GB2138495A

GB2138495A - Automotive ignition systems

Info

Publication number: GB2138495A
Application number: GB08308405A
Authority: GB
Inventors: Heinz Lehning
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1983-03-26
Filing date: 1983-03-26
Publication date: 1984-10-24
Also published as: EP0127205B1; SG41189G; GB8308405D0; GB2138495B; EP0127205A2; DE3475934D1; EP0127205A3

Abstract

A periodic signal provided by a magnetic sensor 3 is compared with two reference potentials by means of two comparators 13 and 15. An output signal provided by one comparator 13 is used to enable the supply of current to an ignition coil 39 whilst an output signal provided by the second comparator 15 is used to switch off the current in the ignition coil. In this way the switching on and off of the flow of current through the ignition coil are made independent of one another enabling more accurate ignition timing to be obtained at all engine speeds. The coil current is regulated by a circuit including an operational amplifier 58 and the reference potential supplied to the comparator 13 is increased as the speed of the engine decreases so that the time period for which the coil current is at its maximum value is a constant proportion of the period of the sensor waveform despite changes in engine speed. To prevent malfunction of the system at low engine speeds and low supply voltage, a switch 69 controlled by a speed detector 70 and a voltage detector 75 are provided to step down the waveform Vsc supplied to the comparator 13. An over voltage protection circuit 84-90 is provided for Darlington transistors 28, 29. <IMAGE>

Description

1 GB 2 138 495 A 1

SPECIFICATION

Ignition circuit This invention relatesto an ignition circuit suitable for use in an automotive ignition system having a distributor which is provided with a magnetic sensor.

Automotive ignition circuits utilising magnetic sensors are known. The magnetic sensor replacesthe contact breaker set within the distributor and provides a periodic waveform output.

The periodic waveform is compared with a reference, level and the result of this comparison is used to control both the dwell period i.e. the time during which currentflows in the ignition coil and the spark, when the coil current is switched off.

The dwell time and the spark are defined by the single reference level and this results in this known circuit having disadvantages. Timing errors arise and result in sparks coming too early at low engine speed. Also since the dwell time cannot be regulated overthe full speed range the known circuit uses more power than is desirable.

This invention seeks to provide an automatic ignition circuit suitable for use with a magneticsensor and in which the above mentioned disadvantages are mitigated.

the said predetermined period following the setting and resetting of the flip-flop.

The gating means maybe blocked by an output signal fed from a monostable, triggered in response to the setting and resetting of the f lip-f lop.

Means maybe provided for maintaining the flip-flop in a reset condition during periods when no periodic signal is provided by the magnetic sensor.

Means maybe provided for adjusting the effective value of the first reference potential level in orderto control the point in time during a cycle of the periodic signal atwhich the supply of current to the ignition coil is enabled.

The Means for adjusting the effective value of the first reference potential may conveniently comprise meansfor adding a control potential to the periodic signal.

The control potential maybe derived from charge storage means arranged to be charged during a first portion of each cycle of the periodic signal and discharged during a second portion.

The charge storage means may be charges in responseto the periodic signal crossing the second reference potential level in a first predetermined direction and may be discharged in responseto the current in the ignition coil attaining a predetermined value, preferably a limiting value.

According to the present invention there is provided 90 Means responsiveto distributor speed and/or igni an automotive ignition circuitfor controlling the supply of currentto an ignition coil and suitable for use in an ignition system in which a distributor is provided with a magnetic sensor, the circuit compris ing a first comparatorfor comparing the amplitude of a periodic signal fed from the magnetic sensorwith a first reference potential level and for providing a first output signal indicative of the amplitude of the periodic signal crossing the f irst reference potential level, a second comparator for comparing the ampli tude of the periodic signal with a second reference potential level and for providing a second output signal indicative of the amplitude of the periodic signal crossing the second reference potential level and means for enabling and disabling the supply of cu rrentto the ignition coil in response to the first and second output signals respectively.

