Ignition coil energizing circuit

An ignition coil energizing circuit is provided which has a signal generator for generating an output signal having a frequency corresponding to an engine rotational speed, a switching circuit connected to an ignition coil, a current detector for detecting a current flowing through the ignition coil, a duty control for supplying to the switching circuit a control signal having a duty cycle corresponding to a duty cycle of an output signal from the signal generator and controlling a conduction state of the switching circuit, and a current control circuit for controlling the switching circuit in response to an output signal generated from the current detector and maintaining a current flowing through the current detector at a predetermined value. The ignition coil energizing circuit also has a timer for generating an output signal when the switching circuit is detected to be on over a predetermined period of time and a current supply circuit for supplying a gradually increasing current to the current detector in response to an output signal of the timer.

BACKGROUND OF THE INVENTION 
The present invention relates to an ignition coil energizing circuit for 
use in the internal combustion engine. 
FIG. 1 is a circuit diagram showing the conventional ignition device 
employed in the internal combustion engine. This prior art ignition device 
includes a signal generator 2 for generating a rotation speed signal which 
represents the rotation speed of a shaft picked up by a pickup coil of 
magnet induction type, for example, an ignition timing detector 4 for 
generating an output pulse representing the ignition timing responsive to 
an output signal applied from the signal generator 2, a series circuit of 
a primary winding 6-1 of an ignition coil 6 connected between a power 
source V.sub.D and the ground, a transistor switching circuit 8 and a 
resistor 10, and a secondary winding 6-2 of ignition coil 6 and an 
ignition plug 12 connected in series between the power source V.sub.D and 
ground. The prior art ignition device further includes a duty cycle 
control circuit 14 for generating a duty cycle signal responsive to an 
output signal applied from the ignition timing detector 4. The duty cycle 
signal serves to determine the period for which the switching circuit 8 is 
kept ON or OFF, and a circuit 16 for driving the switching circuit 8 
responsive to an output signal applied from the duty cycle control circuit 
14. A series circuit of resistors 18 and 20 is further connected in 
parallel with the resistor 10, and an input terminal of a constant current 
control circuit 22 is connected to the junction between these resistors 18 
and 20. This constant current control circuits detects that the current 
flowing to the primary winding 6-1 of the ignition coil 6 reaches a 
predetermined value, controls a driver circuit 16 responsive to this 
current to keep constant the current flowing to the switching circuit 18, 
and supplies to the duty control circuit 14 an output signal used to 
achieve duty control in a succeeding cycle. 
In the ignition device of this type, when engine failure occurs, the 
switching circuit 8 may happen to be kept on by the output signal of the 
driver circuit 16 while the device is rendered inoperative. As a result, a 
steady-state current flows through the primary winding 6-1 and the 
transistor switching circuit 8, damaging them due to heat. In order to 
solve this problem, a circuit may be incorporated in the device in which, 
after the switching circuit 8 is kept on for a certain period of time, the 
additional circuit turns off the switching circuit 8. However, when the 
transistor switching circuit 8 is abruptly turned off, a large current 
flows through the winding 6-2 and the ignition plug 12 is triggered, 
reversing the engine rotation and exhausting unburned gas. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide an ignition coil 
energizing circuit wherein when a coil current flowing for a period longer 
than a predetermined period is detected, this coil current is gradually 
reduced to zero without triggering the ignition plug. 
According to one aspect of the present invention, there is provided an 
ignition coil biasing circuit comprising a signal generating circuit for 
generating an output signal having a frequency corresponding to an engine 
rotational speed, a switching circuit connected to an ignition coil, a 
current detecting circuit for detecting a current flowing through the 
ignition coil and for generating a signal corresponding to the detected 
current, a first control circuit for supplying to the switching circuit a 
control signal having a duty cycle corresponding to a duty cycle of an 
output signal from the signal generating circuit and controlling a 
conduction state of the switching circuit, a second control circuit for 
controlling a conduction state of the switching circuit in response to an 
output signal generated from the current detecting circuit and maintaining 
a current flowing through the current detecting circuit at a predetermined 
value while the switching circuit is being turned on by the control signal 
of the first control circuit, a timer for generating an output signal when 
the switching circuit is detected to be on over a predetermined period of 
time by the first control circuit, and a current supply circuit for 
supplying a gradually increasing current to the current detecting circuit 
in response to an output signal of the timer. 
