I used to be an instructor at an automotive trade school, and I know that
the role of the ballast resistor is both confusing and poorly understood.
Here's my whack at it. I am talking about conventional ignition systems
here--either points or electronic--not capacitive discharge, multiple spark,
or other gee-whiz stuff.
The ignition coil is designed to produce a hot spark even when the available
voltage is as low as 9-10 volts--which is about what it gets during
cranking. The coil should get this 9-10 volts through a straight feed (no
ballast) from the start terminal of the ignition switch or from the starter
solenoid.
When the engine starts and the alternator/generator kicks in, the available
voltage jumps to about 14-15 volts--way more than the coil needs to produce
a hot spark and enough to shorten the life of the coil (even if it does not
feel hot to the touch). The job of the ballast resistor--which should only
be in the line from the run position of the ignition switch--is to reduce
the voltage back down to the 9-10 volts for which the coil is designed.
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