Posted on Feb 6, 2013 10329
The ignition substitute provides a constant power source for the ignition coil. Its frequency, 0.5-1.0 kHz, is that used by an 8-cylinder engine with an idling speed of 650 RPM, and the unit provides a rapid spark at a 17% duty cycle, while staying within the power dissipation limits of the components. The circuit consists of a 555 timer IC configured as an astable free~running multivibrator that is used to drive a high-current npn transistor, such as a 2N6384. The transistor should be heavily heatsinked because it might be drawing several amps over quite a long period of time.
The coil ballast can be from 0.68 to 6.5 0, depending on what"s available. The 2.5-0, 20-W ballast shown works well. All the other resistors can be either 1/4-or 1/2-W devices, and the capacitor, between pins 1 and 5 of the 555, can range from 0.01 to 0.051"F. Do not omit the 100-V, 0.05-I"F capacitor across the transistor; it prevents voltage spikes from damaging the device. Although designed for an 8-cylinder engine, this device can be used with other types. In addition, a neon bulb can be added to the circuit to verify the presence of a spark.