coil voltage

lee9588 · Jun 27, 2008

how many volts should there be going to the coil on a 12 volt system most people say you should use a resistor, but I just checked 4 tractors that I have and they all have 12 volts at the coil I just switched a 6v to a 12v and have been told I need a resistor Any advice?

Anonymous-0 · Jun 27, 2008

It depends on the coil you are using, a coil that measures 1.5 ohms across the primary winding needs a resistor of about 1.5 ohms inline to the coil to work on 12V
a coil that measures aprox 3 ohms across the primary winding does not need a resistor.

One caution when you are measuring voltage at the coil, make sure the points are closed when measuring voltage. A system with an inline resistor will measure voltage dropped from 12V to around 6V-7V at the coil with the points closed.
with the points open, the resistor is not loaded and will not drop the voltage, so you will read full 12V inspite of the inline resistor.

John T · Jun 27, 2008

Lee, the voltage at the coils input should obviously be whatever the coils rated voltage is, like 6 volts on a 6 volt coil and 12 on a 12 WELL DUH.

NOTE a full true 12 volt coil may be labeled "12 volts" or "12 volts NOT for use with external ballast resistor" HOWEVER a coil labeled "12 volts requires external ballast resistor" is in reality a 6 volt coil and requires the ballast (if on a 12 volt tractor) just like it says it does.

Sooooo on a 12 volt tractor you can EITHER use a full true 12 volt coil ORRRRRRRR a 6 volt coil PLUS a series voltage dropping (12 to 6) ballast resistor.

Again, on a 12 volt tractor, if its a true 12 volt coil DO NOT use any ballast, but if its a 6 volt coil, use one.

NOTE if you say you just switched a tractor from 6 to 12 volts and are still gonna use the old 6 volt coil, THE PEOPLE WHO TOLD YOU TO USE A RESISTOR ARE INDEED CORRECT. Unless you also switch to a 12 volt coil that is.

John T

lee9588 · Jun 27, 2008

Thanks for the info where exactly do you measure the ohms across the + and - terminal?

Jerry/MT · Jun 27, 2008

To answer simply-12V. However, since most primary ignition circuits are designed to carry about 4 amps maximum current continuous without overheating, that means the resistance from the battery, through the key switch to the coil to the distributor to ground is something like 3.15 ohms ( 12.6 V/4 amps= 3.15 ohms). The biggest part of that resistance is (or should be) the coil primary winding.

I checked a 6 V coil and it had a resistance of 1.6 ohms so that would say that the it needs another 1.6 ohms to be able to be used on a 12v system and not pass over 4 amps of primary current.

Anonymous-0 · Jun 27, 2008

Yes, check across the + and - terminals. 1.5 ohm needs an inline 1.5 ohm resistor, 3 ohm needs no resistor.

coil voltage

lee9588

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lee9588

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