Ignition coils are basically just copper wire wound around an iron core. Normally, I would expect copper wire, measured with a ohmmeter to show close to zero resistance. However on my vehicle for example, the primary winding is specified at 0.55 Ohms and the secondary at 15.5 kilo Ohms.

Is the winding, the iron core or both that create the resistance, and why?

  • 1
    The wire it self has a resistance. This increases as the wire get longers and decreases as the wire gets thicker. You can find more info at: electronics-tutorials.ws/resistor/resistivity.html. – rana Jan 10 '18 at 15:09
  • Winding and iron core does not change the resistance of the coil. – rana Jan 10 '18 at 15:10
  • @rana Very nice link, has the exact information answering this question. – Robert S. Barnes Jan 10 '18 at 20:22

The primary winding has several hundred turns of copper wire of a given thickness and the length and thickness contribute to the resistance (proportional to the length and inversely proportional to the thickness).

The secondary winding has several thousand turns of a thin copper wire which contribute to its resistance.

The ratio of turns gives the change of voltage ie 12V on the primary to tens of thousands on the secondary.

There are several sources that explain how coils work, this diagram is good and comes from here which is a good source giving a lot of detail.

enter image description here

  • Correct in principle but the multi-kiloohms resistance comes from a resistive ignition cable. Historically, a resistor was inside the cable plug but for a long while the ignition cables itself are carbon resistors. A pure copper secondary circuit has about 1kΩ resistance. – Janka Jan 10 '18 at 20:05
  • So it basically comes from the length and diameter of the wires used in the primary and secondary windings is all.. OK, sounds good. – Robert S. Barnes Jan 10 '18 at 20:16
  • @Janka Ignition cables add to the resistance, but do so for suppression of noise that would interfere with radio communication. A conventional coil (which still uses cables) easily measures up to 10kohm. – Bart Jan 11 '18 at 22:48
  • @RobertS.Barnes Yes, but that is no problem to overcome. The 'resistance' that has to be overcome to ionise the air between the electrodes of the spark plug, in order to support conduction, is an order of magnitude greater. – Bart Jan 11 '18 at 22:51

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