I’m in a kind of similar situation, I have the THCAD-10 and a plasma cutter without voltage divider so am planning to use resistors on wiring from plasma to the THCAD-10 to give appropriate voltage reduction. I’m not an expert in the electronics side of the build but have read several threads on this on linuxcnc forum and elsewhere. Will post my progress when I get to this stage.
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IIRC you can change out some resistors (of the proper high voltage tolerant variety) on the THCAD to change the input range. I probably read that over in the linuxcnc forums.
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OK, so I got a reply back from MESA Electronics. Peter from their company sent this information in response:
You could return the THCAD2 card for credit and re-order a THCAD-300
Another option would be to add an external resistor to the THCAD2 in the negative lead, say 2.4M for 250V full scale.
For safety this should be a HV resistor like:
HVR3700002404FR500 Vishay Beyschlag/Draloric/BC Components | Resistors | DigiKey
The 250 range improves the resolution somewhat (and continuous overvoltages up to 1000V are no problem)
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I went ahead and ordered some of the HV resistors recommended by the rep from MESA (2.4 Mohm for 250V full scale).
Note: the cheapest shipping was $5, while the resistors were only $0.52 each, so I ordered 4 of them. With tax and shipping came to $7.50 or so.
I’d have probably spent some amount for return shipping to re-oder another THCAD, so I figure this approach is as good as the other. Thanks, all, for being so helpful!
PS: If any of you want one of these resistors, I can probably sell and ship one of mine cheaper than you could buy one by itself.
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I understand Peter IS Mesa… owner, engineer, wearer of all hats. He’s also PCW over at linuxcnc.org and is very responsive. You’re in good hands.
Also, be careful!
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@kd2018
Hey, quick question, when Peter says
…does that mean I can just add the resistor to the wiring on its way to THCAD2, on the negative side? Or is he talking about me modifying the THCAD2 itself? I feel reasonably comfortable with the former, and not at all comfortable with the latter.
I would ask Peter for clarification. I have thoughts/questions of my own but I don’t want to muddy the waters… especially in this instance.
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OK, I did ask him, and he wrote back with this:
Also, below is a video showing a review of the Hynade CUT60DN plasma cutter I bought (just arrived yesterday, have not had a chance to video an unboxing of it). Below that is a link to the cutter on Amazon.
Affiliate link to the Hynade cutter on Amazon:
https://amzn.to/3ODO2Zp
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Sounds like what I’ve got planned for mine. I haven’t sourced resistors yet but since I’m in NZ it’s probably not going to be cost efficient to get you to send one to me.
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Also Peter has recommended in the past that if your wiring into a plasma machine without a voltage divider that it would be a good idea to mount the thcad to the plasma machine, or inside it if there’s space. And then run leads from the thcad output to your control box. If I remember correctly I think the idea is it’s just safer not to run that raw 100-200v to your control box.
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Thanks, that’s good advice! Certainly sounds wise to me, though I would not have thought of it on my own.
Hey quick question. Wouldn’t putting the resistor either inside or close to the plasma cutter then mean that the rest of the run of wiring to the control panel would no longer have that high voltage? If so it wouldn’t be absolutely necessary to put the THCAD at the plasma cutter. If not and the voltage is still high in the wire regardless of whether the resistor is in place then I would have to go ahead and put the THCAD on the plasma cutter side.
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I’m not electrical engineer but I seem to remember from physics class that an inline resistor would just reduce the current, you’d need a couple or three resistors to create a voltage divider circuit if you wanted to reduce the voltage.
(Edit)
Wait, there’s voltage drop across the resistor right? I don’t remember the math. Probably an application of ohms law or something…
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I am gathering two things. 1. A floating Z is necessary with plasma for touch off reasons, correct? 2. the design here seems to be for LR2, and I’m working from LR3 platform. Any advice for me on a floating Z design for LR3?
Confession: I feel a bit sheepish because I designed a torch mount in SketchUp and even have it almost done printing, and just started reading more and realizing I guess I needed to have incorporated floating Z into the design.
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You need a float switch OR an ohm sensor for touching off and finding zero… Rather you need some way to find zero before each pierce. Metal moves a lot as you apply heat and releive internal stresses. And depending on your qtplasmac settings a pierce hight might influence the cut height (voltage setpoint) and cut quality.
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OK, thanks. I have to go back to the drawing board. I already designed and printed a torch mount made for magnetic breakaway feature, but I was not taking into account the need for floating Z. I understand now what needs to happen (switch and all).
If you know of any designs for doing all this with a LowRider 3, please let me know. Thanks again!
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I used ohmic, it would be nice to have some give still in case you try to touch off on a dirty spot or get a bunch of gunk on your tip.
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I don’t, I haven’t built the lr3. If you search for floating plasma torch mount over at thingiverse you’ll get some ideas.
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Aaaaaannd no less than 3 models that I had spent several hours on have suddenly disappeared from “the cloud”… One of them a slow work in progress of over 5 months.
Don’t mind me, I just felt like bitchin.