Re: Ignition coil voltage

Posted by John T on August 31, 2019 at 05:40:58 from (45.16.200.171):

In Reply to: Ignition coil voltage posted by Northvale PA on August 31, 2019 at 02:33:30:

Good question, if I have posted this once I have 1000 times over the past nearly 30 years, but here goes again. Save this for future reference.

On a 12 volt tractor you can EITHER

A) Use a full true 12 volt rated coil (NO ballast required) OR

B) Use a 6 volt rated coil PLUS an inline series voltage dropping (12 down to 6) ballast resistor so the coil only receives
the 6 volts its rated for. The ballast drops 6 volts leaving 6 for the 6 volt coil

NOTES

1 A typical full true 12 volt coil may have a LV primary resistance between its + and - terminals around 2.5 to 4 ohms,
many around 3 ohms.

2 A full true 12 volt coil may be labeled "12 Volts" or "12 Volts NOT for use with ballast resistor"

3 A typical 6 volt coil will have a LV primary resistance between 1.2 to 2 ohms, many around 1.5.

4 A coil labeled "12 Volts for use with ballast resistor" is in reality more like a 6 volt coil and requires the
ballast just like it says on a 12 volt tractor or it will overheat and the points burn rapidly

5 If you use a 6 volt coil with no ballast on a 12 volt tractor it will overheat and the points burn quick

6 If you use a 12 volt coil on a 6 volt tractor the spark will be weak

7 If the 12 volt tractor has a ballast installed and in the LV primary ignition circuit you need to use a 6 volt coil
as a 12 volt coil PLUS the ballast would produce a weak spark

A reason some tractors used a 6 volt coil on a 12 volt tractor is they could add a ballast start by pass system so the coil receives unballasted battery voltage when cranking which improves cold weather starting

NOW TO YOUR SPECIFIC QUESTION "Now, I just replaced the coil in my dad’s TO-20 with a 12 volt coil. I converted that tractor to 12 volt years ago. But, I pulled off a 6 volt coil, which I must have replaced at some point over the years. So, what do you guys recommend? Match the charging system or use 6 volt?

FWIW as a retired electrical engineer I recommend: If the tractor was converted to 12 volts and assuming there's no ballast in the ignition circuits LV primary I WOULD USE A FULL TRUE 12 VOLT RATED COIL. A 6 volt unballasted would overheat badly. If you insist on using a 6 volt coil add an inline series voltage dropping (12 down to 6 volts) ballast resistor.

Hope this helps post back any questions

John T

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