US3831571A

US3831571A - Variable dwell ignition system

Info

Publication number: US3831571A
Application number: US00359472A
Authority: US
Inventors: H Weber
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1973-05-11
Filing date: 1973-05-11
Publication date: 1974-08-27
Anticipated expiration: 1991-08-27
Also published as: JPS5517231B2; GB1422763A; DE2418177B2; FR2228956B1; IT1004296B; FR2228956A1; DE2418177C3; SE387715B; DE2418177A1; JPS5014937A

Abstract

An electronic triggering circuit for an ignition system develops electric signals which correspond to the closing and opening of breaker points to supply variable dwell (ratio of on-to-off) pulses to the primary winding of the ignition coil at speeds below a predetermined RPM. The circuit operates to supply constant dwell pulses to the primary winding of the ingition coil at higher engine speeds.

Description

United States Patent [19] Weber [11] 3,831,571 Aug. 27, 1974 VARIABLE DWELL IGNITION SYSTEM [75] Inventor: Howard F. Weber, Scottsdale, Ariz. [73] Assignee: Motorola, Inc., Franklin Park, Ill. [22] Filed: May 11, 1973 [21] App]. No.: 359,472

3,749,974 7/1973 Kissel 315/209 T Primary Examiner-Laurence M. Goodridge Assistant ExaminerRonald B. Cox Attorney, Agent, or Firm-Mueller, Aichele & Ptak ABSTRACT An electronic triggering circuit for an ignition system [52] US. Cl 123/148 E, 315/209 R develops electric s gnal which rr spond t th los- [51] Int. Cl. F020 1/00 g and p n ng of r ak r points to supply variable [58] Field ofSearch 123/148 E; 315/209 T, dwell (ratio of on-to-off) pulses to the primary wind- 315 /2Q9 SC ing of the ignition coil at speeds below a predetermined RPM. The circuit operates to supply constant [56] References Cited dwell pulses to the primary winding of the ingition coil UNITED STATES PATENTS at ig er engine speeds. 3,587,552 6/1971 Varaut 123/148 6 Claims, 3 Drawing Figures 3,605,713 9/1971 LeMasters 123/148 E 42 T H 2:25? Y j 24 26 IO HIGH i AND TIME AND V LTAG TRIGGER I4 4; 7

ug E CURRENT REF LIMI TE R PATENTED M 2 3.3%]... 57 1 MI 1 Bf 2 V2 FIG/ 70% DUTY II CYCLE COIL 22 24 26 I0 I v HIGH I AND TIME AND I MoNo DRIvE VOLTAGE 7 a GATE MULTIPl-Y GATE SMTCH TRIGGER I4 4 0 V TIME cuRRENT LIMITER REE I F/G3 CYCLE OF CIRCUIT oPERATIoN /\ICONSTANT TIME MoNo PULSE B C A DISCHARGE c 36 CHARGE OUTPUT OF TRANsIsToR E 50 l TIME TRANSISTOR 74' I I Is OFF I F I I c 78 CHARGE G l H I I j /c 7s DISCHARGE L OUTPUT OF I TRANsIsToR so i I con. TuRN ON TIME L J l h TIME PAIENTEDmazmu saw an z M ETFMOZOE 1 VARIABLE DWELL IGNITION SYSTEM BACKGROUND OF THE INVENTION The Kettering ignition system currently used in vehicles depends upon the energy storage in the primary of a high turns ratio ignition coil to develop the necessary output voltage to fire the spark plug. This energy level is dependent upon the coil current flowing at the time the coil circuit is interrupted by the breaker points to deliver output spark voltage. The coil current that can be reached during the available time is dependent upon coil primary inductance, primary resistance and voltage.

Variable dwell transistorized ignition circuits operating on the Kettering principle have been proposed in which the current through the primary winding of the ignition coil is turned on only shortly before the ignition point and is turned off at the moment the ignition pulse is desired. At low engine speeds, that is when the pickup pulse source for triggering the ignition system operates at a relatively low frequency, the current is connected to the ignition coil long before the ignition time and a strong ignition pulse is provided. At higher speeds, however, the frequency of the triggering pulses is increased and the current is connected to the ignition coil for increasingly shorter periods of time so that the ignition impulse to the coil is weaker at high speeds so that ignition degrades with increased engine speed.

