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Capacitor explodes on PSU

Sparkymark

Feb 16, 2024
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Hi all, I am a qualified electrician and recently decided to get into electronics. I have been working on a power supply board for a naim mu so 2 all in one speaker. It had 5v standby and I was checking voltages and accidentally shorted positive side of bridge rectifier to heatsink. Obviously went bang. Now when I plug it in it blows a single 400v 4.7uf capacitor and shorts a diode just before it. There is also a smaller circuit board upright on the main board with 2 wires running to it from the transformer, I'm guessing this might be something to do with PFC but not sure. There is a resistor on there that has also got a damaged pad but resistor is still intact and attached to what remains of the pad. Any ideas what could be the issue. I'm thinking maybe shorted at the little board so sending high voltage back thru negative side of capacitor. Any advice very much appreciated. Thanks for reading.
I will try to get some pics for clarity.
 

Sparkymark

Feb 16, 2024
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Feb 16, 2024
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Hi all, I am a qualified electrician and recently decided to get into electronics. I have been working on a power supply board for a naim mu so 2 all in one speaker. It had 5v standby and I was checking voltages and accidentally shorted positive side of bridge rectifier to heatsink. Obviously went bang. Now when I plug it in it blows a single 400v 4.7uf capacitor and shorts a diode just before it. There is also a smaller circuit board upright on the main board with 2 wires running to it from the transformer, I'm guessing this might be something to do with PFC but not sure. There is a resistor on there that has also got a damaged pad but resistor is still intact and attached to what remains of the pad. Any ideas what could be the issue. I'm thinking maybe shorted at the little board so sending high voltage back thru negative side of capacitor. Any advice very much appreciated. Thanks for reading.
I will try to get some pics for clarity.
Just a bit to add on.
Photo 1 shows where the diode and 400v 4.7 cap goes and photo 2 shows the small board and circled resistor that had a bit of the pad come off. Photo 3 and 4 is just the whole board, 5 better lighting. Not sure if any of these will help.
 

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Sparkymark

Feb 16, 2024
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Here's a photo of the small board with damaged resistor pad.
 

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Alec_t

Jul 7, 2015
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Welcome to maker.pro!
Have you checked that all diodes in the bridge-rectifier are still good? A shorted diode would cause destruction of any downstream electrolytic cap.
 

Sparkymark

Feb 16, 2024
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Thank you, I have checked between both ac and negative and both ac and positive and all give me a diode junction. Also checked between both positive and negative outputs and get double reading.
 

Martaine2005

May 12, 2015
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You say that you have a diode junction. Did you reverse the leads and check again?

You probably destroyed the switching transistor and it’s shorted.
 
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Sparkymark

Feb 16, 2024
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You say that you have a diode junction. Did you reverse the leads and check again?
Thanks for the reply, yes I reversed the leads and dont get any readings. I haven't replaced the shorted diode and cap again yet and I'm thinking about powering on again to see what voltage I get at the point where the diode goes. Would this be ok and not damage anything else as it does branch off and go to a triac and 2 mosfets?
 

Martaine2005

May 12, 2015
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If you have an old (or new) incandescent bulb, you can wire it in series with the live mains input.
Obviously 240v bulb!. 100 watts or so.
It’ll light up bright indicating a short. Then with power disconnected, lift a leg or remove the diode then transistor until the short goes and the bulb will light up dim. Then you’ll know whether it’s upstream or downstream of those components.
 

Sparkymark

Feb 16, 2024
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I have just today built a light bulb current limiter. Trying to get a bulb without having to buy a pack of 10 on Amazon. Everywhere sells the LEDs but not incandescent. I will try this when I get one. Thanks
 

Sparkymark

Feb 16, 2024
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OK, upon further inspection I have come across a smaller bridge rectifier aswell as the normal bridge rectifier. I have checked diode junctions and reading OK apart from between the + and - outputs. I am getting a double diode reading both ways and from my understanding of bridge rectifiers I should only get it one way. Am I correct in my thinking on this or is it a different type of rectifier. It is MD10S single phase glass passivated bridge rectifier. This one is halfway between 2 coils/inductors. The main bridge rectifier is after them both. Also a better picture of the small board that had the blown pad on it.
Thanks again.
 

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Sparkymark

Feb 16, 2024
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Hi, I have been doing more research and I'm thinking maybe the transformer has shorted windings. I can't find any other faults. The bridge rectifier I said about is fine as I tested out of circuit. There is a small looks like melted plastic on top of the transformer and from what I've read a shorted winding would create a massive spike in current flow so that could be why I'm having the problems that I'm having. Can anyone point to a source for transformers and how do I find a part number for this one. Thanks again.
 

Martaine2005

May 12, 2015
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Don’t jump the gun, although possible it’s doubtful.
Have you got hold of an incandescent bulb?.

Have you disconnected the transformer secondaries?
Have you disconnected the transformer primary?.
It’s more likely the switching transistor on the heat sink is shorted.
Edit: And of course electrolytic capacitors as they are worked extremely hard in high frequency SMPS.
 

kellys_eye

Jun 25, 2010
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There is a small looks like melted plastic on top of the transformer and from what I've read a shorted winding would create a massive spike in current flow so that could be why I'm having the problems that I'm having.
Nope. Not melted, just some glue. Switching transformers don't fail like that.
 

kellys_eye

Jun 25, 2010
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Remove the main switching device(s) - probably the ones bolted to the big heatsink, replace all the missing/defective components and power up. You should be able to measure the correct DC voltages which will 'prove' the main input side of the circuit is operational.
 

Sparkymark

Feb 16, 2024
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OK, so I have been busy removing and testing pretty much everything before the diode that shorts. Bridge rectifier, to main filtering caps, 2 small HEL 10D471K varistors and I also removed the transformer and tested that. I have sketched a rough diagram for the results. There is 2 small wires that come out of it too and go to the small board, they are pictured at the top of my crappy drawing.
 

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Sparkymark

Feb 16, 2024
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Also been trying to figure out what this SOIC-8 Chip is. Can't seem to find anything in the usual places.
 

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