Stuck Rings Maybe

Sep 30, 2025 Last reply: 3 months ago 43 Replies

I think the coil resistor that gets bypassed on slightly newer vehicles during starting was a hold over of that concept, although I have never thought it through exactly.

------------------------- As I understand it, the coil is designed for 6V and the resistor limits the extra current it would draw at 12V, except during starting when the battery voltage drops and the resistor is shorted out at the ignition switch Start position to boost the spark.

The resistor also speeds up the current rise when the points close, allowing higher RPMs. Coil inductance limits the rate of current rise, at the initial low current little voltage is lost in the resistor, the rest goes to the coil and speeds up its current rise.

The reason inductance limits the rise is because current change creates a changing magnetic field that generates a somewhat weaker opposing current and voltage. This is why motor starting current surge is much higher than running current.

When the breaker points open the current flow from the battery stops, the magnetic field it created tries to keep its opposing current flowing which builds up voltage, like a water hammer in plumbing. This voltage can arc across the open breaker points or the spark plug gap, but the condensor/capacitor wired across the points absorbs some of it and protects the points from rapid erosion, so it jumps the plug gap and ignites the mixture.

That's my attempt too make sense of electrical engineering. Here's another:

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2 and 4 cylinder inline motorcycles and my Ranger have a simpler distributorless variation in which both ends of the coil's high voltage winding produce a spark in two cylinders that are one crankshaft turn apart. It fires on every top dead center instead of every second one. When one cylinder is at the top of the compression stroke and ready to fire the other is at the top between exhaust and intake, where the spark does nothing.

If anyone is interested and knows Calculus this is a good beginner explanation, though the steady state examples it uses are rare, usually the change is an abrupt step, sinusoidal or exponential and the circuit math model is a differential equation.

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The corresponding formula for a capacitor is current = C*dv/dt.

I remember "ELI the ICE man" from my military training in electrical ... plus other extraneous bits and pieces that come in handy from time to time . I doubt I'll ever need to calculate inductive reactance for a vector analysis of the total resistance of a circuit though .

When Mitre reassigned me to developing digital radio I was very relieved to see there weren't any inductors in the circuit, unlike analog radio. They knew I don't have an EE degree but the radio engineers didn't have my experience with computer-interfaced measurement circuits.

A still-active retiree held an amateur radio license class to teach a group who needed to learn the basics of radio. He had been one of the early British radar wizards and had discovered some of the atmospheric properties he taught us.

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