Some prior art electronic ignition systems have provided a constant off time for the coil current, causing an undesirable power dissipation at low speeds or low RPM of operation of the engine. Thisis highly undesirable and creates a heavy drain on the battery of the vehicle in which the ignition system is used. I

It is desirable to employ an ignition system which does not waste power at low RPM and which employs a variable dwell, or ratio of on-time to off-time, which varies in a manner to cause the on time (during which charging current flows through the primary winding of the coil) to be relatively constant up to some preestablished speed of the engine; and which employs a constant dwell at engine speeds above this preestablished speed in order to improve the performance of transistor ignition systems over those previously known.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to pro- In accordance with a preferred embodiment of this invention, input pulses obtained from a magnetic pickup are applied to a monostable pulse generator which produces a train of pulses, each having a fixed duration and occurring at a frequency determined by the RPMs of the internal combustion engine. The output pulses from the monostable pulse generator are applied to a 70 percent duty cycle circuit to initiate a cycle of operation of that circuit. The output of the percent duty cycle circuit and the output of the monostable pulse generator are applied to a time multiplying circuit which produces a variable inhibiting output, the duration of which is a function of the relationship of the width of the pulses from the monostable pulse generator and the output of the 70 percent duty cycle circuit. The outputs of the 70 percent duty cycle circuit and the time multiplying circuit are applied to a coincidence gate which, after termination of the inhibiting signal from the time multiplying circuit, passes the 70 percent duty cycle circuit output to a drive circuit to permit the conduction of direct current through the ignition coil. Thus, the dwell of the signals applied to the drive circuit and, therefore, to the ignition. coil is variable in accordance with the joint operation of the time multiplying circuit and the 70 percent duty cycle circuit. As the engine speed increases, a speed is reached where the pulse width of the pulses from the monostable pulse generator exceed the 30% time of the output of the duty cycle circuit. Then for that and higher speeds, there is no inhibiting signal obtained from the multiplying circuit and the circuit operates as a constant dwell circuit.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a preferred embodiment of the invention;

FIG. 2 is a detailed schematic drawing of the circuit shown in FIG. 1; and

FIG. 3 illustrates various waveforms useful in explaining the operation of the circuits shown in FIGS. 1 and 2.

DETAILED DESCRIPTION Referring now to the drawings, there is shown in FIGS. 1 and 2 a transistor ignition system operating as a variable dwell (variable ratio of on timeto off time of current'flow in the primary winding of the ignition coil of an automobile) ignition system which converts to a constant dwell ignition system at a predetermined engine speed and continues to operate as a constant dwell ignition system at speeds above such predetermined speed. i

FIG. 1 illustrates in block diagram form the circuit components which are used to provide this variable dwell to constant dwell circuit operation. Preferably, a magnetic pickup device (not shown) is positioned within the distributor of a vehicle to produce a sequence of trigger pulses which are applied to an input terminal 10 of the ignition system shown in FIGS. 1 and 2 to control the operation of the system. A suitable pickup device which can be used to produce the trigger pulses to a pulse input terminal 10 may be of the type disclosed in US. Pat. No. 3,390,668, issued to Arthur G. Hufton and assigned to the same assignee of the present application.

Each input trigger pulse triggers a monostable multivibrator 11 into its astable state to produce an output pulse as shown in waveform B of FIG. 3. This output pulse initiates a cycle of operation of a 70 percent duty cycle circuit 12, which preferably is a constant duty cycle circuit of the type disclosed in copending application Ser. No. 308,125, filed Nov. 20, 1972, and assigned to the same assignee of the present application.

percent duty cycle circuit 12 is still in its off condition or in the 30 percent portion of its cycle. If this condition exists, the time multiplying circuit 16 is provided with an enabling output from the AND gate 14 for a time period which extends from the termination of the input pulse from the monostable multivibrator l1 until the 70 percent duty cycle circuit 12 changes to its on state. This time interval is variable since the output pulses from the monostable multivibrator 11 are of fixed duration whereas the total time interval between cycles of operation of the 70 percent duty cycle circuit 12 is longer at low speed operation of the engine and becomes increasingly shorter at higher speed operation.

The time multiplying circuit 16 produces an output inhibiting signal when the 70 percent duty cycle circuit 12 switches to its on state. The duration of the inhibiting signal is a multiple of the time interval which occurred between the end of the monostable output pulse and the turning on of the 70 percent duty cycle circuit 12.

This inhibiting signal from the time multiplying circuit 16 is applied to an AND gate 18 along with the output signal from the 70 percent duty cycle circuit 12. The AND gate 18 also is provided with a third input from a current limiter circuit 20, and this third input normally is an enabling input. The output of the AND gate 18 is supplied to a drive circuit 22, which in turn controls the conduction of a high voltage switch 24 coupled to the primary winding of the ignition coil at a terminal 26.

