US3581726A - Capacitive-discharge system for internal combustion engines - Google Patents

Abstract

The capacitive-discharge system for internal combustion engines comprises a stepup transformer having primary and secondary windings. The capacitor is placed in series with the primary winding. A DC power source including a push-pull full wave oscillator circuit is provided to charge the capacitor. The capacitor is discharged through the primary winding in timed sequence with an operating engine to induce a voltage in the secondary winding sufficient to fire a spark plug.

Description

United States Patent Inventor Alfred Plume, Jr.

Taylor, Mich.

Appl, No. 843,489

Filed July 22, 1969 Patented June 1, 1971 Assignee Mallory Electric Corporation Detroit, Mich.

CAPAClTIVE-DISCHARGE SYSTEM FOR INTERNAL COMBUSTION ENGINES Primary Examiner-Laurence M. Goodridge Attorney-Whittemore, Hu1b'ert& Belknap ABSTRACT: The capacitive-discharge system for internal combustion engines comprises a stepup transformer having primary and secondary windings. The capacitor is placed in 3 Claims 3 Drawing series with the primary winding. A DC power source including US. Cl [23/148, a push pull full wave oscillator circuit is provided to charge 315/209 the capacitor. The capacitor is discharged through the prima- Int. Cl F02p 3/06 ry winding in timed sequence with an operating engine to in- Field of Search 123/148 E; duce a voltage in the secondary winding sufficient to fire a 315/209, 214 spark plug.

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ALFRED PLUME, JR. QM a 7; WWW M ATTORNEYS CAPACITIVE-DISCHARGE SYSTEM FOR INTERNAL COMBUSTION ENGINES BACKGROUND OF THE INVENTION In conventional inductive-discharge ignition systems, the coil develops the high voltage necessary to cause the vehicle spark plugs to fire. Capacitive-discharge ignition systems have been previously proposed. Capacitive-discharge systems have several advantages over conventional systems. One advantage is that such a system has the ability to cause firing of defective spark plugs. Additionally, in such systems the capacitor is not discharged as often when operating at low speeds as it is when operating at high speeds. This allows a relatively long time in which the capacitor achieves a full charge. This is advantageous in that it results in improved cold-weather starting and reduces power consumption by the ignition system while starting.

The present invention provides an improved version of a capacitive-discharge ignition system. In the present system, a controlled rectifier is provided as the switching device to cause discharge of the capacitor. The controlled rectifier is pulsed to conduct by means of a one-shot circuit. The signal from the one-shot circuit is carefully controlled so that it will be sharply applied at the precise desired instant and the circuit is protected from stray signals being inadvertently applied to cause misfiring. Additionally, protective means are provided to shield the controlled rectifier from high peak inverse voltages.

SUMMARY OF THE INVENTION The capacitive-discharge system is provided for an internal combustion engine/It comprises a stepup transformer having primary and secondary windings. A capacitor is provided in series with the primary winding. A DC power source including a push-pull full wave oscillator circuit is provided to charge the capacitor. Discharge means including a controlled rectifier are provided to discharge the capacitor through the primary winding in timed sequence with an operating engine to induce a voltage in the secondary winding sufficient to fire a spark plug. Pulse producing means are provided. A first transistor is coupled between the pulse-producing means and the gate of the controlled rectifier for biasing the controlled rectifier to conduct in the desired timed sequence with an operating engine. A diode is provided between the output of the first transistor and the gate of the controlled rectifier. The diode conducts only when a minimum predetermined voltage is applied thereto. The first transistor is valued to provide the necessary voltage when it is fired to conduct. A second transistor is connected between the output of the first transistor and ground. Means are provided to bias the second transistor to a conducting state when the first transistor is not biased to conduct by a signal from the pulse-producing means and to a nonconducting state when the first transistor is biased to conduct by a signal from the pulse-producing means.

IN THE DRAWINGS:

FIG. l is a schematic view of one embodiment of an electrical switching circuit for a vehicle ignition system in accordance with the present invention.

FIG. 2 is a schematic view of the push-pull full wave oscillator circuit utilized as a power source; and

FIG. 3 is a graph illustrating operation of the oscillator circuit.

Referring to FIG. 1, it will be noted that a stepup transformer I is included in the circuit for providing a voltage sufficiently high to fire a spark plug of a vehicle engine. The transformer comprises aprimary winding 12 and asecondary winding 14. The high voltage output of thewinding 14 is utilized by the distributor (not shown) of the ignition system for firing the vehicle spark plugs.

