Posted by John T on December 19, 2012 at 06:34:57 from (216.249.82.117):
In Reply to: 12 volt coil posted by Buddy Woodson on December 19, 2012 at 05:32:26:
I didnt save them, but several of the fine gents here have posted listings of NAPA and Echlin "12 volt" coil part numbers and I'm sure they will again. DARN GOOD PEOPLE ON HERE
FYI heres a brief treatise on typical old tractor coils
Such if a typical STOCK "old tractor" coil (NOT talkin after market or high energy or high voltage or super coils or MSD or Accel or high performance automotive mind you) if labeled "12 volts" orrrrrrrrrrr "12 volts not for use with ballast resistor" orrrrrrrrrr "12 volts no ballast required":
1) Are designed to operate at 12 volts "nominal" and will still produce a sufficient spark when starting drags battery voltage down to as low as maybe 9 volts but when charging and the battery voltage rises to 14+ they still produce a spark, handle the current and dissipate the heat.......
2) Many (NOT all) typical old tractor original stock 12 volt coils will have a LV primary resistance (between lil + and - terminals) of around 2.5 to 4 ohms (many around 3 ohms), that way the amount of current the points are required to handle and switch isnt too much over 4 amps so they dont burn up prematurely.
3) Contrary to what many lay persons believe who call them coils that have an "internal ballast resistor" YOU WILL NOTTTTTTTTTTTT FIND A DISCRETE STAND ALONE RESIS"TOR" TUCKED AWAY SOMEWHERE INSIDE THE CAN. The way a "12 volt" coil achieves its necessary 3 or so ohms of LV primary resistANCE is by enough wire coil length (more wire or more coil turns) orrrrrrrrr using wire with certain resistANCE per unit length so the resistANCE end to end (+ to -) ends up in the range of 2.5 to 4 or so ohms so the points dont burn up and the coil doesnt overheat and handles the current when 12 volts is applied.
SORRY CHARLIE, DONT DISECT ONE AND EXPECT TO FIND A STAND ALONE "RESISTOR" HIDDEN INSIDE
NOTE they actually did make some very early automotive coils that DID HAVE a stand alone discrete ResisTOR in a seperate part of the can!! Many had like a ring where the internal can portions were seperated, coil in one end resisTOR in other end all by itslef!! HOWEVER in alllllll my years as a used tractor dealer (older tractors) I never saw a coil with such an internal resisTOR inside the can!!!!!!!!
Wire and the coils and turns of wire used in the coil, not being a "perfect" conductor, sure, has some (but low) inherent resistANCE, BUT WIRE IS NOT CALLED A RESISTOR, ITS CALLED WIRE which of course contains some small degree of resistANCE. If you go to Radio Shack and ask for "wire" they show you wire NOT ResisTORS. lIKEWISE, if you ask for a resisTOR, they will show you resisTORS NOTTTTTTTTTT wire lol
NOTE many tractors, 6 orrrrrrr 12 volt, still used the SAME COIL. Its just that if on a 12 volt tractor they added a series voltage dropping (12 to 6) external (NOTTTTT in the can) ballast resistor so it dropped 6 volts leaving 6 for the 6 volt coil
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