No current flows through the coil from the terminal 26 until the switch 24 is switched on by the drive circuit 22. This occurs only when all three inputs to the AND gate 18 are enabling inputs. Such a condition exists when the 70 percent duty cycle 12 is in its on" state and the inhibiting output from the time multiplying circuit l6 terminates. When this occurs, the AND gate 18 causes the drive circuit 22 to turn on the high voltage switch 24 and current flows through the coil. This condition exists until the next pulse from the monostable multivibrator circuit 1] occurs at which time the cycle of operation repeats.

At some high speed the of time of the 70 percent duty cycle circuit 12 becomes equal to the duration of the monostable pulse width. At this and higher speeds of the engine, the time multiplying circuit 16 is rendercd ineffective and no inhibiting signals are supplied by that circuit. The output of the AND gate 18 then follows the constant dwell output signals from the 70 percent duty cycle circuit 12 to operate the drive circuit 22 and the high voltage switch 24 as a constant dwell circuit for such high engine speeds.

After the last trigger pulse appears on the input terminal to the monostable multivibrator 11, the circuit operates to predict the occurrence of another trigger pulse and the high voltage switch 24 remains conductive. This causes current to be continuously supplied through the coil from the terminal 26. If no trigger pulse appears on the terminal 10 to tum off the coil circuit, the transistors in the high voltage switch must dissipate high power for a long period of time. This is undesirable. Thus, it is necessary to turn off the high voltage switch 24 if the time interval between successive pulses on the terminal 10 exceeds the longest interval which would occur in normal operation. For this reason, the output of the monostable multivibrator 11 also is applied to a time limiter reference circuit 28 which is continuously reset by the output pulses from the monostable circuit 11.

If the time interval between output pulses from monostable circuit 11 exceeds the maximum amount which should occur in operation of the system, the time limiter reference circuit 28 causes a current limiter circuit 20 to produce a gradually increasing inhibiting signal to the AND gate 18. A gradual or slow reduction in the output of the drive circuit 22 results, which in turn relatively slowly turns off the high voltage switch 24. The current limiter circuit 20 also operates in response to the current flowing through the switch 24 to limit the maximum current by reducing the drive circuit output through the gate 18 whenever such maximum current is sensed.

It should be noted that although the above description refers to the constant duty cycle circuit 12 as a percent duty cycle circuit, this percentage is an arbitrary one and can be varied in accordance with the particular operating conditions desired in actual applications of the circuit. A 70 percent duty cycle operation for the circuit 12 is one which typically is within the range of operation which would be encountered.

The circuit described in conjunction with the block diagram of FIG. 1 can be implemented in monolithic integrated circuit form as illustrated in FIG. 2 and the operation of the detailed schematic circuit shown in FIG. 2 is given in conjunction with the waveforms shown in FIG. 3 for a better understanding of the system.

Referring now to FIG. 2, the various portions of the detailed schematic diagram depicted therein are provided with the reference numbers which correspond to the circuit functions of FIG. 1 to facilitate correlation between the circuits of FIG. 2 and FIG. 1.

In FIG. 3 waveform A shows the time period for a single cycle of the circuit operation of the circuit shown in FIGS. 1 and 2. This cycle is not a complete cycle of the rotor of the distributor but represents the cycle .required to produce each individual spark in the firing sequence for operating the internal combustion engine with which the circuit is used. The cycle of circuit operation is not of a fixed time duration but is longer for low speed operation of the engine and is shorter for high speed operation. Thus, the time duration which is indicated in FIG. 3 is correct for only a single speed of operation of the engine. It is to be understood that this time frame can be greater or less than that which is illustrated.

The pulse of waveform B is the only pulse of fixed duration which is illustrated in the waveforms of FIG. 3. All of the other time periods illustrated vary with respect to the monostable output pulse of waveform B depending upon the speed of operation of the engine.

The pulses B from the output of the monostable multivibrator 11 are applied through isolating

resistors

30 and 32 to the inputs of the 70 percent duty cycle circuit 12 and the time multiplying circuit 16. The AND gate 14 of FIG. 1 is illustrated as a junction 14 in FIG. 2 since this gate is not a true logic AND gate, but instead comprises a pair of analog inputs to the time multiplying circuit 16 of FIG. 2. The functional operation of this portion of the circuit of FIG. 2, however, is the same as the portion of the circuit of FIG. 1 which has been described in conjunction with the AND gate 14.

When the positive output pulse of the monostable multivibrator circuit 11 is applied to the input of the constant duty cycle circuit 12, it causes an input transistor 34 to be rendered conductive to initiate a discharge cycle of a timing capacitor 36. This discharge cycle is illustrated in waveform C of FIG. 3. The rate of the discharge is controlled by a current source consisting of a PNP transistor 38 connected in series with a current limiting resistor 40 to a source of positive potential (not shown) on the positive battery tenninal 42. The transistor 34 completes the discharge path to ground. The value of the resistor 40 and the bias on the base of the transistor 38 determine the rate of discharge, and the parameters of these circuit components can be changed to vary the rate of discharge.