In an embodiment of the invention the means for enabling and disabling the supply of currentto the ig nition coil includes a set-resetf lip-flop arranged to 110 be set in response to the said f irst output signal and reset in responseto the said second output signal.

Differentiating means may be provided between the set-resetflip-flop and the first and second comparatorsfor differentiating the first and second output signals.

Means maybe provided for preventing a change of state of the flip-f lop for a predetermined period following the setting and resetting in response to the first and second output signals respectively.

The means for preventing the change of state of the set-resetflip-flop may comprise gating means positioned between the differentiating means and the set-resetflip-flop, the gating means being blocked for tion circultsupply potential may be provided for periodically charging the charge storage meansto a reference potential during periods when the distributor speed and/orthe supply potential fall below predetermined values.

The means for enabling and disabling the supply of currentto the ignition coil may include one or more output transistors and means may be provided for protecting the said one or more output transistors against potentials greaterthan a predetermined value.

An exemplary embodiment of the invention will now be described with referenceto the drawings in which:

Figure 1 is a schematic block diagram of an automotive ignition circuit in accordance with the present invention; and Figures 2 - 5 are explanatory waveform diagrams illustrating the operation of the ignition circuit of Fig u re 1.

Referring nowto Figure 1 the ignition circuit shown therein is particularly suitable for production on an integrated circuit chip illustrated by a dashed line box 1,those elements lying within the box 1 being located on the chip. The circuit operatesfrom a supply potential line 2 which is coupled to an automotive battery via an ignition switch (not shown).

A magnetic sensor3 which replacesthe contact breaker set in the automotive distributor has output terminals 4 and 5 which are respectively connected via resistors 6 and 7 to input pins 8 and 9 of the chip 1. A further resistor 10 is coupled between the pins 8 and 9, the resistors 6,7 and 10 being provided in orderto adaptthe amplitude of the signal provided bythe magnetic sensor 3to the integrated circuit chip 1.

The drawing(s) originally filed were informal and the print here reproduced is taken from a later filed formal copy.

2 GB 2 138 495 A 2 The pin 9 is coupled via a resistor 11 to a positive input 12 of a firstcomparator 13 and also to a positive input 14 of a second comparator 15.

Asecond and negative inputterminal 16 of the comparator 13 receives a reference potential from voltage sources 17 of values V, and V4 coupled between the terminal 16 and ground. A second and negative inputterminal 18 of the comparator 15 also receives a reference potential of value V2from one terminal of a voltage source 19, a second terminal of which is coupled through a resistor 20 to the pin 8.

Referring also now to Figure 2, when the cam of the distributor is rotating a cyclic waveform as shown in line A of Figure 2 will be developed between the terminals 4 and 5 of the magnetic sensor 3. In orderto control the dwell time, that is the time during which currentflows through the ignition coil, a feedback voltage Vc derived in a manner to be described is applied to a terminal 21 and is added to the cyclic waveform derived from the magnetic sensor 3 so that 85 anew cyclic signal Vsc is applied to theterminal 12 of the comparator 13 and is compared with the reference potential provided bythe sources 17 atthe negative input 16 of the comparator 13.

The comparator 13 provides a rectangular output waveform having a transition each time the cyclic waveform applied to the positive terminal 12 of the comparator crosses the reference potential applied to the negative terminal 16. This outputwaveform of the comparator 13 is differentiated by a differentiator22 which provides an output pulse on each rising edge of the waveform output of the comparator 13 indicative of the cyclicwaveform crossing the reference poten tial in a first predetermined direction. The differenti ated output pulse provided by the differentiator 22 is fed to one input of an AND gate 23 whose output is connected to a set input 24 of a set-reset f lip-f lop 25.

