Selection of the proper ballast resistor

Question:

How should I choose the proper ballast resistor for my application?

Answer:

The ballast resistor is connected between the power source and the ignition coil to limit the current through the contact points. Six-volt systems don’t need one. With a 12-volt system the rule of thumb was that the voltage present at the battery side of the coil should be between eight and nine volts with the engine running.

Ballast resistors are available in several values and selecting the proper one is less critical than not having one. (Note that many British cars had resistors internal to the coil and 1960s GM cars often used a piece of resistance wire buried in the wiring harness.) Some systems (like GM) used a resistor bypass from the starter solenoid to improve starting when the battery was low.

Thanks for this expanded explanation. As I said, the answer to the fellow’s question was yes, you can go with a slightly different rating on the ballast resistor, but it would not be the best solution.

Understanding resistor function: I would suggest that a better analogy than the one provided would be that a resistor limits the flow of electrical current like a jet limits the flow of fuel in a carburetor. Fuses have a different function.

That's why I prefaced my analogy with “sort of when comparing a ballast resistor to a fuse. Both are designed to limit current; in that way they are similar. The difference is, a fuse is intended to sacrifice itself to save components if the correct amperage is exceeded. A ballast resistor also regulates current, but its primary function is—in this case—to limit the voltage to the ignition points in a 12-volt system.