Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2195793
IGNITION SYSTEM FOR INTERNAL COMBUSTION ENGINES
BACKGROUND OF THE INVENTION
The present invention relates to an ignition system for
internal combustion engines.
Heretofore, ignition systems for internal combustion
engines which supply high voltage to an ignition plug of an
internal combustion engine, have included an ignition system
of the type (AC-CDI method)in which a high voltage generated
by a magneto-generator is accumulated in an ignition capacitor
and the accumulated high voltage charge is discharged to an
ignition plug to induce sparking at predetermined times in
accordance with a crankshaft rotation position signal. This
type of ignition system in which spark discharge is developed
with an alternating current based on resonance between the
ignition capacitor and inductance of the ignition coil is
called AC arc type.
There is also another known type (DC-CDI method) of
ignition system in which a capacitor is charged from a
battery. An example of an ignition system of this type, a
self-excited, contactless ignition system for an internal
combustion engine, is disclosed in Japanese Unexamined Patent
Publication No. 3-85370.
In the self-excited, contactless ignition system, a high
voltage which has been stepped up by a DC step-up circuit is
input in several stages to a main discharge capacitor, and at
a predetermined time in the ignition cycle an SCR gate is
triggered with a signal generated in a timing sensor, thereby
permitting the electric charge accumulated in the main
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discharge capacitor to be discharged to an ignition coil to
produce a single ignition spark. This type of spark discharge
is called the DC arc type.
However, because the ignition system disclosed in the
Japanese Unexamined Patent Publication No. 3-85370 is of the
DC arc type, the spark discharge developed in the ignition
coil occurs only once, so the discharge time is short and it
is at times difficult to effect a satisfactory ignition of the
air-fuel mixture in the engine.
One solution to the above-described problem may be to
dispose a diode in parallel with the SCR in a direction
opposite in polarity to the SCR to provide an AC arc discharge
circuit, instead of a free-wheeling diode connected in
parallel with a primary winding of the ignition coil. In this
case, a series LC circuit comprising an ignition capacitor and
the ignition coil is formed, thereby facilitating oscillation.
When this series LC circuit oscillates, the oscillation energy
laps through the SCR and the DC-DC converter, so that a
stable discharge of the ignition capacitor is obstructed and a
spiky noise-containing discharge waveform (Fig. 4) results.
This gives rise to a new problem in which the voltage to which
the ignition capacitor is charged does not reach a
predetermined optimum level (150v).
Moreover, in the method described above wherein the DC-
CDI type circuit is merely converted into an AC arc discharge
circuit, since the free-wheeling diode is not used, there is
also a problem because the electric current for charging the
ignition capacitor flows through the ignition coil and hence
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CA 02195793 2000-04-04
the mere supply of electric current to the ignition circuit
results in a high voltage accumulation in the secondary side
of the ignition coil.
Another ignition system has been disclosed in Japanese
Patent Publication No.48-12013. However, this ignition system
has the disadvantage that to much high voltage is applied to
the ignition coil during the charging of the ignition
capacitor before the SCR turns on. This accelerates
deterioration of the ignition coil.
SUMMARY OF THE INVENTION
The present invention provides an ignition system capable
of generating a stable AC arc in accordance with the DC-CDI
method and of preventing high voltage from being applied to an
ignition coil while charging of an ignition capacitor before
turning on a thyristor to discharge the capictor. According to
the present invention, a switching device is connected at one
end to a capacitor and at the opposite end to a high voltage
side of a primary coil of the ignition coil, and a reverse ON
circuit is connected in parallel with the switching device.
Preferably, the reverse ON circuit includes a diode, and the
switching device includes a thyristor.