Alead 16 extends from one side of theprimary winding 12 and is connected to one side of acapacitor 18. Alead 20 extends from the other side of the capacitor I8 and is connected at Z to a DC power source (FIG. 2) which is capable of supplying a relatively high voltage, in the neighborhood of 225- 400 volts.

In operation of the system, thecapacitor 18 is first charged to the voltage of the power supply. Thecapacitor 18 is then suddenly discharged through theprimary winding 12 whereby the voltage applied to the primary winding rises to the full voltage of the capacitor, in the present instance about 400 volts, in a very short period of time, for example, two microseconds. The voltage induced in thesecondary winding 14 is sufficient to cause the spark plugs to fire.

The present invention is concerned with the circuitry for causing thecapacitor 18 to charge and discharge and the DC power source. This circuitry includes a solid-state controlledrectifier 22 which is provided in alead 24 which extends between thelead 20 and aground lead 26. Therectifier 22, which may be a silicone-controlled rectifier, has ananode 28, acathode 30 and agate 32. As is well known, a controlled rectifier is a solid-state four-layer device. In its normal state, the controlled rectifier actsas an open circuit that will not pass current. When an appropriate voltage or current pulse is applied to the gate electrode, it will cause the controlled rectifier to be forward biased .to permit current flow. Application of the proper polarity voltage to the controlled rectifier will allow electrons to flow from the cathode to the anode. Reversal of the voltage polarity results in the controlled rectifier being an open circuit. Similarly, when the controlled rectifier is conducting, application of a reverse polarity to the gate electrode will place the controlled rectifier in its original state of an open circuit. Thus, the controlled rectifier can act as a controlled switching diode capable of being switched on and off by application of voltages of appropriate polarity.

In the present invention, thegate 32 is connected to circuitry which provides a pulse which is in timed relation to the engine speed. The pulse,-in the present case, derives fromconventional breaker points 34 which are opened and closed in timed relation to the speed of the distributor shaft. However, the signal may be derived from any suitable source from the engine. One satisfactory method for providing a pulse is disclosed in copending application Ser. No. 447,004, filed Apr. 9, 1965, now U.S. Pat. No. 3,408,993.

The pulsing circuit includes aconventional vehicle battery 36 the negative terminal of which is grounded at 38. A lead 40 extends from the positive terminal of the battery. Anignition switch 42 is provided inlead 40 for turning the ignition system on and off.

A lead 44 extends between theleads 40 and 26. A current limitingresistor 46 and the breaker points 34 are provided in thelead 44. It will be noted that thelead 26 is grounded at 48. Thus, when the breaker points '34 are closed, current will flow therethrough from thebattery 36 to ground. A lead 50 extends from a point between the resistor 46 breaker points 34 to thebase 52 of atransistor 54. Thetransistor 54 forms a portion of what is termed a one-shot circuit" which receives pulses generated by the opening and closing of the breaker points 34 which are proportional to the speed of the engine. The oneshot circuit converts these pulses into a square'wave signal which operates the switching means for charging and discharging thecapacitor 18.

A pair ofresistors 56, 58 are provided in thelead 50. A lead 60 extends from a point between theresistors 56, 58 to theground lead 26. Acapacitor 62 is provided inlead 60. Anothercapacitor 63 is provided in a lead 65 which extends from the other side ofresistor 58 toground lead 26. Theresistor 56 andcapacitors 62, 63 form a filter which removes any oscillations of the input voltage. Theresistor 58 forms a biasing resistor for thetransistor 54. Thetransistor 54 is connected in the common emitter configuration, theemitter 66 being connected to ground lead 26 bylead 68 and thecollector 70 of the transistor being connected to thepotential supply line 40 by alead 72. Aload resistor 74 is provided in thelead 72.

The one-shot circuit also includes asecond transistor 64. Theemitter 76 is connected to ground lead 26 by alead 78. Thecollector 80 is connected to thepotential supply line 40 by alead 82. Aload resistor 84 is provided in thelead 82. Thebase 86 is coupled by acapacitor 88 inlead 90 to thecollector 70 of thetransistor 54. A lead 92 extends from thelead 90 into connection with thepotential supply line 40. Aresistor 94 is provided inlead 92. Theresistor 94 functions as a biasing resistor for thetransistor 64 and also as a time constant device for controlling operation of the transistor.

