Why did this linear PSU fail? - Page 1
Forum section: electronics / beginners

author: Time
author_rank: Frequent Contributor
timestamp: February 07, 2012, 04:33:40 pm
content: This is kind of a newbie question I think...I am a power engineer but the type of power I deal with is different than low voltage power condition circuitry and that sort of thing.I was installing some hall effect based current transducers which required a stable +/- 15V rails for operation. I planned on supplying this voltage with the HBB15-1.5A+G linear supply made by Condor. This seems pretty run of the mill. Datasheet:http://www.slpower.com/data/collateral/Linears_DS.pdfI asked a new hire to take the PSU and attach the appropriate mains cabling to it so I could use it. He went and did this and before I could inspect what he had done he plugged it in to the wall and it failed catastrophically. He had the pins jumpered correctly and applied the AC to the right places but he had not attached the earth ground to the chasis of the PSU.While I would have known to do this I would have not expected such a bad failure upon not doing so. The transformer basically started smoking and getting extremely hot after just a few short seconds. Once power was removed I told him to attach the ground lead and properly fuse the mains line to the PSU. After reapplying power to see what would happen the PSU seemed to function properly but the transformer was still getting hot so I figured despite the voltage rails producing the correct voltages the device was ruined and should no longer be used.Is neglecting to ground the chasis on this particular PSU what caused the problem? I am in the US so it was mains 120V and we jumpered 1-3 and 2-4 and applied the AC to 1 and 4 on the Xfrmer.

author: IanB
author_rank: Super Contributor
timestamp: February 07, 2012, 04:42:20 pm
content: When you jumpered the primary windings in parallel did you have them connected with the wrong phasing by any chance?

author: Time
author_rank: Frequent Contributor
timestamp: February 07, 2012, 04:49:08 pm
content: I am not quite sure what you mean. Its single phase, 120V. The table on the front of the PSU says for 120V mains jumper 1-3, and 2-4 than apply AC to 1 and 4. If its single phase what phasing would I have to worry about?

author: ejeffrey
author_rank: Super Contributor
timestamp: February 07, 2012, 05:38:18 pm
content: You have connected the two windings in parallel, but there are two ways to do that. The correct way they are both winding around the transformer the same direction. This would be called in-phase. The wrong way is where the windings are in opposite directions, aka out-of-phase (by 180 degrees). In the later case, the magnetic fields generated by the two coils cancels, and it operates just like you short-circuited the mains. That would give a result similar to what you describe, except that I would expect it to immediately blow the circuit breaker while doing possibly catastrophic damage to the transformer, not sit there smoking. I guess it depends on how well coupled the two primary coils are. Perhaps there is enough leakage inductance to keep the current to several amps. A shorted turn or secondary winding might cause the behavior you describe, but that would be odd to say the least.The earth ground shouldn't do anything to prevent a fault, it is only for safety after the fault happens. If the ground lead is carrying appreciable current it is a serious problem. Obviously the lack of a fuse is a problem as it would have limited the fault current to a safer value. On the other hand I have no explanation for how changing that could make it work better, even if you still doubt that it is really operating properly.Transformers do heat up even when running at no load due to core losses. It is even conceivable that you could get the right voltage out at zero load, as it doesn't take much current to operate the regulator. If there is some short circuit or incorrect wiring, the voltage would collapse very quickly under load, so that would be easy to test with a suitable pair of power resistors.

author: Time
author_rank: Frequent Contributor
timestamp: February 07, 2012, 06:36:10 pm
content: I actually tested it under load. It doesn't produce the right voltages without load. The winding were connected how it says to connect it on the datasheet it came with and also on the table on the front of the case.Perhaps the failure is not as catastrophic as I thought? or something else is at fault? It seems to function but the xfrmr gets too warm to touch so it makes me nervous about placing it in the end application. I already got a new PSU but I'd like to understand the problem with this one first.I understand what you mean by phasing now. The phasing of the fields produced by the 2 parallel coils. Assuming the jumpers are properly applied to have the correct phasing does it matter where I place the hot and neutral? My intuition is telling me no.

author: siliconmix
author_rank: Frequent Contributor
timestamp: February 07, 2012, 07:43:46 pm
content: wasn't there a thermal fuse in the xfrmr ?

author: SeanB
author_rank: Super Contributor
timestamp: February 07, 2012, 07:57:01 pm
content: It definitely has shorted turn somewhere, or was wired incorrectly. It may just have been a unit that slipped through QC or it may not have been wired correctly in manufacture. I doubt there is a thermal fuse, as then there would be a single 120V winding, not 2 in parallel.