Coil Question 6/12 volt confusion

I see in one catalog I have coil for IH battery ignition. They list one for 6 volt and a similar one (different part number) for 12 volt that states "use with external resister" My question is, are not both these in fact 6 volt coils and the only difference is the labeling on the package for the man who [i:654c4848f0]thinks[/i:654c4848f0] he needs a 12 volt coil for his 12 volt system??????????

Cowman, some time back I answered your question in GREAT DETAIL so I have copied and pasted it below SINCE ITS SUCH A GREAT QUESTION.

Youre correct, a coil labeled "12 volts use with external resistor" assuming the "resistor" drops 12 volts input down to 6 for the coil, I LIKE YOU CONSIDER A 6 VOLT COIL. I bet if you used it a 6 volt tracotr with no ballast of course, it would operate fine and not overheat.

Heres a copy of my previous discussion:


First of all, 6 volt and 12 volt coils ARE NOTTTTTTTTTTTTTTT THE SAME what the parts dude told you ("all coils are the same") IS WRONG AS RAIN. A coil designed and intended for 12 volt NOMINAL use, one that will still produce a spark when battery voltage is reduced or when its raised due to charging, has "around" 3 ohms (range say 2.25 to under 4 ohms) of primary resistance while a 6 volt NOMINAL coil has about half that, "around" 1.5 ohms. THESE VALUES VARY SOMEWHAT BUT THIS GIVES YOU THE IDEA.

NOTE I’m talking typical old points and condenser ignition tractor coils here NOT after market or high energy coil or coils for elec. ignitions.

One of the main design limitation parameters in old tractor Kettering points and coil ignition systems is limiting the current the points must swtich to the 4 amp and less range, otherwise the points burn up too fast.
THEREFORE:
In a 6 volt system, the 6 volt coils low voltage primary winding resistance is around 1.5 ohms, meaning the current (the points must switch) is 6/1.5 = 4 amps
In a 12 volt unballasted system, the coils low voltage primary winding resistance is more like 3 ohms, meaning the current (the points must switch) is 12/3 = again 4 amps.
Now, some 12 volt tractors still used a 6 volt (1.5 ohms) coil BUTTTTTTTTTT they added a series 1.5 ohms EXTERNAL VOLTAGE DROPPING (12 to 6) RESISTOR which dropped 6 volts leaving 6 on the coil as designed for, so the points still only switch 4 amps and alls well.
NOTE: Coils labeled "12 volts" or "12 volts NOT for use with ballast resistor" are 12 volt nominal coils. HOWEVER a coil labeled "12 volts requires ballast resistor" or a coil labeled "6 volts" ARE 6 VOLT COILS and require the ballast on a 12 volt tractor but not if only a 6 volt tractor.
OLD WIVES TALES: Some mistakenly refer to some 12 volt coils as being "internally ballasted" HOWEVER if you disect that coil, on most, you WILL NOT FIND A DISCRETE STAND ALONE RESISTOR HIDDEN AWAY SOMEWHERE INSIDE THE CAN. The primary winding resistance of around 3 ohms is achieved due to more windings/turns or higher resistance wire NOT A RESISTIR HIDDEN INSIDE THERE.
If a coil overheats it’s because its drawing more current then it was designed for and cant effectivelty dissipate the heat.

Nuff said, hope this helps
John T retired Electrical Engineer

Exactly what I was thinking. They are selling ONE coil to two markets. It's like the old hardware store owner that sold two grades of Kerosene from the same barrel. His thought was "some want economy and some want quality and you have to keep everybody happy"

Yes, the vast majority of 12 volt systems in the USA use what you are calling a "6 volt" coil. More then 75% of all cars and trucks with breaker points used that style. Same coil in a Deere 6 volt tractor is used in a Deere 12 volt tractor.

A 12 volt system has to make spark at 9 volts when cranking and at 13-14 volts when running. So various systems use differing strategies to accomplish the task. Ballast resistors, differing values in the primary and secondary windings, etc.

If you look closely, many 12 volt coils for sale also list usage for 6 volt systems.

The so-called "12 volt" coils have a higher resistance in the primary windings inside the coil and thus do not use an external resistor.

I don't see it that way. "6 volt" coils used in 6 volt systems were wired differently. Not the same application. When used in a 12 volt system, the coil is still run on 6-8 volts, just as it is in a 6 volt system machine. Just comes with the added bonus of "over-volting" for better cold starting.

Yeppers, the coil labeled "12 volts for use with external resistor" will work on a 6 volt tractor, and one labled "6 volts" will work fine on a 12 volt tractor IFFFFFF ahead of it you add a series voltage dropping (12 to 6) Ballast resistor cuz then it only sees 6 volts WELL DUHHHHHHHHH lol..................

It may not be quite as simple and that exact, but the above remains basically true.

Fun chatitn with ya

John T

John this is a great discourse on "6 V" and "12v" coils!

I find that 6V coils are generally around 1.6+/- ohms primary resistance while most 12V coils are 3.2+/- ohms. They key is to try to limit the coil primary current to a maximum of ~ 4amps. The required resistance then falls out.

THANKS, I do my best, Yep thats close to the numbers Ive seen on many old typical tractor coils AROUND 3 ohms for 12 volt coils AROUND 1.5 for 6 volt coils are just the numbers I use for general discussion and illustration. Its impossible to cover evey coil and every situation here, there are always a few exceptiosn to the general rule which some will point out for us.

Best wishes

John T