TO-220 exploded...estimated overload?

[eccerr0r]

Curious if anyone had some clues at estimated overcurrent/overpower necessary to cause a TO-220 transistor to smoke and blow up... like how many watts it was trying to dissipate/how hot it got.

TO-220 packages were meant to take some abuse but of course not invincible.

The circuit looked like it was worked on before and either done poorly(ugh) or it had extensive damage prior to the repair attempt. I suspect the previous owner "repaired" it, turned it on, magic smoke came out. The transistor apparently went up in flames and melted the plastic grille above it and the transistor shattered. The unit was subsequently trashed and here I am trying to repair it again. I stuck another random transistor in not caring about balancing as this amp might just be trash but it looks like it's working again with the "new" (unmatched) transistor.

(BTW, the original transistor had an Ft of 8MHz. The transistor I put in has an Ft of 800KHz )

[stj]

Re: TO-220 exploded...estimated overload?

that looks more like physical damage, no signs of a flashover or soot-stain.

[redwire]

Re: TO-220 exploded...estimated overload?

Nothing is burnt or melted which is needed for smoke. The epoxy broke including the backside, which is weird. Maybe a pic of the backside would show something.

There's no scratches on the mounting tab hole, a bolt was never there. Sometimes a gorilla can overtorque the heatsink bolt and warp the tab and it cracks.

I would say it was physical damage but I can't see the any plier marks etc. and the die looks fine.

[R_J]

Re: TO-220 exploded...estimated overload?

The transistor shorted and blew apart, nothing new, happens all the time

[eccerr0r]

Re: TO-220 exploded...estimated overload?

Strange, I didn't think it was mechanical damage but can't be sure. The bolt wasn't all that tight and it did have an insulating kit - and the plastic shoulder washer seemed intact which implies it didn't die to overheat or at least long term overheating which would agree with these observations. (I had a transistor I had under fairly long term overheating and it melted the shoulder washer!)

One fact that is a bit obscured by the photo is that the emitter pin was still in the board, loose; it just so happens to fit right in with the chunk of the epoxy that cracked off and looks like it's stuck in there. Now where is that emitter bond wire...

As for other clues in the circuit, there are plastic insulators on the power resistor wire that shows signs of overheating along the whole length, so I can't conclude solder iron error...

Now I'm baffled to how this died, then again, this wasn't the first time it died and the molten plastic may have been from the first death!

The transistor shorted and blew apart, nothing new, happens all the time

I wonder how much of an overload would cause this kind of damage... 10% over max dissipation? 100%? 1000%?

[budm]

Re: TO-220 exploded...estimated overload?

If over current flown then I expect the Emitter resistor or anything connected to the Emitter to be damaged too.
How is this connected in the circuit?

[eccerr0r]

Re: TO-220 exploded...estimated overload?

They're actually fine, fortunately just this one component fried this time around. It's the upper in a totem pole output with emitter resistors. (output stage of a class AB differential/direct drive audio power amplifier.)

Just wondering how this died so violently while the others did not, including the bottom in the totem pole. Maybe this needs some experimentation...

[R_J]

Re: TO-220 exploded...estimated overload?

Once this transistor blew apart, it acted like a fuse protecting the rest of the circuit. had it just shorted and stayed together, something else would have gone up in smoke.

[eccerr0r]

Re: TO-220 exploded...estimated overload?

Yep the typical "transistor sacrifices itself to protect the fuse" deal... but how much power was it dissipating relative to package/mounting situation, up until that instant of failure (if it also catches fire and/or explodes, that is)

[larrymoencurly]

Re: TO-220 exploded...estimated overload?

I once had a triac inside a Woods brand plug-in ground fault interrupter explode and left a nice copper plating inside the case, over a 3/4" diameter area. It turned out there was a recall on it, but the redesigned replacement failed in 1-2 weeks -- by leaving the electricity running. The 3rd GFI has been working fine for over 20 years.

[eccerr0r]

Re: TO-220 exploded...estimated overload?

Ah crap, blew another IRF640. Overheated after hour of testing.

Where can I get shoulder washers cheap? I'm losing^H^H^H^H^H^H^H melting them left and right...

[petehall347]

Re: TO-220 exploded...estimated overload?

is the idle current ok ?
genuine transistors ?

[eccerr0r]

Re: TO-220 exploded...estimated overload?

Sorry, this is unrelated to the original TO-220 but rather a different one in a different usage (switch mode PSU instead of audio amplifier).

I knew it would dissipate 10+W but had a fairly poor heatsink, not to mention it had to share dissipation with the rectifier diode (another 2-3W). Was asking for it, alas now I need more shoulder washers. I probably can salvage some stuff but the transistor is probably dead. Nylon washer is very dead indeed.

10 watts was 10A and 0.18Ω Rdson at no more than 50% duty cycle, each pulse no more than 1ms ...

[eccerr0r]

overheating carnage

dead, dead, dead!!! Shorted D-S. Got so hot that the solder melted and disconnected itself.

Power source was solar... large solar cell...

Fortunately still have a few more IRF640s to play with but think I may need to get more, killing them left and right.

The diode that was on the same heatsink survived, still good, lives another day.

Need to find a source of cheap shoulder washers... they sure should not be a buck a piece...

[stj]

Re: TO-220 exploded...estimated overload?

real men use berilium oxide washers.

so what is it, a charge controller?

[eccerr0r]

Re: TO-220 exploded...estimated overload?

bring it on, gimme free BeO shoulder washers

It's the first part of a homemade grid tie inverter, it was a success until it melt down and desoldered itself. Due to the expected heat generation I was pondering about paralleling a few transistors but didn't due to laziness, thought the 10W I calculated was "acceptable" but nope this is absolutely not...

(It was successful in lighting a 100W 120V light bulb with DC at full brightness for at least an hour or so before the magic smoke came out; I think I also may have had a laptop plugged in but it was done charging by then. Power source was two 18V "off grid" panels.)

I expect to create more magic smoke in this project, this is only the first batch Especially the second half of the project - the actual inverter and grid tie, if I get the timing wrong, it will definitely create more magic smoke...

[stj]

Re: TO-220 exploded...estimated overload?

a friend is messing with home-made invertors - magic smoke is the main product of such things!

[eccerr0r]

Re: TO-220 exploded...estimated overload?

The good thing so far is that my source of IRF640's are all recycled pulls from somewhere. Bad thing is I'm running out. Worse thing is that I'm running out of shoulder washers, need a new one each time I fry a MOSFET. Maybe I should just live with live heatsinks.

Wish I could use some of the 75N06's I have as I have a few more of them on hand, alas, voltage is too high for them.

Oh. Putting your hand to touch an inductor for heat is not a good idea when there's 120V going through it. That lacquer is not much of insulation... Ouch.

[stj]

Re: TO-220 exploded...estimated overload?

what about not using the hole and instead using a bar with screws on either side to hold it down?

[eccerr0r]

Re: TO-220 exploded...estimated overload?

Might have to resort to that, though there's a reason why people made heatsinks with springs in their assembly mechanisms...

Fortunately my car toolbox has something to fix that, sort of...