After the positive pulse is received, a dwell control section of the control circuit determines when the primary circuit will be switched on, and how long current will flow in the primary winding. The dwell period can thus be varied according to engine speed, improving coil efficiency. At low engine speeds, the dwell period is short. As engine speed rises the dwell period increases.
Ignition coils employed with electronic systems are referred to as low inductance coils, because their primary winding resistance and number of turns is low. Current flow is much higher than with contact breaker systems and reaches its optimum level sooner. A current limiting device within the control circuit limits maximum current flow to a safe value.
Ignition advance according to engine speed and load is provided by centrifugal and vacuum advance mechanisms.