Plasma Cutter repair and Transistor Question

[edb1984]

Hello!

[edb1984]

Update, also found two other smaller transistors gone.

[edb1984]

Photo

[SeanB]

Likely they are all the same IGBT, though you will not be able to fix the one bottom right back down to the PCB material as the whole pad has been ripped off. Replacing them all, and the 4 driver transistors likely will get this running again. Just check the cremated transistors, they might be a different type than the 2 non cooked ones, you should still be able to see the markings on the char, so see if they are the same or different.

[EPTech]

Hi there,

These modern welders and plasma cutters are actually switch mode supplies that are in constant current mode. The assembly you are showing looks like a H-bridge configuration that switches the primary side of the transformer. The rails marked + and - are coming from the rectified voltage, off the capacitor bank right? The 2 braided wires on the bottom are going to the "big" switch mode transformer, right? In that case you can almost certainly say that all 4 transistors are the same. Be careful though with the polarity of your replacement diodes. They need to be inversely polarized to the current flowing through the H-bridge. They are there to protect the IGBT's from reverse high voltage that occurs at switch-over of the transformer primary.

It may be one of the diodes has been shorted, causing a short circuit in the H-bridge. There can only be either one pair of transistors on at any given time. It may also be the IGBT driver is defective, putting an IGBT permanantly ON. As said, I would also swap out the IGBT drivers. They are the IC's connected to the gate, the smaller trace on the transistor, probably via some series resistor. They are probably located on the controller board. Follow the small wires .

Good hunting.