Happy New Year George, WE pretty well beat this to death last week lol but Im glad to try n help with your questions.....
YOUR QUESTION
Is it not true that the 12 volt coil cuts the 12 volts down to 6 in order to work 6 volt ignition systems.
NOOOOOO Most (but not all) 12 volt coils are designed n rated to operate on full 12 volt input and one major design consideration that makes them 12 n NOT 6 volt is their primary resistance being enough (around 3 ohms) to limit the amount of current the points have to switch to around 4 amps or less so they dont burn up too quickly. A 12 volt coil has around 3 ohms primary resistance between its lil + and - terminals (regardless if caused by more wire or an internal seperate discrete ballast resistor) while a 6 volt coil only has around half that or 1.5 ohms. They ARE NOT THE SAME AND NOT BOTH 6 VOLT COILS
Sooooooo to get 3 ohms of primary resistance (a 6 volt coil just has 1.5 ohms) in the 12 volt coil versus only half that used on a 6 volt coil MOST (not all) simply have more length of wire or turns so they have twice the primary resistance so the points current is held to around 4 amps or less.
Nowwwww there are a few 12 volt coils that do indeed work as you suspect, they have a seperate internal compartment and a seperate discrete internal ballast resistor to reduce the 12 volts down to 6 and the current through the coil/winding portion of the coil down to that 4 amp or so figure so the points dont burn up quickly. These appear to be the fewwwwww Ive seen that have a ring on the can which is that seperate compartment where the resistor is housed. jd even showed some with the ressitr on top n very seperated but those were pretty old units n NOT like ones Ive seen on most thirties thru fifties Deere n IHC n other tractors n maybe not even 12 volt coils but 6 volt units.
jd did an excellent job of posting good correct info n pics that were internal ballasted coils (actually had an internal ballast) buttttt many were real old and many were 6 volt coils and one could even make out that ring or seperated internal component I mentioned. Stillllllll most allllllll 12 volt coils I ovserved over manyyyyyy years of a used tractor dealer were NOT internally ballasted but instead were 12 volt coils with 3 ohms of primary resistance because they had more wire length/turns of primary winding and DID NOT have the outer ring or groove or the top compartment like jd's pics indicative of any seperated internal compartment housign a resistor .
SOOOOOOO again a 12 volt coil IS NOT a 6 volt coil. The 12 volt has around twice the resistance so the points current is limited to arund 4 amps or less. Andddddd it dont matter if that resistance is more wire/turns (like most) or a seperate internal ballast resistor (like the ones jd showed) SIMILAR a 6 volt coil is designed for 6 volts n only has around half the resistance (1.5 ohms) and if you apply 12 volts it will overheat plus the points burn up quuicker.
Thats why if you convert from 6 to `12 volts you must EITHER use a full true 12 volt coil (3 ohms resistance n 4 amps current) or add a series voltage dropping (12 to 6) ballast resistor so the coil only sees the 6 volts it was designed for and it n the points carry the same 4 amps.
Finally the 4 amps and 1.5 (6 volt coil) or 3 ohms (12 volt coil) is an approximation n not every coil in the world is exactky that. I try to explain in a few sentences here what it takes books to fill and I spent years in college n the practice to learn so dont dare try n expect it perfect for all in the world......
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