Resistor placement

Jim

Gerald J.

The resistor has no effect on the coil voltage when the points close because an inductor takes zero current at the instant the points close. The current rises (exponentially) according to the L/R ratio and the resistor limits the peak current. The use of the resistor also allows a more rapid rise in current good for high speed engines because the low voltage coil will have lower inductance giving a better L/R ratio with the external resistor for very high speed engines.

I suppose the resistor could be wired between the coil and the distributor too, but never is though if a coil has an internal resistor is switched in polarity it might. If the resistor was between the coil and the points it could confuse someone trouble shooting the ignition system expecting the points terminal to go to ground with the points closed and might effect an electronic tachometer connected there also. I've not tried putting the resistor there, my gas 4020 uses the resistor shorting by a special contact on the starter solenoid for better starting. Originally that resistor was hidden in the ignition switch and sometimes in iron wires that look like copper wires.

Gerald J.

rvirgil ...Kansas

In a high speed engine the use of a coil with resistor will run better because the L/R ratio of the combination will allow a much faster rise of the coil primary current when the points close so that the dwell time can be shorter and still have good ignition power. The output ignition power depends on the peak current just before the points open. The inductance of the coil primary causes that current to rise relatively slowly, but using twice the voltage with a resistor will allow that current to rise considerably faster and so allow the engine to run considerably faster. This probably has no detectable effect below 6,000 or 8000 RPM on an 8 cylinder engine, but for high speed engines the resistor is very important.

Gerald J.