Electronics > Beginners
Most good transformers I pull are "shorted", whats going on?
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Syllith:
Okay, dumb question time. Let's say I go to an electronics store and pick up some transformers. I can read the resistance across the primary and I'll get like 50-200 ohms. Seems normal. Hook it up, it works fine. But, I repair a lot of rectifiers and converters at work and sometimes I'll pull some transformers just to play around with them. But, literally every single one I pull, I'll check every combination I can think of, and I get less than .5 ohms on every wire. And of course, when you hook this up, it shorts out (I use a variac and slowly ramp the power up for safety). I can tell its shorting because the variac has some resistance in the knob and you can hear the coil whine get louder. So I stop before I do any damage. Do this same thing on a transformer I bought, it works fine. What's going on here? These were pulled out of known working units. Why is there no resistance and how was it working to begin with?

I have 2 theories. Either these are designed to have pulses sent through them (which is evident by me checking it with the scope while its hooked up), rather than a straight sine wave, and that's preventing me from hooking it up to the variac. Second theory is something someone else suggested and they said there needs to be a load on the other end so the transformer doesn't act as the load itself. I personally don't think this is the case but I could be wrong. Transformers should be able to operate perfectly fine without a load right? Not really sure whats going on but hopefully someone can shed some light. Thanks
MagicSmoker:
A picture is worth a 1000 words and all that, but it sounds like you are trying to apply 60Hz AC to a switchmode power supply (aka SMPS) transformer.

Mains transformer (ie - those designed for 50/60Hz power) tend to require lots of turns per volt so their primaries exhibit a substantial resistance reading; SMPS transformers, on the other hand, tend to have few turns per volt so trying to identify which winding is the primary with just a DMM is all but impossible.

Syllith:
I see. So my first theory was correct. I did take one apart one time and it had very few turns. Like 2 or something. And these transformers are in fact coming out of SMPS's. So just to be clear, you said you can't identify which is the primary because of the little turns it has. Does this also mean there's no way to use these as a regular transformer because it has so little turns? I'd have to send a pulsed signal to it?
MagicSmoker:

--- Quote from: Syllith on November 27, 2019, 05:36:48 pm ---I see. So my first theory was correct. I did take one apart one time and it had very few turns. Like 2 or something. And these transformers are in fact coming out of SMPS's. So just to be clear, you said you can't identify which is the primary because of the little turns it has. Does this also mean there's no way to use these as a regular transformer because it has so little turns? I'd have to send a pulsed signal to it?

--- End quote ---

Yeah, trying to discriminate between, say, a 10 turn winding and a 2 turn one with just a DMM is pretty much impossible because they simply don't read resistance under 1 ohm reliably.  That said, you can't really reuse SMPS transformers, anyway, unless you design the rest of the converter around it... And it has a turns ratio somewhat close to what you need.

magic:
It's not even about pulsed vs sinewaves, it's about frequency. They are intended for very high frequencies and therefore they can get away with small size, few turns and low primary inductance.

It's the same reason you can't use 60Hz transformers on 50Hz mains, but taken to another level.
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