THCAD-10 series resistor values

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23 Jun 2022 15:36 #245692 by chowderhead
Hi, All:

I'm planning on using a THCAD-10 on a plasma welding (not cutting) system that applies a 6kV HF start and is expected to run ~20V during welding. I'll be connecting directly to the power lugs on the welder and read here that 1M series resistors are used in both positive and negative circuits for plasma cutters, which apply ~15-30kV startup potentials. I have two questions, one is to satisfy my curiosity, the other practical:

1) How do the 1M series resistors keep the THCAD-10 from frying when there's 15-30kV across the inputs?
2) Should I use 500k resistors since only a 6kV startup potential is applied on the welder? I want as high resolution as I can get.

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23 Jun 2022 16:17 #245696 by tommylight
Plasma welding?
In that case we need to know how the pilot arc works exactly and what does it use to strike the arc.
Plasma cutting strikes the arc between the nozzle and the electrode on inside the torch, while cutting is done between electrode and material. Meaning the THCAD never sees the full brunt of the 20KV ignition on both leads, hence not a "big" deal to protect from it.
Tig pilot is between electrode and material, so is the welding, so attaching the THCAD to it would most probably mess it up, although i never tried it, well yet! :)
All that out of the way, there is always a way to wire THCAD safely to all those, wire it directly to the rectifier output inside the box/welder/plasma/tig, as all of those must have protection against those 6 to 20KV going back to the rectifier that is usually rated at 1000V.
I have several (6 or 7 or 9 or...) of THCAD300 working with high frequency/high voltage start plasma's, despite all the warnings that they should not be used in those cases, they are all fine and dandy working 10-12 hours a day, every day, except for some of them on Sundays. They are all wired to the rectifier.

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23 Jun 2022 16:18 #245697 by tommylight


Thanks for all your hard work - LinuxCNC is a gift to humanity!

I wholeheartedly agree.
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24 Jun 2022 14:56 #245792 by chowderhead
Ah, hadn't thought about where the arc is started. The system I'm using starts the arc inside the nozzle then transfers the plasma to the workpiece when welding power is applied, which sounds like the same method used for plasma cutting.

I like the idea of connecting to the rectifier. Can you provide guidance on identifying it? I have no schematics.

And, seriously, LinuxCNC is awesome, second only to Debian in my list of favorite FOSS. I love working with it and am regularly blown away by how flexible it is.

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24 Jun 2022 16:16 #245796 by chowderhead
I think I found the rectifier. Right after the transformer, stuck to the biggest heatsink in the box. Even has a couple of nice taps for me to use.

Just to confirm, to maximize resolution, the series resistance should be set per the equation in the THCAD manual for the welder's open circuit voltage (65V), yes? Is it necessary halve it and install a resistor on each of the legs?

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24 Jun 2022 16:17 #245797 by tommylight
Take some pictures of the insides and post them here.

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24 Jun 2022 16:36 - 24 Jun 2022 16:42 #245799 by chowderhead
There's only a top cover, so here's a top view. I think the rectifier is the at the top of the pic, the board with the blue wires attached to it above the transformer.

 
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Last edit: 24 Jun 2022 16:42 by chowderhead. Reason: Add picture?

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