My understanding of the ballast resistor is as follows. A coil works at something like 9 volts(for arguments sake – it might not be exactly but bear with me) so the ballast resistor is used during normal running conditions to drop the voltage from the car/bike electrical system to the required voltage as imperial stated. However, during heavy load conditions on the battery such as cranking the engine over , pulling massive current the voltage across the battery drops so the coil is supplied straight from the battery (reduced voltage remember)so that it continues to function and provide a strong spark.
GSX750 Ratfighter / GSX600F / Beamish250 / Honda70 Rat
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