Abstract:
A capacitive discharge ignition system for an internal combustion engine and for use with a generator powered current source comprises a storage capacitor, at least one power thyristor for discharging the storage capacitor to transfer energy to at least one coil triggered in synchronism with an internal combustion engine with which the ignition system may be associated. A triggering circuit biases the power thyristor to discharge the storage capacitor. A shut-off circuit comprises a solid state device for switching the output of the generator to ground interrupting the flow of current to the power thyristor. A circuit sensing current flow in the triggering circuit generates a control and gates the solid state device into conduction in response to the control signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to capacitive discharge ignition systems in which a storage capacitor is charged by a generator or alternator and is discharged by a silicon-controlled rectifier (SCR) or thyristor switch triggered conducting in synchronism with the engine. 
     2. Description of Related Art 
     The Problem of turning SCRs off in capacitive discharge ignition systems is well known. Once triggered to conduct, the SCR remains conducting until the current source stops and the triggering pulse is removed. A number of patents have addressed shutting down the anode-to-cathode current from the source charging the storage capacitor. These have all related to direct current to direct current converter power supplies which can be electronically shut-down. See, for example, U.S. Pat. Nos. 3,583,378; 3,800,771; 3,838,328; and 4,069,801. The output of an alternator or generator cannot be switched off as with direct current converter power supplies. The output being an alternating current will periodically pass through zero. At this time, the SCR will shut down. However, it is desired to shut down the SCR immediately, that is, without waiting for the next polarity reversal of the alternator or generator. The applicant&#39;s circuit is directed to the shutting off of the power SCR in a capacitive discharge ignition system supplied by a generator or alternator. 
     U.S. Pat. No. 4,522,184 specifically addresses these shortcomings, but does so at the cost of added circuit complexity. The present invention reduces power loss and provides simplified circuitry. 
     SUMMARY OF THE INVENTION 
     This invention is an improvement in circuits for capacitive discharge ignition systems for internal combustion engines which ignition systems are charged by a generator or alternator. Capacitive discharge ignition systems comprise a storage capacitor charged through a diode or full wave rectifier. At least one main silicon-controlled rectifier (SCR) or other suitable thyristor is provided for discharging the storage capacitor through the primary of an ignition coil. A triggering circuit provides a gate pulse to the main SCR gate in synchronism with the internal combustion engine. A shut-off circuit includes a solid state device for grounding the output of the generator or a full wave rectifier connected to the output of the generator during the discharge of the storage capacitor. 
     The triggering circuit comprises a second SCR that is triggered by a pick-up pulse generated, for example, by the rotation of the engine crank shaft. The second SCR then discharges a triggering capacitor to the gate of the main SCR. The storage capacitor is discharged in response to the firing of the triggering circuit. 
     The LED of an optocoupler is placed in series with the triggering circuit. The phototransistor of the optocoupler is placed in series with the shut-off circuit. When the phototransistor conducts, the shut-off circuit charges a holding capacitor at the gate of a switch, preferably an FET switch. Preferably, a zener diode limits the voltage on the capacitor to less than the breakover voltage of the FET gate. A discharge resistor is placed across the holding capacitor. 
     After the SCR is triggered by the triggering circuit, the main storage capacitor starts to discharge. At this time, the triggering current shunted through the optocoupler gates the phototransistor into conduction. Almost immediately, the holding capacitor at the FET gate is charged from the storage capacitor to the fully conductive level of the FET gate and the FET shorts the output of the full wave rectifier to ground. As the storage capacitor discharges, its charge falls below the charge on the holding capacitor before the storage capacitor is fully discharged. However, the holding capacitor keeps the voltage at the FET gate above the threshold voltage until after the storage capacitor has completely discharged and, therefore, the SCR has turned off. 
     Previous techniques required the sensing of the discharge current directly which wasted considerable power (˜100 watts, 50 amp×2 volts) since the required sensing circuitry added significant impedance to the discharge loop. By sensing only the triggering current to the power device rather than the primary discharge, the wasted power is reduced to approximately 0.1 watt (0.1 amps×1 volt). 
