Motorola Radius PM400 Installation Manual - Page 27
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Operation of a Conventional Ignition System
19
The battery is connected to the primary winding of the coil through the ignition switch. The primary
circuit is returned to the battery through the breaker points, which are bypassed by the condenser.
The points are normally closed. As the cam shaft is rotated by the engine, it's lobes or corners open
and close the points in proper synchronization with the piston in each cylinder.
With the ignition switch on and the breaker points closed, coil primary current builds up at a rate
determined by the coil inductance.
When the breaker points open, primary current decreases and, by self-induction, an electromotive
force is induced in the primary which is many times greater than the battery voltage. The high voltage
inducted in the coil secondary causes a spark across the distributor rotor-to-spark plug wire gap and
then across the spark plug gap for a short interval of time when the breaker points open. The
condenser reduces arcing of the points.
The secondary circuit of the ignition coil, including the distributor rotor gap and the spark gap, is the
main source of ignition interference. The lead inductance and stray capacitance provide a tuned
circuit. Because the discharge of the circuit is through a low resistance (ionized spark gap), the circuit
tends to oscillate. The frequency and amplitude of oscillation vary as current changes in the spark
gap.