Contactless igniton circuit for internal combustion engines

The present invention relates to a contactless ignition circuit for internal combustion engines wherein a primary short-circuiting current flowing through the primary winding of an ignition coil is made to flow through a power transistor controlled to be conducted and interrupted by a programable unijunction transistor and a potential dividing type capacitor is connected to the anode electrode so as to operate this programable unijunction transistor when the wave height value of the primary short-circuiting current flowing through the above mentioned primary winding becomes maximum so that a spark plug connected to the secondary winding will be operated to ignite at a high sensitivity.

This invention relates to a contactless ignition circuit for internal 
combustion engines particularly wherein a primary short-circuiting current 
flowing through the primary winding of an ignition coil is made to flow 
through a power transistor controlled to be conducted and interrupted by a 
programable unijunction transistor and a potential dividing type capacitor 
is connected to the anode electrode so as to operate this programable 
unijunction transistor when the wave height value of the primary 
short-circuiting current flowing through the above mentioned primary 
winding becomes maximum so that a spark plug connected to the secondary 
winding will be operated to ignite at a high sensitivity. 
Further, in the present invention, as no current source coil is used as 
before, there is no fear of causing an accident of melting off by a surge 
voltage and the circuit formation and setting structure can be simplified. 
There is already known an ignition circuit for internal combustion engines 
wherein a primary short-circuiting current of the primary winding for 
ignition is interrupted and conducted by a mechanical contact connected in 
parallel with a capacitor for extinguishing arcs with the rotation of the 
crankshaft of the engine. However, in such ignition circuit, there have 
been problems that it is difficult to set the positions by which a 
mechanical contact and an interruption and conduction contrlling cam are 
fit and the wear of the contact is so remarkable that the durability of 
the contact is low. Further, there is naturally a limit in the structure 
to the reduction of the entire shape and many complicated steps are 
required to assemble it. 
On the other hand, there are provided ingition circuits wherein a voltage 
induced by a current source winding is accumulated in a capacitor and this 
accumulated electric charge is discharged to the primary winding of an 
ignition coil by a silicon controlled element and wherein a primary 
short-circuiting current induced in the primary winding of an ignition 
coil by the rotation of the flywheel magnet is made to flow through a 
power transistor, this power transistor is conducted and interrupted by 
the trigger of a silicon controlled element and the trigger of this 
element is controlled by a resistance branch circuit of an induction 
current flowing through the primary coil. 
However, in the former ignition circuit, the places of setting the silicon 
controlled element and trigger coil can be made small without limit, the 
contact is not worn and the life on the circuit is long but there are 
defects in that the weight is increased and the current source winding is 
melted and cut by a surge voltage. The latter ignition circuit is of a 
subswitch system using a thyristor and small signal transistor, the 
trigger level of the thyristor is so high that the response sensitivity is 
low and therefore there is an inconvenience that no ignition can be made 
at the time of a low speed. 
The present invention is to solve such various problems in the conventional 
ignition circuit and particularly has it as an object to provide a novel 
contactless ignition circuit for internal combustion engines wherein a 
primary short-circuiting current flowing through the primary winding of an 
ignition coil is controlled to be conducted and interrupted through a 
power transistor, this controlling operation is made by a programable 
unijunction transistor and small signal transistor, the trigger of the 
programable unijunction transistor is adjusted by a potential dividing 
circuit of a resistance and capacitor and the programable unijunction 
transistor is controlled to be on and off at a high sensitivity with the 
adjusted trigger level so that an efficient ignition can be made even at 
the time of a low speed rotation of the flywheel magnet.

