GYS 40FV + THCAD-10 wiring

Where would it be recommended to tap into this plasma cutter to gain the arc voltage signal? I have only partial understanding of this machine, so am not confident where is common (and don't really want to test my theories with live voltage!). Am I simply after HT- and HT+, and connect wherever convenient? If they are equivalent I'm tempted to use piggyback spade connectors on the HT-/HT+ to the right of the photograph/schematic for reversibility. I don't want to mess with the (assumedly) higher current connections at the front of the machine if I can help it.

I have 2W 500V metal film 1 megaohm resistors to stick on the negative and positive sides, which I will fit inside the case, sending out only the reduced signal to the THCAD which is mounted in the control cabinet.

My plan is to run the switch and signal through a single multi-core 0.5mm^2 CY cable back to the controller - I assume that's ok...let me know if that's a bad idea!

I have verified the switch on the torch is doing what I expect and it is simply connected via spades which I can nicely piggyback off (or discard the torch switch altogether). I'm not sure what the 'safety' circuit which also goes out to the torch is doing though - initially I thought the torch had parallel NC+NO switching (or possibly a switch in the trigger guard), but actually the safety pair of wires just appears shorted irrespective of the trigger state.

Thanks in advance

I have added some labels on the attached image - it lines up well with the schematic.

Thanks

I've added a detail shot of the torch black cable meeting the circuit board - do you think I can use the unoccupied male spade connector there? It appears (and tests as) common, and looking at the circuit board it appears to share the massive trace, but I also get continuity with the red so quite paranoid...!

Awesome, I've been putting this job off for a while, but now confident enough to get on with it

That's why I was putting it off!

I think the surgery was a success - I've got ~2 meg across my signal output and the switch seems to function. Verification may have to come later, I'm not really set up for a cut.

I'm getting ~10x the voltage I was expecting - the test here is just a multimeter, with me hand-cutting a bit of thin scrap metal (using a stand-off to set the cutting distance).

I checked if I had omitted a resistor, but that was not the case. Both resistors read around 0.99Meg. Is it because the multimeter has a crazy-high impedance, i.e. I need to connect a 100k resistor across the +/- and read the voltage across that?



The remote switch seems to function anyhow - about 4.3V across that (haven't checked current yet).