Ignition Coil ballast Resistor

[Bob Bidonde]

The question of what is the function of an ignition coil's ballast resistor often comes up, so this is an explanation of what it does.
Simply put, the ballast resistor reduces surging / high current flow into the coil's primary winding to protect the coil from damage and service life reduction.

When starting the engine, the starter motor causes a high surge current in the electrical system because it draws 175+ Amps from the battery to crank the engine. Once the engine starts, the generator / alternator surges current into the electrical system to charge the battery. The surge currents would also go through the ignition coil, but the ballast resistor drops the voltage and so reduces the surge current going into the coil's primary windings.

The ignition coil actually runs with less voltage than the battery / generator / alternator produces because of the ballast resistor.

I hope this simplified explanation makes sense to you all.