I'm working on a friends inoperative Miller 300 Amp DC inverter Tungsten Inert Gas (TIG) machine. These machines convert 240v single phase A/C to DC, then use IGBT chips to generate controlled DC output. Rated output for this machine is 250A @ 30 VDC. The actually welding amperage range is 5-300 amps, wit maximum open circuit voltage at 95 volts. Typically you would set a maximum output current on a panel, then further control that output with a footpedal (full pedal = maximum set output current).
The control boards seem to communicate well with the front panel. There are no faults set. There are no signs of any component overheating, nor any sign of blown capacitors. Note: Factory repair is priced in the $1000 of dollars. My suspicion is some key high power components are not functioning correctly. I'd like to verify this, and I was hoping to verify this in circuit. I'm reasonably active on the welding forums and component failure is a known cause for non-function.
Tools I have..
- Old analog two channel oscilloscope, with new aftermarket probes (x10, x1), a device I have little experience with.
- Typical $100 multimeter, nothing fancy.
- Fluke 36 Clamp Meter (rated at 600v, 600A AC, 1000A DC)
I'd like to check the diode bridge rectifier, the IGBT chips and the output diodes in circuit. My initial thought was to just use the oscilloscope and look at the interim signals.
And then I pause, to think. Is this even safe? I don't want to hurt myself, or any tools or any devices being tested. Can I safely probe up to a 240 VAC signal with the oscilloscope? Is there an isolation device available? If not, is there an alternative way to read a full rectified 240v output, or troubleshoot a switching power supply board with the above rated power?
Best Answer
I used to repair welding machines for a living. First thing I would do was to disconnect the things from the power completely. Then I'd disconnect the big diodes and SCRs, etc. and just test them with a meter. 95 times out of 100 one or more of the big semiconductors failed open (you'll know if they fail shorted).
You might have to disconnect one side of them and it could get tedious with 3 phase welders but with such a ratio I considered it worth it instead of troubleshooting with wave-forms right off.