New Project with Mesa 7I96 and THCAD-05

The relay circuit will have a much higher (say 20X) sense current threshold
than the 24K resistor +THCAD, so if there is very significant leakage current
it would be expected to work better. A lower value sense resistor would
do the same thing while retaining the adjustable threshold , filtering
of the THCAD circuit.

That said, if the relay circuit never doesn't miss
a probe contact event because of high contact resistance, then the
THCAD circuit is unnecessarily complex for that setup.

PCW wrote:

That suggests leakage currents as I mentioned. That is if you measure the voltage across the sense resistor with a DVM, you will see that the THCAD is correctly measuring the voltage which is registering the increasing leakage currents.

I am sure that the THCAD is reading correctly.

So if I understand correctly, you are saying that making the resistor value much lower may make that leakage problem go away?

Lowering the sense resistor value doesn't make the the leakage problem
go away it just makes it harder to saturate the measuring circuit

If fact a relay circuit could trip on leakage current, it just has a higher threshold

If you lower the resistance you need to increase the power rating of the sense resistor
to accommodate full plasma voltage or add diode isolation

I'm not sure I understand then what is causing said leakage, other than electrolysis? 

That would be my guess

PCW wrote:

That would be my guess

I always figured it had something to do with providing the power supply with the torch voltage. I do know that with the diodes in line in the orientation shown in the diagram, it doesn't happen.