In the present invention, when the coil current continuously flows over a 
predetermined period of time, the gradually increasing current which 
overlaps the coil current is supplied to the current detecting circuit. 
Since the current flowing through the current detecting circuit is made 
constant by the constant current control circuit, the coil current is 
reduced to zero without triggering the ignition plug.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 is a block diagram of one embodiment of an ignition device including 
an ignition coil biasing circuit according to the present invention. The 
ignition device in FIG. 2 has a signal generator 2, a timing detector 4, 
an ignition coil 6, a transistor switching circuit 8, a resistor 10, an 
ignition plug 12, a duty control circuit 14, a driver circuit 16, 
resistors 18 and 20, and a constant current circuit 22 which are connected 
in the same manner as shown in FIG. 1. This ignition device further has a 
timer circuit 24 which is connected to an output terminal of the duty 
control circuit 14. The timer circuit 24 generates an output signal when a 
signal of high level is continuously generated for a predetermined period 
of time by the duty control circuit 14 and a current supply circuit 26 is 
connected to supply a gradually increasing current through a resistor 20 
to ground in response to an output signal of the timer circuit 24. 
FIG. 3 is a detailed circuit diagram of the duty control circuit 14, the 
driver circuit 16 and the constant current control circuit 22 of the 
ignition device shown in FIG. 2. 
The duty control circuit 14 includes a control circuit 100, a NOR gate 120, 
a voltage storing circuit 140, a sawtooth wave generating circuit 160, a 
reference voltage generating circuit 180 and a comparator 200, and the 
driver circuit 16 includes a NOR gate 122. 
An output signal is supplied from the ignition timing detector 4 to the 
control circuit 100 and NOR gate 120. First and second control signals 
from the control circuit 100 are supplied to a voltage storing circuit 140 
to control the level of voltage stored in the voltage storing circuit 140. 
A sawtooth wave generating circuit 160 generates a sawtooth wave signal 
whose slope changes according to a voltage signal from the voltage storing 
circuit 140 and having a period determined by third and fourth control 
signals from the control circuit 100. An output signal from the sawtooth 
wave generating circuit 160 is compared by a comparator 200 with a 
reference voltage signal from a reference voltage generating circuit 180. 
When it detects that the output signal of sawtooth wave generating circuit 
160 is larger than the voltage signal of reference voltage generating 
circuit 180, the comparator 200 supplies a logic signal "1" to the NOR 
gate 120. An output signal of the NOR gate 120 is supplied to the timer 24 
and the NOR gate 122 which controls the conduction state of transistor 
switching circuit 8 while being controlled by an output signal from a 
control circuit 22. 
The control circuit 100 includes a NOR gate 101 connected to receive an 
output signal directly from the timing detector 4, a NOR gate 102 
connected to receive an output signal from the timing detector 4 through 
an inverter, a flip-flop circuit 103 of reset dominant type having a reset 
input terminal at which an output signal from the timing detector 4 is 
received via an inverter, and an inverter 104 for inverting the output 
signal of NOR gate 102. The other input terminal of NOR gate 102 is 
connected to an output terminal Q of the flip-flop circuit 103. The set 
input terminal of the flip-flop circuit 103 is connected to an output 
terminal of a comparator 202 for comparing a reference voltage V.sub.REF1 
with a sawtooth wave signal applied from the sawtooth wave generating 
circuit 160 and generating a high level output signal when it detects that 
the sawtooth wave signal is larger than the reference voltage. The other 
input terminal of the NOR gate 101 is connected to the output terminal of 
the comparator 202 through an inverter. 
