accidental fireball production using SMPS

[conundrum]

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

 

[Doug Miller]

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.

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

 

[conundrum]

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

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

 

[Art]

Flippin 110/220 switch aparently in wrong position would be the first hint
of the cause of this abberition. Cheers

 

[conundrum]

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

No, it got removed *after* the fireworks.

-A

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

 

[Pete Wilcox]

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

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.

 

[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.

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