BRIEF DESCRIPTION OF THE DRAWINGS
Other aspects, features and advantages of the present
invention will become more apparent from the following
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detailed description when read in conjunction with the
accompanying drawings, in which:
Fig. 1 is a circuit diagram of an ignition system for
internal combustion engines according to an embodiment of the
present invention;
Fig. 2 is a diagram which shows the characteristics of
voltages applied to a gate of a thyristor and to an ignition
capacitor represented in relation to a crankshaft rotation
position signal;
Fig. 3 is a diagram representing changes over time of a
primary voltage on an ignition coil of an ignition system of
this embodiment; and
Fig. 4 is a diagram representing changes over time of a
primary voltage of an ignition coil of a conventional ignition
system.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
An embodiment of the present invention will be described
hereinunder with reference to an example ignition system for
two-wheeled motorcycles. It should be understood that the
principles described can be applied to an ignition system for
any internal combustion engine.
As shown in Fig. 1, the ignition system is of the DC-CDI
type and comprises an ignition circuit 10, a DC-DC
converter 20, an ignition timing control circuit 32 and a
voltage detecting circuit 36. A DC voltage supplied from a
battery 1 as a DC power supply is switched on or off by a
switch 2. The DC-DC converter 20 serves as a step-up circuit
which receives the DC voltage supplied from the battery 1 as
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an input voltage. In the DC-DC converter 20, the input
voltage is stepped up to a DC voltage of several hundred volts
by means of a transformer 21, a switching transistor 23 and a
current detecting circuit 24. The DC voltage thus stepped up
by the DC-DC converter 20 is used as a power supply for
ignition in the ignition circuit 10, in which circuit the DC
voltage is further stepped up by an ignition coil 15 to a
spark-dischargeable level. The stepped-up high voltage is
supplied to an ignition plug 16.
The DC-DC converter 20 includes the transformer 21, a
diode 22, the switching transistor 23, the current detecting
circuit 24 and a Zener diode 25. An input end of a primary
coil 21a in the transformer 21 is connected to the collector
of the switching transistor 23, and the emitter is connected
to ground through the current detecting circuit 24. A
terminal end of the primary coil 21a is connected to the
cathode of a diode 31 whose anode is connected to the
switch 2. Thus, an electric current is supplied to the
primary coil 21a of the transformer 21 by the switching
transistor 23 which is controlled by the value of a voltage
applied to the base thereof. The current (primary current)
flowing through the primary coil 21a is detected by the
current detecting circuit 24. When the primary current has
reached a predetermined current value, the current detecting
circuit 24 turns off the switching transistor 23.
An input end of a tertiary coil 21c in the transformer 21
is connected to a terminal end of a secondary coil 21b in the
transformer 21. The terminal end of the secondary coil 21b is
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also connected to ground. A terminal end of the tertiary coil
21c is connected through a resistor to the base of the
switching transistor 23, so that an electric current generated
in the tertiary coil 21c when current is applied to the
primary coil is fed back to the base of the switching
transistor 23. Consequently, when the primary current is
applied, an electric current is induced in the tertiary coil
21c of the transformer 21 in a direction that increases the
base current on the switching transistor 23. When this
current reachs a predetermined value, the base current is
connected to ground by the current detecting circuit 24, so
that the switching transistor 23 is turned off.
When the switching transistor 23 is turned off, the
voltage developed in the tertiary coil 21c also reverses in
polarity and the base current flows to ground through the
tertiary coil 21c, so that the switching transistor 23 turns
off rapidly. As a result, electromagnetic energy induced by
the primary current is transferred from the secondary coil 21b
through a diode 22 to an ignition capacitor 12.
When the transfer of the electromagnetic energy is
complete, the primary current begins to flow again in the
primary coil 21a and the events described above are repeated
until the charge voltage of the ignition capacitor 12 reaches
a predetermined level. In this way oscillation of the DC-DC
converter 20 is accomplished and a stepped-up AC voltage is
produced. This AC voltage is rectified by the diode 22
connected to an end of the secondary coil 21b in the
transformer 21, whereby the stepped-up and rectified DC
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voltage is accumulated in the ignition capacitor 12. The
Zener diode 25 is connected between the base and the collector
of the switching transistor 23 to protect the switching
transistor 23.
The electric circuit for supplying the electric current
from the DC-DC converter 20 to the ignition capacitor 12 via
diode 22 forms a first closed circuit.
