Sorry, I missed yesterday's updates, but we needed today's higher time-resolution scope images anyway.

There certainly isn't any ringing visible, just a clean discharge of energy stored in the inductance., The shape of the discharge of the negative spike is what is expected when an inductance is discharged through a resistance. Fencer transformers that I've seen (not huge numbers, but several models from one (Australian?) manufacturer) had a lot of turns, so I would expect fairly high distributed capacitance. I see no evidence of capacitance in the shape of that curve - certainly not a hint of ringing. A shorted turn would act much like a shunt resistance.

The fact that the voltage on channel 2 is so small suggests that that is where there shorted turn is. I would expect that, since the wire on the secondary is usually quite fine and that is where the high voltage, most likely to punch a hole in the wire's insulation is.

If I understand the test arrangement correctly, the current through the primary would have been about
1.5 V (cell) minus 0.6 V (across winding) divided by 0.5 ohms = 1.8 A
I would think that magnitude of current should be plenty to store sufficient energy to see some ringing.

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I should have clarified originally - I tossed out the terming "ringing" without explanation. The "ringing" expected is usually a sinusoid that declines in amplitude after a few cycles as the energy stored in the winding inductance gets shifted back and forth between the inductance and the capacitance formed among the turns of the winding. The winding resistance plus losses in the iron core dissipate some of the energy each half cycle, so the amplitude declines. A shorted turn prevents significant ringing because most of the energy will be dissipated in that turn as the magnetic field from the initial charge is first "collapsing."

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Digital oscilloscopes are a big help for things like this. I often say rude things about digital scopes, partly because they have some shortcomings but partly to annoy digital scope fanboys (easy targets), but this is a case where they really are superior.

 

I think that it only seems to behave like a transformer with a 1:2 ratio and the fact that there is no ringing shows that the transformer is faulty.

Les.

Thank you for your response, unfortunately a faulty transformer seems to be the only reasonable conclusion.

Happy Xmas and thanks to all who responded.

Best regards.

 

I hope that the holidays went well and I would appreciate some further comments on the faulty transformer.

Partly out of curiosity and partly out of wanting to replace it I disassembled the transformer, see attached photo. The primary coil has two layers of 34 turns of 1.1mm wire and the secondary has four layers of approximately 160 turns of 0.2mm wire. The ratio then between the primary and secondary is 640/68 which is 9.411 or approx 1:10 as predicted.

To be compatible with the control circuit presumably any replacement transformer would have to have a similar primary coil along with a similar turns ratio or is there some other requirement for a pulse transformer such as in this application. For example, would a relatively standard 220V to 20V mains transformer be suitable?

Best regards