     In previous techniques, grounding the output of the generator was the only function of the FET. Preferably, in this invention, the FET functions as a linear voltage regulator biased through a series of very low power zener diodes. This approach eliminates the need for a costly and failure prone high power zener diode across the storage capacitor. As the storage capacitor charges to the desired maximum voltage, the gate of the FET will be biased to turn conducting causing it to conduct current and thereby limiting the voltage of the storage capacitor. The double duty out of the shorting FET reduces the cost and complexity of the circuitry needed for proper operation of the system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Further features and other objects and advantages of this invention will become clear from the following detailed description made with reference to the drawing in which  FIG. 1  is a schematic diagram of the circuit according to one embodiment of this invention having an optocoupled phototransistor in the gating circuit configured to sense the triggering current pulse to the power thyristor and turns on the shorting FET. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawing, there is shown a capacitive discharge ignition system for one cylinder of an internal combustion spark ignition engine. Each component to the right of the dashed line is replicated the number of times there are cylinders to be served. For purposes of illustration of the instant invention, the circuit for one cylinder will suffice. 
     The storage capacitor  11  is charged by the output of the full wave rectifier  10  to store energy between each spark plug firing. The alternating current input to the full wave rectifier is taken from a generator  9  (or alternator) associated with the engine. The energy stored on the storage capacitor  11  is discharged to the primary of a step-up coil  12 . The secondary of the step-up coil is connected across the spark plug  13  (or plugs) as shown. Discharge of the storage capacitor  11  to the coil  12  is gated by SCR  14 . The zener diode network  15   a ,  15   b ,  15   c  and FET  23  limit the voltage across the storage capacitor  11 . The low power zener network and FET biased in the linear mode for voltage regulation costs less and is more reliable than the older style power zener placed directly across the storage capacitor. 
     A triggering circuit for triggering the SCR  14  into conduction comprises a trigger capacitor  16  charged through resistor  20  and diode  22  and a trigger SCR  18 . The basic characteristics of an SCR are well known: a positive pulse at the gate will start anode cathode current flowing and that flow will continue until two conditions are simultaneously met; namely, removal of the positive pulse at the gate and drop of the anode cathode current below the holding current level for the SCR. 
     A magnetic pick-up coil  19  or similar pick-up device provides a synchronized pick-up pulse to the gate of the trigger SCR  18 . The trigger SCR then discharges capacitor  16  to the gate of the main SCR  14  driving it into conduction. The trigger capacitor and trigger SCR amplify the pick-up pulse. They also provide isolation between the pick-up pulse (which may not fall off rapidly) and the main SCR. The discharge of the trigger capacitor is completed before the magnetically induced trigger pulse is removed from the gate of SCR  18 . This is important to the quick turn off of the main SCR  14 . Turn off of the power SCR  14  has remained a problem even with the isolation provided by the resistor through which the trigger capacitor is charged, trigger capacitor and trigger SCR as described above. Even if the gate pulse is removed, SCR  14  will continue to conduct as long as there is a source of anode cathode current. The generator and full wave rectifier  10  can supply that current even after the storage capacitor is discharged. 
     What follows is a description of the improvement for ensuring shut-down of SCR  14  by grounding the output of the full wave rectifier, or the generator as the case may be, during just the right time period. An LED  28  of optocoupler  27  is arranged in series with the discharge of the trigger capacitor  16 . The LED polarity is such that only the trigger capacitor  16  discharge current is sensed and not the capacitor charging current which flows through the diode  22 . Current through the LED comprises a signal for gating field effect transistor (FET)  23 . The polarities of the LED and charging diode eliminate the possibility of an undesired turn on of FET  23 . FET  23  is arranged to ground the output of the full wave rectifier but not to discharge the storage capacitor. Diode  21  prevents discharge of the storage capacitor  11  when the FET  23  is conducting. With the embodiment shown in the drawing, the output of the full wave rectifier is grounded. If the output of the generator is grounded, it will be necessary to provide back-to-back FETs between the output and ground in order that both positive and negative output pulses are diverted away from the full wave rectifier and, thus, the storage capacitor. In this embodiment, the diode  21  is not required as its function would be served by the full wave rectifier itself. 