The flip-flop 25 has an output 26 connected to a base 27 of one transistor28 of a pair of Darlington connected outputtransistors 28 and 29, the transistor 105 29 having an emitter 30 which is connected to an output pin 31 of the ignition circuit. The transistors 28 and 29 have collectors 32 and 33 respectively, the collector 32 of the transistor 28 being connected to a pin 34 of the integrated circuit whilst the collector 33 of 110 the transistor 29 is connected to a pin 35. The pins 34 and 35 are in operation coupled to the supply potential 2 via load resistors 36 and 37 respectively. The output pin31 is coupled to a switch 38 for switching current flow through the ignition coil 39. The current switch 38 115 also consists of two Darlington connected transistors and 41, the transistor 41 having its collector electrode 42 connected to one terminal of the ignition coil 39 a second terminal which is coupled to the supply potential 2, while emitter electrode 43 of the transistor 41 is connected to one terminal of a resistor 44whose otherterminal is connected to ground reference potential.

Currentthroughthe ignition coil 39 isturned off by means of the comparator 15which comparesthe cyclicsignal provided bythe magneticsensor3 and appearing between the pins 8 and 9 differentially with the reference potential provided bythe source 19. The comparator 15 also provides a rectangular output waveform having a transition edge each time the cyclic waveform crosses the value of the reference potential.

The output waveform of the comparator 15 is differentiated in a differentiator45 which provides an output pulse for each negative going edge of the rectangularwaveform indicative of the cyclic waveform failing belowthe value of the reference potential. Each output pulse provided by the differentiator 45 is connected to one input of an AND gate46whose output isfed via an OR gateCtothe resetinput48of the flip-f lop 25. Each pulse atthe reset input 48 of the fl ip-fl op 25 is operative to reset the fl ip-flop and toturn off the transistors 28 and 29 and hence thetransistors 40and41 andtocutoffthecurrentthroughthe ignition coil 39thereby producing an ignition spark.

In orderto protectthe ignition circuit againstnoise the setand reset inputs 24and 48 respectively& the flip-flop 25 are connected to respective inputs ofarrOR gate 49whose outputtriggers a monostable multivibrator50which in turn has an output51 connected.to a second input52 of theAND gate 23 and to a second input53 of theAND gate46. Each time the flip-flopr 25 changesstatethe monostable 50 istriggered and provides an output pulsewhich blocks each of the AND gates 23 and 46 during the period of the output pulse of the monostable 50 which is typically 250 microseconds. This prevents any further changes of state of theflip-flop 25 during this 250 microsecond period and provides the flip-flop 25 with some noise immunity.

The ignition circuit of the invention as shown in Figure 1 provides accurate ignition timing at different speeds with relatively low power consumption by regulation of the dwell time in accordance with the engine speed. The dwell time is soughtto be controlled in such a mannerthat the time period T, during which currentthrough the ignition coil 39 has reached a limiting value is kept at a constant proportion, typically8% of the period T of the cyclic waveform provided by the magnetic sensor 3.

The potential Vcapplied totheterminal 21 and addedtothe signal provided bythe sensor3 is dependent upon engine speed and the effectofthis added engine speed dependent potential is effectively to vary the reference potential level forthe comparator 13 in dependence upon engine speed. In waveform, A as shown in Figure 2 the lineV2 represents thefixed value of the reference potential forthe comparator 1.5 provided bythe source 19, the line V,, 1 represents'the effectivevalue ofthe reference potential ofthe comparator 13 for high engine speed,whilstthe rine Vw2 represents the effective value ofthe reference potential forthe comparator 13 at low engine speed.

As can be seen the effective value of the reference potential for the comparator 13 moves. up and down the waveform provided by the magnetic sensor3 as the engine speed decreases and increases. The range of on-timing, that is the range of variation intime at which currentth rough the ignition coil 39 is switched on by the output from the comparator 13, is defined by the lines of R, and R2- At low engine speed the sensor waveform crosses the low speed reference level V.,,2 at one range extremity R2 whilst at high speed the waveform crosses the h ig h speed reference level Vc 1 atthe R 3 range extremity R,. Coil current is turned off at each downward crossing D by the sensorwaveform of the reference potential V2.

