Big Ohmic Problem ???

Hey, I've been battling an issue with probing for quite a while. In halwatch I can watch my voltage and it fluctuated a lot! It would jump around several times a second and fluctuate by up to 9 volts (this occured no matter if it was touching the material or not, and the torch wasn't even on). My thc did the same thing. I unplugged the "shield" from both thcad boards and the fluctuating voltage went from large fluctuations to + or - .05v. The other problem is that while cutting the voltage rises (faster when I'm cutting at 65 amps than at 45 amps). It will shortly get up to above the threshold where it will be above 23.5v full time whether it's touching the plate or not. If I unplug the work lead from the hypertherm, the voltage falls back to 0. It acts like it's charging a capacitor of sorts in the plasma cutter and slowly bleeding down. Any thoughts???

I just went through and grounded EVERYTHING. I ran a wire from my table to the metal mounting plate in my box and to electrical ground. I also grounded the box, I ran a wire from each shield on the thcad boards to the box and a wire from each shield to the v- on the 24v power supply that powers my Ohmic circuit. My negative ohmic wire is connected to the work clamp on my hypertherm.

The wires coming out of the 24v power supply caught fire this evening when I did my first test cuts. Any ideas?

If you have large voltage variations with THCAD shield grounded, this suggest that the plasma table/work ground is not the same as earth/frame ground.

If your 24V supply wires caught fire, this suggests that the 24V supply DC side
was not floating

Thanks for the response. The voltage variation was before I grounded the table. I got everything grounded today and the voltage variation went away. What it does now is maintain a charge after a cut. It can be at zero volts in an idle state before I fire the torch. If I fire the torch in the air it'll show voltage until the torch shuts off and then it goes right back to zero. If I actually cut with the torch. It will still show 22+ volts after the torch is done firing and just sitting there. It slowly bleeds down like a battery that is draining. The problem is when it ends up holding a charge above 23.5v my ohmic doesn't see it drop back below so it doesn't know where to pick up the material. What do you mean when you say "floating"?

Floating means there is no electrical connection between the 24VDC output and ground or line voltages.

I know that my ground clamp fell off while the machine was scribing. I don't have a relay hooked into my Ohmic set up so it has a connection 24/7. I wonder if it would have happened if I had it hooked into a relay so that it's off during cutting. I'm wondering if when the ground clamp fell off, it took the path of the wire from my frame to my to the metal panel in my box and from the metal panel to the shield of the thcad then to the 24v- of the power supply where those little shield wires were and that caught them on fire because it was too high of current? I put one of those tiny wires between the v- and v+ sides of the power supply and plugged it in to see if it would catch on fire (I removed it from my box first) and it just automatically shuts the power off to the power supply so it won't catch on fire if it grounds out. Should I have a relay that brakes connection to my ohmic wires when the torch fires so that if the ground clamp to the plasma cutter isn't hooked in for some reason that it can't take the path to ground through those smaller wires?

There is no way to get significant current in the 24VDC wires unless the 24V supply is not
floating (likely 24V common is connected to ground), or you have accidentally connected
one of the 24V leads to frame ground.

I would ask one of the successful users of Ohmic probing what specific power supply they
used

How are you doing Ohmic sensing?
Are you using a second THCAD in your circuit?
If you don't and do not have isolating relays, flames could be expected...

According to the diagram doesn't the 24v common have to go to the table ground to complete the circuit? I thought that all of the components and the table should be grounded to one another in a round about way. Is this not the case?

I am using two separate thcad boards for thc and Ohmic.

Use a Meanwell Hdr-15-24 power supply.

That's what I had, that's the one that had wires catch on fire. It melted part of the case in it so I ordered a different power supply to replace it. I ordered a mean well 20-24