Tips diagnosing plasma cutter tripping breaker at power on

[Signal32]

Hello,

Have a cheapo 50A plasma cutter with pilot arc. All of the sudden it started tripping the breaker at power on (with no load applied). Happens at 110V and 220v input.
If I disconnect one cable from the main power supply to the mosfet / diode board, then the device works (display is fine, air engages) except doesn't deliver any current and pilot arc doesn't work.
Nothing on any board seems blown, I've done basic tests on the MOSFETs and diodes in-circuit and all appear fine.
Seems maybe like the MOSFET switching circuitry is not working causing lots of power draw which trips the breaker.

Any tips for troubleshooting this ?
Pictures: https://imgur.com/a/h7gNpV3

Thanks!

[rob77]

connect a big incandescent lightbulb in series with the device. if it stays fully lit something is shorted

[strawberry]

common faults for all these things :
-relay welding is usual (weak relay and ill-timing cause it to weld)
-bridge rectifier (factory defects + voltage transients)
-MOSFETs/IGBTs usual (main reason is small heatsink + buggy protection)
-rectifier rarely
-AUX supply usual (factory defects)
-PCB rarely (factory defects)
-resistors sometimes (resistance doubling... for no reason. factory defects)
-power cord strain relief usual (compress cable to the point it shorts line)
...

in couple years repaired some hundreds
similar schematics to welding inverter but need to know how it operates and what it should be or not
need do full inspection/repair otherwise it will soon lead to the next failure/repair and start it all over again with much more blown parts

LEDs are to look fancy or for some factory check. I assume that one red LED means there is 9..24V on AUX supply rail depending on brightness

[Stray Electron]

  The first thing that you should check is to try it on a different AC circuit or a different breaker. Most breakers get weak and will carry less and less current if they're tripped too much.  If you have another breaker in your box that is the same size, just swap them and see what happens. Or go buy another breaker and you can keep it as a spare if the old one proves to be ok.

  You are using properly sized plugs, outlets and WIRING aren't you?  I've seen a lot of amateur welders install larger breakers and try to use circuits that weren't built to carry the loads that they're putting on them.  That's a good way to burn your house down! 

   In every jurisdiction that I know of, it's illegal to install a larger breaker than what the wiring and outlets are designed for for exactly that reason.

[Signal32]

connect a big incandescent lightbulb in series with the device. if it stays fully lit something is shorted

Good idea ! I can try this with a heater. But won't this possibly damage it further ?

  The first thing that you should check is to try it on a different AC circuit or a different breaker.

Already tried on 4 different breakers - both 220 and 110v - same thing. Also this happens at no load where it should be drawing like 0.2A. No mismatched plugs or other funny business.

in couple years repaired some hundreds

Thanks for the tips, I guess I need to fully disassemble it.  Curious, how long does it typically take you to repair one ?
 

[rob77]

connect a big incandescent lightbulb in series with the device. if it stays fully lit something is shorted

Good idea ! I can try this with a heater. But won't this possibly damage it further ?

it should be a light bulb (non-linear resistance) , and no it will not damage it further. the light bulb will limit the current.
when cold the lightbulb has low resistance - so it will allow inrush current, but if the current stays high, it will heat up the filament and resistance of the filament goes up limiting the current to the tested device.
it's a pretty common practice when repairing switch-mode power supplies with unknown fault.

[PaulAm]

In every jurisdiction that I know of, it's illegal to install a larger breaker than what the wiring and outlets are designed for for exactly that reason.

Depending on where you live, there is a very specific exception to this in the US for manual welding machinery.  You have to look for specific details in the code, but the conductors can be sized  for 50-70% of the equipment's rated draw.  This is also coupled with labeling requirements and a requirement to downsize the breaker if any other use is allowed on the circuit.  This generally only applies to larger equipment.   Typically, a 100A breaker for a welder could use #6 instead of the usual #4 or #2 (depending on wire, insulation and temp rise).

Something like a plasma cutter is generally too small to bother with this.  I run mine off a standard 240V 30A line with #10 conductors.

Something must be going around, my plasma cutter died the other day.  It's waiting for me to open up and check it out.

[Stray Electron]

   There's no exception is the US for using breakers that are larger than the rated capacity of the wiring or the outlets.

   The exception that you're referring to is to allow the use of welders (and some other equipment) that draw slightly more (20% IIRC) than the rated capacity of the breakers, wiring and outlet. But the breaker, wiring and outlet must still be matched in terms of current capacity. The NEC allows this  specifically for welders because they are only run for short duty cycles.

[strawberry]

ARC200 ZX200 ...
https://elektrotanya.com/EasyArc_zx7-200_igbt_inverter_welder.pdf/download.html
spareparts:
one similar unit could be differences in control circuit https://www.aliexpress.com/item/32908913945.html