Electronics > Beginners

transformer testing (toroid)

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jaunty:
posting this under dummies because I am really not getting the results i'm expecting.

I have a known good new toroid for mains use and I am trying to evaluate whether my old one was shot or not.
Since i'm terrified of mains voltage in general i thought it might be safer just to supply 1/10th mains voltage but in DC from my power supply. So I hooked the power supply up to my primaries and connected my meter on the first secondary pair i wanted to test and SURPRISE! as soon as i turned the voltage up at all the current was going off the scale! Seems to me that the current should stay at ZERO.

So - is there likely a fault in my transformer (got the same result off the known good one though) or am I going about this all wrong and not realized it's foolish to use DC for this - and the current drain is due to the core being energized with DC?

is there a better way to do this? i already 'ohmed' out all the leads on both transformers and both seem identical - no shorts or unexpected opens - but I figured that's not telling the whole story and if I ran some juice through it - it might tell me a more accurate story (?)

help? and thanks

Jwillis:
You can not determine if a transformer is working with a DC voltage. Transformers must have an alternating current. If you check your primary winding it will have a very low resistance . It would be like shorting your power supply. So of course the Amps would go through the roof.

jaunty:
well that answers that then - thanks - i thought you could maybe use the windings a bit like a voltage divider - passively somehow - i guess not then. Thanks

jaunty:
so - at any rate -  IS there a preferred method of 'cheating' and doing this without using full mains current? or there's no way around it?

james_s:
You shouldn't be terrified of mains voltage, just respect it and don't go poking around the primary side with your fingers while it's plugged in. The voltage is not going to leap out and bite you.

If you want to test a transformer with low voltage you can use the low voltage AC output of another transformer, just keep in mind that if you feed that into the secondary you can step it up to a dangerous voltage coming out what was intended to be the primary.

Another way to test a transformer is just use a multimeter to check the resistance and see if any of the windings are open or the secondary leaking to the primary. If anything is shorted it will get very hot when powered up, so if it's not doing that and none of the windings test open then it's probably fine. Transformers rarely fail.

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