[CWPASADENA]

With point type distributors, the maximum amperage the points can handle reliably for long life without excessive pitting or burning is about 5 amps. With a 6 volt system, when limiting the current to 5 amps you need a coil with a resistance of no less than 1.2 ohm. This comes from using the formula, "Voltage=Current x Resistance" (V=IxR) which is one of the basic electrical formulas.
Model A Coils are about 1.2 Ohm, so with 6 volts, the resultant current is 5 amps.
With a 12 volt system, if you want to limit the current thru the points to 5 amps, then the coil must have a resistance of about 2.4 Ohm. (V=IxR)
Now, general speaking, 12 volt coils come 2 ways, with a built-in ballast resistor and without a built-in ballast resistor. A 12 volt coil without a built in ballast resistor will usually be marked that it is to be used with an external ballast resistor.

A 12 volt coil with a built-in ballast resistor will have a total resistance of about 2.4 ohm. A 12 volt coil that does not have a built-in ballast resistor will usually have a resistance of apx. 1.2 Ohm (same as a 6 volt coil). A ballast resistor is also usually about 1.2 Ohm so the total resistance of the coil and ballast resistor is 2.4 ohm. Therefore, if you are converting a 6 volt system to 12 volts, you do not have to change the coil, you only have to add a ballast resistor.

Now, starting in 1932 and continuing thru 1948, With the new V-8 engine, Ford used a 3 volt coil and a ballast resistor. These resistors were made with ni-chrome wire, the same as you would find in a toaster. When ni-chrome wire is cold, it has no resistance but as you pass current thru it and it becomes hot and its resistance will increase. With this system, when first starting the car and the resistor is cold, the coil will be receiving full battery voltage and the spark to the plugs will be hot and strong to assist in starting, especially in cold weather. Keep in mind, the starter on the V.-8 engine when cranking will take a lot of current and pull the battery voltage down to may be 4-5 volts so during starting cold, the voltage to the coil is also in the range of 4-5 volts. After the engine starts and the ballast resistor heats up, the current thru the points will be reduced to a manageable amount and the points will not burn.

This system worked well except on a hot day when the engine was hot. If you stalled the car, it may crank but not start due to not enough voltage to the coil to produce a hot enough spark when cranking the engine. In the "old days" when this happened, you could take a coin and reach under the dash and short across the ballast resistor and get enough voltage to the coil to produce a hot enough spark to start the engine. If you would wait a few minutes, the resistor would cool enough so its resistance would be reduced enough to allow enough voltage to the coil to get a good hot spark.

After going to 12 volts, most manufacturers used a system that used a coil and a separate ballast resistor. During cranking, the ballast resistor was by-passed and the coil would receive full battery voltage, so you had a good hot spark for starting.

Now, please understand, this is a generalization of how basic point type ignition systems work. Resistance values may be slightly different for different applications. Higher performance coils may have a little less resistance allowing a little more current and a hotter spark but usually with these applications, you are changing the points a little more often.

Anyway, sorry for the long-winded explanation but this is the basics of the point type ignition system.

Chris W.