Can bad caps make a fuse blow?

Sectorseven

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I was testing a semi-working power supply. Went to plug it in and all of a sudden the fuse went. Upon closer inspection there are some bulging caps, but the fuse is all the way on the AC side literally right after the power input, so would the caps cause this?
 
Electrolytics usually just develop a high ESR, but ceramic caps can blow shorted, and if a cap is physically damaged or dies violently or from overvoltage it can blow shorted. A blown-shorted cap, yeah, probably will blow fuses.
 
If it's a switcher I'd say you probably have issues on the high voltage side with some shorted semiconductors at a minimum. Switchers typically shut down when there is an overcurrent issue on the low voltage side.

RJ
 
Yes it's a switcher, might even be a PC power supply. Someone gave it to me not working and I wanted to get some practice on it. I just basically ran a power cord from the AC inputs to the wall. Hope I didn't do wrong by that, anyway it worked the first time I tried it.

Caps on the low voltage side definitely have to be replaced. As for the blown fuse, I guess I should start by testing the bridge rectifier and move on to the MOSFETs? There's also the filter capacitor of course, but visually it doesn't look damaged, for what it's worth.
 
Yes it's a switcher, might even be a PC power supply. Someone gave it to me not working and I wanted to get some practice on it. I just basically ran a power cord from the AC inputs to the wall. Hope I didn't do wrong by that, anyway it worked the first time I tried it.

If it's an ATX or newer unit, you'll have to short PS-ON to ground to get anything but 5VSB working. Of course, if you know you're blowing fuses, you're not that far yet...

Caps on the low voltage side definitely have to be replaced.

As others have said, failures in LV won't blow fuses -- the switching transistors would explode first, and even that is fairly unlikely. Swap 'em anyway.

I guess I should start by testing the bridge rectifier and move on to the MOSFETs?

Definitely. Dig through the ENTIRE HV section looking for shorts -- there is one there somewhere.

There's also the filter capacitor of course, but visually it doesn't look damaged, for what it's worth.

If it really bothers you, lift a leg and check it for continuity -- it should read open after settling. No need for an ESR check -- a dried filter cap won't blow fuses.
 
DO NOT PLUG IN SWITCHING POWER SUPPLIES WITHOUT A LOAD.

They *can* and *have* self destructed by powering them up without a load. They *require* a load to work properly.

Now if you want to troubleshoot them, try my repair logs. There's an entry specifically on that.

RJ
 
That's a really useful guide. The only thing I found on the high voltage side that's questionable is a very large resistor which keeps coming up OL on the meter. I'll desolder one end and retest it. Bridge rectifier and all the semiconductors tested fine, at least in circuit.
 
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