12v conversion. Is a resistor needed if using a 12v coil?

Tornik550 · Aug 20, 2017

I just did the 12v conversion on my 46 Farmall H. I used a new 12v coil that I had laying around from a different project. Is it necessary to use a resistor going into the coil in my case? Any advantages or disadvantages in this situation?

Steve@Advance · Aug 20, 2017

You need to check the resistance across the + and - terminals of the coil, out of circuit. Need to have around 3 ohms resistance for use without a resistor. Even though the coil may say 12 volt, some still need a resistor, it is the way the coil is wound internally.

If it has around 1.5 ohms, it will need a 1.5 ohm resistor added.

There is no advantage or disadvantage to either, as long as the resistance equals 3 ohms, it will work.

JMOR · Aug 21, 2017

(quoted from post at 17:20:29 08/20/17) You need to check the resistance across the + and - terminals of the coil, out of circuit. Need to have around 3 ohms resistance for use without a resistor. Even though the coil may say 12 volt, some still need a resistor, it is the way the coil is wound internally.

If it has around 1.5 ohms, it will need a 1.5 ohm resistor added.

There is no advantage or disadvantage to either, as long as the resistance equals 3 ohms, it will work.

ood job, Steve! Cut to the chase, without the usual BS!

John T · Aug 22, 2017

I agree with Steve. On a 12 volt tractor you can use EITHER a full 12 volt rated coil without a ballast OR a 6 volt coil PLUS an external series voltage dropping (12 down to 6) ballast resistor so it drops 6 leaving 6 for a 6 volt coil.

Some old farm tractors could have used the same (6 volt) coil on a 6 or 12 volt tractor, its just that on 12 volts they added the series ballast.

A typical old farm tractor 12 volt coil has around 2.5 to 3.5 ohms LV primary resistance while a typical old farm tractor 6 volt coil has around 1.2 to 2 ohms

A coil labeled 12 volts or 12 volts NOT for use with ballast is a 12 volt coil

A coil labeled 12 volts for use with or requires ballast is in reality more like a 6 volt coil....

NOTE as this is an old FARM TRACTOR forum IM NOT TALKING ABOUT OLD CAR COILS some of which old American cars and trucks used 6 volt coils and some used 12 volt.

John T

JDEM · Aug 22, 2017

Most setups with points and "12 volt" systems call for a total of 3 to 3.5 ohms. It can be a 1.5 ohm coil and 1.5 ohm
resistor, or just a 3 ohm coil. Get out our ohm-meter and check your coil.

12v conversion. Is a resistor needed if using a 12v coil?

Tornik550

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Steve@Advance

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JMOR

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John T

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JDEM

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