Front Mount Coil Math: FYI

Wattage = (V x V) / r or ( I x I) x r

Normal 6 V Coil with Ballast (3 x 3) / 1.0 = 9.0 Watts

6 V Coil w/o Ballast ( 6 x 6) / 1.0 = 36.0 Watts

6 V Running w/o B-res (7.5 X 7.5) / 1.0 = 56.25 Watts

I think it can be said will some certainty that the wattage must be (less than) < 36 watts or the COIL will fail. This gives us a Max. current of ~ 5 Amps

12 Volt Supply requires 2.4 ohms external
13.5 Volt (Running) requires 2.7 ohms external

(The regular Ballast Resister is .7 ohms Cold and about ~ 1.5 Ohms @ 3 Amps) SO you need 2 ea., in “series” (one after the other), and you are good for GO.

Example 12 V Coil , 3 ohms Primary The form of the coil and its heat dissipation are ~ equal to the 6 volt , so
you shouldn’t exceed 36 Watts IMHO (12 x 12) /3 = 48 Watts

Running ( 13.5 x 13.5) /3 = 60.75 Watts

If we want to limit the Heat to 36 watts we need a Max. current of ~ 3.5 amps

This calls for a ~1.0 Ohm External
Resistor (The regular Ballast Resister is .7 ohms Cold and about ~ 1.5 Ohms @ 3 Amps)

SO you need 2 ea., in “Parallel” (On top of each other) and you are good for GO.

Hope this helps

Jim in AZ Thanks

I would suggest that the 6 Volt (Front mount) ballast resister reduces the current "too much"
and that's the reason its hard Starting. The Voltage measured across the coil is only ~2.4V (when the points are new) and ~2.4 Amps. This makes the system marginal

If you buy(have)2 Ballast resistors and
connect them in "Parallel" (side by side). The current will be limited to ~ 3.5 Amps. The difference in "spark" during starting/runing will be considerable. JMHO

I hope you guys paid attention here, because there is going to be a quizz on this stuff tomorrow..
Happy Motoring
Claus

thanks
Gary D

Thanks,
Gary

Don't buy Rat shack resistors (they are a fixed
value of R). The Ballast increases resistance with an increase in current. (.7 ohm cold and
~1.5 ohms @ 3 amps)

6 volt system, 6v coil = 2 in parallel
12volt System, 6v coil = 2 in series
12volt system, 12v coil= 2 in parallel

Only on Front Mount,

Hope this helps,

Two Ballast resisters in series may (most likely) reduce the current flow to much. 12V Coils are manufactured with recommended External Ballast Resistors (.7 ohm coils) and others, those that are 3.0 Ohm or have special heat sinking capabilities.

Ballast Resistors are designed to operate at/or in the current limiting portion of their characteristics (the point that the resistance increases exponentially with respect to current) i.e. The ballast resistor used with the Front Mount N’s has a characteristic curve that starts at 2.4 amps, at that point an increase in voltage applied will not result in a corresponding increase in current. It is important to operate the resistor sufficiently “up” the characteristic curve (they run hot) so that a decrease in Voltage Applied (during Starting) will not result in a corresponding decrease of current. What that means is that selecting the correct ballast resister for the coil circuit involves V(voltage) vs. I (current) plots or access to that information (not likely unless a MIL spec exists)

To answer your question, use the ballast resistor recommend by the coil manufacture.
If its generic or unknown; measure the coil and if its 3 ohms no ballast is required (for a can coil) ... Try an Automotive Ballast resistor (ford or GM) If you use a ballast resistor that
has to "Low" a value it will burn-up, not your
COIL. So start low and move up

Sorry

Wrong! Wrong Edit:: If you use a ballast resistor that has a Low Current/High Resistance value it will not burn your COIL.

Hate it when the mind has a "Senior Moment"