What prevents the magnetic field in the transformer of a battery ignition system from un-collapsing and reverting to using primary coil? Is it as simple as the functional battery?

When the car is shut-down, does the backwards process happen such that the transformer (distributor)'s intended low to high voltage happens in reverse?

  • 3
    \$\begingroup\$ The primary circuit gets interrupted ... That's how an inductive kick works. Nowadays it's done with transistors acting as switches. Back in the day, it was done mechanically using "points". \$\endgroup\$
    – Kyle B
    Aug 20, 2022 at 1:07

1 Answer 1


The ignition moment is the collaps of the magnetic field. There is no field left in the transformer, that could revert.

Some ignition circuits use forward conversion. In such systems there is no permanent current in the coil anyway.

If the car is shut down while a voltage across the primary coil is present, this voltage would decay slowly compared to the immediate turn off of the transistor during an ignition. So the current ramps just down, probably giving some last dozen volts on the secondary side.

It is also possible, that the shut down quickly turns off the transistor and a last fractional ignition is generated. That depends on the circuit implementation and is more likely in pre 1990 cars.


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