[SOLVED] measuring capacitance of a transformers primary coil

Getting 2 different readings when secondary coil is shorted or open when using an LCR meter to measure primary capacitance of an HEI ignition coil. This parameter is not provided by the manufacturer. There are 2 windings, primary and secondary are not connected to each other as in other automotive ignition coils.

Trying to input measurements into a SPICE simulation but it would be far more accurate with the coils parasitic properties. When secondary is open reading is ~645uF then when secondary is shorted reading is ~430uF. Which is the correct capacitance value of the primary coil?

Hi,

Which is the correct capacitance value of the primary coil?

Click to expand...

I assume neither.

Please tell us your exact measurement circuit.
Tell us the expected circuit of the ignition coil.
And the manufacturer and tyoe of your capacitance meter.

We can just guess:
You are measuring primary to primary.
If now the ignition coil is a standard coil without other internal parts (like a capacitor) then the expectable parallel capacitanceshould be in the high pF range up to low nF range. (mind that there could be a series capacitance, too)

70uF and more is definitely too high....

Klaus

KlausST said:

Hi,


I assume neither.

Please tell us your exact measurement circuit.
Tell us the expected circuit of the ignition coil.
And the manufacturer and tyoe of your capacitance meter.

We can just guess:
You are measuring primary to primary.
If now the ignition coil is a standard coil without other internal parts (like a capacitor) then the expectable parallel capacitanceshould be in the high pF range up to low nF range. (mind that there could be a series capacitance, too)

70uF and more is definitely too high....

Klaus

Click to expand...

Coil is a generic/universal HEI https://www.ebay.com/itm/301940310137/ and LCR meter seems to be the culprit as its an inexpensive LCR meter model BM4070 however fluke 177 reads overload on its capacitance range. Seems ordinary capacitance meters cannot be used to measure the parasitic properties of this transformer. The measurement circuit consisted of nothing more than a pair of leads from the LCR attached to the primary end of the transformer.

The HEI is intended to be in a circuit to generate high voltage DC for some bottle or film capacitors. Which feature is needed from an LCR meter to obtain valid measurement?

Getting 2 different readings when secondary coil is shorted or open when using an LCR meter to measure primary capacitance of an HEI ignition coil...

Click to expand...

Let me guess. a 2-3cm length of wire will have capacitance of 2-3pF. The primary is perhaps about a 1000 turns (say about 5-10m), and the expected capacitance is around a few nF. The expected value is higher because of inter-turn capacitance.

It is not a simple experiment to measure the capacitance of the coil, because

1. Your LCR meter wants the two ends to be connected to the two ends of the capacitor. In this case, one end is one terminal of the coil and the other end to be connected to the ground (and this is not available here).

2. you can measure the capacitance between the pri-sec windings by connecting one lead to one terminal of the primary and the other lead to any terminal of the secondary.

3. The best you can do here is the use a lock in amplifier to get the capacitance of the primary and secondary separately. This capacitance will be frequency dependent.

4. First you need to make an "equivalent circuit" and include the parasitic elements in the model. Then use this model to experimentally measure the capacitance.

I may be wrong, but I do not think there is any other way to go...

2. you can measure the capacitance between the pri-sec windings by connecting one lead to one terminal of the primary and the other lead to any terminal of the secondary.

Click to expand...

Most ignition coils, apparently also the linked device, don't have separate windings. They are wired as autotransformer, having two primary and one secondary terminal.

Obviously there's no way to simply measure a winding capacitance at these devices. An LCR meter with selectable operation frequency could be set to a frequency far above the winding SRF to give an estimation of winding capacitance. Having coupled windings and different SRF of primary and secondary winding makes it questionable to get any useful results.

But the original poster claimed that...

primary and secondary are not connected to each other...

Click to expand...

(from post #1)

Thanks, but did they measure interwinding capacitance? That's the only useful measurement that can be made with a regular LCR meter. It can be expected in a few 100 pF range.

What's wrong with measuring self-resonant frequency? I connect a signal generator with a volt or two sine wave and scope coil current. HEI is around 1:85 turns ratio.
A decent SPICE model is difficult without knowing the leakage inductances, parasitic pri-sec and pri or sec coil-core capacitance values as well.
In the end, your spark plug wire's resistance, firing voltage are highly variable so modelling is of limited use. Driver transistors have 385V clamps built-in.
Ignition coils can get damaged from too fast a rise time on the primary. I believe you end up hitting the leakage inductance hard and can get breakdown in the winding's insulation. So slow turn-on, like IGBT's or mosfets with higher gate resistance (and fast turn off) are preferred.