The voltage storing circuit 140 includes a current source 141, a switch 142 
and a capacitor 143 connected in series between a power source terminal 
V.sub.D1 and the ground, and a switch 144 and a current source 145 
connected in series between both ends of capacitor 143. Switches 144 and 
142 are controlled by first and second control signals applied through the 
NOR gates 101 and 102 of the control circuit 100. Voltage is charged into 
the capacitor 143 and this charged voltage is generated as a voltage 
signal in the voltage storing circuit 140. 
The sawtooth wave generating circuit 160 has an amplifier 161 for receiving 
a voltage signal from the voltage storing circuit 140, an npn transistor 
162 whose base is connected to the output terminal of amplifier 161, and a 
constant current source 163 whose first current path includes the 
collector-emitter path of the transistor 162 and whose second current path 
includes a capacitor 164. One end of the capacitor 164 is grounded while 
the other end thereof is connected to a switch 165 which is controlled by 
an output signal from the inverter 104 of the control circuit 100, and to 
a collector of an npn transistor 166 whose base is connected to an output 
terminal Q of the flip-flop circuit 103 and whose emitter is grounded. 
The reference voltage generating circuit 180 is provided with an amplifier 
181 for receiving voltage signal from the voltage storing circuit 140, an 
npn transistor 182 whose base is connected to the output terminal of 
amplifier 181, a constant current source 183 whose first current path 
includes the collector-emitter path of transistor 182, and a constant 
current source 184 whose first current path is connected in series with a 
second current path of the constant current source 183 and whose second 
current path is connected to an output terminal OT via a resistor 185. 
This output terminal OT is grounded via a resistor 186 and connected to a 
power source terminal V.sub.D via a resistor 187. The reference voltage 
generating circuit 180 further includes a resistor 188 and a capacitor 189 
connected in series, and an npn transistor 190 whose base is connected to 
the junction between the resistor 188 and capacitor 189, whose collector 
is connected to a power source terminal V.sub.D3, and whose emitter is 
connected to the output terminal OT via a resistor 191. 
The control circuit 22 includes a comparator 221 for comparing a reference 
voltage V.sub.REF2 with a voltage appearing at a junction RP between the 
resistors 18 and 20 and generating a high level output signal when the 
voltage at the junction RP is larger than the reference voltage, an npn 
transistor 222 whose base is connected to the output terminal of 
comparator 221, whose collector is connected to a constant current source 
circuit 223, and whose emitter is grounded. The constant current source 
223 has a first current path grounded through the collector-emitter path 
of the transistor 222 and a second current path grounded through the 
resistor 188 and capacitor 189. 
In the case of this ignition device, an output voltage of a battery 240 for 
supplying current energy to the ignition coil 6 is supplied through a 
voltage stabilizing circuit 241 as power source voltage necessary for 
operating the ignition circuit. 
FIG. 4 is a detailed circuit diagram of the current supply circuit 26 of 
the ignition device shown in FIG. 2. The current supply circuit 26 
comprises an npn transistor 260 the base of which is connected to the 
output terminal of the timer 24 through an inverter 261 and the emitter of 
which is grounded, a capacitor 262 connected between the emitter and the 
collector of the transistor 260, a constant current source 263 connected 
between a power source terminal V.sub.B and the collector of the 
transistor 260, and an npn transistor 264 the base of which is connected 
to the collector of the transistor 260, the collector of which is 
connected to the power source terminal V.sub.B and the emitter of which is 
connected to a junction RP of the resistors 18 and 20 through a resistor 
265. 
The timer 24 is reset by a signal of "1" from the NOR gate 120 and supplies 
a signal of high level in response to a signal of "0" from the NOR gate 
120 when the period of the signal of "0" reaches a predetermined value. 
The operation of ignition device shown in FIG. 3 will be described with 
reference to signal waveforms shown in FIGS. 5A to 5I. 