Line B of Figure 2 illustrates currentth rough the ignition coil at low speed whilst the waveform at line C 70 illustrates currentthrough the coil at high speed. At timet, current begins to flowthrough the ignition coil, attime t2 current reaches its limiting value and attime t3 current ceases to flow. Thetimescales of the waveforms B, and Care of course different but the figures clearly illustrate thatthe current limiting time Tr remains a substantially constant proportion of the period T of the sensorwaveform.

Referring once again to Figure 1 a potential divider formed by series connected resistors 54 and 55 is connected between the emitter electrode 43 of the transistor 41 and ground. The junction between the resistors 54 and 55 is coupled through a resistor 56to a negative input 57 of an operational amplifier 58 a positive input 59 of which receives a reference 85 potential of value V3 from a band gap reference circuit 60. The operational amplifier 58 has a first output 61 which is operative to turn on a current generator 62.

The current source 62 is connected to a pin 63 of the integrated circuit 1 and is operative to discharge a capacitor 64 which is connected externally of the integrated circuit 1 between the pin 63 and ground. A second current source 65 is connected in parallel with the current source 62 between the pin 63 and ground and is operative to charge the capacitor 64. The current source 65 is turned on by a leading edge of the rectang u la r waveform fed via a connection 66 from the output of the comparator 15.

The instantaneous potential appearing on the capa citor 64 is applied via a voltage follower 67 to the 100 terminal 21 where it is added to the waveform provided by the magnetic sensor 3 and is operative to change effectively the reference level of the compara tor 13 and hence to control the dwell time. Typically the current source 62 which discharges the capacitor 64 generates a current which is approximately six times that generated by the current source 65 which charges the capacitor 64.

In operation the current source 65 is firstly turned on by means of the output waveform fed from the comparator 15 and this current source 65 charges the capacitor 64. A current flowing through the ignition coil 39 builds up and consequentlythe potential applied tothe inputterminal 57 of the operational amplifier 58, which is representative of thevalue amplitude of the currentflowing through the ignition coil also increases. The reference potential applies to the inputterminal 59 of the operational amplifier 58 is set ata value representative of the coil reaching its maximum, i.e. limiting, value and when this limiting 120 value is reached the operational amplifier 61 will provide an output signal at its output 61 which switches on the current source 62 which commences to discharge the capacitor 64.

The operational amplifier 58 is also utilized to 125 regulate the coil current by providing a second output 68 connected to the base electrode 27 of the transistor 28. In this way currentthrough the output transistors 28 and 29, and hence the coil current flowing th rough the ignition coil 39, are regulated.

GB 2 138 495 A 3 A problem can arise at lowspeed and lowsupply voltage. Underthese conditions it is possiblethatthe currentflowing through the ignition coil 39 may never reach its limiting value. Consequently in each cycle of the waveform provided bythe magnetic sensor 3, the outputfed from the comparator 15will turn onthe currentsource 65 to chargethe capacitor64 but becausethe coil current never limits,the current source 62will never beturned on.The effectof this is illustrated in Figure 3 to which reference will now be made.

In Figure 3 and considering firstly Figure 3Athe level of the potential V4 is indicated, together with the level V4 +V, representing the actual potential applied to the input 16 of the comparator 13 and derived from the voltage sources 17. The waveform Vr,, is the sensor waveform applied between the inputterminal 14 and 18 of the comparator 15 whilstthe waveform Vsc is the waveform appliedtothe positive inputterminal 12 of the comparator 13. Figure3B shows the waveform outputprovided bythe comparator 15whilstthe waveform shown in Figure 3C illustratesthe increasing potential on the capacitor 64. Ignition coil current is illustrated in Figure3Dandthe Line Undicatesthe limiting value of the coil current. As can be seen in Figure 3A the effect of the steadily increasing charge on the capacitor 64 is to cause the overall level of the waveform Vscto rise until it no longer crosses the level Von applied to the negative input terminal 16 of the comparator 13. The points P1, P2 and P3 at which the waveform Vc crosses the level V.n represent the turning on of the coil current caused by an output of the comparator 13 and as can be seen the point P3 will be the lastturn on of this coil current. Thereafter the coil currentwill not be turned on and the ignition circuitwill ceasetofunction properly.