An oscillation stop circuit 27 is connected to the base
of the switching transistor 23 and receives a voltage
detection signal from the voltage detection circuit 36 which
detects both output voltage of the DC-DC converter 20 and
charge voltage of the ignition capacitor 12. The oscillation
stop circuit 27 controls the base of the switching transistor
23 so as to stop oscillation of the DC-DC converter 20 when
the charge voltage of the ignition capacitor 12 reaches a
predetermined level. In this way the oscillation of the DC-DC
converter 20 is controlled.
The ignition circuit 10 includes the ignition capacitor
12, thyristor 13, diode 14, ignition coil 15 and ignition
plug 16.
One end terminal of the ignition capacitor 12 is
connected to an output terminal of the DC-DC converter 20,
while an opposite end terminal of the capacitor 12 is
connected to ground. Thus, the ignition capacitor 12 is
connected between the output terminal of the DC-DC converter
20 and ground, whereby the DC voltage stepped up by the DC-DC
converter 20 is accumulated in the ignition capacitor 12. In
order that the electric charge accumulated in the ignition
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capacitor 12 may be discharged to ground via a primary coil
15a of the ignition coil 15 at a predetermined time, a
thyristor (SCR) 13 is used as a switching device. The
thyristor 13 is connected in a forward direction between the
ignition capacitor 12 and the ignition coil 15. That is, the
anode serves as an input terminal of the thyristor 13 and is
connected to one end terminal of the ignition capacitor 12,
while the cathode services as an output terminal of the
thyristor 13 and is connected to primary coil 15a of the
ignition coil 15.
In parallel with the thyristor 13 and in a direction
opposite in polarity to the thyristor there is connected the
diode 14 which acts as a reverse ON circuit. More
specifically, the anode of the thyristor 13 is connected to
the cathode of the diode 14, while to the cathode of the
thyristor 13 is connected to the anode of the diode 14. An
ignition signal generated by the ignition timing control
circuit 32 is input to the gate of the thyristor 13 via a
diode 33 and a resistor 34. The ignition signal switches the
thyristor 13 between an ON state in which the anode side and
the cathode side of the thyristor 13 are electrically
connected at a low resistance and an OFF state in which the
anode and the cathode are electrically disconnected.
The cathode of the thyristor 13 and the anode of the
diode 14 are connected to the primary coil 15a of the ignition
coil 15, while a secondary coil 15b of the ignition coil 15 is
connected to one end of the ignition plug 16. The opposite
end side of the ignition plug 16 is connected to ground. In
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this arrangement, when a predetermined voltage is applied to
the gate of the thyristor 13 by the ignition signal output
from the ignition timing control circuit 32, the thyristor 13
turns ON, so that the electric charge accumulated in the
ignition capacitor 12 flows to ground via the thyristor 13 and
the primary coil 15a of the ignition coil 15. That is, a high
voltage is induced in the secondary coil 15b of the ignition
coil 15 as the electric current flows through the primary coil
15a of the ignition coil 15, which high voltage is applied to
the gap in the ignition plug 16, so that a spark discharge
occurs across the plug 16.
Once the electric current flowing through the primary
coil 15a of the ignition coil 15 exceeds a predetermined peak,
a voltage having an inverted polarity, namely, an
electromotive force created by self-induction, is generated in
the primary coil 15a. This voltage having the opposite
polarity is charged to the ignition capacitor 12 through the
ground connection. The electric charge thus accumulated in
the capacitor 12 has a polarity opposite to the polarity of
the electric charge supplied by the DC-DC converter 20. Since
the diode 14 is connected in parallel with the thyristor 13
and in a direction opposite to the forward direction of the
thyristor 13, the electric charge accumulated in the ignition
capacitor 12 is permitted to flow to a lower potential
successively through the ignition plug 16, ignition coil 15
and diode 14, that is, to one end of the capacitor 12.
Consequently, after an initial spark discharge by the ignition
plug 16 using the electric charge stored in the capacitor 12
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by the DC-DC converter 20, a second spark discharge by the
ignition plug 16 can occur using the electric charge of the
electromotive force generated by self-induction in the primary
coil 15a.