Responsive to the rotation movement of engine shaft, an output signal 
corresponding to the rotation speed or rotation angle of engine shaft is 
generated from the signal generator 2, as shown in FIG. 5A. Responsive to 
an output signal of the signal generator 2, the timing detector 4 
generates a pulse signal which rises when the output signal reaches a 
predetermined level and falls when the output signal becomes zero in 
level, as shown in FIG. 5B. When the output signal of timing detector 4 
becomes low in level, the flip-flop circuit 103 is reset to generate a low 
level output signal through the output terminal Q thereof. Since a low 
level output signal is generated from the comparator 202 at this time, 
both of the NOR gates 101 and 102 generates signals "0" to thereby leave 
both of switches 142 and 144 open. Therefore, the amplifier 161 biases the 
transistor 162 with a bias voltage corresponding to the charged voltage of 
the capacitor 143, whereby current corresponding to this bias voltage 
flows through the collector-emitter path of transistor 162 causing the 
charging current to flow into the capacitor 164. Namely, the capacitor 164 
is charged at a rate corresponding to the charged voltage of capacitor 
143, as shown in FIG. 5C. When the output signal of the timing detector 4 
rises at the same time when the charged voltage of the capacitor 164 
reaches the reference voltage V.sub.REF1, the flip-flop circuit 103 is set 
to generate a high level output signal from the output terminal Q thereof, 
whereby the transistor 166 is rendered conductive to discharge the 
capacitor 164 to zero level. 
When the output signal of the timing detector 4 rises before the charged 
voltage of the capacitor 164 reaches the reference voltage V.sub.REF1 with 
the engine being accelerated, a signal "1" is generated from the NOR gate 
102 to close switches 142 and 165 for such a period as shown in FIG. 5E 
and to charge capacitors 143 and 164 as shown in FIGS. 5D and 5C. When the 
charged voltage of capacitor 164 thus comes to the reference voltage, a 
high level output signal is generated from the comparator 202 to set the 
flip-flop circuit 103, whereby a signal "0" is generated from the NOR gate 
102 and the transistor 164 is rendered conductive, causing the capacitor 
164 to be discharged to zero level. 
Since a higher voltage is now charged in the capacitor 143, a sawtooth wave 
signal which rises at a sharper slope is generated from the sawtooth wave 
generating circuit 160 in a successive cycle and this sawtooth wave signal 
is controlled so as to reach the reference voltage V.sub.REF1 when the 
output pulse of timing detector 4 rises. 
Where the charged voltage of capacitor 164 comes to the reference voltage 
V.sub.REF1 before the output signal of timing detector 4 rises with the 
engine reduced in speed, for example, a high level output signal is 
generated from the comparator 202 and a signal "1" is generated from the 
NOR gate 101 to close the switch 144 for a period shown in FIG. 5F, 
whereby the capacitor 143 is discharged a little as shown in FIG. 5D to 
thereby lower the charged voltage thereof a little. On the other hand, a 
high level signal is generated from the comparator 202 but a reset signal 
is supplied to the flip-flop circuit 103 so that the flip-flop circuit 103 
is kept reset. When the output signal of the timing detector 4 rises 
thereafter, the flip-flop circuit 103 is set to render the transistor 166 
conductive and to discharge the capacitor 166 to zero level. 
When the output signal of the timing detector 4 falls, the capacitor 166 is 
charged at a rate responsive to the charged voltage of capacitor 143 and a 
sawtooth wave signal component is generated from the sawtooth wave signal 
generating circuit 160 in the same way as described above. Namely, a 
sawtooth wave signal having a slope corresponding to the charged voltage 
of the capacitor 143 and synchronized with the pulse signal of the timing 
detector 4 is obtained. 
