U.S. patent number 3,831,571 [Application Number 05/359,472] was granted by the patent office on 1974-08-27 for variable dwell ignition system.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Howard F. Weber.
United States Patent |
3,831,571 |
Weber |
August 27, 1974 |
VARIABLE DWELL IGNITION SYSTEM
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.
Inventors: |
Weber; Howard F. (Scottsdale,
AZ) |
Assignee: |
Motorola, Inc. (Franklin Park,
IL)
|
Family
ID: |
23413942 |
Appl.
No.: |
05/359,472 |
Filed: |
May 11, 1973 |
Current U.S.
Class: |
123/610; 123/644;
315/209R |
Current CPC
Class: |
F02P
3/0453 (20130101) |
Current International
Class: |
F02P
3/02 (20060101); F02P 3/045 (20060101); F02p
001/00 () |
Field of
Search: |
;123/148E
;315/29T,29SC |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Goodridge; Laurence M.
Assistant Examiner: Cox; Ronald B.
Attorney, Agent or Firm: Mueller, Aichele & Ptak
Claims
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.
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.
Description
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.
This is highly undesirable and creates a heavy drain on the battery
of the vehicle in which the ignition system is used.
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 pre-established
speed of the engine; and which employs a constant dwell at engine
speeds above this pre-established 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 provide an
improved electronic ignition system for an internal combustion
engine.
It is a further object of this invention to provide an ignition
circuit operating as a variable dwell type at engine speeds below a
predetermined speed and operating as a constant dwell type at
engine speeds above such predetermined amount.
It is an additional object of this invention to provide an improved
variable dwell electronic ignition system.
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 70 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" time to "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.
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 U.S. 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.
The output of the 70 percent duty cycle circuit 12 and the output
of the monostable multivibrator 11 are applied to an AND gate
circuit 14, the output of which controls the operation of a time
multiplying circuit 16. So long as the output pulse from the
monostable multivibrator circuit 11 exists, the time multiplying
circuit 16 is prevented from operating by the output of the AND
gate 14. When the output of the monostable multivibrator 11
terminates, a cycle of operation of the time multiplying circuit 16
is initiated provided the 70 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 11 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
16 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 11 occurs at which time the
cycle of operation repeats.
At some high speed the "off" 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 rendered 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 10 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 "turn 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 70 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 terminal 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 70 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. In the
circuit 16, however, there is no feedback from the collector of the
transistor 76 to the base of the 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 11
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 nonconductive. 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 12
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 50 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 70 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 80 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 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 turn-on 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 parallel-connected 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 90 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 turn-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 110 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
111 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 111 and 80 is
derived from the terminal 42 through resistors 115, 116 and 118
coupled directly to the collector of the transistor 111 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 114 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 maintain the charge on the capacitor 114. Under
normal operating conditions, the intervals during which the
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 same 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.
* * * * *