Re: 12 volt front coil

Posted by MarkB_MI on December 26, 2007 at 19:20:26 from (216.234.120.7):

In Reply to: 12 volt front coil posted by Gary Alford on December 26, 2007 at 07:50:06:

I know better than to jump into the "true 12 volt coil" dispute again, but here I go.

Short version: It makes absolutely no difference whatsoever whether the resistance in a 12 volt points and condensor ignition is external or internal to the coil. And if the resistance is internal to the coil, it makes no difference whether the resistance comes from the coil winding or some resistance element. So, there are no "true 12 volt" coils. There are coils that require external resistors and there are coils that do not require external resistors. Call them what you will.

Now, the long version:

First, make the assumption that all ignition coils have an identical inductance. Now, there are coils that have non-standard inductance, but these are intended for hotrodders using high performance distributors; we are not talking about these coils.

OK, so the spark energy released by a coil is whatever energy was stored in the coil at the instance the points open. That energy is equal to one-half the inductance times the square of the current through the coil. So, if the inductance of several different coils is identical, then whichever coil has the greatest current will have the greatest energy.

But what controls the current through the coil? Well, in steady state it is ohm's law: current equals voltage divided by resistance. We know the voltage applied by the battery is 12 volts. (Actually, it's not; a "12 volt" system is really around 14 volts, but never mind that.) The resistance is the TOTAL resistance from the positive battery terminal to the negative battery terminal. That includes the primary wiring, any external resistor, the resistance of the coil windings, and other resistance in the coil, and a whole bunch of other things such as the points, key switch, splices, etc. So IT DOESN"T MATTER where the resistance comes from, it all gets added up in the end. If the TOTAL resistance is the same, the coil current is the same, and the coil energy is the same.

OK, so someone is likely to say "yeah, but what about AFTER the points open". Well at that instant you have an RLC (resistor/inductor/capacitor) circuit. and yes its behavior is going to vary depending on the resistance in the circuit. But once again it is the TOTAL resistance in the circuit, NOT just the internal or external resistance alone. And that total resistance is going to be the same as it was BEFORE the points opened, with the exception that the points are no longer in the circuit (the current that was passing through the points now passes through the condensor).

OK, flame away.

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