What's that sound? BOSOZ PS blowing up?

[lgreen]

Call me an idiot, but I'm getting back to work on my BOSOZ and "accidently" shorted the +60V power supply lead to ground. This on a perfectly good and working power supply board. Damn!

What was the loud POP! that I heard? It came from the power supply board. I assume it was the MOSFET part of the + regulator blowing up, but I'm not sure. There might have been a little burn smell too, I can't really remember I was in too much shock.

Anyway, that leads to the question- how do you tell if the MOSFET is blown up? It looks quite normal. Is this the logical thing to replace? How do you tell if it was a capacitor or something else?

Its only taken me a year so far, I plan on finishing in 2005 sometime at this rate.

[tiroth]

You could conceivably have blown anything from the rectifier through the caps to the MOSFET, but probably you just popped the MOSFET. If you test with a multimeter you can confirm it is popped if there is a short...but be aware that "no short" does not necessarily mean "transistor ok."

Don't get too discouraged. Blowing stuff up is part of the fun.

[wuffwaff]

HI Igreen,

normally the mosfet blows. I had the same thing twice (touched the heatsink with the lid from my ( slimline ) case, causing a nice spark plus "bang".

I would replace it and test with a variac if it works again.

William

[janneman]

What's the output voltage on the 'blown' supply?

If it is still as before, the 'pop' probably was the discharge pulse of the supply output cap. Power fets are notoriously difficult to blow that fast. You really have to work on it .

Jan Didden

[lgreen]

I don't know what the output voltage is, as I am afraid to turn it on again. Should I fire it up and check the output voltage, for all I know it could be working.......but lets assume the mosfet is fried, won't turning it on damage more items?

[janneman]

Of course you turn it on with the amp disconnected! I'm sure you thought of that!
Other than that, I would turn it on and measure the output. If it is above the original (60V?), load it with a few 100 ohms (not too long, those load resistors will get HOT, use at least 20 or more watts and just for long enough to read the meter),
to make sure it is properly loaded, and if it gets nicely to 60V you're home free. If not, it either is above or below the 60V. That's when the real sleuthing begins.

Edit: If it's above the 60V, chances are the mosfet is blown. But I doubt that.


Jan Didden

[Nelson Pass]

Every once in a while you get a tiny piece of wire or
solder in the wrong place and it makes that noise and
smell. Then afterward everything works fine.

I remember that on Threshold 800A control boards we sprinkled
tantalum decoupling caps (far more than needed) and
as the years went by occasionally one would short.
They do that - I don't use tantalums anymore.

Anyway, you couldn't tell which one it was, but you could
drive the board with a 10 amp supply. You would hear
a popping noise, and the short would be gone.

[lgreen]

ahhh........ how do I put this, the thing was working just fine till the red wire touched the case, at which point we had the pop. So I don't think its a little wire...but maybe its working, guess I'll fire it up (obviously with the amp part disconnected... I thought of that!) and see where we are. I think I have sufficient competance to fix this but just hate to put in the time...want to fix the right channel preamp board and get in the blue lights!

The caps are not tantalum, but what if a capacitor blew out how would one figure that out?

Then again, I am using a tantalum cap on a different supply in the same preamp, like a 10 uF 25 volt tantalum as the load cap on a 5.7 volt LM317 power supply, hope this doesn't go, but I did this on purpose because the app note specifically says to use a tant cap of this value. I have heard of those having "spectacular" falilures in the field though.

[tiroth]

Could the pop have been from the spark?

The easiest thing to do is check for a short in the FET, then add some current limiting resistors in the supply and then power up, with a variac if possible, just as people have said here. If you get excessive current draw, stop and check for shorts. Otherwise, proceed as directed.

[lgreen]

Don't think it was from the spark, ohhhh, I've had one of those happen before (on my Gainclone-- if you look at post#3 you can see the burn mark on the top surface of the faceplate near the middle LED) when I accidently did the same thing, that time it was through a tiny little baby wire that was supposed to connect V+ to a LED through some resistors and diodes. Left a nice burn mark on the spot where it touched. Ive tried to be extremely careful since then but oooooops. I am sooo stupid! This time, no burn mark and I believe the sound came way to the left of the contact point, right inside the PS.

Good point on the resistors, what value do you think I should use to provde current limiting? Are we talking 5 ohms or 100 ohms or more?