Thecollector 80 of thetransistor 64 is connected by a lead 96 to thebase 52 oftransistor 54. Aresistor 98 is provided inlead 96 between the lead 82 and thebase 52. Theresistor 98 acts as a feedback device for feeding back part of the output signal fromtransistor 64 to the base of thetransistor 54.

The output of thetransistors 54, 64, forming the one-shot circuit, is fed to a second pair oftransistors 100, 102. These transistors form part of a switching circuit which includes the controlledrectifier 22.

The base 104 of thetransistor 100 is connected to the collector oftransistor 64 by means of thelead 96 which is connected to thelead 82. Thecollector 106 oftransistor 100 is connected to thepotential supply line 40 by alead 108. Theemitter 110 oftransistor 100 is connected to thecollector 112 oftransistor 102 by alead 114. Aresistor 116 is provided inlead 114. Theresistor 116 is a current limiting device and functions to prevent reverse or leakage current from damaging the components.

Thebase 118 of thetransistor 102 is connected to thebase 86 oftransistor 64 by means of alead 120. Theemitter 122 oftransistor 102 is connected to ground lead 26 by alead 124.

Alead 126 extends from the lead 114 from a point betweenresistor 116 andemitter 112 into connection with thegate 32 of the controlledrectifier 22. Adiode 128 is provided in thelead 126. Thediode 128 will not conduct until the voltage thereacross reaches a predetermined level. For example, in one circuit, the diode was designed to conduct at 0.67 volts. The proper voltage for conduction of thediode 128 is provided at the exact point of turning one of thetransistors 100, 102 off, and simultaneously turning the other of these transistors on, as will be later described.

Thediode 128 emits a square-wave pulse to the controlledrectifier 22 to cause conduction of this rectifier. Afilter capacitor 129 is provided inlevel 131 betweengate 32 andground lead 26. Thecapacitor 129 functions to filter out fake frequencies resulting from point bounce and any other transient frequencies which may appear at thegate 32.

Adiode 130 is provided in the lead 24 between theanode 28 of therectifier 22 and thelead 20. Thediode 130 is a high voltage, high current, high speed device. The voltage drop thereacross is very rapid. This prevents high peak inverse voltage being applied across therectifier 22 and thus is a protective device for the rectifier.

A pair ofdiodes 132, 133 are provided in a lead 134 which extends between leads 20 and 26 across therectifier 22. It will be noted that thediodes 132, 133 are oriented to conduct in the opposite direction with respect to thediode 130 andrectifier 22. The function of thediodes 132, 133 is to shut out any high voltage negative pulses which are reflected from the transformer or any other source. As is well known, such transformers tend to ring after the initial surge of current has flowed therethrough upon discharge of thecapacitor 18. Such ringing may cause high voltage negative pulses which are of sufficient magnitude to cause damage. A resonant circuit is placed in series with thediodes 132, 133. The circuit consists of aninductance 136 provided in thelead 134 and stray capacitance of the circuit which is present across theinductance 136. The resonant circuit functions to provide a signal to cause the controlledrectifier 22 to discontinue conduction after thecapacitor 18 has been discharged. The circuit also damps the reflected pulses from thetransformer 10 after the transformer has been activated to produce the required voltage for firing a spark plug, and recharges thecapacitor 18 to some degree.

Operation of the entire circuit of FIG. 1 may now be understood. The circuit will be considered in its quiescent condition just before an input pulse is applied to thetransistor 54. in this condition,transistor 64 is biased for conduction, and a voltage just above ground potential appears at the collector oftransistor 64. This voltage is applied to thebase 52 oftransistor 54 via theresistor 98. Consequently, thebase 52 oftransistor 54 is not sufficiently positive to turntransistor 54 on. Transistor is also in its off condition. As a result of the conduction oftransistor 64, thecapacitor 88 is charged to the polarity indicated.

Thetransistor 102 is also biased to conduct at this time. It will be noted that thecollector 112 is connected to thepotential supply line 40 throughtransistor 100. Therefore, the only current which will flow through the collector-emitter circuit oftransistor 102 is that which leaks throughtransistor 100 while it is in the off state. The result of this is that the battery voltage always appears acrosstransistor 100 and when this transistor is biased to conduct, the peak voltage of the signal from the one-shot circuit is reached very quickly and applied to the gate of controlled rectifier virtually instantaneously.

