US 3859572 A
A pair of transistors are connected to opposite ends of the magnet coil to be energized. A control pulse directly turns one transistor on and simultaneously fires a single shot multivibrator and its output turns on the other transistor. Current builds up in the coil during the single shot multivibrator time out period. The transistor controlled by the single shot multibivrator turns off when the single shot multivibrator times out. Coil current decreases but at slow rate as it circulates from one side of the power supply through a diode connected to one end of the coil and through the transistor connected to the other end of the coil back to the same side of the power supply. The dropping of the control pulse turns off the other transistor and the coil current rapidly decays to zero because the power supply is connected across the coil in a reverse direction via another diode connected to the other end of the coil and the other side of the power supply.
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Description (OCR text may contain errors)
[ MAGNETIC COIL DRIVER CIRCUIT  Inventors: Steven D. Keidl, Rochester; James F. Nelson, Pine Island, both of Minn.
 Assignee: International Business Machines Corporation, Armonk, N.Y.
 Filed: Mar. 16, 1973  Appl. No.: 342,109
 US. Cl. 317/1485 R, 3l7/DIG. 4  Int. Cl. II0lh 47/00, HOlh 47/32  Field of Search 317/DIG. 4, 148.5 R
 References Cited UNITED STATES PATENTS 3,206,651 9/1965 Proulx 3l7/l48.5 R
Primary Examiner-L. T. Hix Attorney, Agent, or FirmDonald F. Voss INPUT ,1 TERMINAL 51 Jan.7, 1975  ABSTRACT A pair of transistors are connected to opposite ends of the magnet coil to be energized. A control pulse directly turns one transistor on and simultaneously fires a single shot multivibrator and its output turns on the other transistor. Current builds up in the coil during the single shot multivibrator time out period. The transistor controlled by the single shot multibivrator turns off when the single shot multivibrator times out. Coil current decreases but at slow rate as it circulates from one side of the power supply through a diode connected to one end of the coil and through the transistor connected to the other end of the coil back to the same side of the power supply. The dropping of the control pulse turns off the other transistor and the coil current rapidly decays to zero because the power supply is connected across the coil in a reverse direction via another diode connected to the other end of the coil and the other side of the power supply.
7 Claims, 2 Drawing Figures PRE DRIVER f +Vs DRIVER PATENTED H915 3.859.572
INPUT TERMINAL I? 15 INPUT A CONTROL SIGNAL B Vss 25 C Vx D Vy
RIVER crno T MAGNETIC coir.
,Q'QBACKGROUND OF THE INVENTION lLField of the Invention This invention relates .to magnet coil driver circuits and more particularly to ariimproved magnetic coil driver circuithaving fast pick and drop out times and i still more particrlilarly to r'nagnet 'coil driver circuits usingt the magnetic coil circulating current principle for energy .tr'ansfejrll The invention finds particular utility for energizing magnet coils of print hammers or punch actuatorsfwhich operate at relatively high repetition rates." 1
2. Description of the Prior Art ln the past, most magnet coil driver circuits have used a combination of transistors with resistors and caciently. The efficiency of the magnet driver circuit of 25 the present invention is improved because without a re-' sister in the circuit, the full power supply voltage is available for both energizing and de-energizing the magnet coil.
SUMMARY or THE INVENTION The principal objects of the invention areto provide an improved magnet coil driver circuit which: (a) operates with relatively little energy dissipation; (b) operates at relatively high repetition rates;'(c) operates efficiently; (d) is made up of-component s adapted to be easily packaged; and (e) is relatively inexpensive.
The foregoing objects are 'achieved by selectively connecting the full power supply voltage across the.40
magnet coil. This quickly energizes the coil and then after the coil is energized to a desired current value as determined by a time delay one side of the power sup ply is disconnected from the coil. The coil current slowly decays but continues to circulate to provide a magnetic field of sufficient strength over a'period of time to the desired work to be performed. The coil current is then quickly removed from the coil by connecting the full power supply voltage across the coil in a reverse direction. The time delay can-beintroduced in many ways depending upon the machine or system incorporating the magnet coil. For example, in some instances, the delay can be developed by a time delay circuit and in other instances microprogramming can be used if available. Desired time delays can be generated based upon a microprogram routine having repetitive micro instruction cycle times where the'number of repetitive micro instruction cycle times determines the amount of time delay.
BRIEF DESCRIPTION OF THE DRAWINGS 60 T1 to go positive, as shown by waveform C, to forward DESCRIPTION With reference to the drawings and particularly to FIG. I, the invention is'illustrated by way of example as magnet driver circuit for energizing magnet coil 10.
-Magnet coil canbe any magnet coil such as the type used for print and punch magnet actuators in H0 devices for a computer system. The magnet coils in such devices are operated in response to a command or control signal which in this instance is shown as waveform A in FIG. 2 and is applied to input terminal in FIG. 1. The input terminal 15 is connected to both predriver and single shot multivibrator 25. The output of single shot multivibrator is connected to predriver and the outputs of pre-drivers 20 and 30 are connected to the bases of transistors T1 and T2 respectively.
