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The voltage provided to a car's ignition coil whilst cranking is less than the nominal battery voltage of 12 volts due to the high current drain to the starter. That may be as low as 8 volts so the ignition coil is rated at 8 volts. Once the engine fires, the terminal voltage rises back to the nominal 12 volts which is too much for the coil so a ballast resistor is switched in to reduce the coil voltage to its rated 8 volts. All well and good.

Bosch market an ignition coil called a "Blue Bosch" which has an internal ballast resistor.
Please help me to understand how that works - how is the ballast switched in/out? Is there some voltage sensitive device within the coil? Zener diode? A circuit diagram would assist.

Thanks.
 
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The voltage provided to a car's ignition coil whilst cranking is less than the nominal battery voltage of 12 volts due to the high current drain to the starter. That may be as low as 8 volts so the ignition coil is rated at 8 volts. Once the engine fires, the terminal voltage rises back to the nominal 12 volts which is too much for the coil so a ballast resistor is switched in to reduce the coil voltage to its rated 8 volts. All well and good.

Bosch market an ignition coil called a "Blue Bosch" which has an internal ballast resistor.
Please help me to understand how that works - how is the ballast switched in/out? Is there some voltage sensitive device within the coil? Zener diode? A circuit diagram would assist.

Thanks.
The voltage provided to a car's ignition coil whilst cranking is less than the nominal battery voltage of 12 volts due to the high current drain to the starter. That may be as low as 8 volts so the ignition coil is rated at 8 volts. Once the engine fires, the terminal voltage rises back to the nominal 12 volts which is too much for the coil so a ballast resistor is switched in to reduce the coil voltage to its rated 8 volts. All well and good.

Bosch market an ignition coil called a "Blue Bosch" which has an internal ballast resistor.
Please help me to understand how that works - how is the ballast switched in/out? Is there some voltage sensitive device within the coil? Zener diode? A circuit diagram would assist.

Thanks.
Solved:
It transpires that it's nothing to do with starting, but to reduce the coil's inductive time constant.
 
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