Posted by John T on July 30, 2008 at 18:37:57 from (66.244.97.31):
In Reply to: Another Coil Question posted by SuperA-Tx on July 30, 2008 at 15:45:14:
TX, if that coil inded has 3 ohms of primary winding resistance as measured between its lil + and - terminals, its suitable for 12 volts NO EXTERNAL BALLAST IS REQUIRED. If you use a 3 ohm (12 volt) coil PLUS a Ballast the coil current is reduced and the spark will be weaker !!!!!!!!!!!
A 12 volt coil has typically around 2.5 to 3.5 ohms of primary winding resistance while a typical 6 volt coil is more like 1.25 to 1.75 ohms. This limits the amount of current the points have to switch to around 4 amps and that points switching current is the limiting factor (to reduce premature points burn up) and why the coil resistance or the sum series of a coil plus any external ballast is such that 4 amps is around the max points current. Ohms law makes that equate to the 3 primary resistance ohms on 12 volt coils or 1.5 ohms on 6 volt coils.
The alternative is to use a 6 volt coil (say 1.5 ohms) PLUS an external series voltage dropping (12 to 6) Balalst Resistor also around 1.5 ohms so again on a 12 volt tractor the points current is around 4 amps I = E/R = 12/3 = 4 amps. NOTE one advantage of a 6 volt coil plus an external balalst on a 12 volt tractor is the ballast can be by passed while starting/cranking for a hotter startign spark, such cant be accomplished if theres no external ballast which can be temporarily by passed.
NOTE just cuz that coil is rated and is capable of rising to 40,000 volts DONT MEAN IT FIRES THE PLUG AT ANY SUCH VOLTAGE. The necessary voltage which the coil must rise to in order to arc jump current across its gap is a function of 1) The plug gap 2) The medium (compression and fuel) in which it fires. All else (compression and fuel and plug gap) being equal that HV coil wont fire at any higher voltage then the stock coil!!!
This high energy high voltage coils are more for when you use an elec ignition and run a wider plug gap in which case it indeed requires a higher voltage to fire. That coil dont hurt nuttin when used with points (if its 3 ohms) and if its more efficient due to better magnetic materials and improved mutual inductive coupling you can have less heat losses and discharge slightly more energy across the plug gap then with the stock coil but to get the most bang for the buck if you couple up a HV high energy coil with a faster more positive elec switch (replaces points) and run a wider plug gap you get a higher energy ignition.........
BOTTOM LINE if the coil is indeed 3 ohms (like a stock true 12 volt coil) primary resistance NO BALLAST IS REQUIRED AND USE OF ONE WILL MAKE A WEAKER SPARK. If it were a 6 volt (1.5 ohms) coil on a 12 volt tractor then YES a ballast is required.
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