transformer testing (toroid) - Page 1
Forum section: electronics / beginners

author: jaunty
author_rank: Regular Contributor
timestamp: January 20, 2020, 05:23:26 am
content: 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

author: Jwillis
author_rank: Super Contributor
timestamp: January 20, 2020, 05:31:37 am
content: 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.

author: jaunty
author_rank: Regular Contributor
timestamp: January 20, 2020, 05:57:36 am
content: 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

author: jaunty
author_rank: Regular Contributor
timestamp: January 20, 2020, 05:59:07 am
content: 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?

author: james_s
author_rank: Super Contributor
timestamp: January 20, 2020, 06:00:56 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: January 20, 2020, 06:23:22 am
content: Quote from: jaunty on January 20, 2020, 05:59:07 amYes, use an incandescent bulb in series with the primary.That way if you wire it up wrong you won't blow a fuse or fry the transformer.A bulb will flash bright initially due to the magnetizing current then barely glow at all until you load the secondary.

author: jaunty
author_rank: Regular Contributor
timestamp: January 21, 2020, 04:21:52 am
content: yeah i just have this visceral, irrational fear of electricity - at least in large quantities - mostly because of some bad experiences i had when i started out playing around with scopes and the like when i was about ten. It's one of those things where simply because i'm so nervous about it - I'm far more likely to make a mistake or do something stupid as a result. I'm much happier trying to troubleshoot TTL stuff or other circuits than working on power supplies or replacing a power inlet module. but anyway - that's my thing i guess - so hooking up a transformer directly to the wall is definitely not something i relish doing. But we'll see - maybe if i can set up a test rig to ensure the leads don't slip out of their intended positions etc - maybe I can be talked into it.Yeah i already 'ohmed out' all the windings and everything seemed fine - but it seemed to me (based on something i read maybe?) that just verifying continuity might not properly represent what was going on with the transformer in situ in a real world situation. Or maybe that's just another irrational fear.

author: jaunty
author_rank: Regular Contributor
timestamp: January 21, 2020, 04:22:45 am
content: tautech : yeah i normally do that anyway - at least when powering up something i worked on extensively for the first time

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 21, 2020, 04:41:22 am
content: You could use a general purpose step down transformer to test another transformer at less than full mains voltage. For example, a common 50VA 24VAC hvac control transformer could be used to power the transformer under test, output should the ratio of 24 / 120 * the expected output. Another, more desirable method is a variable transformer, a.k.a. Variac or Powerstat. Having one in an enclosure with outlet, switch and volt & ammeter is a great addition to any test bench.All that said, my first thought is why bother. If there is no sign of over-temp or physical damage, an ohmmeter check should show open or shorted windings. A cheap meter will make checking for shorts more difficult but it can be done.Use an inline switch with proper fusing and there is little to worry about. I have a switched, fused power strip on the bench with a big pilot light so I know when it’s on. Turn off, confirm, plug in, turn on. My usual configuration for testing a transformer with leads is a proper line cord set and wire nuts. PC mounts I always solder pins to a cord set. No alligator clips with mains. Period. Not worth the less than a minute it takes for either soldering or twisting wire nuts. I don’t fear HV, I respect it and know what it can do.IMO, wiring direct to a plug and relying on an upstream circuit breaker will eventually lead to an unpleasant surprise when inserting that plug into an outlet.

author: Jwillis
author_rank: Super Contributor
timestamp: January 21, 2020, 05:18:24 am
content: The way tautech says ,with a incandescent bulb is best . You don't want to keep tripping breakers to many times it weakens them .Checking for Coil to core and coil to coil is easy.Checking if one coil is shorted itself internally is a little more complex. You really can't do it with just an ohm meter. You have to calculate the number of turns based on the voltage and core area and core material or the VA . Then you can calculate the total length of the wire . Copper wire has a resistance for every size at 1000m . You can look that up. But it would take a milliohm meter to get an accurate measurement. This can get a ball park idea of the coil to see if the short is skipping windings. Taking an inductance measurement and comparing it to what the inductance should be is the most accurate. if the inductance is far to low for the size of coil on a core theirs a short.

author: jaunty
author_rank: Regular Contributor
timestamp: January 21, 2020, 06:30:10 am
content: Quote from: Jwillis on January 21, 2020, 05:18:24 amok that's DEFINITELY useful thanks

author: tautech
author_rank: Super Contributor
timestamp: January 21, 2020, 07:45:44 am
content: Quote from: jaunty on January 21, 2020, 04:22:45 amGood on you ! It's good simple insurance not to let the magic smoke out.Still you must use some discretion as to when you can use a dim bulb tester as some SMPS preregs don't like them at all. Some later Phillips CRO's apparently also.FYI here's a post I did some years back of my dim bulb tester:Quote from: tautech on June 30, 2014, 01:15:58 amFor a transformer like your one in question it helps to understand a little of how and why they are wound like they are and if all secondary windings are step down they'll naturally have less turns and be thicker wire, not that you can see them as they'll most likely be terminated onto tails. Still if they weren't identified the winding with the highest R will be the primary and other low R windings secondaries.You need watch for tapped windings especially on the primary side like those for different mains voltages for international use.Don't be scared of these things just apply the thing between your ears and a dollop of safe practice and have fun.