I Messed Up - Torroidal Power Supply - Page 1
Forum section: electronics / general

author: metrologist
author_rank: Super Contributor
timestamp: August 05, 2016, 09:17:39 pm
content: I have a moderately large DC power supply, 80V 20A. It can be wired for either 120V or 240V. The instructions were rather simplistic and I blindly followed them...There are two reds and two blacks on the primary. The secondary is already wired to an unregulated dual output rectifier circuit with large filtering caps.I wanted to wire for 240V and the instructions said to take any red and a black and connect them together, then take the remaining red and black and connect them to the two hots. I did not quite get that "a" black had to be "the" specific black, and the way they came out of the torroid was not obvious. Naturally, we know why I'm posting...OK, after taking a deep breath, I threw the switch, the lights went dim, and my torroid started making rice krispy sounds. I saw a slight puff of smoke, and quickly shut the mess off. Then it dawns on me that there are two individual primaries that needed to be wired in series, and all I did was wire them in parallel reverse polarityI undid the mess and wired it up correctly and it seems to power up fine. None of the components on the output seemed to be blown up, and the DC voltage with moderate load seems just fine.But, I saw smoke and I'm sure something bad has happened. I suspect some of the winding insulation has burned, and I am sitting on a ticking time bomb before it melts down an destroys the stuff on the output. Does this seem about right? Or is this just a bit Chicken Little?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2016, 09:33:32 pm
content: You plugged it in unfused?

author: Red Squirrel
author_rank: Super Contributor
timestamp: August 05, 2016, 09:50:53 pm
content: I was testing a UPS to see if I can repair it, I narrowed it down to possibly the transformer but it was somewhat uneducated guess. I did the mistake of hooking up what I assumed with the primary side directly to 120v to see if I get voltage on the other end, but I got nothing. I then tested the ohms and got nothing - was open... I should have tested the ohms FIRST. I don't know if I blew it, or if it was indeed already blown, and indeed problem. I'm going to go with it was already blown.UPS transformers make great door stops though, for when you open windows in summer and don't want doors to randomly slam.

author: metrologist
author_rank: Super Contributor
timestamp: August 05, 2016, 11:01:00 pm
content: Quote from: T3sl4co1l on August 05, 2016, 09:33:32 pmThere's a dual 30A circuit breaker on the panel. Does that count? I better install that 10A switch breaker, huh?

author: rdl
author_rank: Super Contributor
timestamp: August 05, 2016, 11:12:52 pm
content: Circuit breakers are to protect the wiring in the walls. They're too slow to protect devices plugged into the wall outlets.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2016, 11:39:02 pm
content: You need a 8-10A fuse to protect a transformer that size, and actually have it protected.Breakers may operate too slowly to save a transformer, certainly one triple the rating! Which explains your observation, more or less.Tim

author: metrologist
author_rank: Super Contributor
timestamp: August 06, 2016, 12:53:59 am
content: I bought a two pole, red rocker style 10A circuit breaker switch. Is that good enough. (like this one:http://www.digikey.com/product-detail/en/te-connectivity-potter-brumfield-relays/W33-S4B1Q-15/PB262-ND/254480)And should I just buy a new power supply? I know, it's an impossible question to answer. Who knows what could have happened. Does connecting the windings in reverse polarity make the load even worse, or provide some cancellation of EMF induced current?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2016, 01:15:42 am
content: Hmm, maybe. I still don't like the idea of relying on a fragile mechanical breaker for protection. Good old cartridge fuses are cheap and reliable, and you should never have to replace them (or flip a breaker) in properly designed equipment.And the faster clearing speed of a cartridge fuse means less damage to the transformer -- no sizzle to worry about.Tim

author: David Hess
author_rank: Super Contributor
timestamp: August 06, 2016, 07:10:57 am
content: Quote from: metrologist on August 06, 2016, 12:53:59 amI have had power transformers degrade over time and fail later leading to fires after abuse like that so I would definitely consider it a safety hazard.QuoteThat is what happened; the 120 volt primary windings connected anti-parallel so the inductance mostly cancelled out and it looked like a short to the power line. I am amazed that the 30 amp circuit breakers did not trip.What is the resistance of one of the primary windings? Maybe that plus the wire run resistance was enough to limit the current so that the circuit breakers did not immediately trip.

author: metrologist
author_rank: Super Contributor
timestamp: August 06, 2016, 12:58:55 pm
content: You's are not instilling any piece of mindBut I guess there is a silver lining in here...

