5x10 Home built Plasma CNC - in progress - Mesa 7I96, THCAD

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22 Dec 2021 11:16 - 22 Dec 2021 11:17 #229715 by snowgoer540
If you are using a power supply for THCAD ohmic sensing (really any ohmic sensing method), then it should be a stand alone, fully isolated power supply.  All of the diagrams here show a THCAD-5, but it can be done with a THCAD-10 as well, just would require a different scaling resistor.  So, I would match the model you used for arc voltage as you would have a backup if it ever stopped working.

That said, overall, I would advocate for doing your own research and drawing your own conclusions.  Plasma is more complicated than it seems, and there is no one size fits all solution to anything here.

For example, if you choose to use a pull-down resistor, why not calculate the correct value for your plasma cutter rather than blindly following values that were used for a setup that does not mimic yours?  Here is a good resource for another method of contact loading, as well as an example of how to calculate for your setup:  linuxcnc.org/docs/devel/html/plasma/qtpl...html#qt-contact-load

Phill also did a write up on THCAD vs Relay ohmic sensing:  forum.linuxcnc.org/38-general-linuxcnc-q...-05?start=460#216759

Several of us have moved away from "hypersensing" in favor of relay ohmic sensing to the diagram shown in the PDF.  The setup is way less complicated especially for someone new to the scene, and there is way less fiddling to get it to work, no components to install, it's cheaper, and its not bothered by noise/interference, and I've found it to be way more reliable, etc.

Either way, it's an enjoyable journey ( most of the time :laugh: )
Last edit: 22 Dec 2021 11:17 by snowgoer540.

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22 Dec 2021 11:24 #229717 by rodw

well I could easily change out the 12v power I put in for 24v power. I just picked 12v from inexperience. a 24v power supply will run all my npn stuff. easy peasy switch out, it will also fix up the issues you mentioned of being more reliable and less noise prone..

The pull down resistor just goes in parallel with the everlast for arc ok? or in series.

So fortunate to be able to stand on the shoulders of the giants that came before us and not have to learn this stuff the hard way.


I had to learn it all the hard way! And man it was hard!
the pulldown Resistor goes from the input to field power negative. (or to 0 volts for the purists)

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