Quote:

Automotive coils are current-driven devices. In the days of points, if the ignition system were left powered up with the points in the closed position (the key left on with the engine not running), the coil would effectively short out the battery and the coil would overheat. The ballast is a series resistor which limits the current in the coil. The ballast resistor also drops some of the voltage but voltage reduction is not its primary function.



I agree with this.

i dont agree with the post that says a ballast resistor is supposed to change resistance with temperature. yes, resistors DO change resistance as the temperature changes but I dont believe thats what makes a ballast resistor a ballast resistor. and i dont believe this is a something the system needs or is desirable- its a physical propert y of the resistor and I bet, if they could make a more stable resistor as a reasonable cost, they would but since slight changes in resistance due to temperature changes isnt critical, they dont worry about it

afaik, a ballast resistor is just a big resistor that can handle more power than your typical 1/4 or 1/8 watt resistor. Ballast resistors need to be big and bulky to dissipate heat because the current draw through the coil generates lots of heat. as the resistor will limit the current to keep the coil from melting, the ballast resistor heats up and needs to be big and stout so IT wont melt as well.