Low-Inductance Pulse Coil Tester

A shorted turn is the fault that hides from every static measurement. Resistance moves by a fraction of a percent, low-frequency inductance barely changes, and the coil passes both tests before failing in service. The pulse test exists because it looks at the coil in motion rather than at rest.

Product overview

This is a single-model instrument for coils between 0.1 µH and 1 mH, with a peak test voltage from 10 V to 1000 V and a voltage accuracy of ±(1% of setting + 2.0 V). It samples at 200 MHz with 10-bit resolution and completes a single-pulse test in 18 ms.

The front panel carries a 7-inch 65K colour display with the waveform and the comparison result, a keypad at the right and the test connectors along the lower edge. The case is 430 mm wide and 28 kg, which is a bench instrument with a substantial charging supply inside it. Power draw is below 150 W with no load and below 1000 W when driving a test.

How the pulse coil tester works

A small capacitor is charged to the set voltage and discharged into the coil. The coil and the capacitor form a parallel resonant circuit, and the coil rings at its self-resonant frequency with an envelope that decays according to the losses in the winding and the core.

If one turn is shorted, that turn behaves as a coupled secondary with a low resistance across it. It draws energy out of the oscillation on every cycle, so the envelope collapses faster and the waveform shape changes. The instrument samples the decay, compares it with a stored reference from a known-good coil, and decides whether the difference exceeds the limits set for the chosen criterion.

The criteria matter because different faults show up differently. An area comparison catches a general loss of energy; an area difference comparison is more sensitive to a local change; jitter detection catches an unstable contact; the corona second derivative and the peak ratio criteria look for the high-frequency signature of a discharge. The breakdown voltage mode steps the voltage upward and applies the same comparison at each step, so the voltage at which the insulation starts to fail can be located rather than only detected.

How to select the right coil tester

The first question is the inductance. Below about 10 µH the oscillation is fast and the voltage across the part is small, and only an instrument with a fast sampler and four-terminal sensing will measure the coil rather than the leads. This model’s 0.1 µH lower limit is the reason it exists in the range.

The second is the test voltage. It has to be above the working voltage of the coil and below the level that damages a good one, and the stepped breakdown mode is how that level is found for a new design. A coil for a low-voltage converter and one for a mains filter need very different settings from the same instrument.

The third is the fault to be caught. A turn-to-turn short, a corona discharge and a partial discharge produce different signatures, and the detection mode has to match the fault. Running all seven criteria on every part is possible but slows the test and increases the chance of a false reject on a marginal coil.

The fourth is the fixture and the handling. A coil with fine wire and a small former needs a fixture that contacts it repeatably; the pulse test is sensitive to contact resistance in a way that a DC resistance measurement is not, because the contact sits inside the resonant circuit.

Measurement limitations and selection considerations

The test is comparative, not absolute. It says whether a coil differs from the reference, not what its inductance is, and a reference taken from a bad coil will pass bad coils. The reference has to be captured from a part that has been checked independently, and it has to be recaptured when the wire, the core or the winding machine changes.

The waveform depends on the fixture as much as on the coil. Lead length, contact pressure and the position of the coil in the fixture all change the ringing, which is why the fixture belongs to the test and why a coil measured in a different fixture is not comparable.

The test voltage is a compromise. A higher voltage finds more faults and stresses the insulation more; a lower one is safer and less sensitive. The stepped breakdown mode is destructive by design, so it is used on samples rather than on production parts.

Finally, the instrument charges a capacitor to a high voltage and discharges it into an inductor. The energy per pulse is small, but the current is not, and the interlock and grounding provisions on the rear panel are part of the safety of the test rather than an accessory to it.