Meas THCAD-10 noisy signal

Thanks Peter. Yes we have 50 Hz here. The multimeter reads a stable 96.4 volts on AC. I might grab a power resistor today and see if it stablises as DC with a bit of a load on it

Well this is strange. The noise is due to the grounding of the THCAD to the control case.

The lower amlplitude signal is without grounding. The large amplitude is connected to the case



Here is a close up of the transition. Very strange.


I'm certain this was not happening previously so something has happened to cause this. Any ideas?

For now, I won't ground the THCAD to the case.

I still think this looks like a missing ground
That is this is pretty much the signal you would expect if you measure the ground of an ungrounded
piece of equipment vs earth ground (basically the high frequency components of your line voltage)

The THCAD input should be wired

INPUT+ to plasma voltage divider low side
INPUT- to plasma voltage divider high side
INPUT shield to plasma source frame ground/ plasma voltage cable shield

Peter, this might be the case if it were a new installation but this has been in service for a while. On Saturday, the signal went from this



To this



You can see the plasma cutter is not even establishing an arc and retries a couple of times becasue it is not giving itself an ArcOK. Its not Gcode pulling the trigger multiple times.

Its really very frustrating and clearly nothing to do with your hardware.

All of the grounds are wired as per your instructions (and the manual) and have been double checked many many times.

The noise is there even if the plasma cutter is turned off but if the internal Plasma wires leading to the THCAD are removed, the noise stops. I kinda think something has broken in the plasma power supply.

I might double check the plasma power supply is still earthed to mains power in case something has come adrift.

Forgot to mention, the control box earth was in place in the previous images.

Andy thanks. I wondered about that too. I did check continuity from the plasma case to the appliance earth pin and it checked out OK. I might try another power point. I have to use an extension lead to get 15 amp power to my plasma. I might try a different power point and extension lead.

I also wondered if there was an earth fault in my house wiring...

Please keep the ideas coming as I'm feeling quite despondent here.

Maybe, maybe not. I did remove THCAD scaling resistor installed inside the cable backshell that doubled the sensing scale from 10v to 20v. I wondered if I had a dry joint problem so I resoldered the wire just in case. I will check there are no bare wires touching the shield.

I'm pretty sure I saw the fault before I changed this cable.If I put the resistor back, it will be at the control box end where its easy to get to.

Check for voltage differential between grounds with your multimeter and/or scope.

Also check reistance between grounds.

Long shot.