TIP50 power transistor failure

While working on my Metrologic HeNe laser I'm pretty sure I killed the TIP50 power transistor on the board. The TIP50 is an NPN power transistor rated at 400 volts and 1 amp. I ran it without its heatsink to see if it would get hot, and indeed it does. It needs/needed to be replaced anyway due to having two broken leads, but I'm wondering if the TIP50 has any kind of thermal shutdown that will reverse itself upon cooling, or if the it just dies when it overheats. I haven't seen any reference to such a feature in any of the data sheets so I suspect it doesn't have it.

I was hoping to ditch the heatsink entirely because it's part of the reason the transistor's leads got broken in the first place. I guess that won't be possible, however, considering how hot it became.

I suspect it's probably dead, because the laser shut off and then wouldn't power on at all, not even the power light. Turns out I popped the 750mA fuse, so seems like the component might have shorted out internally due to the excessive heat.

Not a big deal if I killed it, it's a 1-dollar part, can order them online. Plus, with a replacement part I can have the leads be longer, which would make them more resistant to being broken in the future.

Other thing I wonder is, do you suppose the HeNe power supply would still work if I replaced the TIP50 with an NPN power transistor rated at 400 volts but higher current, maybe 5 or 10 amps?

Sure, as long as the parameters of the new device are as good or better than the TIP 50.
Give it a try.

If you're going to replace it with a different type, the MJE13009 is a good one to go with. They are pin
compatible and the 13009 is superior in almost every respect. They can be harvested from many different
SMPS devices and other equipment. I can even send you a couple if you PM me your address.

I don't see anything in there to suggest it's the transistor. I hope you're not relying on "it got hot without the heatsink" as the source of your deduction, because that's like saying "I took the hamster's food away and he got hungry, so the hamster is defective."

You said the fuse blew. Start there. Is the input shorted? If so, find out through what.

First thing I would do would be to remove the transistor and see if it is internally shorted! If not, then find the short on your board.

What else would it be? Laser was running just fine, then it quit. Nothing changed, except the transistor smelled like it was burning up. It was so hot I couldn't touch it.

ETA after post: I wasn't running the laser inside its case, I was running it on the ceramic tile countertop. Totally dry, no metal anywhere, no water. I made sure of it before I ever set the unit there. End edit.

I did do a basic resistance test from lead to lead on the transistor and all I got was a dead short. I made sure that I wasn't contacting the test probes.

I'll check the rest of it though, just to be sure.

Regardless, the transistor was damaged due to its broken leads, which presented a strong argument to replace the part altogether. I know that I can wire it up better than it was, such that damage via mechanical jolt would be virtually impossible, so I believe I will go ahead and do that.

Definitely do appreciate your concern though, and if anyone has any more advice on the matter I will gladly take it. I want to LIVE, after all.

Be sure your mains is wired correctly and that hot and neutral aren't reversed at the socket. Likewise check the plug on the laser isn't reversed.


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Marco Polo said:

I did do a basic resistance test from lead to lead on the transistor and all I got was a dead short.

Click to expand...

While removed from the circuit, yes? Then it's dead. Might be a failure of the transistor, or it might be whatever drives it though.

What is the resistance on the mains input?