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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062
Hi all.
I just had something unusual happen. Someone brought in a power supply for a FS laptop, which had stopped working. Input fuse open, but I couldn't find anything wrong with DC tests and the onboard fuse was OK.
Removed output lead as my measurements indicated a potential intermittent short here and then plugged in PSU.
Everything seemed fine for about .3 seconds then there was a HUGE BANG and a brilliant flash of light from the mains plug and the supply. Turned round to see what appeared to be a white fireball (afterimage from the plug possibly) leaving several vertical smoke trails about 4 feet from the PS (in the centre of the room).
It must have vanished about a tenth of a second, the smoke trails lasted a lot longer. (about 3-4 seconds)
Looking at the power supply, several components had been damaged including a metal film resistor with the side blown out.
In case anyone wondered how the PSU was arranged, it was placed with the mainboard facing upwards. The heatsink next to the resistor had plastic film and formed a triangular compartment roughly in line with the direction of the smoke trails. There were two capacitors in this area with a 4mm gap between the last, and the heatsink.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
My flatmate's plugpack supply for her "Unwired" wireless modem failed again a little while ago. I pulled apart the solid-state 6V 3A supply, and found a 7N60 MOSFET had failed.
I replaced the MOSFET with something near enough, and not finding any other faults, replaced the fuse and put it into an extension lead so I could turn it on remotely (as one does with iffy 240V equipment).
Upon flicking the switch, I was presented with lots of smoke, some blobs of molten metal, and the usual earshattering 50Hz 'splatter-splatter'. Before I could turn it off, the apartment's circuit breaker had already blown.
Later inspection revealed the line filter had arced over, active-to-neutral, but there carnage of just about every component on the board. The circuit traces on the bottom got hot enough to smell and boil some soldermask, too.
My first mistake was my diagnosis that only the switching transistor had failed in the flyback converter. My second mistake was replacing the 1A fuse with a 10A I picked out of the wrong box accidentally. More care is needed next time.
Unfortunately, I've never seen those little black bricks explode... they always seemed to have very calm failure modes ('Click.)
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