GYS 40FV + THCAD-10 wiring

As a follow up question, assuming I'm correct in saying I was just using the multimeter incorrectly*, doesn't this imply if you disconnect the THCAD you're sending a high DC voltage into the cabinet? I get that the current is choked down a lot by the 2Meg of series resistance, but I'm not used to sending high voltages down signal cables. I'm sending it down a 0.5mm2 CY cable, which is rated to 300V/500V (the other 2 cores in the CY cable are the 4.3V torch on switch...).

*currently assuming I wasn't doing much voltage dividing since I effectively had the multimeter as the load (instead of the 100K of the THCAD) and it has very high impedance

Answering my own question, yes, I had neglected to actually complete the voltage divider circuit...I put a 100K resistor between the 1Megs on + and - and measured the voltage across it - World's most boring youtube video:



If the raw arc voltage peaks at (say) 300V then I'm assuming since I=V/R the cable is carrying 300/(2.1e6) =>0.14mA. As I was saying before, I'm not experienced in running large signal voltages* (and correspondingly low currents), so if anyone has any words of wisdom I'm all ears (I have googled around to no very satisfying results). I'm tempted to use a dedicated shielded cable inside the cabinet from the connector to the THCAD for this signal, or at very least run as a twisted pair.

*ok, only large when THCAD not connected assuming 1Meg resistors are inside the plasma chassis, which they are...and the THCAD should never be disconnected. Does it show that I did a CE marking course and am looking for fringe cases...?!

tommylight wrote:

I always use a shielded cable from THCAD to the electronics box.
And do not worry to much about the resistors, at 1MO each, not much current is passing through them.
Worry about the part before them, that should be properly wired and away from the HV/HF parts of the plasma.

The resistors couldn't be any closer to the points I've spliced in and are well away from everything

Another tip: connect the THCAD input shield to the + input so any common mode voltage from the plasma + (table)
drives the shield.

PCW wrote:

Another tip: connect the THCAD input shield to the + input so any common mode voltage from the plasma + (table)
drives the shield.

So float (NC) the cable shield at the plasma end, and jumper across +IN and SHIELD on the THCAD?

I have the THCAD in the cabinet and am running via a panel connector, so would it make sense to ground the shield on the internal cable inside the cabinet and carry the +IN to shield connection back to the external cable on one of the cable cores? Alternatively I can just bridge the respective cable shields.

I think the best would be to ground the shield at the plasma end
and then leave it unconnected at the THCAD end, then connect the +
wire to THCAD+ and THCAD shield.

PCW wrote:

I think the best would be to ground the shield at the plasma end
and then leave it unconnected at the THCAD end, then connect the +
wire to THCAD+ and THCAD shield.

Ah, gotcha...too clever!