TO30 12v Coil?

A resistor whether internal to the coil or external is for the SOLE PURPOSE to reduce voltage to the ignition POINTS!

If a full12 v is hitting the points then the points will have a somewhat shorter life span.

The resistor has absolutely nothing to do with the coil itself.
 
"A resistor whether internal to the coil or external is for the SOLE PURPOSE to reduce voltage to the ignition POINTS!"

More accuretly, a "primary resistor" or "ballast resistor" limits CURRENT through the points. CURRENT is what heats and burns ignition points.
 
?A resistor whether internal to the coil or external is for the SOLE PURPOSE to reduce voltage to the ignition POINTS!
If a full12 v is hitting the points then the points will have a somewhat shorter life span.
The resistor has absolutely nothing to do with the coil itself. ?



Points don?t care whether 6 or 12 volts. Points, and coil primary do care about the current flowing through the circuit.

A classic tractor coil has internal RESISTANCE, it does not have an internal RESISTOR. The difference between a 6 volt (1 1/2 ohms primary resistance) and a direct 12 volt-no external resistor required (3 ohms primary resistance) coil is in the type and/or length of wire used to wind the primary of the coil. There is no separate resistor in series with the primary enclosed in the can of direct 12 volt coil.

With a 6 volt battery, points will see 6 volt battery voltage when open and near 0 volts when closed. With a 12 volt battery (regardless of coil/resistor combination) points should see 12 volts when open and near 0 when closed.

As Bob writes, the points care about current, not voltage. Points ignition are designed for about 4 amps. According to ohms law, current equals voltage divided by resistance. So with fixed resistance of the coil primary (1 1/2 ohms) and fixed voltage of the battery (12 volts), an added resistor (1 1/2 ohms) is the one variable used with a 1 1/2 ohm coil in a 12 volt system to limit the current.

Thus the current limiting resistor has much to do with the longevity of a 1 1/2 ohm coil in 12 volt system since excess current that damages points also passes through the series coil primary causing it to overheat as well.
 

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