power consumption approximately constant, e.g., one cycle in four with currents of 9 amps., every other cycle with 12 amps, and five cycles in six with 16 amps.
1. A self-regulating power supply for a load driven by alternating current, comprising:
an alternating-current source;
a load circuit connected across said source for energizing a load detrimentally affected by direct-current components;
normally nonconductive gate-controlled two-way electronic valve means in said circuit in series with the load, said valve means being provided with gating means enabling same to be triggered into a state of conductivity terminating upon a polarity reversal of said source;
a control circuit connected between said source and said gating means for generating a train of trigger pulses normally establishing said state of conductivity at the beginning of the first half of each cycle of said source, said control circuit including capacitive means chargeable during said first half for energizing said gating means during the immediately following second half of a cycle;
sensing means in said load circuit for determining the average magnitude of a load current flowing therethrough; and
switch means in said control circuit responsive to said sensing means for suppressing said trigger pulses upon said average magnitude exceeding a predetermined threshold, thereby inhibiting initiation of conductivity of said valve means for at least one full cycle of said source, said capacitive means being effective independently of said switch means to establish such conductivity in the second half of a cycle upon conduction of said valve means in the first half thereof.
2. A power supply as defined in claim 1 wherein said load is a cooker for the direct heating of foods.
3. A power supply as defined in claim 1 wherein said generator control circuit includes detector means for maintaining substantial coincidence between said trigger pulses and the zero crossings of the source voltage.
4. A power supply as defined in claim 1 wherein said switch means comprises a transistor with an input circuit connected to said sensing means and with an output circuit including said capacitive means.
5. A power supply as defined in claim 4 wherein said sensing means comprises a resistor in series with the load, said input circuit being connected across said resistor.
6. A power supply as defined in claim 5 wherein said input circuit includes diode means for rectifying the voltage drop across said resistor and variable impedance means for adjusting the magnitude of the rectified voltage.
7. A power supply as defined in claim 6 wherein said variable impedance means comprises a potentiometer connected to supply a reference voltage in bucking relationship with the output of said diode means.
8. A power supply as defined in claim 6 wherein said variable impedance means comprises an autotransformer inserted between said resistor anfl sa id d iodf m eans.