"No resistor" for different ign. coil theory. Logic.

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Well-Known Member
Jun 30, 2021
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12 volts/3 ohm(OEM coil)=4 amp.
So, do 4 amps go thru primary circuit of ign. coil???
Because only about 1% of time current goes thru.
Let us say 1% at idle, at 10k RPM it will be 10%
If this is so average current is only 0,4 amps at 10k RPM. But nobody rides at 10k RPM, 5k would be highest for me.
What I am trying to say here, even if you use 1ohm coils, current will not be high and it will probably be OK.
On top of that, PCMs and ignition modules have inner resistor to prevent death of module in case of short.
Disclamer: I cannot promise your module will survive my logic.;)


Well-Known Member
Mar 26, 2020
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No. See response to my post saying 75% of power wasted.

The following post correctly stated that due to the coil being an inductor, the current in the coil is a function of time.

Basically the current will rise when power is applied to the coil, when power is turned off, the magnetic field breaks down to produce a spark.

Below is graph showing the current. In a points system the flat part (primary saturation) of the line depends on rpm.

The maximum value of the current in the coil is limited by the coil"s primary resistance and any resistors in series with it (unless the circuit supplying the current is a constant current source).

The electronic ignition system would be designed so the coil reaches saturation at all usuable rpms so the spark strength is the same across the rpm range.

Now it may well be that the time power is applied to the coil is constant so at low rpm, the average current is a lot less than at high rpm, (imagine the pulse shape below being far apart at low rpm but very close to each other at high rpm) but the maximum current is the same.

Hence I cannot agree with your 1% calculation for maximum current.


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