When a positive pulse is applied to thebase oftransistor 54, as by opening thepoints 34, thetransistor 54 is turned on. Itscollector 70 then foes to a potential just above ground potential. The voltage across thecapacitor 88 suddenly goes negative, and the capacitor begins to discharge through the conductingtransistor 54. The voltage oncapacitor 88 turns off thetransistor 64 and causes itscollector 80 to go positive. This turns thetransistor 100 to the on state. The positive voltage accruing at thecollector 80 oftransistor 64 is also applied via theresistor 98 to thebase 52 oftransistor 54 and holds that transistor on even after the input pulse has terminated.

When the voltage across thecapacitor 88 suddenly goes negative, and the capacitor begins to discharge through the conductingtransistor 54, the voltage atcapacitor 88 also turns off thetransistor 102.

It will thus be appreciated that initiallytransistors 64 and 102 are conducting whiletransistors 54 and 100 are not conducting. Upon the application ofa positive pulse to the base oftransistor 54, thetransistor 54 begins to conduct. Simultaneously,transistors 64 and 102 are turned off whiletransistor 100 is turned on. Conduction oftransistor 100 causes the required voltage to appear acrossdiode 128 thus causing this diode to conduct. lt will be appreciated that this diode will conduct at the exact point of turning thetransistor 100 on and turning hetransistor 102 off. Conduction ofdiode 128 causes the controlledrectifier 22 to begin conducting thus discharging thecapacitor 18 resulting in a high induced voltage in secondary winding 14 to cause sparking ofa spark plug.

Aftercapacitor 88 has fully discharged, thebase 86 oftransistor 64 is again made positive andtransistor 64 begins to conduct again. Thecollector 80 oftransistor 54 then goes negative nearly to ground potential, and this potential is again applied byresistor 98 to thebase 52 oftransistor 54 turning that transistor off.Transistor 100 is also turned off and remains off so long as the near-ground potential remains at thecollector 80 oftransistor 64. At the same time,transistor 102 is again caused to conduct as thecapacitor 18 is recharged bringing the entire system back to the original state.

The controlledrectifier 22 will remain in the on condition so long as thetransistor 100 conducts. The length of time that thetransistor 100 conducts is determined by the R-C time constant ofresistor 94 andcapacitor 88. Thecapacitor 88 will, of course, begin to charge to the battery voltage through theresistor 94 as soon as thetransistor 54 begins to conduct. The time necessary for this to occur is such that a voltage of sufficient value will have been developed in the secondary winding 14.

Referring now to FIG. 2, thepower circuit 142 includes an amplifier which comprises a pair ofNPN transistors 144, 146, connected in circuitry to form an amplifier of the push-pull full-wave type. Thetransistors 144, 146 are connected in the common emitter configuration. Theemitters 148, 150 are connected vialeads 152, 154 to alead 156. Thelead 156 is grounded at 158.

Thelead 156 is connected to the center of a lead 160 which extends in one direction to one side of a bias winding 162 and in the other direction to one side of a second bias winding 164. The other side of winding 162 is connected to thebase 166 of transistor 144 vialead 168. Aload resistor 170 is provided inlead 168. The other side of the winding 164 is connected to thebase 172 oftransistor 146 vialead 174. Aload resistor 176 is provided inlead 174.

The negative side of thebattery 36 is connected at point Y. The positive side of thebattery 36 is connected to the center of a lead 178 at the point X. Thelead 178 is connected betweenleads 168, 174. Aresistor 180, 182 is provided in thelead 178 on either side of the battery connection X. Alead 184 extends fromlead 178 to the center of the primary winding 186 ofstepup transformer 188. One side of the winding 186 is connected to thecollector 190 of transistor 144 vialead 192. The other side of the primary winding 186 is connected to thecollector 194 oftransistor 146 vialead 196.

The secondary winding 198 of the transformer 188-has acenter tap ground 200. One side of the winding 198 is connected to the positive terminal of ajunction diode 202 vialead 204. The negative terminal ofdiode 202 is connected to the positive terminal of asecond diode 206 vialead 208. Alead 210 extends from the negative terminal ofdiode 206 and is connected at Z to thelead 20. Similarly, alead 212 extends from the other side of the winding 198 to the positive terminal of adiode 214. The negative terminal ofdiode 214 is connected to the-positive terminal of adiode 216 vialead 218. Alead 220 extends from the negative terminal ofdiode 216 into connection withlead 210. Thediodes 202, 206, 214, 216 serve as a rectifying circuit. A pair of diodes is placed in series in each of the circuits in order to prevent breakdown of the diodes as a result of peak inverse voltage. v

Alead 222 extends fromlead 210 to ground. Thelead 222 is connected to lead 210 between the connection oflead 220 and the point Z. Acapacitor 224 is provided inlead 222.