This problem isovercome byconnectingthe pin 63 tothe potential source 17through a switch 69which is normally open. The switch isclosed in responseto an output pulse provided by a speed detector 70 which provides output pulses in response to being triggered at a triggering input 71 by means of the output waveform fed from the comparator 15. The speed detector 70 provides output pulses following trigger- ing after either a delay time T1 orT2. The delaytime T1 orT2 is chosen independence upon supply voltage as follows. An input pin 72 of the circuit 1 is coupled to the supply potential 2 through a resistor73. A Zener diode 74 is connected between the pin 72 and ground potential. Avoltage detector 75 is also connected to the pin 72 and detects whether or notthe Zener diode 74 is limiting, which limiting takes place at normal supplyvoltage.

The voltage detector75 controls current supplied by a current source 76 connectedto a control input 79 of the speed detector 70. The control input 79 is also connected to a pin 78 of the integrated circuit chip 1. An external capacitor77 is coupled between the pin 78 and ground.

The currentsource 76 and the capacitor77 arethe time determining elements of the speed detector70. At low supply voltages the voltage detector75 controlsthe currentsupplied bythe source76to a first relatively high value sothatthe speed detector operates on delay time T1 which is dimensioned such 4 GB 2 138 495 A 4 that output pulses provided bythe speed detector 70 di3appear at a frequency at which the putputwaveform provided bythe magnetic sensor 3 is sufficiently large thatthe critical situation illustrated in Figure 3 5 does not occur. At speeds higherthan this critical frequencythe speed detector 70 will be retriggered before an output pulse is provided so that no output pulseswill occur.

Wherespeedfalls belowthis critical value sothat periodically an output pulse is provided bythe speed detector70this pulse operatesto closethe switch 69 thereby connecting the capacitor 64to the potential source 17.This results inthe capacitor64 being chargedtothe reference level of the potential source 17 each timethatan output pulse is provided bythe speed detector70. The effect of periodically charging the capacitor 64to the reference potential of the source 17will nowbe described with referenceto Figure4.

In Figure4, Figure 4A illustratesthe magnetic sensorwaveform V,, Figure 4C illustratesthe output ofthecomparator 15 Figure4D showsthe output pulses provided bythe speed detector70,the period T,, representing the period of the output pulses of the speed detector during which the switch 69 is closed.

Figure 4E shows the potential on the capacitor 64 whilst Figure 4F illustrates the cu rrentthrough the ignition coil 39. This current through the coil as shown in Figure 4F once again can be seen notto rise to the limiting value IL. During the period Tc, during which the switch 69 is closed due to the speed detector 70 not having been reset fora period TI, the potential on the capacitor 64 is held atthe potential V,,n of the source 17. The effect of this on the waveform applied to the positive inputterminal 12 of the comparator 13 is illustrated in Figure 4B. Thewaveform Vc can be seen to be stepped down in level at points P. which correspond to the closing of the switch 69 sothatthe waveform does not continue to rise above the levels of the reference potentials V4 + V4 and V, but continues 105 to cycle aboutthose levels. The ignition circuit keeps its high sensitivity and normal operation at low supply voltage and speed.

At normal supply voltage the speed detector 70 is switched to a much longertime constantT2 and as will 110 now be seen this longer time constant enables the speed detectorto detectvery low speeds e.g. cranking speeds to ensurethatthe ignition circuit can follow acceleration and deceleration at such low speed.

When the voltage detector 75 detects limiting of the Zener diode 74the current supplied bythe current source 76 is switched to a much lower value so that the delay time of the speed detector 70 adopts them uch longer time consta ntT2. Typically the speed detector 70 will provide outputs at delay T1 at distributor speeds of 430 revolutions per minute whilstthe delay time T2 becomes operative at distributor speeds of 60 r.p.m.