The comparator 200 compares a reference voltage of reference voltage 
generating circuit 180 with a charged voltage of the capacitor 164 and 
generates an output signal as shown in FIG. 5G. Therefore, an output 
signal shown in FIG. 5H is generated from the NOR gate 121. The low level 
signal of this NOR gate 121 is supplied through the NOR gate 122 to the 
transistor circuit 8 to render the circuit 8 conductive. Therefore, 
current shown in FIG. 5I flows through the primary winding 6-1, transistor 
circuit 8 and resistor 10. When this current becomes larger than the 
predetermined value and a voltage appearing at the junction between 
resistors 18 and 20 becomes larger than the reference voltage V.sub.REF2, 
the comparator 221 generates a high level output signal, whereby a low 
level signal is generated from the NOR gate 122 to make the transistor 
circuit 8 nonconductive, interrupting the current flowing through the 
primary winding 6-1. As a result, the comparator 221 produces a low level 
signal, causing a high level output signal to be produced from the NOR 
gate 122. Thus, substantially a constant current will flow through the 
primary winding 6-1. The constant current may continuously flow through 
the primary winding 6-1 until an output signal from the timing detector 4 
changes from the high to low level. The low level output signal from the 
timing detector 4 causes current flowing to the primary winding 6-1 to be 
interrupted rapidly, inducing an extremely high voltage in the secondary 
winding 6-2 to trigger the ignition plug 12. 
The reference voltage generating circuit 180 controls the duty cycle of 
output signal applied from the NOR gate 121 by adjusting the level of 
reference voltage supplied to the inverted input terminal of comparator 
200 according to the charged voltage in the capacitor 143, power source 
voltage V.sub.D and the time for which the transistor circuit 8 is made 
conductive. When the power source voltage V.sub.D rises, for example, 
potential at the output terminal OT also rises causing the reference 
voltage applied to the comparator 200 to be risen. When the charged 
voltage of the capacitor 143 rises, an increased amount of current flows 
through the collector-emitter path of the transistor 182 and therefore, an 
increased amount of current also flows to the constant current circuit 
184, whereby current flows through the resistor 185 in the direction shown 
by an arrow, thus causing potential at the output terminal OT to be 
lowered. Namely, when sawtooth wave of high frequency is generated, the 
duty ratio of output signal of NOR gate 120 is controlled so as to have a 
larger value. 
When the time period during which the transistor circuit 8 is made 
conductive becomes longer, that is, when the time period during which the 
high level signal is generated from the comparator 221 becomes longer, the 
time period during which the transistor 222 is rendered conductive also 
becomes longer and the capacitor 189 is charged to a higher voltage, 
whereby an increased amount of current flows through the collector-emitter 
path of transistor 190, causing potential at the output terminal OT to be 
risen. Therefore, the reference voltage supplied to the comparator 200 
becomes higher and the duty ratio of output signal of NOR gate 120 becomes 
small. 
Assume that engine failure occurs when the charged voltage of the capacitor 
164 is higher than the voltage at the output terminal OT and is lower than 
the reference voltage V.sub.REF1. A signal of low level is generated from 
the comparator 202 and therefore the capacitor 164 is not discharged. The 
charged voltage of the capacitor 164 is kept at the constant level as 
shown by the level of the waveform in FIG. 6A. A high level signal is 
continuously generated from the comparator 200 and a signal of "0" level 
is generated from the NOR gate 120 as shown in FIG. 6B. The transistor 
switching circuit 8 is thus rendered conductive and the constant current 
flows into the primary winding 6-1 as shown in FIG. 6C. 
When detecting that the output signal from the NOR gate 120 is kept at a 
low level over a predetermined period of time, the timer 24 generates a 
signal of high level as shown in FIG. 6D. The output signal of high level 
from the timer 24 is inverted by an inverter 261 and supplied to the base 
of a transistor 260 which is in turn rendered nonconductive. The current 
from the constant current source 263 flows to the capacitor 262 which is 
gradually charged. As a result, the base voltage of the transistor 264 
gradually increases, and the conduction resistance of the transistor 264 
is gradually reduced. A gradually increasing current flows to ground 
through the collector-emitter path of the transistor 264, and the 
resistors 265 and 20, as shown in FIG. 6E. This current is combined with 
the current flowing through the primary winding 6-1, thus increasing the 
potential at the junction RP. As described above, when the potential at 
the junction RP increases, the duration for which the voltage of high 
level from the comparator 221 is generated is made long and the duration 
for which the transistor switching circuit 8 is turned off is prolonged. 
