Standard automotive ignition coil has been used in conjunction with a dual timer 556 and MOSFETs employed to handle the power. This can be termed as an electronic ignition. In the dual timer section one half is being used for frequency generation and the other part switches the oscillator section with breaker points getting “closed” and “open”. .
The frequency generator section in this circuit produces a rectangular waveform having frequency of 200 Hz. that drives the MOSFET. IRF 740 has a V-dss of 400 Volts and I-d of 10 Amperes sufficient margin in terms of Voltage and Current rating shall keep the switcher cool and efficient.
Pin 8 and pin 12 are being driven by the breaker contact that in turn produces an inverted signal at pin 9. As pin 9 and 4 have shared logic state, this determines the state of pin 5 that drives the FET.
The frequency determining elements are 15 K, 4K7 resistors and 0.33 micro- Farad capacitor. The duty cycle is 4 milli seconds on the +Ve and 2 milli seconds for the –Ve . During positive transition the FET gates are high enabling the ignition coil current to rise to 4 Amperes level. A Burst of 80 milli Jules of Energy is released in the spark plug when the pin 5 goes to ground state thus turning off the switcher.
A 12 volt Zener Diode as shown in the electronic ignition circuit ensures that Voltage at FET Gate does not go below -0.7 V or above +12 V. A 200 Volt , 5 watt zener is employed to lengthen the Spark Gap. A shorted plug shall work with this circuit however, care must be taken to suppress voltage transients by using MOVs or TVSs such as 1.5KE200A in place of the Zener or in addition.
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