Interesting Failure

[magistertc]

This Enermax is pretty old ... for P4 systems. Dead when I got it, blows fuses. I tried several times to troubleshoot, no joy, probably in switcher circuit?

So, getting ready to part it out, I removed the mosfet switcher with the heat sink and then FINALLY took it off the heat sink. Surprise!

This thing had passed ok when tested, but that was only with~5V diode test on DMM. Seems like it was overloaded some time in the past, but cleverly kept its condition hidden until now.
TC

[Quaddro]

Re: Interesting Failure

If it's show burn mark, that's mean that component is broken.
And yes, shortage in primary section will blow the fuse, or trip your electricity meter.

Check that components once more again, with analog multimeter. And replace it.
and also check surround components, like diodes etc.

[PeteS in CA]

Re: Interesting Failure

Yeah, that'll do it. Maybe there was some trapped particle that partially punctured the insulator. Maybe some assembler-barbarian mishandled the device and insulator when assembling it. Maybe there was a line voltage spike that broke through or around the insulator. Whatever it was, it took quite a while for the short to happen.

[petehall347]

Re: Interesting Failure

heat-sink dont look the best . i would be using mica after being sure sink is flat .. silpad ok but still needs to be flat .

[Longbow]

Re: Interesting Failure

This is a classic failure due to a sharp burr on the metal surface that cut through the mica, or a failure of the mica insulator. You can see where the sharp point cut through to the heat sink, and the resulting heat radiated outward from that point. May have been combined with over-tightenting of the mounting bolt. Good example. Encourages people to flatten the rear surface of metal case transistors before mounting.

[magistertc]

Re: Interesting Failure

Actually, I believe this PS was overloaded, the plastic around the mounting hole looks scorched. Crack in case, path to ground for 300V pulses, metal eroded. Mini-flashover.

I used a mica insulator with the new switcher, runs ok at about 150W, possibly good to 200-250. Not suitable for newer systems with heavy 12V load, it's going to become a bench supply, keeping the load under 150.