Another lead 226 extends fromlead 210 in parallel withlead 222. The lead 226 is also grounded. Aresistor 228 is provided in lead 226.

Aninductance 230 is provided inlead 210 beyond the connection thereof withlead 222. Theinductance 230 serves to stop spikes inlead 210 in eitherdirection A diode 232 is provided betweeninductance 230 and the connection point Z. Thediode 232 functions to prevent application of reverse voltage on thecircuit 142.

Operation of thecircuit 142 may now be understood. The transistors 144,146 in conjunction with thetransformer 188 form a DC to AC inverter. When the battery voltage is applied by closure ofswitch 42, one of the transistors will conduct while the other will go into cutoff. Assuming that transistor 144 conducts, the expanding field in the primary winding 186 of thetransformer 188 caused by the conduction of transistor 144 will develop a forward bias in the base-to-emitter winding 162 of transistor 144 and will maintain transistor 144 in conduction.

When saturation is reached, the magnetic field becomes stationary and there are no longer any moving lines of force to maintain the induced-bias voltage. The transistor 144 will then cease conducting and the magnetic field oftransformer 188 will collapse. As this field collapses, it induces a voltage of the opposite polarity that placestransistor 146 in conduction and transistor 144 is cut off. Withtransistor 146 conducting through the opposite half of the primary winding 186, the magnetic field reverses polarity. As the field reaches saturation, the cycle repeats. Transistor 146' becomes cut off and transistor 144 conducts and the magnetic field of primary winding 186 of thetransformer 188 reverses polarity. Consequently,transistors 144, 146 act as an oscillator with the voltage on the secondary winding 198 of thetransistor 188 appearing as a sine wave.

The high AC voltage developed in the secondary winding 198 of thetransformer 188 is rectified bydiodes 202, 206 and 214, 216. Theresistor 228 andcapacitor 224 along with theinductance 230 function to safeguard the oscillator circuit.

Theresistor 228 provides a load for the charge on thecapacitor 224 which cyclically discharges at certain times in operation of the circuit as, for example, when an internal combustion is being started and the vehicle spark plugs are not yet firing. The time constant prevents the voltage of the circuit from going above, for example, 600 volts when 2,000 volts are available at the output of thetransformer 188. Thecircuit comprising resistor 228 andcapacitor 224 eventually fade out after the spark plugs of the vehicle engine begin firing because the impedance of the remaining circuit falls below the impedance of the resistor. Thecapacitor 224, in combination with theinductance 230, is a safety device to prevent the oscillator circuit from ever seeing a full short as it might, for example, when thecapacitor 18 discharges. This prevents complete stopping of the oscillator which would necessitate restart.

The desirability of this arrangement may be understood by study of the curve illustrated in FIG. 3. The curve represents engine speed plotted against output voltage which is sustained by the distributor. As will be noted, a maximum of 600 volts is possible at zero r.p.m. This would be the condition at startup of the vehicle. With the engine speed at 200 rpm, the voltage would be approximately 400 volts. As engine speed increases, the voltage drops somewhere to 225 volts and will remain relatively constant up to speeds of over 10,000 rpm. In one embodiment, a relatively steady output voltage was available at engine speeds of up to l9,000 r.p.m., which is a circumstance only rarely encountered in an internal combustion engine. Thus, the sparking voltage is sufficient over a broad spectrum of engine speeds.

What I claim as my invention is:

1. In a capacitive-discharge system for internal combustion engines comprising a stepup transformer having primary and secondary windings, a capacitor in series with the primary winding, a DC power source to charge the capacitor, discharge means including a controlled rectifier to discharge the capacitor through the primary winding in timed sequence with an operating engine to induce a voltage in the secondary winding sufficient to fire a spark plug, pulse-producing means, a first transistor coupled between said pulse-producing meansand the gate of saidcontrolled rectifier for biasing the controlled rectifier to conduct in timed sequence with an operating engine, a diode between the output of the first transistor and said gate, said diode conducting only when a minimum predetermined voltage is applied thereto, said first transistor being valued to provide the necessary voltage when it is biased to conduct, and a second transistor connected between the output of the first transistor and ground, means biasing the second transistor to a conducting state when the first transistor is not biased to conduct by a signal from the pulseproducing means and to a nonconducting state when the first transistor is biased to conduct by a signal from the pulseproducing means. v

