Which THCAD to use for Plasma cutter without voltage divider

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10 Jun 2021 20:55 #211732 by alchemy
I'm starting a CNC plasma build and have definitely decided on Linuxcnc as my controller but am totally new to Linux so learning rapidly!
I'm about to order the Mesa 7176 and THCAD but not sure which THCAD I should use. My plasma cutter is a Razorcut45 which doesn't have a voltage divider so I gather I can either use THCAD-300 or make myself a voltage divider to use THCAD-5 or THCAD-10. I've read that with a suitable voltage divider I will get better resolution from the THCAD-5 or THCAD-10 by using the whole voltage range.
Any advice would be appreciated before I buy my THCAD.

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10 Jun 2021 21:12 #211734 by tommylight
Do not know about the plasma, some info would be helpful or a PDF...
THCAD300 = can be used with no divider, not for use with HV/HF pilot arc plasma (it can be used, but risk of messing up high)
THCAD 10 = can be used for all plasma, needs just adding a 1MOhm resistor to each wire between plasma and THC (2 resistors total)
THCAD 5 same as THCAD 10 but i do not know the value of resistors ( venturing a guess at 2MOhm per side)

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10 Jun 2021 21:52 #211739 by Clive S

I'm about to order the Mesa 7176 and THCAD


Do you mean a 7i96 - as a 7i76 will require a 5i25 or 6i25 Or do you mean a 7i76e ?

This might also be of interest to you linuxcnc.org/docs/devel/html/plasma/qtplasmac.html

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11 Jun 2021 01:26 #211765 by alchemy
I was thinking of 7i76e, but again any advice on this vs 7i96 would be appreciated too.

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11 Jun 2021 04:59 #211769 by alchemy
this is the plasma I'm planning on using.
www.xcelarc.nz/product/razor-cut-45-plasma-cutter/

I know it's commonly used with the Langmuir crossfire CNC with their own THC system and from what I've read this is connected to full arc voltage. I'm tending towards the THCAD 10 but I'm sure there are others in the forum with a lot more knowledge on this than me...

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11 Jun 2021 05:47 #211774 by phillc54
My preferences would be:

THCAD-10 - if you decide later to add a CPC port you can still use the same card.

7i96 - unless you think you may need more I/O or that you may need MPG's for jogging.

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11 Jun 2021 07:01 #211779 by alchemy
Thanks, sounds like a good way to go

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13 Jun 2021 03:11 #211912 by alchemy
I've ordered the Mesa 7i96 and THCAD-10 so am now definitely going down this route.

I've done some basic reading on voltage divider circuits and from what I can see if I use 2 resistors of same value I will halve the voltage but will need to reduce it a lot more to get down to the 10v max range. Can anyone direct me to a voltage divider circuit diagram suitable for converting raw arc voltage to a suitable range for THCAD-10. My first step will be to see what raw arc voltage my machine has when cutting.

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13 Jun 2021 05:40 #211914 by phillc54
I use a THCAD-10 with two 1MΩ resistors, one in each leg.

That gives a full scale reading of 210V and a divider ratio of 21:1

linuxcnc.org/docs/devel/html/plasma/qtpl...c.html#qt_mesa-thcad

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13 Jun 2021 07:22 #211922 by alchemy
Thanks, tommylight gave me the same advice, I was just confused as it didn't seem to match with what I'd found on voltage divider calculator.

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