Blue Bosch Ignition Coil Poser

The Blue Bosch ignition coil has an internal "ballast" resistor but no extra terminal.
How does that work? What's it for?

Hyphensmythe said:

What's it for?

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To keep the coil from burning out. Most every points ignition 12V car has a form of ballast resistor to cut the coil voltage to ~8 volts when running, either the old square type mounted to the firewall or a stainless steel wire instead of copper from the ignition switch to act like a resistor. Often if you take a engine wiring harness apart, the coil wire will be folded back on its self to get a long enough wire for the correct resistance.

Thanks for that. I understand.
The ballast resistor is bypassed when cranking so that the reduced battery terminal voltage, ~8 volts, is applied directly to the coil. When the engine fires the starter is released, the battery voltage returns to nominal and the ballast resistor is placed in circuit to keep the coil voltage down to 8 volts rather than the 13.5 volts alternator output. If the resistor is internal to the coil it's in circuit permanently so what use is that? There must be more to it.

Hyphensmythe said:

The ballast resistor is bypassed when cranking so that the reduced battery terminal voltage,

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That is true in a points distributor, but an electronic or HEI type uses the full voltage all the time with an internal ballast, if I remember right. Bosch makes a lot of different coils so did you chose the right one for the application? Not trying to start a fight just asking.

It would seem that you could eliminate the ballast resistor if the electronic driver sensed the coil current and shut off the coil voltage when the desired current was reached.
Of course a free wheeling diode would be needed to keep the coil current flowing until it's interrupted to generate the spark.

If it has an internal, unswitched, ballast resistor, then it could be to reduce the inductive time constant of the coil?

eg. If it were a 6V coil plus a resistor with equal DC resistance so the same steady-state current at 12V as with the coil alone at 6V, the coil will reach a given current (ie. 90%) rather faster that if fed directly at the lower voltage.

In an ignition system that presumably helps maintain a strong spark at high RPM?

(My knowledge of the principle dates back to mechanical teleprinter days - Creed machines used something like a 6V electromagnet on an 80V supply with a ballast resistor (or barreter) to overcome the magnet inductance and allow it to track the 50 or 75 baud data).

crutschow said:

It would seem that you could eliminate the ballast resistor if the electronic driver sensed the coil current and shut off the coil voltage when the desired current was reached.
Of course a free wheeling diode would be needed to keep the coil current flowing until it's interrupted to generate the spark.

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I think that this is what would be done.

Stepper motors were often fed with ballast resistors so that they could be wound with low inductance to keep the time constant low. It is common now to have current control so that the losses in the ballast resistor are removed.

On modern production cars, the ECU handles all of that.

It makes perfect sense to fit some electronics in a coil for an older car, so that it is low inductance / resistance and the current limiter means that it will run from cranking voltage.

In fact, it would be quite easy to fit enough electronics in a coil to reasonably accurately predict when the next spark is going to be needed, and to only turn the coil on a short time before that.

rjenkinsgb said:

If it has an internal, unswitched, ballast resistor, then it could be to reduce the inductive time constant of the coil?

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Yes! Your Creed baud rate experience is relevant still. Well done, and thank you..