Operation of the ignition circuit with the speed detector70 atvery low speed is illustrated in Figure 5 125 to which reference will now be made.

In Figure 5, Figure 5A illustrates the sensorwave form Vn together with the waveform Vsc applied to the positive input 12 of the comparator 13. The levels L,, L2 and L3 represent the spark level set by the potential V2,130 the ON level forthe current source 65 and the on level for the current source 62 respectively. Waveform 513 shows the output of the comparator 15, Figure 5D shows the potential on the capacitor 64, whilstthe waveform 5E illustrates the currentth rough the ignition coil 39.

A problem atvery lowspeedsuch as cranking speed isthatthe amplitude of the sensor waveform V, becomesso small that the comparator 15 switches the, current source 65 on for such a short period during each cycle that the capacitor 64 remains substantially totally discharged. As can be seen in Figure 5A it is possible thatthe sensor waveform may not cross the reference]P-vel V4 + V,. In Figure 5C the speed detector 70 can beseento providean outputpulse which commences ata, delay time T2 aftertriggering and continues until retriggered by another output from the comparator 15. In Figure 5D the capacitor 64 can be seento bechargingfora short period following the end of the period T, In th[swaythe voltage level Vsc can rise until itbecomes equalto V4 +V, so that comparator 1,1 can switchthe coil current on. As soon as the current is at its limiting value, capacitor 64 is discharged again by currentsource 62. The capacitor 64 can be seen to be prevented from remaining permanently discharged as. during each period Tc during which output pulses provided bythe speed detector70 the switch 69 closes and the capacitor 64 is restored to the reference level V4. The ignition circuit can in this way follow any acceleration or deceleration atverylowspeed.

The ignition circuit has several protection devices. A circuit80 is also triggered bytheoutput signal fed from the comparator 15. In the absence of an output waveform from the comparator 15, i.e. when the distributor is not rotating, an output 81 is fed f rom the ci rcuit 80 th roug h a second in put of the OR gate 47 to the reset in put of the flip-flop 25 to maintain that fl ip-flop in a reset condition and thereby prevent cu rrent flowi ng th roug h the Ignition coil 39. The ignition coil is therefore prevented from being burnt out whilst the distri butor is not rotati ng. A power-on reset ci rcuit 82 is also provided and th is circuit has an output 83 also cou pled to a thi rd i nput of the OR gate 47. The poweron reset circuit 82 senses the initia 1 application of the supply potential 2 and provides an output pu Ise to re-set the fli p-flop 25 u ntil such time as a sensorwavefarm is provided bythe magnetic sensor 3 duringthe starting of the engine.

An overvoltage protection circuit is provided forthe Darlington connected transistors 28 and 29. A pair of resistors 84and 85 are connected in series between the supply potential 2 and ground. Thejunction of these resistors is coupled to pin 86 of the chip 1. A Zener diode 87 is coupled between the pin 86 and a base electrode 88 of a pair of Darlington connected transistors 89 and 90. The transistor 90 of the Darlington connected pair is coupled to a further pin 91 of the chip, which pin is connected through a resistor 92 to the pin 35 to which the collector electrode 33 of the transistor 29 is connected.When the potential at the pin 86 exceedsthe breakdown voltage of the Zener diode 87 the pin 91 which is coupled to the collector electrode of the transistor 90 is pulled down the clamps the potential atthe pin 35 to a j _z safe voltage at which the output transistor 28 and 29 can still operate. The voltage at which this protection commences is dimensioned by the values of the resistors 84 and 85. The transistors 40 and 41 ofthe switch 38 are protected by means of a further Zener diode 93 mounted on the integrated circuit chip and connected between a pin 94 and the output pin 31 of the chip. Two series connected resistors 95 and 96 are connected between the collector electrode 42 of the transistor41 and ground potential and the pin 94 of the chip is coupled to the junction of the resistors 95 and 96.