When the electric current flowing in the primary coil 15a
at the instant of the second spark discharge exceeds a
predetermined peak, a voltage having a further inverted
polarity is generated in the coil 15a, so that the ignition
capacitor 12 is charged and the electric charge accumulated
therein flows to ground along the same path as the electric
charge accumulated in the capacitor 12 by the DC-DC converter
20, whereby a third spark discharge by the ignition plug 16
can occur. Thus, in the primary coil 15a of the ignition coil
15, an electric current flows which gradually attenuates while
alternating its direction of flow. Consequently, as a primary
voltage Vout shown in Fig. 3 occurs in the primary coil 15a, a
secondary voltage stepped up in proportion to the primary
voltage Vout is induced in the secondary coil 15b. As a
result, a plurality of spark discharges occur across the gap
of the ignition plug 16 and an AC arc is achieved.
The electric circuit for conducting an electric current
from the ignition capacitor 12 to the ignition coil 15 via the
thyristor 13 and from the ignition coil 15 to the capacitor 12
via diode 14 forms second closed circuit.
The ignition timing control circuit 32 controls the
ignition timing of the ignition plug 16 in accordance with the
crankshaft rotation position signal provided by a timing
sensor 5. More specifically, the timing sensor 5 detects a
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rotation position of a crankshaft of the engine (not shown) on
the basis of the position of a magnet on a flywheel attached
to the crankshaft, and in accordance with a signal provided by
the timing sensor 5 the ignition timing control circuit 32
outputs an OFF signal to the oscillation stop circuit 27 for
the DC-DC converter 20 and an ignition signal to the gate of
the thyristor 13 in the ignition circuit 10, each at a
predetermined time. Using those signals the control circuit
32 controls the DC-DC converter 20 and the ignition circuit 10
as will be described below. The diode 33 connected between
the ignition timing control circuit 32 and the gate of the
thyristor 13 is a protective diode for preventing the high
voltage applied to the ignition coil 15 from flowing to the
ignition timing control circuit 32. Further, a diode 35,
which is connected in parallel between the ignition timing
control circuit 32 and ground, is a protective diode for the
control circuit 32.
More detailed operation of the ignition system according
to this embodiment will be described with reference to the
diagram of Fig. 2. The characteristics shown in FIG. 2 were
obtained using an oscilloscope to plot changes over time of a
gate voltage VSG of the thyristor 13 and a terminal voltage VC
of the ignition capacitor 12. Those changes are expressed in
relation to changes over time of the crankshaft rotation
position signal. As to the measurement ranges used on the
oscilloscope in the measurement of the gate voltage VSG and
the terminal voltage VC, the axis of abscissa, or voltage
axis, is set to 2V/div for VSG and 50V/div for VC, and the
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axis of ordinate, or time base, is set to 1 ms/div for both
VSG and VC.
As shown in Fig. 2, when the crankshaft rotation position
signal output by the timing sensor 5 deflects to the positive
side, an ignition signal is output from the ignition timing
control circuit 32 to the gate of the thyristor 13 as
indicated by an upward arrow at the leading edge of the
crankshaft rotation position signal. As a result, the gate
voltage of the thyristor 13 which is normally OFF, with a
positive voltage applied to the anode side and a negative
voltage applied to the cathode side by the electric charge
accumulated in the ignition capacitor 12, is driven high.
Consequently, the thyristor 13 is switched from OFF to ON, and
the anode and cathode of the thyristor 13 assume an
electrically conductive state at a low resistance so that the
electric charge stored in the ignition capacitor 12 is
conducted to the primary coil 15a of the ignition coil 15 via
the thyristor 13. By this conduction of electric charge,
namely, by the electric current flowing through the primary
coil 15a, a predetermined high voltage is induced in the
secondary coil 15b of the ignition coil 15 and hence a spark
discharge occurs across the gap of the ignition plug 16.
