An inductive ignition coil is basically a step up transformer, in which the ratio of windings between the primary coil and secondary coil determines how much the primary voltage is multiplied by.

I am wondering if the ratio of the primary to secondary coil resistances, will give you the same ratio as that number of windings?

EDIT 26 Oct. 2016

I have a feeling this might be possible, as I stumbled across some interesting info, but don't have the patience to figure it out myself at the moment:

Coil Inductance Calculator

and this:

Wheeler’s formulas for inductance of air core coils

I bet there's something similar for iron core coils.

  • If you know the circular mils of the wire and could determine the ohms per foot or meter. But for instance I could make a 10 turn primary our of 6ga or 10ga both ten turns but lower resistance on the 6ga. Oct 24, 2016 at 18:16
  • Would the winding ratio be proportional to the length of the wires in the windings? If it is, and you have access to a fancy cable tester (something like Fluke LinkRunner, that can measure distance to a fault (open)) you might be able to get a close enough length on the wire in each winding to tell something useful. What are you trying to learn?
    – dlu
    Oct 25, 2016 at 6:24
  • @dlu basically I'm just curious. Oct 25, 2016 at 7:11
  • Do you understand the difference between resistance and inductance? Because in your question you smoothly move from one to another.
    – Agent_L
    Oct 26, 2016 at 13:15
  • And no, ignition coil is not a "basically a transformer". It's a combination of inductor and transformer. Inductor stores energy in magnetic field (just like capacitor does in electric field) while real transformers are built to minimize such storage. Which means that ignition cold makes a poor (lossy) transformer.
    – Agent_L
    Oct 26, 2016 at 13:20

5 Answers 5


Unfortunately no.

This is because the primary and secondary windings don't use the same wire. The primary uses a few turns of a thick wire. While the secondary uses many turns of a thinner wire. The reason for the thinner wire is that when the voltage is stepped up and current is stepped down and a thinner wire can carry the necessary current.


IMHO the only reliable way of telling the winding relation would be to feed an low voltage alternating current with known amplitude into the primary winding and measure the amplitude of the secondary. Attention: You could easily generate a potentially fatal tension, please do this only if you are experienced with high tension and can apply the necessary safety precautions.

Just some background information on the idea of measuring of the resistance:

Assuming you will use a digital multimeter it is quite possible that you will have problems measuring the resistance precisely. The reason is that the multimeter tries to adjust the tension dynamically thereby interfering with the inductance. The result will be the multimeter showing wildly varying values.

If you want to measure the resistance accurately you better use a fixed tension source with an protective resistance. The result will be a simple voltage divider, albeit since it is not guaranteed that both primary and secondary winding have the same wire the difference in specific resistance of the wire would permit no statement about the number of windings. You can calculate the secondary resistance of a voltage divider by applying the following formula:

enter image description here

  • 1
    A much safer way to do this test is to feed a moderate AC voltage (but not enough to cause injury - less than 50V) through the secondary, and measure the voltage in the primary - the primary voltage will be small, of course. An electrical "accident," even with a severely damaged ignition coil that won't generate enough voltage to fire the engine, can be anything between very painful (i.e. you will feel it for a couple of days, not a few minutes) and fatal - don't go there unless you are absolutely sure you know what you are doing and what precautions to take.
    – alephzero
    Oct 25, 2016 at 2:13
  • @alephzero I was also thinking about this, but I cannot be sure if the person doing this can tell the primary from the secondary winding. Therefore I advice the person to treat everything as potentially dangerous tension
    – Martin
    Oct 25, 2016 at 7:58
  • Why an AC current as opposed to a DC current? Don't ignition coils normally operate on DC current? Oct 26, 2016 at 11:28
  • It isn't strict DC, it is switched DC. upload.wikimedia.org/wikipedia/commons/thumb/a/ae/… Transformers do not work on constant DC, they "work" only on changes in the current. Therefore the need for AC
    – Martin
    Oct 26, 2016 at 11:43

You could avoid the shock risk mentioned by @Myself, perhaps at the loss of some precision in the measurement, by applying the known source to the secondary winding. That way you're using the coil as a step-down transformer, instead of a step-up transformer.

