Exploded transistor

[ben7]

Re: Exploded transistor

Well yes, there is that approach as well :P

[stj]

Re: Exploded transistor

why not just check the rectifiers, diodes and ceramic caps while it's off, instead of stressing the transistors in a way that may kill them!

[ben7]

Re: Exploded transistor

no, i had taken the fan off to keep it out of the way, and it didnt work anyway. the fan might have made it click a little slower, but it sounds like it is coming from possibly one of the 2 transistors i replaced. though my meter is not trms, so i dont know if that 25vac is actually accurate or not.

i could hook it up to my scope though.

The ticking might actually be coming from the transformer. That is typical when a switch mode power supply runs at a low duty cycle (because of a very light/non-existent load).

Try putting a load on it and see if the ticking goes away.
Otherwise, it's possible the ticking could be coming from a snubber capacitor that is breaking down.

[jasonbay13]

Re: Exploded transistor

no, i had taken the fan off to keep it out of the way, and it didnt work anyway. the fan might have made it click a little slower, but it sounds like it is coming from possibly one of the 2 transistors i replaced. though my meter is not trms, so i dont know if that 25vac is actually accurate or not.

i could hook it up to my scope though.

[ben7]

Re: Exploded transistor

so anyway, i grounded ps_on and checked the 12v voltage, whilst it clicking away. measured 12.1vdc and 25vac. might this be a shorted rectifier as ben7 has suggested?

What about the other voltages? It might be instability because of a low load on one of the voltage outputs. That leads to my question, do you have a load on the PSU now?

Edit: Well, not instability, but rather a very low duty cycle.

[jasonbay13]

Re: Exploded transistor

so anyway, i grounded ps_on and checked the 12v voltage, whilst it clicking away. measured 12.1vdc and 25vac. might this be a shorted rectifier as ben7 has suggested?

[stj]

Re: Exploded transistor

check any ceramic caps for shorts too.

[ben7]

Re: Exploded transistor

so status update. i replaced the blown transistor with a c1815, and the c5027 switching npn was infinite resistance through all combinations on my meter. replaced that with a 13007. plugged it in and it has voltage to the 5v. but it was making a clicking noise, kinda like the rad meter on fallout :P, whats it called a geiger counter? i unplugged it after that in fear of exposions. any reason its ticking?

It might be ticking because of a short circuit. Check to make sure none of the secondary side rectifiers have been damaged (shorted) from the catastrophic transistor failure.

[jasonbay13]

Re: Exploded transistor

so status update. i replaced the blown transistor with a c1815, and the c5027 switching npn was infinite resistance through all combinations on my meter. replaced that with a 13007. plugged it in and it has voltage to the 5v. but it was making a clicking noise, kinda like the rad meter on fallout :P, whats it called a geiger counter? i unplugged it after that in fear of exposions. any reason its ticking?

[stj]

Re: Exploded transistor

the resistor is usually inside thick tubing.
and i'v never seen a bad transistor, just bad caps and open heaters.

maybe the 110v lamps stress the transistors harder.

[ben7]

Re: Exploded transistor

Take apart the base of one of those curly CFL light bulbs. Those usually have a bad cap or an exploded transistor in them.

+1

I've also seen popped/overheated film capacitors and resistors that have holes in them (burnt out).

One had a transistor that had popped in half :P

Also, I hate fusible resistors, I think they are a fire hazard when overloaded, but not severely overloaded. They are flameproof, but it doesn't stop nearby items from catching fire. Even the plastic cfl lamp base WILL burn if given a heat source. It just won't (well, shouldn't) continue burning on it's own.

[canadaboy25]

Re: Exploded transistor

Take apart the base of one of those curly CFL light bulbs. Those usually have a bad cap or an exploded transistor in them.

[tom66]

Re: Exploded transistor

you can do over 1w in that type of package.
take a look at ZTX753!!

1W in a TO-92? Good luck on that. The standard thermal resistance for that package is about 150C/W. Sure, if your ambient is 0C you might be able to do it. But for all other reasonable applications, stay around 0.5W maximum.

There are extended-height TO-92 packages that may also have additional copper inside of the package; this may increase the power dissipation rating. But a standard TO-92 cannot dissipate a watt. The ZTX75x specify a rating of 175C/W, so even that cannot dissipate a watt unless you are at 25C and can tolerate a 200C junction... that's extremely optimistic... If you heatsink it sure - but heatsinking a TO-92 is silly, you should use a TO-220 or a surface mount package (D2PAK, etc) if you want a heatsunk device.

[stj]

Re: Exploded transistor

the blown one has a logo that looks like mitsubishi

[budm]

Re: Exploded transistor

It looks like the Chinese specmanship.
This C945 by STANTON is rated at 250mW.
So if you look around it will come up with lots of numbers.
I would trust the Japanese spec of the (2S)C945.

[ben7]

Re: Exploded transistor

you can do over 1w in that type of package.
take a look at ZTX753!!

It's rated for 200C vs the 2SC945's rating of 125C. Also, once it goes above 130C, the maximum allowable power dissipation decreases.

I'd never run parts over about 100C, unless it is a resistor. The hotter parts run, the less reliable they will be. (Though most people don't seem to care about that...)
(Also, reminds me of the minimum load resistors in PSUs, right up against cheap capacitors ... the extra heat helps kill the caps faster!!!)

That's as far as I will argue.
-Ben

[stj]

Re: Exploded transistor

you can do over 1w in that type of package.
take a look at ZTX753!!

[ben7]

Re: Exploded transistor

That may well be true, however in this case it isn't. Look at the data sheets for the two. They are not the same transistor. That doesn't mean a 2sc945 can't be substituted for a c945 in some circuits, but the 2sc has a significantly lower power dissipation limit and a greater current gain. 2sc is also rated for 50v, the c is 60v.

According to a datasheet I found, the C945 has a breakdown of 50V. The 60V rating is the collector to base voltage rating.

Also, they have different Hfe classifications. (O, Y, GR, BL)
I did find that the C945 is rated for 400mW, and the 2SC945 is 250mW.
The ratings might differ from manufacturer to manufacturer. Either way, 400mW seems a little much for a heatsinkless TO-92 package!

[stj]

Re: Exploded transistor

it's also one of the most common japanese transistors on the planet.

[SteveNielsen]

Re: Exploded transistor

That may well be true, however in this case it isn't. Look at the data sheets for the two. They are not the same transistor. That doesn't mean a 2sc945 can't be substituted for a c945 in some circuits, but the 2sc has a significantly lower power dissipation limit and a greater current gain. 2sc is also rated for 50v, the c is 60v.