Testing pulse transformer with an oscilloscope

Pulse transformers are little different from regular AC transformers. In AC transformers magnetic flux alternates between negative and positive values in core while in pulse transformers magnetic flux is unipolar. Pulse transformers are used in TV, computer power supplies. If you need to test pulse transformer it may be almost impossible to do this with ohmmeter even digital one because windings of impulse transformer have very low active resistance except high voltage ones.

One way to test is to measure inductance of windings and compare them to values in specifications, but not everyone has ability to measure inductance and not always you have transformed specifications near by. So there is another way to test pulse transformers which may be acceptable to almost everyone radio-hobbyist. By using low frequency generator which works at resonance frequency of contour which consists of external capacitor and pulse transformer winding.

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For this you done even need separate frequency generator as usually it is build in oscilloscope for calibration purposes. Usually this generator works at frequency 1…2kHz. But again testing and finding resonance isn’t always handy. So another way is simpler. Connect transformer to generator as follows:

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In this case you have to measure signal in test point (TP). Differentiated signal on winding should have amplitude near to signal generator amplitude. Then pulse transformer can be considered to be OK otherwise if there is no pulses then it can be that at least one winding is short-circuited.

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It can be situation that measured signal amplitude is very low comparing to source signal amplitude. This is also a result of short-circuit in one of windings.

Such testing method is very handy because you can test pulse transformers without taking them out of circuit. Simply disconnect or desolder one lead of primary winding and connect it to calibrating generator of oscilloscope. The according to measured signals you can diagnose failures like diode failure that is connected to secondary winding, or simply windings are short-circuited because of overheat and so on.

4 Comments:

  1. Engr. Kishwar Kumar

    Good

  2. a result of short-circuit in one of windings.

  3. hi, very nice answer

  4. this was really useful to me.

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