Transistors T1 and T2 are conventional NPN switching transistors with the collector and emitter of transistor T2 connected to the +Vs terminal of the power supply and to one end of coil 10 respectively. The collector and emitter of transistor T1 are connected to the other end of coil 10 and to the ground potential terminal of the power supply, respectively. The end of coil 10 connected to the emitter of transistor T2 is also connected to the cathode'of diode D2. The anode of diode D2 is connected to the ground potential .terminal of the power supply. The other end of the coil, i.e., th end connected to the collector of transistor Tlis also con- I "nected to the anode of diode D1. The cathode of diode D1 is connected to the +Vs terminal of the power supply. TransistorsTl and T2 are normally off and thus diodes Dl'and D2 are reverse biased and no current norrnally flows in coil 10.
fWhen a positive going signal is applied to input terminal 15,.single shot multivibrator 25 fires and both pre- 'drivers 20 and 30 generate a positive going signal to Current rapidly builds up in coil 10, see waveform E,
7 until single shot multivibrator 25 times out. The current a peak value in coil 10 is proportional to the time period of single shot multivibrator 25, i.e., i,, (t) Vs(t)/L where t equals the time period of the single shot multivibrator 25, Vs is the power supply voltage and L is the inductance of coil 10. Transistor Tl turns off when single shot multivibrator 25 times out. The signal from single shot multivibrator 25 is represented by waveform B in FIG. 2. This causes the collector voltage of transistor bias diode D1 whereby current flowing in coil 10 continues to flow but decreases as represented in waveform E at a relatively slow rate-for the time td. The current circulating in coil 10, under the conditions just mentioned, is from the +Vs terminal of the power supply, through transistor T2, through coil 10, and through diode D1 back to the +Vs terminal of the power supply.
i(t-)/t (VT2:+:"VD1)/L The currentdecay occurs slow enough that the electromag'net performs its intendedfunction. i; v V
The coil .10 is d e-energized rapidlyjbyturningoff transistor T2 This occurs whentheinput' signal goes to a down level as represented by waveform A.:With T2 off, its emitter voltage goes. negative,.as representedby waveform D, to forward biasdiodeDZ. Thisplaces the power supply across coil in; a reverse direction and the current therein decays rapidly as'iti reverses direction as seen-in waveform E.
Various other techniques may be used to control the turn off of transistor T1 without departing from the spirit of the invention. The time delay, of course,'effects the amount of current conducted by transistors T1 and T2. Hence, if for some applications it is necessary to maintain current in the magnet coil for relatively long periods of time, intermittent time delays can be used, i.e., multiple single shots or multiple delay circuits. Thus the coil currentwould decay afterthe first time delay times out but would again increase when the next time delay is initiated. In this way the coil current could be maintained for a relatively long period of time without exceeding the current ratings of transistors T1 and T2. The same type of action can be achieved with microprogramming control. One microprogram control routine can develop one time delay period and a subsequent microprogram control routine or the same one can be called at a later time to initiate another time delay period. The magnet current, if desired, can be maintained over a longer period of time by adding inductance in series with the magnet coil to decreasethe amount of decay of the circulating current.
From the foregoing it isseen that the invention provides an improved magnet coildriver circuit which because of its fast response time can operateat relatively high repetition rates. Further, the invention keeps power dissipation toa minimum, because the power is primarily reactive power and the truepower is limited to the power consumed ,bythe transistors T1 and T2 and diodes D1 and DZ which isminimal.
What is claimed is; a J
1. A magnetic coil driver a source of power supply voltage,
, circuit means including, first and second current switches for connecting said magnet coildirectly across said power supply voltage in one direction, and first and second unilateral current conducting "devices responsive 'to being forward biased for connecting said magnet coil across said power supply voltage'in a'reverse direction, and I control means for'selec'tively operating said circuit means to first'simultaneously turn said first' and second current switches on to'conn'ect'sfaid magnet coil directly across said power supply in one direction and thereby energize said coil whereby the coil current rises jlinearlyqand reaches a level deter-. mined by said power supply. voltage, the length of time saidcoil 'is'energized,"and the'inductance of said coil and after a predetermined time delay turn off said first current switch to allo w currentcircu- 4 letting in saidcoil to decay wherebysaid circulating c'urrent maintain's said coil energized witha holding current and :then turn off said second eurrent' switch prior to said circulating currentdropping belowholding current level whereby said first and -*secondunilateralcurrent conducting devices connectsaid' magnet 'coil across said power supply in r saidnreverse direction to:-de-.energize said magnet -coil.w
I 2. The magnet coil driver circuit of claim 1 where said first and :second current switchesare transistors. 3. The-magnet coil driver circuit of claim 1 where said first and second unilateral current conducting devices are diodes.
4. The. magnet coil driver circuit of claim 1 wherein said control means includes a single shot multivibrator for generating said predetermined time delay. -5. A magnetic coil driver circuit comprising a source of-power supply voltage, a first transistor connected to one side of said power supply voltage and to one end of said magnet coil, a second transistor connected to the other side of said power supply voltage and to the other end of said magnet coil, whereby when said first and second transistors are turned on said, power supply voltage is across said magnet coil in one direction 'to cause an energizing current to flow in said coil increasing linearly to a level determined by the powersupply voltage, the inductance of said coil and the length of time said first and second transistors are turned on, first diode connected to said oneend ofsaid coil andto said other side of said power supply voltage to conduct current upon being forward biased by said first transistor turning off to de-energize said coil,. f I ,second diode connected to said other end of said co il and ,tosaid one side of said powersupply voltage to conduct a circulatingtcurrent for holding said coil energized upon being forward biased by Q said second transistor turning off, and
:c on trolmean's fm simultaneously turning ,on said first and second transistors, said control means including time delay means to set the level of energizing current: in said coil by turning off saidfirst transis- .tor after a predetermined time delay and means for gturning off said second transistor at a predetermined time after said first transistor has been turned, off whereby saidenergiz ing current circulat es said coil flowing from said one side of said power supply through said first transistor, through said coil, andthrough said second diode back to saidone side of said power supply from the time said second transistoris turned. off until said first turned off said coil becomes de-energized as said first diodeconducts current to to said other side of v Po upp y. i
',.6. The' magnet coil drivercircuit of claim 5 wherein said ti'medelay means is a single shot multivibrator.
7 The magnetcoil drive circuit of claim 5 wherein said first and second transistors are NPN'transistors.
' transistor is turned off and after said first transistor I