When the transistor 34 is rendered conductive to commence the discharge of the capacitor 36, the current flow through the capacitor 36 is reversed from the direction which it previously was flowing and causes a negative bias to be applied to the base of an output transistor 44 for the constant duty cycle circuit 12. The transistor 44 is then rendered nonconductive and the potential on its collector rises to near the full positive potential available on the terminal 42 which is coupled to the collector of the transistor 44 through a collector load resistor 46. This positive potential is fed back through a coupling resistor 48 to the base of the transistor 34 to maintain the transistor 34 conductive following termination of the input pulse from the monostable multivibrator 11. This is illustrated by a comparison of waveforms B and C of FIG. 3 which shows that the capacitor 36 continues to discharge through the transistor 34 after termination of the pulse shown in waveform B.

When the transistor 44 is rendered nonconductive, an NPN transistor 50 coupled to the collector of the transistor 33 is rendered conductive to cause a near ground potential. to appear on its collector. The collector of the transistor 50 is connected through an isolating resistor 52 to the junction 14, so that as long as a positive pulse appears at the output of the monostable multivibrator 11, the junction 14 remains at a positive potential.

After the capacitor 36 has completed discharging to the point where the base of the transistor 44 is forward biased relative to its emitter, the transistor 44 then is rendered conductive. The drop in potential on the collector of the transistor 44 fed back to the base of the transistor 34 once again renders the transistor 34 nonconductive. The capacitor 36 commences charging in the opposite direction, at a rate controlled by the parameters of a PNP current source transistor 54 and a resistor 56, through the base-emitter junction of the transistor 44. This charging rate is illustrated in waveform D of FIG. 3; and for the purposes of the discussion of the preferred embodiment, the charge rate is selected to be approximately percent of the total timing cycle shown in waveform A, while the discharge rate of the capacitor 36 comprises 30 percent of that cycle. This ratio is determined by selection of the relative values of the resistors 40 and 56, with the resistor 56 having the higher resistance in the example given.

Control of the current conduction of the

current source transistors

38 and 34 is effected by a divider circuit consisting of a resistor 64, a resistor 58, a PNP transistor diode 60 and another resistor 62 connected in series between the source of positive potential and ground. In addition to providing the bias for the

current source transistors

38 and 54, this circuit also supplies a corresponding bias to an additional pair of PNP

current source transistors

66 and 68 in the time multiplying circuit 16. The

transistors

66 and 68 operate in a manner similar to the operation of the

transistors

54 and 38 in the constant duty cycle circuit 12 and supply currents of values determined by the relative values of a pair of

resistors

70 and 72 connected in series with the

transistors

66 and 68, respectively, to the resistor 64.

The time multiplier circuit 16 is similar in operation to the operation of the constant duty cycle circuit 12 and includes an input transistor 74 and an output transistor 76 which correspond functionally to the transistors 34 and 44, respectively. ln the circuit 16, however, there is no feedback from the collector of the transistor 76 to the base of they transistor 74; so that the conductivity of the transistor 74 is determined solely by the relative values of potential applied to its base and emitter. The equilibrium state of the time multiplying circuit 16 just prior to the application of each pulse from the monostable multivibrator l 1 is a state in which both the transistors 74 and 76 are conductive and the charge storage capacitor 78 is at an equilibrium condition (the same potential at both ends), with no charging taking place in either direction. During the time interval when the transistor 44 is conductive, the transistor 50 is non conductive. This causes a relatively high positive potential to appear on its collector, and this potential is applied by way of the resistor 52 to the base of the transistor 74 so that the transistor 74 is held conductive.

When the next pulse from the monostable multivibrator 11 is applied to the base of the transistor 34, the transistor 44 becomes nonconductive and causes the transistor 50 to be conductive to drop the potential on the collector thereof to near ground potential. This does not cause the transistor 74 to be made nonconductive at this time, however, since the positive pulse from the monostable multivibrator also is applied to the junction 14 simultaneously with its application to the base of the transistor 34. Thus, there is no change in the state of operation of the time multiplying circuit 16 so long as the pulse from the monostable multivibrator circuit 11 remains. At the time the pulse from the monostable multivibrator circuit 11 terminates, however, the transistor 74 is rendered nonconductive provided that the

transistors

34 and 50 also are nonconductive at this time. This is true so long as the capacitor 36 is in the discharge cycle of operation illustrated in waveform C. Thus, the transistor 74 is rendered nonconductive, as indicated in waveform F from the time that the pulse from the monostable multivibrator (waveform B) terminates until the next charge cycle of the capacitor 36 begins (waveform D).