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2016, 01:22:23 pm
content: Quote from: David Hess on August 06, 2016, 07:10:57 amYes, that sounds like what's happened.A similar thing would have happened, if the two windings were in series and one was backwards but the current would have been lower as the resistance would be double.

author: David Hess
author_rank: Super Contributor
timestamp: August 06, 2016, 02:16:44 pm
content: Quote from: metrologist on August 06, 2016, 12:58:55 pmGood, I almost had a house burn down because of this.You are relatively safe because the transformer is inside of what is effectively a fire shielded enclosure. Add some appropriate circuit protection on the line side like a fuse or fast circuit breaker. Then run a supervised long term test over a couple of weeks where you check the temperature of the transformer at the end of each run. I suppose you could do a high potential test but I am not sure if it would reveal anything useful.QuoteWhat silver lining is that?

author: metrologist
author_rank: Super Contributor
timestamp: August 06, 2016, 03:31:55 pm
content: I learned something about transformers.

author: David Hess
author_rank: Super Contributor
timestamp: August 06, 2016, 06:40:11 pm
content: Quote from: metrologist on August 06, 2016, 03:31:55 pmTrue wisdom would be learning from someone else making this mistake but the opportunity does not always present itself. At least it turned out well except for the transformer and your wallet and who knows? The transformer may have survived to operate for its expected operating life or longer now that we know it is not weak.

author: johansen
author_rank: Super Contributor
timestamp: August 07, 2016, 03:54:16 am
content: I wouldn't worry about the transformer, unless the insulation changed color.the crackling sound is the windings expanding due to thermal expansion causing the mylar and other plastics to creak as they shift a bit..

author: VK5RC
author_rank: Unknown
timestamp: August 07, 2016, 07:20:08 am
content: If it will be powered up and unsupervised for long-ish periods I would ditch it, have a look at the impedances of the primaries, comparing one to the other, also fire each side up individualy to see if you get the same voltage out on the secondaries, if out by much, I would ditch it. I nearly went insane trying to parallel some secondaries in a big toroid. No smoke but I was thinking of starting a fuse factory. HiHi.

author: Kilrah
author_rank: Unknown
timestamp: August 07, 2016, 07:21:32 am
content: Quote from: johansen on August 07, 2016, 03:54:16 amHow can you see insulation color deep down in the coil, which is where most of the heat buildup would happen?

author: SeanB
author_rank: Super Contributor
timestamp: August 07, 2016, 08:50:12 am
content: Use an insulation tester to see if there is increased leakage current between the primaries. If there is ( and on a good transformer this will be over 1G of resistance at 1kV DC test voltage) then you have damaged carbonised insulation, and the transformer is now a candidate to rewind or scrap.

author: Assafl
author_rank: Frequent Contributor
timestamp: August 07, 2016, 01:27:38 pm
content: Quote from: metrologist on August 05, 2016, 09:17:39 pmSomething doesn't seem right. It seems each red and black form a winding - where either the red (or the black) always has the "Transformer Dot" (the symbol used in a schematic to assign the winding direction). In that case it wouldn't matter if Coil1Black is Connected to Coil2Red or Coil2Black to Coil1Red. In both of these - the Dots are in series (properly).Perhaps the trafo is wired incorrectly?

author: metrologist
author_rank: Super Contributor
timestamp: August 07, 2016, 01:47:09 pm
content: Quote from: Assafl on August 07, 2016, 01:27:38 pmOh, you're right. I'm not sure why I was saying parallel. For 240V, it would need to be wired series.So what it really means is that I connected one primary together (shorted) and blasted the other primary with 240V. The lights dim anyway when I first click it on because of the large filter caps.

author: electr_peter
author_rank: Unknown
timestamp: August 07, 2016, 09:19:45 pm
content: Quote from: metrologist on August 07, 2016, 01:47:09 pmThanks for clarification. I also thought that you may have connected 120 winding to 240V and shorted the other winding, but your "parallel connection" explanation threw me off.

author: Assafl
author_rank: Frequent Contributor
timestamp: August 08, 2016, 11:40:56 am
content: Quote from: metrologist on August 07, 2016, 01:47:09 pmLOL. If you shorted the second primary then the filter caps are not the reason your lights dimmed.Hint - it is the shorted primary, acting as a shorted secondary.... In fact, I'd gamble that your secondaries are perfectly fine. I'd test the primaries with a Megger, though...

author: metrologist
author_rank: Super Contributor
timestamp: August 08, 2016, 03:10:46 pm
content: I'll have to see if there is a place that will test it. It powers a machine that is not used that often, and is somewhat supervised when running.

author: qno
author_rank: Frequent Contributor
timestamp: August 08, 2016, 07:39:13 pm
content: Next time you do something like this put an light bulb in series with the transformer and do not load the power supply.The light bulb will light up and slowly dim due to the rush-in current of the power supply if all is ok.Else the light bulb will act as an current limiter and indicator that "something is wrong"