The invention has been described byway of example only and modifications may be made without departing from the scope of the invention.

Claims

1. An automotive ignition circuit for controlling the supply of curreritto an ignition coil and suitable for use in an ignition system in which a distributor is provided with a magnetic sensor,the circuit compris ing a first comparatorfor comparing the amplitude of a periodic signal fed from the magnetic sensorwith a first reference potential level and for providing a first outputsignal indicative of the amplitude of the periodicsignal crossing the first reference potential level, a second comparator for corn paring the ampli tude of the periodic signal with a second reference potential level and for providing a second output signal indicative of the amplitude of the periodic signal crossing the second reference potential level and meansfor enabling and disabling the supply of curreritto the ignition coil in response to thefirst and second output signals respectively.

2. An ignition circuit as claimed in claim 1 wherein the means forenabling and disabling the supply of curreritto the ignition coil includes a set re-set f lip-f lop arranged to beset in response to the said first output signal and reset in response to the second output signal.

3. An ignition circuit as claimed in claim 1 or 2 105 wherein differentiating means is provided between the setre-setflip-flop and thefirst and second comparatorsfor differentiating the first and second outputsignals.

4. An ignition circuit as claimed in any preceding claim wherein means is provided for preventing a change of state of the flip-flop for a predetermined periodfollowing the setting and resetting in response tothefirst and second output signals respectively.

5. An ignition circuit as claimed in claim 4 wherein the means for preventing the change of state of the set re-setflip-flop comprises gating means positioned between the differentiating means and the set re-set flip-flop the gating means being blocked forthe said predetermined period following the setting and reset ting of the flip-flop.

6. An ignition circuit as claimed in claim 5 wherein the gating means is blocked by an output signal fed from a monostable circuit triggered in response to the setting and re-setting of theflip-flop.

7. An ignition circuit as claimed in any preceding claim wherein means is provided for maintaining the flip-flop in a resetcondition during periods when no periodicsignal is provided bythe magnetic sensor.

8. An ignition circuit as claimed in any preceding GB 2 138 495 A 5 clairnwherein means is provided for adjusting the effective value of thefirst reference level in orderto controlthe pointin time during a cycle of the periodic signal atwhich thesupplyof currenttothe ignition coil isenabled.

9. An ignition circuit as claimed in claim 8 wherein the means for adjusting the effective value of the first reference potential comprises means for adding a control potential to the periodic signal.

10. An ignition circuit as claimed in claim 9 wherein the control potential is derived from charge storage means arranged to be charged during a first portion of each cycle of the periodic cycle and discharged during a second portion.

11. An ignition circuit as claimed in claim 10 wherein the charge storage means is a capacitor.

12. An automotive ignition circuit as claimed in claim 10 or 11 wherein the charge storage means is charged in response to the periodic signal crossing the second reference potential level in a first predetermined direction and discharged in response to the current in the ignition coil attaining a predetermined value.

13. An ignition circuit as claimed in claim 12 wherein the capacitor is discharged in response to the current in the ignition coil attaining a limiting value.

14. An ignition circuit as claimed in anyone of claims 10-13 wherein means responsive to distributor speed andlor ignition circuit supply potential is provided for periodically charging the charge storage means to a reference potential during periods when the distributor speed and/orthe supply potential fall below predetermined values.

15. An ignition circuit as claimed in any preceding claim wherein the means for enabling and disabling the supply of currentto the ignition coil includes one or more output transistors means being provided for protecting the transistors against potentials greater than a predetermined value.

16. An automotive ignition circuit substantially as herein described with reference to and as illustrated in Figure 1 of the drawings.

Printed in the united Kingdom for Her Majesty's Stationery Office, 8818935, 10184, 18996. Published at the Patent Office, 25 Southampton Buildings, London WC2A lAY, from which copies may be obtained.