  • 1
    This would work provided that the voltmeter has the necessary precision of measurement.
    – Zaid
    Oct 25, 2016 at 5:55
  • I cannot be sure that the person doing this can tell the primary from the secondary winding. Therefore (IMHO) it is safer to assume that the generated voltage is dangerous
    – Martin
    Oct 25, 2016 at 8:24
  • @Myself I know the difference between the primary and secondary coils, at least on my car. Oct 26, 2016 at 9:08
  • @RobertS.Barnes You do, but not necessarily everyone who reads my answer
    – Martin
    Oct 26, 2016 at 9:18

There were already good answers, I'd like to add some points and also write something about the inductance.

Here is a sketch of a typical coil:

enter image description here


There are in principle two reasons why there is no correlation between resistances and windings. Therefore, keep in mind that resistance depends on length and diameter (gauge) of a wire.

First, as already said by others, if different wires are used for both windings and the gauge is unknown, the length of the wires can not be calculated from the resistances.

Second, the length of a single loop of a winding depends on the diameter of the loop. In the sketch, the primary winding is put around the secondary, giving the entire primary winding a larger diameter. And each winding consists of several layers, the outer having a larger diameter than the inner. So, even if the length of wires is known, the number of windings can not be determined

@resident_heretic wrote about autotransformers. They use the same gauge for both windings, but since the secondary part has much more turns, it is possible that the secondary part is thicker and has a higher effective diameter.
And there may be ignition coils of this type out there, but they are by far not the only type. I guess the classic two-wire type is still (more) common.


Now, in your edit, you ask about inductance, which you would have to measure first. I guess devices which can measure the huge inductivity of asecondary coil are rare. However:

First, inductance depends on the number of windings, but also on the diameter of the coils. So same problem as above.
Second, it depends on the permeability µ_r. Looking at the sketch above, one could use the permeability of iron for the secondary winding. But for the primary, it's a function of the permeabilities of the iron core, the secondary winding and the insulation between primary and secondary windings.

In reality, it is much more complicated, since the shape of every component plays a role, the permeability of inner layers play a role for the outer layers of the same winding and so on.

The inductivity depends on much more unknown parameters than the resistance, rendering the possibility to calculate winding ratio impossible.

The only reliable way is to put a small AC voltage on the primary and measure the voltage secondary - though that can be difficult and dangerous.

  • Great answer! Why an AC current as opposed to a DC current? Don't ignition coils normally operate on DC current? And as others suggested, wouldn't it be safer to input to the secondary and read the primary? Oct 26, 2016 at 11:29
  • @RobertS.Barnes If you want to use it as a transformer then you need AC, transformers can't operate on SC. Ignition coils operate on pulsed DC, and that pulsing makes it "not DC at all".
    – Agent_L
    Oct 26, 2016 at 13:23

The ignition coil is not a conventional step up transformer. The type of transformer used is called a autotranformer. This type of transformer is only one continuous winding where both the primary and secondary are on the same winding. The primary connection is tapped near the bottom of the coil. The secondary output connection is at the top of the coil. The very bottom of the coil is their shared common ground. Both coils share the same core.

Since they share the winding using a very sensitive ohmmeter (10M-Ohm/Volt or better) you could determine the differences in coil resistance thereby determining the step up ratio by measuring each coil from the common ground.

Transformers are very efficient devices 97% of the energy sent into the primary winding is induced into the secondary winding.

![autotransformer ]1

  • While it is (almost) correct what you write about autotransformers, not every car has them. Google is full of images of "classic" coils, even with opened ones showing the different windings.
    – sweber
    Oct 26, 2016 at 10:28

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