Author Topic: Variable Power supply blew up.  (Read 3157 times)

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Offline zerorisers

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Variable Power supply blew up.
« on: November 05, 2016, 01:53:59 am »
So... I raised up enough money for a small lab bench power supply 60v 5 amps. After about a week a mosfet and 3 resisotrs blew up. found matching resistors to put them in place. waited a month for the mosfets to come in (Ordered 10 off of banggood I believe) after I installed the new mosfet I turned it on and now both mosfets are blown. At first what I thought caused the issue was the fact the leads for the fan where shorted during manufacturing...  I will post pictures later tonight of the carnage at first, and the carnage now after I messed it up a bit more.
 

Offline janoc

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Re: Variable Power supply blew up.
« Reply #1 on: November 05, 2016, 02:03:09 am »
Not sure what you are after (do you need help troubleshooting it?), but:

a) Don't buy mosfets from Banggood, AliExpress, eBay and similar sources - most are counterfeit and who knows what you have got. A lot of people received parts that blew up at 50% of the rated voltage/current or less.

b) If the original mosfets blew, then there is likely a reason for that. If you haven't found (and fixed) it, then simply putting new transistors in will make it go kaboom again. Setting your wallet on fire is likely simpler method of achieving the same result :)

c) Fan leads being shorted means two things - the fan wasn't spinning, so the PSU was likely running hot, the manufacturing quality of that supply is likely fairly suspect.

 

Offline zerorisers

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Re: Variable Power supply blew up.
« Reply #2 on: November 05, 2016, 02:34:25 am »
Mostly need help troubleshooting. They asked for a video showing what went wrong, also I kinda didn't have an NTC on hand nor enough money for it, so I put a link in place instead. (Probably the most retarded thing I could do but I was desperate)

https://drive.google.com/file/d/0B8rOHVwhGx0dbFZXa0NUYmFONEU/view?usp=sharing
There is the video I made.

I'm not the best with any of this, and my ADD makes it a little difficult to try and reverse engineer.

I will note that these sites are not reliable for mosfets. It was the IRF840 if that makes any difference.
 

Offline Seekonk

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Re: Variable Power supply blew up.
« Reply #3 on: November 05, 2016, 03:47:14 am »
After a repair, put a 100W lamp in series with the AC power to test.  Likely what was wrong the first time was still bad after the replacements were put in.
 

Offline janoc

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Re: Variable Power supply blew up.
« Reply #4 on: November 05, 2016, 08:00:45 am »
Uh jeeze, 500MB video download? Can't you upload it on something like Youtube or Vimeo, for example?

Anyway, it would likely be better to take detailed pictures of the supply, both sides of the PCB. That would be much easier than trying to figure the circuit out from a grainy and shaky video.


 

Offline Alex Eisenhut

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Re: Variable Power supply blew up.
« Reply #5 on: November 05, 2016, 01:01:02 pm »
60V 5A is *not* a small lab supply.
 

Offline zerorisers

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Re: Variable Power supply blew up.
« Reply #6 on: November 07, 2016, 11:08:42 am »
The model of the power supply is the STP6005.
I am just staring at the board figuring out why there is so much current flowing through the mosfets. they are in a push pull configuration driving a transformer. But if thats what was drawing all the current then surely other small packages would have blown aswell. seems to me like both mosfets are activating at the same time causing the main capacitors to short through them. I will try to keep up this analysis until I figure it out.
 

Offline The Soulman

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Re: Variable Power supply blew up.
« Reply #7 on: November 07, 2016, 11:32:26 am »
Under what conditions did it fail the first time?
 

Online peteb2

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Re: Variable Power supply blew up.
« Reply #8 on: November 07, 2016, 12:52:14 pm »
Looks like this unit, your average, obscure "One Hung Low" brand as Dave would say... There's one on ebay that model number and spec outputs :

http://www.ebay.com/itm/60V-5A-Precision-Variable-Adjustable-Digital-Regulated-DC-Power-Supply-US-O9B5-/162163845806

Seriously though it's obviously a Switchmode and could well have failed due to out-of-spec components or something as simple as the fan not being able to cool things down fast enough if it was being operated up around 5A or the fan not actually working...

I have repaired many a switchmode that reveals the obvious damage as having detonated its own switching transistors (which drive that inductor unit that looks like a transformer) but in just about every repair i had to hunt down the "out of the way resistors"  that had also failed too and replace them with the right value which can be at times real hard to know.

To help out the OP here does anyone have a schematic of this bench PSU because that's going to be a good start...

