Electronics > Repair

Stick welder repair

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dog80:
They brought me a Miller GoldSeal 160i that had two blown diodes.

The board is this:


The blown diodes are on the other side, somewhere between the two big heatsinks



As you can see, part of the board is burned out.


The diodes were already removed when I got the board on my hands so I don't know what type they were. I installed a couple of 1N4007 that I had handy, turned the welder on and BOOOM! the diodes blew again in milliseconds.


The traces on the board that lead to the diodes are thin, so I don't think these diodes handle too much current. So either there's higher voltage there that the 1000V the 1N4007 are rated for, or there is a short somewhere else on the welder. Also, since part of the board was missing, I am not sure about the direction of the diodes. I installed both in the same direction as the schematic on the board. Could the other diode be in the opposite direction?

digsys:
Turn the board over and see if the Cathodes are connected (or use a multimeter if that's not possible).
ALL diodes seem to face the same way, which was a* convention in older days ie ICs, diodes, colour codes etc all orient the same.
Most likely there is a short elsewhere, but if that is too difficult, you'll HAVE to trace the circuit.

Psi:
I would check all the fets/igbts for shorts.  That's what normally fails when stuff like this blows up.

If they're blown make sure the gate drivers are ok before replacing with new ones, or they may smoke again.

dog80:
That's weird. I removed the diodes and tested them with a multimeter and they seem ok. Also there are no burn marks on them or anything. But when I turned the damn thing on there was a loud bang and some electrical flash coming from the diode area.

I did some probing here and there and the lowest resistance I measured was something like 20 KOhm

digsys, the two diodes do not share anything.

digsys:
In that case you may have introduced a NEW fault. The burnt area is now conductive, carbon deposits from the
flash / burn. So we may be able to explain the 2nd incident, but not the 1st. You'll need to grind away ALL black / burnt bits.
You'll definitely need to trace where each end of both diodes goes to.

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