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Testing a NST with a DMM? Answered

I have been looking for a good (and cheap) NST for a while but i dont want to risk spending too much on a NST that doesnt work. I know that my local scrap yard has a large supply of old NSTs that are either there because both or one side doesnt work, they were upgraded, or someone needed the money and/or space. i also know that they would frown upon me wiring each one up and testing each.

so onto the real question: is there a way to test a NST with just a multimeter and possibly a battery (i know thats how some people test for flyback transformer pins) or if checking continuity would be enough or even another way?

Just so everyone knows i have spent quite a while searching for someone using this technique but didnt find anything, also im looking for a way to know for sure if one or both sides are working.

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The old-school neon sign transformer is, in fact, just a transformer,

https://en.wikipedia.org/wiki/Transformer

so an ohmmeter (or multimeter in resistance measuring mode) can "see", erm "sense", into each winding for to measure its DC resistance. So that test should be pretty definitive.

In contrast to the just-a-transformer topology, there may be other things out there which are more complicated internally. For example, a typical modern flyback transformer, will often include transformer, plus voltage multiplier, i.e. a network of diodes and capacitors, and everything is entombed in potting compound. So it's kind of a mysterious "black box". There's no way to see into it with your eyes, to see what wire goes where. Also the voltage multiplier network prevents an ohmmeter "seeing" into the secondary of the transformer. What's worse is the potting compound is so tough it makes it impossible to take it apart without breaking the insides. Well, maybe not impossible, but kind of like cutting apart a rock.

For that reason, the testing for a flyback transformer with integrated voltage multiplier, must necessarily be more complicated. E.g. tests like pulsing current into the primary winding, and looking for high(er?) voltage at the output.

BTW, I think I've got an old NST, one wired for mains power as delivered
in the former U.S. That is to say, it is wired for 110 VAC, 60 Hz.
Upon request, I can take a picture of the plate, and measure the
resistance of its windings, in ohms, just to prove the old-style NST is wired the way the way I imagine it to be.

i know that MOST old NSTs are center tapped so it would be nice to get at least one reading from each side and the case to know how the two sides compare. the only reason i dont want to just use continuity is because i dont know what the resistance of each side is and is both coils are melted internally they could show a similar resistance and not work either way, on the other hand if i had a similar "base" reading it might prove to be useful for this.

My NST also has a center-tapped secondary winding, the center of which is connected to the metal case of the transformer, for some reason. I measured about 16000 ohm for the full length of the secondary winding, and about 8000 ohms for each half of that winding, as measured with one probe on the center-tap.

I measured about 15 ohms across the primary winding.

Some pictures, and a hand-drawn circuit diagram, of this are attached.

neon-sign-transformer-wiring-resistance-05.jpgneon-sign-transformer-wiring-resistance-04.jpgneon-sign-transformer-wiring-resistance-03.jpgneon-sign-transformer-wiring-resistance-02.jpgneon-sign-transformer-wiring-resistance-01.jpg

this information i feel will be very useful, and thank you for the pictures.

If the resistance of the coils is ok than the only concern is insulation.
You don't want a HV transformer with faulty insulation sticking your house full - and it is hard to remove that stink (speaking from experience here...).
The HV side should have quite high resistance and the primary far less.
How much exactly depends on the power level and type of the transformer.
If it looks rusty and faulty don't bother, look for those that were kept dry and have no corrosion.