Inverter welder help

[Dannyx]

Good day folks. It's been a while since I've actually repaired anything, so today I have something I haven't worked on before: a friend of mine gave me this small inverter welder to look at. It powers on, but doesn't work.

I took the cover off and immediately noticed the problem: two dead G40N60 transistors, along with some SMD resistors in what I think is the gate drive for the trannies. One IGBT is dead-short, the other reads fine, but it's missing a chunk of the casing, plus its SS14 diode is also gone, so it's safe to say we'll be replacing both.

The SMD resistors are by far the biggest issue, since I'm not able to make out what value two of them are. I can see 5R1 on one pair, but the other is completely gone. All I can see is a 1 and a 0, but nothing in between....if I had to hazard a guess, I'd say it's 150, but that could be totally random....

The welder is an ESAB MMA 300 if it matters at all. Without a schematic, I was hoping someone may recognize the circuit and offer some approximation to those values, since it looks "generic" enough and I recall seeing this shape and size under other brand names too on the shelves...it may not even be worth repairing it, given how expensive those transistors can get, especially since there's the risk of blowing them again. It's almost identical to THIS one, except it's got an ammeter at the front. The blurry schematic does actually show those 5R1 resistors in the gate drive on the far right of the page there, but the schematic is not clear enough to work out what IGBTs it uses, so I'm not sure if those values would work in mine as well. Of course, I'm grasping at straws, but this would mean the blown resistors are 20R ones, which comes close to the 150 value I'm betting on...

[stj]

so it's just a stick welder, not even a mig?
those things are sooo cheap now that it's not worth the time.

[Dannyx]

Originally Posted by stj

so it's just a stick welder, not even a mig?
those things are sooo cheap now that it's not worth the time.

Yes, it's a stick welder - tiny too. It's like a small UPS

I'd hazard a guess it's the dust buildup that killed it: there was a LOT of dirt caked on the legs of the two transistors. It's not the fluffy dust you get on the inside of electronics or PCs - it was more like dust that got moist and turned into mud which then dried off....that kind...

[stj]

the crap may even be conductive,
remember before welding you clean the surfaces with a grinder!!
metal dust!!!!

[Dannyx]

Originally Posted by stj

metal dust!!!!

+ fan on the back to help suck it in

[petehall347]

needs an air filter then

[stj]

needed an air filter - not any more!

[CapLeaker]

my take on it is a 750, as being 75 Ohm.

[Dannyx]

Could be...what's the worst that can happen

Now that I look at it more carefully, it could be a 7 after all, the way the line seems to curve ever so slightly...

[petehall347]

be better buying a new welder and adding a filter
then maybe looking at values of unknown resistors .

[CapLeaker]

Originally Posted by Dannyx

Could be...what's the worst that can happen

Now that I look at it more carefully, it could be a 7 after all, the way the line seems to curve ever so slightly...

Nothing. The gate drive resistor just limits the current. Common to see up to 100 Ohm resistor there.

[Dannyx]

Originally Posted by CapLeaker

Nothing. The gate drive resistor just limits the current. Common to see up to 100 Ohm resistor there.

So I guess if it's too high, the gate would be insufficiently driven and the transistor would overheat (linear region ???).

If it's too low = too MUCH base current...

[CapLeaker]

Yeah, the gate resistor has to be selected. All depends on the IGBT, switching time, etc...
However, yours look like 750 to me, which is 75 Ohms.