Ballast resistor

[oldredford]

If you run a stock 6-volt system, why is it you need a ballast resistor on a 6-volt coil?

[rockfla]

It prevents too much voltage (heat) from prematurely burning out your points and also "basically" extends the life of both your points and coil for longevity of the circuits performance.

[tubman]

Actually, the Ford ignition system runs on about 4 volts. At startup, the resistor is cool and offers no resistance so the coil gets a full 6 volts. After running a while, the resistor heats up and develops enough resistance to cut the coil voltage down. Although it is called a "ballast resistor", the Ford resistor functions differently than a regular ballast resistor, so the term is kind of a misnomer.

[oldredford]

Explain to me why a 12 volt car with resistor the points don't prematurely fail on 6 volts. Points are points correct, regardless of voltage.

[tubman]

Current (amps), not volts are what "burn out" points. Normally, a set of points will tolerate up to 4 amps before premature "burning" sets in. That's why 6 volt and 12 volt systems run coils with different resistance factors. You do remember Ohm's Law, don't you?

I (amps)=V (volts)/R (ohms)

To get 4 amps across the points in a 6 volt system, you need 1.5 ohms resistance through the coil. For a 12 volt system, you need 3 ohms. With 12 volt systems, the manufacturers sometimes used a 3 ohm coil. However, old habits are hard to break (and 6 volt coils were available and cheap), so they used a 6 volt coil with a 1.5 ohm ballast resistor and called it good. Then, they got the idea to bypass the ballast resistor during starting so the system got 8 amps for a short while, which aided starting and really didn't harm the components.

[rotorwrench]

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Coils aren't strictly made for any particular voltage. The can type coils were mostly 1.5 Ohm primary types. Ford used them in the model A era but Henry was keen to use a front mount distributor on the V8 engine since it was direct drive to the cam. I'm not sure how Henry Ford got to know Marion Mallory but it's known that they were friends. Mallory worked with the Ford electrical engineer to develop the front mounted distributor and the coils that attached to it. These coils were using the same principle as the can type coil but the shape and packaging made them different. Due to this difference, they found that a 0.8 Ohm ballast resistor controlled the heat well enough to give an acceptable spark and give good reliability to both the coil and the breaker assembly to include the condenser. It was efficient enough to give a good hot spark at the lower amperage.

The dual point breaker controlled the dwell and the vacuum brake gave a decent amount of load control. It was actually pretty ingenious the way it functioned and was very reliable in service. Henry was already passed on to the ages by the time the Holley Load-O-Matic system was developed in the 8BA era. The can type 1.5 Ohm coils functioned just fine with no ballast but their load control was just good enough for a stock motor. If you add compression or change carburetors then all bets are off.

[Terry,OH]

Points are not alike. They have current ratings and passing excessive current through the contacts will cause them to fail. The Ford coil is not 6V. That is why there is a resistor. Many of the 6V aftermarket coils from the same era as your early Ford also use the ignition resistor in one form or another.

[FlatheadTed]

,A 6 volt Generator can produce up to 15 volts and if you accidentally leave your ING you could burnt up your coil ,so the resister is needed as a cushioning facture and upping voltage for starting and reducing it for running .All wire connections must be good .