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accidental fireball production using SMPS

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, flat on a painted wooden desk. 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.

Anyone else had this happen?

regards, -A

Reply to
conundrum
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North American power supply (120V 60Hz) on UK mains (240V 50Hz)?

--
Regards,
        Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.
Reply to
Doug Miller

Sounds like there was a problem with the PSU that your tests didn't reveal - perhaps one of those capacitors had failed short circuit.

Reply to
CWatters

Possible. I did swap over one of its capacitors to another PSU though as the original had been damaged by removing the case (grr) and it seemed fine.

Best guess was that the switching transistor had partially failed and plugging it in caused it to short as the capacitor fully charged.

Interesting effect though, one wonders if a shaped discharge chamber with a metal film resistor at one end connected to a switch and fairly large capacitor would generate a repeatable fireball with longer lifetime.

-A

Reply to
conundrum

Reply to
Art

reveal -

You mean from another PSU? If you put the new one in backwards that would explain a lot.

Reply to
CWatters

No, it got removed *after* the fireworks. :)

-A

"Bother" said Pooh, as he was decapitated by the "kawoosh"...

Reply to
conundrum

Yup, had an over-voltaged electrolytic go off like a rocket on me before. The can goes off like a bullet; the "fireball" that you witnessed may have been the innards that folow the trajectory of the can burning as they go.

Cheers, Pete.

Reply to
Pete Wilcox

Nope, but good thinking. The only damage I could find was an SMD resistor near the main drive transistor, that resistor with the side blown out (I have another from a broken similar SMPS with identical damage) and the pcb track next to it.

wish i'd had a camera, it looked like something out of "The Twilight Zone" !

regards, -A

Reply to
conundrum

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