The Ford Barn

tubman · 02-26-2025, 12:42 PM

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.

02-26-2025, 12:42 PM	#5
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 11,629	Re: Ballast resistor 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.