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Feb15-12, 07:35 PM
Sci Advisor
PF Gold
jambaugh's Avatar
P: 1,776
The coil is an induction coil and its purpose is to create a voltage spike sufficient to jump the spark gap in the spark plug. So the connection from the coil is producing electrical spikes in time with the engine. The voltage is quite high. The capacitor keeps DC voltage from flowing through the circuit and the resistor is a ballast resistor. Its high resistance prevents much current from flowing through (even AC current from the pulse).

The resistor's voltage rating is not too big an issue given the pulsed input.

The whole purpose of this circuit is to provide a moderate voltage low current signal to the tachometer. Inside the tach. should be a circuit converting frequency to (small) proportional current which deflects the meter in proportion to the engine speed.

What is your ultimate issue/goal/problem?