During the time that the transistor 74 is rendered non-conductive, the capacitor 78 is charged through a charge circuit including the resistor 70, the current source transistor 66, and the base-emitter junction of the transistor 76. When the transistor 76 is conductive, the potential on its collector is near ground potential and an output transistor 80 for the time multiplier circuit is rendered nonconductive causing the potential on its collector to be high. This is indicated in the initial portion of waveform I which illustrates the output potential on the collector of the transistor 80.

When the capacitor 36 of the constant duty cycle circuit l2 commences charging, the output transistor 50 for the constant duty cycle circuit again is rendered nonconductive, causing a positive potential to appear on its collector. This in turn causes the transistor 74 once again to be rendered conductive initiating the discharge cycle of the capacitor 78 which is illustrated in waveform H of FIG. 3. When this discharge cycle commences it causes the bias on the base of the transistor 76 to be negative with respect to the ground potential on its emitter, thereby driving the transistor 76 to a nonconductive state and rendering the transistor 80 conductive. This latter condition is illustrated in the center portion of waveform I.

The capacitor 78 discharges at a rate determined by the parameters of a discharge circuit including the current source transistor 78 and the resistor 72. As illustrated in waveforms G and H, the rate of discharge of the capacitor 78 is indicated as longer than the rate of charge from the same potential. The total length of time for the capacitor 78 to discharge to the point where the base of the transistor 76 once again is forward biased relative to its emitter is determined both by the rate of discharge and by the final charge which the capacitor 78 reached during the time interval that the transistor 74 was nonconductive. This final charge level varies in accordance with the duration of time that the transistor 74 conducts, so that the total discharge time period also varies in accordance with the maximum charge reached by the capacitor 78 during the charge portion of the cycle of operation.

For high speed operation of the engine, the constant duty cycle circuit ultimately reaches a point where the length of time to discharge the capacitor 36, as shown in waveform C, becomes equal to or less than the fixed width or time duration of the pulse from the output of the monostable multivibrator shown in waveform B. When this occurs, there is no time when the transistor 74 is rendered nonconductive since there then is a continuous overlap between the pulses from the monstable multivibrator 11 and the positive output of the transistor 50 applied to the junction 14. Then the capacitor 78 always is at an equilibrium state in which both the transistors 74 and 76 are conductive, and the transistor 80 is continuously non-conductive. In such a situation, the waveform I then is continuously at the positive potential throughout the entire cycle of operation. This only occurs for a predetermined speed of the engine relative to the width of the output pulses from the monostable multivibrator. The significance of this operation will become apparent from the subsequent description of the operation of the remainder of the circuit.

As stated above in conjunction with the description of operation of the block diagram circuit in FIG. 1, it can be seen that the output of the constant duty cycle circuit 12 and the output of the time multiplying circuit 16 both are applied to respective inputs of an AND gate 18. That AND gate 18 is illustrated in' FIG. 2 as comprising three

diodes

82, 84, and 86. The diode 86 is connected to the output of the current limiter circuit 20, the operation of which will be described subsequently. At the present time, assume that the diode 86 is back biased with a positive potential applied to its cathode. This operates to enable the AND gate 18.

Whenever all three

diodes

82, 84 and 86 are reverse biased with positive potentials applied to their cathodes, a positive potential is applied from the output of the AND gate 18 to forward bias an NPN input transistor. Thus, the transistor 88 is rendered conductive only when both the transistors and 80 are nonconductive. Examination of waveforms E and I indicates that this occurs only during the time interval indicated in waveform J as coil turn-on time.

The circuit operation can be considered to be such that the primary control of the conduction of the input transistor 88 in the drive circuit 22 is obtained from the collector of the transistor 50 in the constant duty cycle circuit 12. In the absence of the time multiplying circuit 16, the transistor 88 would be rendered conductive for percent of the duty cycle of operation established by the circuit 12.

The discharge time interval for the capacitor 78, however, operates to cause the transistor to be conductive during a portion of the time that the output transistor 50 is non-conductive. This causes a near ground potential to be applied through the diode 84 to the base of the transistor 88 causing it to remain nonconductive until the capacitor 78 discharges to the level where the transistor 76 becomes conductive and I the transistor 80 once again becomes nonconductive,

as indicated in waveforms H and I. Thus, the time multiplying circuit 16, through the AND gate 18, inhibits the turning on of current through the primary winding of an ignition coil 90 connected to the terminal 26 until the discharge of the capacitor 78 is complete.