     The circuit for controlling the on-time of the FET  23  will now be described. The trigger capacitor  16  is charged through full wave rectifier  10  and is discharged through trigger SCR  18 . In parallel with the diode  22  is the current-sensing circuit comprising the LED  28  of an optocoupler  27 . During discharge of the trigger capacitor  16 , current flows through the optocoupler LED  28  turning on output phototransistor  29 . 
     Almost immediately after the phototransistor  29  begins conducting, capacitor  32  is charged from a capacitor  40  to the voltage of a zener diode  34 . The higher positive voltage at the gate of the FET  23  increases the current flow from the source to the drain of the FET shorting the full wave rectifier to ground. Since the FET gate has very high input impedance, the capacitor  32  does not discharge through the gate. Also, charge on the capacitor cannot reach the breakover voltage of the FET gate because it will first discharge through the protective zener diode  34 . As the storage capacitor  11  discharges, the voltage on capacitor  40  drops down below the voltage across capacitor  32  and, therefore, it can no longer supply current to capacitor  32 . Were the FET to cease conducting at this time, it would do so before the storage capacitor  11  had fully discharged. Nothing would be gained by the added circuitry because the holding circuit for SCR  14  would never be cut off. The FET must remain conducting until after the capacitor  11  fully discharges. The RC time constant of the capacitor  32  and resistor  33  are selected to delay the time when the voltage on capacitor  32  drops below the threshold gate voltage of the FET. The FET is held conducting just beyond the time of complete discharge of the storage capacitor  11  and ring-out of the coil  12 . The SCR  14  turns off and the storage capacitor can begin recharging for the next cycle. 
     The positive shut-off of the SCR  14  makes the circuit much more versatile. For example, a diode  35  can be placed in parallel with the primary to lengthen the duration of the spark. The shut-off of the SCR is less sensitive to the output voltage of the full wave rectifier. There is no need to attempt to synchronize the nodes in the generator output with the discharge of the storage capacitor. Also, the generator output may be increased without regard to the holding current rating of the SCR  14 . 
     In a particular embodiment ignition for which the circuit according to this invention is useful, the time between spark firings is 5 milliseconds. The time between polarity reversals of the alternator or generator is between 1 and 2 milliseconds. A 1 or 2 millisecond delay before recharging of the storage capacitor is considered very undesirable. With the SCR shut-off circuit according to this invention, discharge of the storage capacitor would be expected in about 40 microseconds. The gating circuit of the FET switch is designed to hold the FET in conduction for about 200 to 300 microseconds to allow for coil ring out. Hence, recharge of the storage capacitor can begin after about 200 to 300 microseconds as opposed to 1 to 2 milliseconds. The times given in this paragraph are exemplary only. The time between firings depends on the number of cylinders being served by the ignition system and the speed of the engine. The time between polarity reversals also depends upon the speed of the engine as the alternator is driven by the engine. Preferably, the capacitor  32  and resistor  33  at the gate of the FET  23  switch to ground the output of the alternator for about 100 to 200 microseconds after the storage capacitor has discharged. 
     It is possible to modify the circuit shown in the drawing somewhat. For example, it may be possible to use a bipolar transistor in place of the FET. However, if this is done, it must be recognized that the capacitor  32  will discharge both through the resistor  33  and the base of the transistor. This can be considered in the design of the timing circuit with some difficulty since the voltage current relationship through the base is not linear. Moreover, if the temperature of the transistor is not held constant, the current voltage relationship for the base will not be held constant. It is also possible to substitute an optocoupled device having a photo SCR rather than a phototransistor. In this case, attention must be paid to selection of the components such that the current in the photo-SCR falls below its holding current enabling shut-off. 
     It is a significant advantage of this circuit over the prior art including U.S. Pat. No. 4,522,184 that the series diodes needed to sense the discharge of the storage capacitor through the power thyristor have been eliminated. These diodes carried the full 50 ampere discharge current and caused considerable power loss (˜100 watts). 
     Having thus defined my invention with the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.