As a result, the coil current, the waveform of which is shown in FIG. 6C, 
gradually decreases as opposed to the increase in the current flowing from 
the current supply circuit 26. When the current flowing from the current 
supply circuit 26 reaches the predetermined level, the comparator 221 
produces a signal of high level without any cooperation of the coil 
current. The coil current is thus cut off. In this way, since the coil 
current flowing through the primary winding 6-1 gradually decreases to 
zero, a large voltage is not induced at the secondary winding 6-2 when the 
coil current is cut off. Therefore, the ignition plug 12 may not be 
triggered. 
FIG. 7 is a circuit diagram of a modification of the driver circuit 16 and 
the constant current control circuit 22. The driver circuit 16 has an npn 
transistor 30 whose collector and base are respectively connected to the 
power source terminal V.sub.D through resistors 31 and 32 and whose 
emitter is connected to the base of the transistor of the switching 
circuit 8, and npn transistors 33 and 34 whose collectors are connected to 
the base of the transistor 30 and whose emitters are grounded. The 
collector of the transistor 33 is connected to the output terminal of the 
duty control circuit 14. 
The constant current circuit 22 has an npn transistor 40 whose collector is 
connected to the power source terminal V.sub.C through a resistor 41 and 
whose emitter is connected to the base of the transistor 34 and the duty 
control 14, an npn transistor 42 whose emitter is connected to the 
junction of the resistors 18 and 20 through a resistor 43 and whose base 
is connected to the power source terminal V.sub.C through a resistor 45, a 
constant current source 46 connected between the power source terminal 
V.sub.C and the collector of the transistor 42, and a diode 47 and a 
resistor 48 which are connected in series between ground and the base of 
the transistor 42. 
In the circuit shown in FIG. 7, when the current flowing through the 
primary winding 6-1 becomes greater than the predetermined value and when 
the potential at the junction RP of the resistors 18 and 20 becomes higher 
than the voltage drop across the resistor 48, the transistor 42 is 
rendered nonconductive. The current flows from the constant current source 
46 to the base of the transistor 40 and the transistor 40 is rendered 
conductive. As a result, the voltage of the power source terminal V.sub.C 
is applied to the base of the transistor 34 through the resistor 41 and 
the collector-emitter path of the transistor 40 to turn on the transistor 
34. The transistor 30 is therefore rendered nonconductive and the 
transistor switching circuit 8 is turned off, decreasing the coil current. 
By this decrease, the potential at the junction of the resistors 18 and 20 
falls below the predetermined value and the transistor 42 begins to be 
conductive. The base current of the transistor 40 thus decreases and the 
transistor 40 begins to be nonconductive. As a result, the transistor 34 
is rendered nonconductive. In this case, if the transistor 33 is kept 
nonconductive in response to an output signal from the duty control 
circuit 14, the transistor 30 is rendered conductive and the switching 
circuit 8 is turned on, increasing the coil current flowing through the 
primary winding 6-1. The coil current is substantially kept constant. 
Assume that engine failure occurs at a certain timing, a signal of low 
level is generated from the duty control 14, and the transistor 33 remains 
nonconductive as described before. A constant coil current flows through 
the primary winding 6-1. When the timer 24 detects that the output signal 
from the duty control circuit 14 is kept at a low level over a 
predetermined period of time, the timer 24 generates an output signal of 
high level. The current supply circuit 26 sends an output signal of high 
level. The gradually increasing current from current supply circuit 26 
flows to ground through the resistor 20, as shown in FIG. 6E in response 
to the output signal of high level from the timer 24. The potential at the 
junction RP of the resistors 18 and 20 thus increases and the transistor 
42 gradually begins to be turned off. The transistors 40 and 34 also begin 
to be conductive gradually while the transistor 30 and the switching 
circuit 8 begin to be nonconductive gradually. As a result, the coil 
current gradually decreases to zero as shown in FIG. 6C. Even in this 
case, when the coil current is cut off, the current is not induced in the 
secondary winding 6-2, thus preventing triggering the ignition plug 12.