The particular form of the drive circuit 22 and high voltage switch 24 which control the conduction through the ignition coil 90 is not important, and the circuit which is shown in FIG. 2 is illustrative of the type of circuit which can be used. So long as the emitter-follower transistor 88 is nonconductive, an NPN transistor 92 controlled by the transistor 88 also is nonconductive. The collector of the transistor 92 is coupled to the base of a transistor 94 which then is rendered conductive for this state of operation. This in turn causes an NPN emitter follower transistor 95 to be rendered nonconductive. The emitter follower transistor 95 is coupled to the high voltage NPN switching transistor 24 to render that transistor nonconductive so long as the transistor 88 is nonconductive.

When the transistor 88 conducts, the conductive states of all of the

transistors

92, 94, 95 and 24 then change. The transistor 92 conducts, and the transistor 94 is rendered nonconductive which in turn causes both of the

transistors

95 and 24 to conduct. When the high voltage switching transistor 24 conducts, current flows from the terminal 42 through the primary winding of the ignition coil 90, the terminal 26 and the transistor 24 through its emitter resistor 96 to ground. The

duration of time over which this current flows is illustrated in waveform J.

The circuit continues in this state of operation until the next pulse from the monostable multivibrator 11 occurs. Then the output state of the constant duty cycle circuit 12 changes to cause ground potential to be applied through the diode 82 from the transistor 50 to the base of the transistor 88 causing it to become nonconductive and the high voltage switch transistor 24 once again is rendered nonconductive. The collapse of flux which then occurs in the primary winding of the ignition coil 90 is applied to the secondary winding to produce the desired spark.

The time duration during which current flows through the primary winding of the ignition coil 90, as shown in waveform J, is selected to be sufficient to provide a proper ignition spark. The parameters of the charge and discharge cycles of the constant duty cycle circuit 12 and the time multiplier circuit 16 preferably are selected to cause the multiplier ratios and the constant duty cycle ratios to be such that the coil 90 turnon time of waveform J becomes a constant on time over the limits of the lower speed range determined by the fixed width of the output pulses from the monostable multivibrator 11. Once that a speed is reached where the time multiplier circuit 16 ceases to operate, the turn-on time of the coil becomes a constant duty cycle time rather than a constant on time as described previously To maintain stable operating voltages in the circuit and further to provide voltage protection for the dwell circuitry, a zener diode 100 is connected between ground and through the resistor 64 to the positive voltage supply terminal 42. A second zener diode 101 is coupled between ground and a resistor 102 connected to the positive voltage supply terminal 42 and establishes the collector potentials for the

transistors

92 and 94 in the driver circuit 22. In addition, another zener diode 104 is connected across the collector and emitter of the transistor 95 to establish a maximum voltage limit on the base of the transistor 24 that keeps the output transistor 24 within its safe operating area during the turn on or turn off conditions of operation of the circuit.

Protection of the output transistor 24 from high voltage transients produced during the collapse of the flux in the coil 90 is provided by a pair of parallelconnected reversely poled diodes 106 and 107.

The portion of the circuit which has been described thus far is all that is necessary for normal operation of an engine to provide electronic ignition for the spark plugs of the engine. It will be noted, however, that after the occurrence of each trigger pulse or output pulse from the monostable multivibrator 11, the circuit is in a condition in which the transistor 24 is conductive and current flows through the primary winding at the ignition coil 90. This current flow is initiated at a time determined by a prediction of the circuit as to when the next trigger pulse from the monostable multivibrator 11 will occur. If no trigger pulse occurs as predicted to turn off the coil circuit, the output transistor 24 will dissipate high power for a long period of time. This is undesirable both as a waste of power from the battery coupled to the terminal 42 and from the standpoint that it is harmful to the transistor and could result in its destruction.

Therefore, it is desirable to provide a circuit to slowly turn off the current through the coil if no input pulse is received from the monostable multivibrator 11 within a time interval which is greater than the longest interval which should occur in normal operation of the system. A slow tum-off is required to avoid a rapid collapse of flux in the coil 90 which would produce a false ignition pulse. This is the function which is performed by the time limiter reference circuit 28. The pulses from the output of the monostable multivibrator circuit 11 which are applied to the bases of the

transistors

34 and 74 also are applied through a coupling resistor to the base of an NPN transistor 111 which also is connected through a resistor 112 to ground. In the absence of any pulses from the monostable multivibrator 11, the base of the transistor 111 is at ground potential and the transistor 111 is not conductive. Its collector potential then rises to a positive value, provided the transistor 80 is nonconductive. Whenever a pulse from the monostable multivibrator 11 appears, however, the transistor 1 1 1- is biased into conduction and causes a near ground potential to be applied to the base of a normally nonconductive PNP transistor 113. The collector potential for the transistors 1 1 1 and 80 is derived from the terminal 42 through resistors 115, 116 and 118 coupled directly to the collector of the transistor 1 11 and through a diode 119 to the collector of the transistor 80.

