12v ignition coil vs. 6v coil

I understand how an ignition coil works, primary winding to secondary winding, and I understand some have more resistance so are called "internal resistor" coils, but wonder what the difference is between a true 6v coil and a true 12v coil? Are the 12v coil windings of heavier gauge metal, are there additional windings over the 6v unit, is there better insulation coating on the 12v windings, or what? Except for the additional heat generated, it seems like a 6v coil without an external resistor would work in a 12v situation, much like a 6v coil will work when using an 8v battery in a 6v system.

--Lee

True 12 Volt coils have more primary turns/windings.

As to using a 6 Volt coil on 12 Volts, in addition to it getting HOT, it doubles the current through the breaker points.

It's pretty much accepted that in common practice ignition points typically switch between 3 and 4 Amps, and when you start to get above that, they get hot, burn, pit, etc..

Also, you wrote " some have more resistance so are called "internal resistor" coils".

NOT by anyone but the uninformed.

While there were some such oddities 80 or so years ago, I don't believe you will find ONE single example in the modern era of a coil labeled "internal resistor".

Coils requiring an EXTERNAL resistor are so labeled, but you won't see one labeled "INTERNAL resistor".

Thank you for the explanation, Bob. --Lee

I agree with Bob. A coil designed for 12 volt use has around twice the LV primary winding resistance as a 6 volt coil. Since one of the limiting factors in point ignition systems is the amount of current the points must pass and switch and that amount is typically (older stock farm tractor coils NOT talkin high energy after market) around 3 to 5 amps, that means in a 6 volt coil: R = E/I = 6/4 = 1.5 ohms primary resistance or in a 12 volt coil R = E/I = 12/4 = 3 ohms LV primary winding resistance.

In order that the 12 volt coil have more LV primary winding resistance it might have more turns of wire or wire with a higher resistance, but usually in stock old farm tractors the coil would NOT have any stand alone internal discrete ballast resistor, but instead simply more wire length or turns or higher resistance wire then a 6 volt coil.

Sure, there were a few older cars that indeed had a coil with a compartment inside where a stand alone discrete ballast resistor was tucked away, but that's NOT typical in stock old farm tractors.

Also there can be exceptions to the above, but typical LV primary winding resistances of old stock 6 volt farm tractor coils are 1 to 2+ ohms while many stock old farm tractor 12 volt coils have around 2.5 to 4+ ohms. High energy high voltage after market and performance coils can be different

If you put a 6 volt coil unballasted in a 12 volt tractor it could overheat plus the points burn up sooner (would be switching 8 instead of only 4 amps). If you put only 6 volts on a 12 volt coil the points will last longer and it wouldnt get very hot but the spark would be weak.

John T