2. A capacitive-discharge system as defined in claim 1, and further characterized in that said pulse-producing means includes a one-shot circuit, said one-shot circuit comprising third and fourth transistors cross-coupled by means including a capacitor for translating input pulses to a square wave signal, said first transistor being connected to the collector-emitter circuit of said fourth transistor and biased to conduct when said third transistor conducts, said second transistor being connected between said third and fourth transistors and biased to conduct when said fourth transistor conducts.

3. In a capacitor-discharge system for internal combustion engines comprising a stepup transformer having primary and secondary windings, a first capacitor in series with the primary winding, a DC voltage amplifier including an oscillator, a second capacitor connected to the output of the amplifier to be charged thereby, a resistor in parallel with said capacitor, said resistor providing a discharge path for the capacitor'during operation of the amplifier under no-load conditions, the output of said DC power source being connected to said first capacitor to charge said first capacitor, discharge means to discharge the first capacitor to the primary winding in timed' sequence with an operating engine to induce a voltage in the secondary winding sufficient to fire a spark plug, pulse producing means, a first transistor coupled between said pulse producing means and the discharge means for actuating said discharge means, a voltage device between the output of the first transistor and said discharge means, said voltage device conducting only when a minimum predetermined voltage is applied thereto, said first transistor being valued to provide the necessary voltage when biased to a conducting state, and a second transistor connected between the output of the first transistor and ground, means biasing the second transistor to a

Claims (3)

1. In a capacitive-discharge system for internal combustion engines comprising a stepup transformer having primary and secondary windings, a capacitor in series with the primary winding, a DC power source to charge the capacitor, discharge means including a controlled rectifier to discharge the capacitor through the primary winding in timed sequence with an operating engine to induce a voltage in the secondary winding sufficient to fire a spark plug, pulse-producing means, a first transistor coupled between said pulse-producing means and the gate of said controlled rectifier for biasing the controlled rectifier to conduct in timed sequence with an operating engine, a diode between the output of the first transistor and said gate, said diode conducting only when a minimum predetermined voltage is applied thereto, said first transistor being valued to provide the necessary voltage when it is biased to conduct, and a second transistor connected between the output of the first transistor and ground, means biasing the second transistor to a conducting state when the first transistor is not biased to conduct by a signal from the pulse-producing means and to a nonconducting state when the first transistor is biased to conduct by a signal from the pulse-producing means.

2. A capacitive-discharge system as defined in claim 1, and further characterized in that said pulse-producing means includes a one-shot circuit, said one-shot circuit comprising third and fourth transistors cross-coupled by means including a capacitor for translating input pulses to a square wave signal, said first transistor being connected to the collector-emitter circuit of said fourth transistor and biased to conduct when said third transistor conducts, said second transistor being connected between said third and fourth transistors and biased to conduct when said fourth transistor conducts.

3. In a capacitor-discharge system for internal combustion engines comprising a stepup transformer having primary and secondary windings, a first capacitor in series with the primary winding, a DC voltage amplifier including an oscillator, a second capacitor connected to the output of the amplifier to be charged thereby, a resistor in parallel with said capacitor, said resistor providing a discharge path for the capacitor during operation of the amplifier under no-load conditions, the output of said DC power source being connected to said first capacitor to charge said first capacitor, discharge means to discharge the first capacitor to the primary winding in timed sequence with an operating engine to induce a voltage in the secondary winding sufficient to fire a spark plug, pulse producing means, a first transistor coupled between said pulse producing means and the discharge means for actuating said discharge means, a voltage device between the output of the first transistor and said discharge means, said voltage device conducting only when a minimum predetermined voltage is applied thereto, said first transistor being valued to provide the necessary voltage when biased to a conducting state, and a second transistor connected between the output of the first transistor and ground, means biasing the second transistor to a conducting state when the first transistor is not biased to conduct by a signal from the pulse-producing means, and to a nonconducting state when the first transistor is biased to conduct by a signal from the pulse-producing means, and an inductor in series with said second capacitor, said inductor being located between said first and second capacitors and functioning, along with said second capacitor, to prevent a full short across the voltage amplifier during discharge of said first capacitOr.

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