Thus, whenever either of the transistors 80 or 111 are rendered conductive, a near ground potential is applied from the collector of those transistors to the base of the transistor 113 to render it conductive. At all other times the transistor 113 is nonconductive. Whenever the transistor 113 conducts, it applies a charging current to a capacitor 114 (typically 20 microfarads) to rapidly charge the capacitor through a low impedance path. Whenever the transistors 111 and 80 both are nonconductive, the capacitor 1 14 discharges through a high impedance resistor 116 connected in parallel with the capacitor 114. Under normal operation of the circuit the transistor 113 is rendered conductive in each cycle of operation at least for the duration of the output pulses from the monostable multivibrator 11 to maintainthe charge on the capacitor 114. Under normal operating conditions, the intervals during whichthe transistor 113 is nonconductive are insufficient to permit the capacitor 114 to discharge to more than a small amount through the resistor 116.

The junction of the capacitor 114 and the emitter of the transistor 113 is connected through a coupling resistor 120 to the base of an NPN control transistor 122. After the first pulse from the multivibrator 11 and for normal operation, the bias applied to the base of the transistor 122 from the capacitor 114 is sufficient to maintain a forward bias on the base of the transistor 122 and it is conductive. When the transistor 122 is conductive, current flows through its emitter circuit which includes a transistor diode 124 connected in series with a pair of

resistors

125 and 126 to ground. The junction of the transistor diode 124 with the resistor 125 is coupled to the base of a further current control transistor 127 which has the sanne current flowing through it so that the transistor 127 is conductive for normal operation of the circuit. Whenever the transistor 127 is fully conductive, an NPN control transistor 129, the base of which is coupled to the collector of the transistor 127, is rendered nonconductive, and a positive potential appears on its collector. This positive potential is applied to the cathode of the diode 86 in the AND gate 18, reverse biasing the diode 66; so that the transistor 88 of the driver circuit 22 responds to the inputs applied through the

other diodes

84 and 82 in the AND gate 18.

If input pulses from the monostable multivibrator 11 do not occur within a pre-established minimum time interval, the transistor 113 remains continuously nonconductive; and the charge on the capacitor 114 slowly reduces as it discharges through the resistor 116. This linearly reduces the forward bias of the transistor 122 until it becomes nonconductive. The conductivity of the transistor 127 follows that of the transistor 122 to linearly increase the forward bias applied to the transistor 129 to linearly increase its conduction. As the transistor 129 is rendered increasingly conductive, the diode 86 is increasingly forward biased to linearly reduce the conductivity of the transistor 88 to a state of nonconduction irrespective of the inputs to the

diodes

82 and 84 in the AND gate 18. Thus current flow through the primary winding of the ignition coil 90 is relatively slowly reduced and terminated and the circuit then is in a standby condition ready for the next pulse from the monostable multivibrator 11.

Current limiting is also effected by the circuit 20 when the output power switching transistor 24 attempts to conduct a current greater than a pre-established mount to which the current limiter circuit 20 is adjusted. To accomplish such current limiting, the emitter of the switching transistor 24 supplies the coil current through a resistor 131 to ground. The junction of the emitter of the transistor 24 with this resistor is coupled through a resistor 130 to the emitter of the transistor 127. The amount of current supplied by the transistor 127 is determined by the circuit elements connected to the transistor 122, as previously described.

Whenever the current flowing from the emitter of the transistor 24 exceeds the current supplied by the transistor 127, the emitter of the transistor 127 is provided with an increasing reverse bias to reduce its conductivity. This in turn causes the potential on its collector to rise. so that the transistor 129 is rendered conductive (but not saturated) in an amount determined by the magnitude of the current supplied by the transistor 24 causing the reverse bias of the transistor 127. As the transistor 129 commences conduction, a linear reduction in the magnitude of the signal applied from the AND gate 18 to the base of the transistor 88 occurs. This causes the conductivity of the transistor 88 to be limited or reduced to reduce the drive signal applied to the output transistor 24 thereby reducing its conductivity. This in turn results in effecting the desired current limiting once the maximum current to which the circuit is adjusted has been reached.

