The present invention relates to an ignition circuit for an internal combustion engine and, particularly, to such a circuit of the current cut-off type.
FIG. 4 is a circuit diagram of an example of a conventional ignition circuit of the type which includes a signal coil 1 for producing a positive and negative angle signal in synchronism with an engine rotation and a pair of parallel CR circuits 2, 3 and 4, 5 for noise suppression. The ignition circuit further includes a comparator 8 and an ignition control circuit 15 having an output connected to a primary coil of an ignition coil 16 whose secondary coil is connected to an ignition plug 17. The CR circuits have one ends connected together and to one end of the signal coil 1. The other ends of the CR circuits are connected through a backward connected diode 6 and a forward connected diode 7 to an inversion input of the comparator 8, respectively. A diode 9 is connected between the inversion input of the comparator 8 and a grounding point and a pair of series connected resistors 10, 11 and 12, 13 are inserted between a power source and the grounding point, respectively, a junction between the resistors 10 and 11 being connected to the inversion input of the comparator 8 and a junction between the resistors 12 and 13 being connected to a non-inversion input of the comparator 8. A resistor 14 is connected between an output of the comparator 8 and the non-inversion input thereof. The resistors 10 to 14 function to set the input level of the comparator 8 and constitute, together with the comparator 8, a bistable flip-flop circuit. The ignition control circuit 15 receives an output of the comparator 8 to calculate an ignition timing.
In operation, when the engine rotates, an angle signal such as shown by (a) in FIG. 5 is produced by the signal coil 1. The comparator 8 responds to a positive portion and a negative portion of this signal to change its output from H level to L level and from L level to H level, respectively. That is, when the voltage applied to the inversion input of the comparator 8 exceeds a voltage level V1 determined by the series resistors 10 and 11, the output of the comparator 8 becomes L level and, when it exceeds a voltage V2 determined by the series 12 and 13, the output becomes H level, as shown by (b) in FIG. 5. The output of the comparator 8 is supplied through the ignition control circuit 15 to the ignition coil 16 as shown by (c) in FIG. 5 and, at a current cut off time, a high voltage produced in the secondary coil of the ignition coil 16 is supplied to the plug 17 to form a spark discharge thereat.
The voltage level V2 is preliminarily set as being lower than the voltage level V1 by a suitable selection of resistances of the resistors 10 to 14, so that the ignition coil 16 is prevented from being supplied with a continuous current. That is, a magnitude of the angle signal produced by the signal coil 1 is proportional to engine rotation, the larger the magnitude being the higher the engine rotation as shown by (a) in FIG. 6 and otherwise as shown by (b) in FIG. 6. Therefore, in order to ensure that the ignition coil 16 is cut off at a low voltage during a low engine rotation such as during an engine starting or idling time, it is necessary to make V2 lower than V1 so that the circuit operation is performed on the negative portion of the angle signal.
The bistable flip-flop composed of the comparator 8 and the associated resistors is relatively complicated due to a relatively large number of circuit elements and associated wirings and expensive due to, particularly, the use of the comparator 8.
In order to solve this problem, it is necessary to avoid the use of the expensive comparator 8, firstly. FIG. 7 shows a flip-flop circuit which may be used as a substitution thereof. The flip-flop circuit shown in FIG. 7 comprises a pair of NPN transistors 41 and 42 and resistors 43 to 46 connected in a well known manner. The operation of this flip-flop is well known. Although this circuit is inexpensive, the problems of the necessity of relatively large number of circuit elements and the complicated wiring are left as they are.