 

Offline zerorisers

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Re: Variable Power supply blew up.
« Reply #9 on: November 07, 2016, 01:19:50 pm »
Under what conditions did it fail the first time?
After about a week or two of use I turned it on after getting home from school and it blew up.  It was one of the two drive mosfets. At first I suspected faulty manufacturing as the fan leads where shorted by the board, but now Its getting strange. With how the tracks are setup it seems as though both mosfets where on at the same time.

I decided to try and turn it on and look with an oscilloscope. First turned it on without the probes attached, no explosion, turned it off, waited for capacitor discharge, attached probe, turn on breaker goes off. All without the extra control circuitry (front panel PCB) seems strange that that happened. but I did kinda take out the NTC (because I had no more money for a replacement, nor could I find it on ANY other board around me)

EDIT: I think I have an idea of why the breaker went off... I didn't float the scope so the gfi actually went off. or at least I assume.

http://www.ebay.com/itm/222181044607?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
where I Purchaced the item from.
« Last Edit: November 07, 2016, 02:05:21 pm by zerorisers »
 

Online peteb2

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Re: Variable Power supply blew up.
« Reply #10 on: November 07, 2016, 02:13:59 pm »
You've gotta be careful here when scoping a switchmode. It's likely there is no isolation from mains for the MOSFETS and chances are high the entire side of the circuit, the bridge rectifier etc, and the MOSFETS that are driving the inductor-transformer are all at mains potential. The inductor's windings provide isolation such that the DC voltage output has no effective connection to Mains.  Poking your scope in there sends your scope to the same potential or grounds it all (via the probe screen clip) and bang go more parts... not to mention that there's a real level of danger working on such a thing. Just be very careful as i am sure you know, run your scope off an isolation transformer maybe and use one hand only holding the probe to the circuit put the other in your pocket not on the powered up supply case...

OK.. safety message over... but seriously it isn't worth it coping a belt (shock) off this thing...

My next question is have you checked out how a switchmode works, basic principles at least... I don't want to be telling Granny to suck eggs sort of thing but knowing what's meant to happen in basic terms is always a good start. Some place on your supplies PCB there be a switchmode management ic. With luck the One Hung Low company won't have scratched off its number as they so often do for some weird reason. I'd do a Google for its .pdf and see if any of the data reveals how your Power Supply has been designed.

Is there a tiny chance you already have a schematic for it someplace like in the back of the owner's manual? Is your the supply the one in my link on ebay?

http://www.ebay.com/itm/60V-5A-Precision-Variable-Adjustable-Digital-Regulated-DC-Power-Supply-US-O9B5-/162163845806?autorefresh=true&autorefresh=true

...oops scratch that i see your link. Looks like the same maker, different housing.





« Last Edit: November 07, 2016, 02:19:53 pm by peteb2 »
 

Offline zerorisers

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Re: Variable Power supply blew up.
« Reply #11 on: November 07, 2016, 02:29:01 pm »
I didn't mind the manual. Its probably in the bin. Ill try to look for it after school tomorrow.



I kept both hands away from the board and I even put a face shield from a motorcycle helmet in front of it in the case it blows up. :P safety third.

I will look more into switch-mode supplies tomorrow after (or during) school. And see if I can get a feel for how they work.
 

Offline janoc

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Re: Variable Power supply blew up.
« Reply #12 on: November 08, 2016, 11:04:50 am »
http://www.ebay.com/itm/222181044607?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
where I Purchaced the item from.

I have found what seems like yours on DealExtreme:
http://www.dx.com/p/stp6005-universal-adjustable-dc-power-supply-w-4-5-screen-grey-408269

I love that "High quality components make sure steady operation system" slogan - with a nice photo of noname cheapo electrolytic capacitors and the choke flapping in the breeze right next to it  :-DD

That seems to match what you have in the video, only with different brand of capacitors (and who knows what else).

BTW, that shattered part you have at the start of the video that looks like a capacitor is a thermistor, probably protection against inrush current.

I would probably complain to the seller you have got this from first and try to get a replacement or refund. You may be even still covered by the eBay buyer protection if you have got this recently.

I am not sure I would bother trying to repair something like this - it is certainly built to a price and who knows what issues are there in the design if it managed to open both of the FETs at the same time blowing itself up. There certainly isn't much of a safety margin there. It could have been a fluke because of a manufacturing defect or an out of spec component but it could be also a design problem or the cheapest components the factory managed to source.  You could be spending a ton of time and money fixing something that isn't economically fixable. Switchers can be a pain to troubleshoot even when you know they were designed properly.
« Last Edit: November 08, 2016, 11:09:08 am by janoc »
 


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