In FIG. 1, reference numeral 1 denotes an ignition coil having a primary 
winding 1a and secondary winding 1b connected as illustrated through an 
iron core 1c. That is to say, both windings 1a and 1b are made by doubly 
winding a small amount of a thick lead wire and a large amount of a thin 
lead wire on the same iron core 1a and one terminal of each of them is 
connected to a common ground. Further, a spark plug 2 is connected between 
both terminals of the secondary winding 1b. The circuit relating to this 
secondary winding 1b is the same as that of the conventional one. To both 
ends of the above mentioned primary winding 1a, a series circuit 
consisting of a resistance 3 and rectifying diode 4 is connected in 
parallel and the collector and emitter of a transistor 6 of two 
Darlington-connected transistors 5 and 6 forming a power transistor 7 are 
connected respectively as illustrated. Further, to both ends of the above 
mentioned primary winding 1a, a voltage controlling resistance 8 and the 
collector and emitter of a small signal transistor 9 are connected as 
illustrated. To a connecting neutral point P.sub.1 of the resistance 8 and 
the collector of the transistor 9, the base of the above mentioned 
transistor 5 is connected. Reference numeral 10 denotes a current 
adjusting resistance. The base of the transistor 9 and the grounding 
terminal of the primary winding 1a are connected with the terminal on the 
other side. Reference numeral 11 denotes a programable unijunction 
transistor in which the cathode is connected to the base of the above 
mentioned transistor 9. Its anode is connected to a connecting neutral 
point of a series circuit consisting of a resistance 12 and capacitor 13 
connected in parallel with the above mentioned primary winding 1a. The 
resistance 12 and capacitor 13 form a potential dividing circuit and the 
capacitor 13 operates to charge and discharge electricity. Further, the 
gate of the above mentioned programable unijunction transistor 11 is 
connected to a connecting neutral point P3 of a potential dividing circuit 
consisting of resistances 14 and 15 connected in parallel with the above 
mentioned primary winding. By the way, it is necessary to set the 
capacitance of the above mentioned capacitor 13 in advance to be of such 
size as to operate the programable unijunction transistor due to the 
correlation with the potential of the above mentioned gate when the wave 
height value of the primary short-circuiting current flowing through the 
primary winding 1a becomes maximum. 
The operation of the contactless ignition circuit of the above mentioned 
formation shall be explained in the following. 
First, by the rotation of the flywheel magnet, a primary short-circuiting 
current I will be induced in the primary winding 1a of the ignition coil 1 
and will be divided as in I.sub.0, I.sub.1, I.sub.2 and I.sub.3 into the 
above mentioned four circuits connected in parallel with the primary 
winding 1a. By the way, the wave form of the current produced in the 
primary winding 1a at this time is shown in FIG. 2(a). At this time, the 
power transistor 7 will not be interrupted. That is to say, a 
predetermined voltage will be obtained at the points P.sub.2 and P.sub.3 
by the above mentioned divided currents I.sub.2 and I.sub.3 but, at the 
beginning of charging the above mentioned capacitor 13, the voltage at 
both ends of the capacitor 13 will not be high enough, therefore the 
potential at the point P.sub.2 will be lower than the potential at the 
point P.sub.3 and the above mentioned programable unijunction transistor 
12 and small signal transistor 19 will not operate. Therefore, when the 
potential at the point P.sub.1, that is, at the base rises, the collector 
and emitter of the transistor 6 will conduct, the above mentioned primary 
winding 1a will be short-circuited and such primary shortcircuiting 
current as is shown in FIG. 2(a) will be fed to it. On the other hand, the 
charged voltage of the capacitor 13 charged with the above mentioned 
divided current I.sub.3 will rise and, at a fixed voltage value determined 
by the time constant of its capacitance and the above mentioned resistnce 
value 12, the anode voltage of the programable unijunction transistor will 
become higher than the gate voltage in respect of the time. The relation 
between such anode voltage and cathode voltage is shown in FIG. 2(b). At 
the above mentioned fixed voltage value at which both voltage values are 
equal, the programable unijunction transistor 11 will conduct, such 
current I.sub.A as is shown in FIG. 2(c) will flow into its cathode side 
and the small signal transistor 9 will be driven by this current I.sub.A. 
When the collector and emitter of the small signal transistor 9 conduct, 
the base potential of the transistor 5 forming the above mentioned power 
transistor 7 will fall, this transistor 5 will be off and the current 
flowing to such I.sub.p as is shown in FIG. 2(d) will be interrupted. 
Thus, by such interrupting operation of the power transistor 7, a high 
voltage will be induced on the secondary winding side. What is important 
here is to set in advance the values of the above mentioned capacitor 13 
and resistance 12 so that, when the primary short-circuiting current 
becomes substantially to be of the maximum value (time point t), the above 
mentioned programable unijunction transistor 11 will be triggered. At the 
time point t at which the potential (anode potential) at both ends of the 
capacitor 13 exceeds the gate potential, this programable unijunction 
transistor 7 will be off and the interrupting operation will be 
effectively made. Here, the sensitivity of the gate trigger of the 
programable unijunction transistor 11 is so sufficiently higher than of 
the one adopting the subswitching system of the conventional thyristor or 
transistor that, even with a minute current, the trigger is easy. 
Therefore, even if the rotation of the flywheel magnet is low, if an 
expected relative potential difference is formed between the above 
mentioned points P.sub.2 and P.sub.3, the operation of the programable 
unijunction transistor will be able to be made possible. Thus, its on-off 
signal transistor 9 will sharply rise and the power transistor 7 will be 
efficiently and quickly controlled. Thus, in this contactless ignition 
circuit, the same as in the conventional point type igniting means, an 
ignition in a low speed range can be smoothly made and the internal 
combustion engine can be quickly and efficiently started. 
As explained in detail in the above, according to the present invention, a 
primary short-circuiting current flowing through the primary winding of an 
ignition coil is conducted and interrupted by a power transistor 
controlled by a programable unijunction transistor and small signal 
transistor and the trigger of the above mentioned programable unijunction 
transistor is controlled by the time constant of a potential dividing 
circuit consisting of a resistance and capacitor so that, even if the 
flywheel magnet rotates at a low speed and the current induced in the 
above mentioned winding is minute, the programable unijunction transistor 
will be operated at a high sensitivity, the power transistor will be able 
to be interrupted quickly and reasonably by a signal sharp in the rising 
operation, a high voltage will be generated in the secondary winding and 
an efficient ignition will be able to be made. Further, as no such current 
source coil as in the conventional type is used, there is no fear of 
causing an accident of melting off by a surge voltage and the circuit 
formation and setting structure can be simplified.