Ohmic probe interfacing

[chainds]

I am not sure what forum to put this in, because it is in between the torch and the control electronics. What do people do with respect to actually hooking the shield on the torch up to the BOB input pin when implementing ohmic probing? It seems to me that there is potentially bad things that can happen that might allow the negative voltage on the plasma machine power supply make it to the input pin. How about isolating the pin with a relay during cutting? External zener? I am thinking it would be good to have something that is less expensive to replace than the BOB. I cannot find any discussions addressing this, nor have I ever ran across one. Does anyone have thoughts or inputs? Thanks!


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[rodw]

Try the circuit in this article I wrote for the LinuxCNC documents

Plasma Cutting Primer for LinuxCNC Users

You need two pins as we keep the power totally disconnected unless we are probing.. Works a treat! Good luck

[Phokem321]

Hi . Dear I have hyd thx with 125A hypertherm . HYD got E3 error.can you tell me what is this about ?

[Cole2534]

Try the circuit in this article I wrote for the LinuxCNC documents

Plasma Cutting Primer for LinuxCNC Users

You need two pins as we keep the power totally disconnected unless we are probing.. Works a treat! Good luck

Rod, thanks for that write-up it was very helpful.

I have a followup question- my breakout board has but one output relay that I'm using to fire the torch. Would it be safely possible to use a 3 pole relay to interlock the probing circuit from the torch circuit? Tie probe leads to a pair of NC contacts and the torch to the NO contacts. Thus the probe would be hot when the torch is off and isolated when the torch is on? The circuit would certainly work, I'm just not sure if it would provide ample isolation.