I claim: 1. An electronic ignition system for charging and discharging an ignition coil to produce a spark to operate an internal combustion engine, including in combination:

Circuit means for conducting direct current through the ignition coil to charge the same in response to a control signal;

pulsing means for producing pulses of a predetermined duration at a frequency proportional to engine RPM;

coincidence gate means with at least first and second inputs, and an output coupled with said circuit means and producing said control signal on the output thereof upon coincidence of a predetermined relationship of signals on said first and second inputs thereof;

constant duty cycle means having an input coupled with said pulsing means and having an output, said duty cycle means responsive to said pulses for producing an output signal on the output thereof indicative of a first state for a predetermined precent of the time interval between the beginning of successive pulses from said pulsing means and indicative of a second state the remainder of said time interval between said pulses, the output of said duty cycle means coupled with the first input of said gate means;

time multiplying means operated in response to said pulsing means and said constant duty cycle means, having an input coupled with said pulsing means and further coupled with the output of said duty cycle circuit means, and having an output, said time multiplying circuit means producing a gate inhibiting signal on said output, said gate inhibiting signal commencing with the output of said constant duty cycle means changing from an output indicative of said first state to an output indicative of said second state and said inhibiting signal having a variable time period which has a predetermined relationship with the time interval extending from the end of a pulse from said pulsing means until the output signal of said duty cycle circuit means changes from said first state to said second state; and

means for coupling the output of said time multiplying means with the second input of said gating means, said gating means producing said control signal upon coincidence of an output indicative of said second state of operation from said constant duty cycle means and termination of said gate inhibiting signal from said time multiplying means.

2. The combination according to claim 1 wherein said time multiplying means includes means for causing said variable time period of said gate inhibiting signal to be a predetermined multiple of the time period extending from the end of a pulse from said pulsing means until the output of said duty cycle circuit means changes from an output indicative of said first state to one indicative of said second state, no gate inhibiting signal being produced whenever said pulse from said pulsing means has a duration greater than the duration of an output signal indicative of said first state on the output of said constant duty cycle means.

3. The combination according to claim 1 wherein said pulsing means includes a monostable multivibrator, the output of which is coupled with the inputs of said constant duty cycle means and said time multiplying means.

4. The combination according to claim 1 wherein said coincidence gate means has a third input and further including a time interval measuring means coupled with the output of said pulsing means and having an output coupled with the third input of said coincidence gate means, said time interval measuring means normally enabling said coincidence gate means and producing an inhibiting signal on the output thereof in response to a predetermined time interval between successive pulses from said pulsing means.

14 the output of said pulsing means and having an output coupled with said current limiter means for causing said current limiter means to apply an inhibiting signal to the third input of said gate means to terminate said control signal in response to a predetermined time interval between successive pulses from said pulsing means.

Claims

1. An electronic ignition system for charging and discharging an ignition coil to produce a spark to operate an internal combustion engine, including in combination: Circuit means for conducting direct current through the ignition coil to charge the same in response to a control signal; pulsing means for producing pulses of a predetermined duration at a frequency proportional to engine RPM; coincidence gate means with at least first and second inputs, and an output coupled with said circuit means and producing said control signal on the output thereof upon coincidence of a predetermined relationship of signals on said first and second inputs thereof; constant duty cycle means having an input coupled with said pulsing means and having an output, said duty cycle means responsive to said pulses for producing an output signal on the output thereof indicative of a first state for a predetermined precent of the time interval between the beginning of successive pulses from said pulsing means and indicative of a second state the remainder of said time interval between said pulses, the output of said duty cycle means coupled with the first input of said gate means; time multiplying means operated in response to said pulsing means and said constant duty cycle means, having an input coupled with said pulsing means and further coupled with the output of said duty cycle circuit means, and having an output, said time multiplying circuit means produciNg a gate inhibiting signal on said output, said gate inhibiting signal commencing with the output of said constant duty cycle means changing from an output indicative of said first state to an output indicative of said second state and said inhibiting signal having a variable time period which has a predetermined relationship with the time interval extending from the end of a pulse from said pulsing means until the output signal of said duty cycle circuit means changes from said first state to said second state; and means for coupling the output of said time multiplying means with the second input of said gating means, said gating means producing said control signal upon coincidence of an output indicative of said second state of operation from said constant duty cycle means and termination of said gate inhibiting signal from said time multiplying means.

5. The combination according to claim 1 further including current limiter means coupled with said circuit means and a third input of said gate means and responsive to current in excess of a predetermined value in said circuit means for applying a signal to the third input of said gate means to effect a change in said control signal.

6. The combination according to claim 5 further including a time interval measuring means coupled with the output of said pulsing means and having an output coupled with said current limiter means for causing said current limiter means to apply an inhibiting signal to the third input of said gate means to terminate said control signal in response to a predetermined time interval between successive pulses from said pulsing means.