Once the electric current flowing through the primary
coil 15a of the ignition coil 15, i.e., the primary current,
exceeds a predetermined peak, the voltage developed across the
primary coil 15a reverses polarity and is accumulated in the
ignition capacitor 12 as an electric charge having an opposite
polarity. This electric charge of opposite polarity
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accumulated in the capacitor 12 is discharged via diode 14, so
that a second spark discharge occurs across the ignition
plug 16. Further, when the primary current in the ignition
coil 15 exceeds a predetermined peak due to the electric
charge of opposite polarity, the ignition capacitor 12
accumulates a third electric charge having the same polarity
as that of the electric charge stored by the DC-DC
converter 20. Since the thyristor 13 is held ON, the electric
charge accumulated in the ignition capacitor 12 is once more
discharged via the thyristor 13 and a spark discharge occurs
across the ignition plug 16. In this way the cycle of charge
and discharge of the capacitor 12 is repeated as is the
inversion in polarity of the electric charge accumulated
therein, whereby a spark discharge can be generated repeatedly
across the ignition plug 16. Thus, it is possible to achieve
a stable DC arc in accordance with the DC-CDI method.
As shown in Fig. 2, the terminal voltage VC of the
ignition capacitor 12 oscillates alternate-currentwise, and
after a period tl from the start of discharge, it drops to
nearly zero volts. The period tl corresponds to the spark
discharge time of the ignition plug 16. In Fig. 2 it is shown
that both the gate voltage VSG of the thyristor l3 and the
terminal voltage VC of the ignition capacitor 12 attenuate
while oscillating substantially in an alternate-currentwise
mode during the period tl. The period t2 shown in the same
figure indicates the period during which the voltage based on
the ignition signal is applied to the gate of the thyristor
13 .
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At the leading edge of the crankshaft rotation position
signal referred to previously, the ignition timing control
circuit 32 outputs not only an ignition signal to the gate of
the thyristor 13 but also a HALT signal for the DC-DC
converter 20 to the oscillation stop circuit 27. Upon receipt
of the HALT signal the oscillation stop circuit 27 controls
the base of the switching transistor 23 to stop the
oscillation of the DC-DC converter 20. Consequently, a
stepped-up voltage is not induced in the secondary coil 21b of
the transformer 21 and the DC-DC converter 20 turns off. The
quiescent period of the DC-DC converter 20 is represented by
t3. The period t3 is determined by a timer circuit which
includes a resistor and a capacitor, which are the preferred
components of the oscillation stop circuit 27. More
specifically, upon output of the HALT signal from the ignition
timing control circuit 32, the timer circuit is initialized
and the base of the switching transistor 23 is controlled by
the oscillation stop circuit 27 until predetermined time t3
has elapsed. The quiescent period t3 has the following
relation to the discharge period tl and the thyristor 13 ON
period t2 . tl < t2 < t3
Upon lapse of the quiescent period t3, control by the
oscillation stop circuit 27 is terminated, so that the
oscillation of the DC-DC converter 20 is resumed and a
stepped-up voltage is induced on the output side of the DC-DC
converter. Then, as shown in Fig. 2, the stepped-up voltage
is accumulated little by little in the ignition capacitor 12
and this charging continues until the voltage detected by the
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voltage detecting circuit 36 reaches the predetermined level.
More specifically, the voltage detecting circuit 36 monitors
the terminal voltage VC of the ignition capacitor 12 and, when
VC has reached the predetermined level, the voltage control
circuit outputs a HALT signal to the oscillation stop
circuit 27. Upon receipt of the HALT signal the oscillation
stop circuit 27 controls the base of the switching transistor
23 which stops the oscillation of the DC-DC converter 20 in
the same way as when a HALT signal is received from the
ignition timing control circuit 32. As a result, the
oscillation of the DC-DC converter 20 is stopped and therefore
the stepped-up voltage in the transformer 21 is no longer
output by the DC-DC converter 20. However, between the
secondary coil 21b of the transformer 21 and the ignition
capacitor 12, the diode 22 is connected in the forward
direction from the transformer 21 toward the ignition
capacitor 12, so that the electric charge stored in the
ignition capacitor 12 is prevented from flowing back to the
transformer 21. Thus, the electric charge accumulated in the
capacitor 12 is stored until the thyristor 13 is switched from
OFF to ON.
