Posted by John T on April 25, 2020 at 08:09:27 from (45.16.200.171):
In Reply to: confused over coils posted by Charlie M on April 25, 2020 at 06:32:57:
Charlie, great question myself as well as others have covered this extensively over the past thirty years, but no harm in re educating everyone.
1) On a 12 volt tractor you can use EITHER a full true 12 nominal volt rated coil orrrrrrrrrrrrrr a 6 nominal volt rated coil PLUS an external series voltage dropping (12 down to 6 volts) ballast resistor. If you used a 6 volt coil on a 12 volt tractor (with no ballast) it can overheat badly PLUS the points burn quickly
2) A Coil designed to operate at 12 nominal volts can produce a spark if during starting voltage is reduced to 10 or so volts ORRRRRRRRRRRR if under a high charge battery voltage may be 14+. It will operate, produce a spark and not overheat if operated at those brief lowered starting or higher charging voltages.
3) A so called 12 volt coil may be labeled "12 Volts" orrrrrrrr "12 Volts NOT for use with ballast" orrrrrrrrr "12 Volts no ballast required" etc etc HOWEVER BEWARE A coil labeled "12 Volts for use with or requires external ballast" is more like a 6 volt coil and as it says requires an external series voltage dropping (12 down to 6) ballast or it can overheat!!!!!!!!!!!!!
4) "Most" old tractor 12 Volt coils DO NOT have an internal stand alone discrete "resistor" tucked away inside the can even if they may be referred to by some as "Internally Ballasted" I am aware some old cars (maybe tractors???) used coils that indeed had stand alone discrete "resistors" (maybe temperature compensated) in the bottom of the metal can some with a noticeable ring around the perimeter.
5) A typical 12 Volt coil may have a LV primary winding resistance of 2.5 to 4 (many around 3) ohms.
6) A typical 6 Volt coil may have a LV primary winding resistance of around 1.2 to 2+ (many around 1.5) ohms.
7) Some tractors might use the SAME 6 volt coil on EITHER a 6 or 12 volt tractor, its just that on a 12 volt they add the external series voltage dropping (12 down to 6) ballast resistor.
8) Some 12 volt tractors used a 6 volt coil with an external ballast PLUS A START BALLAST BY PASS SWITCH/SYSTEM to improve cold weather starting.
FINALY TO YOUR SPECIFIC QUESTION " I'm planning to put it on a 12 volt system so I assume I still need an external resistor."
YES if the coil is 6 volt rated and you want to use it on a 12 volt tractor you need the series voltage dropping ballast or it could overheat and the points will burn quickly
"Can someone educate me on what I have and am I correct needing the additional resistor on a 12 volt system"
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