In Fig. 2, the period from the foregoing restart of
oscillation of the DC-DC converter 20 until stopping the
oscillation by the voltage detecting circuit 36 is represented
by t4. It is seen that during the charge period t4, the
terminal voltage VC of the ignition capacitor 12 rises
gradually.
If the voltage supply from the DC-DC converter 20 is cut
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off after the terminal voltage VC of the ignition capacitor 12
has reached the predetermined level, the capacitor 12 stores
the charge until arrival the next ignition timing pulse. The
charge storage period of the capacitor 12 is represented by
the period t5 in Fig. 2. After the crankshaft (not shown)
rotates 360° CA from the previous ignition time, the crankshaft
rotation position signal deflects in the positive direction
and a high signal is generated, whereby an ignition signal is
output by the ignition timing control circuit 32 to the
thyristor 13 in the ignition circuit 10. At the same time, a
_H_AT~T signal is output by the ignition timing control circuit
32 to the oscillation OFF circuit 27. As a result, the
thyristor 13 is switched from OFF to ON and the oscillation of
the DC-DC converter 20 stops, as described above.
Consequently, the electric charge accumulated and stored in
the ignition capacitor 12 escapes to the ground side of the
primary coil 15a of the ignition coil 15, thus inducing a
spark discharge across the ignition plug 16.
According to this embodiment, as set forth above, one end
terminal of the ignition capacitor 12 is connected to the
anode of the thyristor 13 whose cathode is connected to the
ignition coil 15, and its opposite end terminal is connected
to ground. Further, the cathode of the diode 14 is connected
to the anode of the thyristor 13, and the anode of the diode
14 is connected to the cathode of the thyristor 13. With this
construction, the charge and discharge of the ignition
capacitor 12 is repeated with a repeating inversion in
polarity of the electric charge accumulated in the capacitor
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12. Thus, a spark discharge across the ignition plug 16 can
be repeatedly achieved. It is therefore possible to achieve a
stable AC arc in accordance with the DC-CDI method.
Accordingly, it is possible to achieve a long sparking period
by an AC arc whose attainment by the DC-CDI method has
heretofore been difficult.
In this embodiment, moreover, the first closed circuit in
which the ignition capacitor 12 is charged by both DC-DC
converter 20 and the diode 22, and the second closed circuit
in which an electric charge shifts between the ignition
capacitor 12 and the ignition coil 15 so as to induce repeated
charges and discharges of the capacitor 12 through both the
SCR 13 and the diode 14, are separated from each other, so
that a series LC circuit comprising the capacitor 12 and the
ignition coil 15 is not formed.
Therefore, the charging of the capacitor 12 by the DC-DC
converter 20 can be effected stably up to a predetermined
voltage level.
Further, according to this embodiment, the opposite end
terminal of the ignition capacitor 12 is connected to ground,
so when an electric charge is not stored in the capacitor 12,
the output voltage of the DC-DC converter 20 developed by on-
off operation of the switch 2 does cause a flow of electric
current in the ignition coil 15 unless an ignition signal is
fed to the gate of the thyristor 13.
For example in Fig. 1, an AC arc as described above can
be obtained even in a circuit configuration wherein the
position of connection of the parallel thyristor 13 and diode
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~14 with the series-connected ignition coil 15 and ignition
plug 16 is transposed. However, this circuit configuration
has two problems:
(1) Since the capacitor for ignition and the ignition coil
are electrically interconnected at all times, the ignition
coil is supplied a voltage as high as several hundred volts
from the ignition capacitor over a longer period of time than
in the circuit configuration adopted in the above embodiment,
and as a result deterioration in electrical insulation
characteristics of the ignition coil is accelerated.
(2) Since the juncture between the primary coil and the
secondary coil is not connected to ground but is connected to
the parallel circuit of the thyristor and the diode, it is
necessary to provide a new terminal for connection to the
parallel thyristor-diode circuit, that is, a condutor for
connection to the terminal is also required, which increases
the number of components and the production cost.
These problems can be avoided by adopting the circuit
configuration of the embodiment shown in Fig. 1.
Additionally, according to this embodiment it is possible
to prevent a high voltage from being applied to the ignition
coil before turning on the thyristor.
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