Converting a THCAD-300 to a THCAD-10?

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01 Apr 2020 23:01 #162398 by Uthayne
My first plasma cutter was a pilot arc, cheap chinese unit with no divided voltage output. Since now I am using a Hypertherm Powermax 45xp, I would like to convert my THCAD-300 to a THCAD-10 to get better use out of it, if possible.

Can this be done? Is it just as simple as replacing some resistors on the input side of the card?
I have an oscilloscope and I can recalibrate the card afterwards.

Reading the docs, it seems like the current to voltage chip is the only difference between the -5 and -10. Both resistor networks are 100kOhm.

The input circuitry of the THCAD-5 consists of a current to voltage
converter with a 50 uA full scale range. The input circuitry of the THCAD-10 consists of a
current to voltage converter with a 100 uA full scale range. The THCAD-5 and THCAD-10
have 100K of total input resistance in their input divider section giving them a full scale
input ranges of 100K*50uA = 5V for the THCAD5 and 100K*100uA = 10V for the THCAD10.

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01 Apr 2020 23:28 - 01 Apr 2020 23:30 #162407 by PCW
The 10 through-hole resistors on the THCAD-10 and THCAD-5
are 10K .1% . The 10 through hole resistors on a THCAD-300
are 300K 1% HVR (3500v rated)

The THCAD-5 input current to voltage converter feedback resistor is 49.9k
0.1% The THCAD-10 and THCAD-300 feedback resistor is 24.9K 0.1%
Last edit: 01 Apr 2020 23:30 by PCW.
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01 Apr 2020 23:38 #162409 by Uthayne
Excellent, thanks for this information!

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04 Apr 2020 16:45 #162688 by Uthayne
Just following up, I switched out the 300k resistors for 10k resistors and recalibrated the board. Seems to be promising with the frequency values I measured.

0v 118.2kHz
10V 920.8kHz

Took a measurement at 7.0V and it was spot on as well.

Not the prettiest soldering job...

Thanks PCW!
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04 Apr 2020 18:44 #162696 by jay1st
why all the kapton tape ?

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04 Apr 2020 18:50 #162697 by Uthayne
I previously had this mounted inside the plasma cutter and didn't want it shorting against anything
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04 Apr 2020 20:07 #162700 by Chris1978
Hi, having the same problem, want to convert my THCAD 300 to a 10, ive got the resistors but dont know how to recalibrate, how did you recalibrate yours?

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04 Apr 2020 20:24 #162701 by Uthayne
I used an external power supply and an oscilloscope, but you could use a 9 volt battery and halscope. Here's someone that did just that
forum.linuxcnc.org/plasma-laser/35065-thcad-10?start=40
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05 Apr 2020 14:01 #162769 by Chris1978
Hi,

Im ready with soldering, is a Fluke 179 Multimeter also okay for calibration?

And for Calibration i connect my multimeter on FO+ and FO- power up the THCAD 300 (now 10) and connect the powersupply with 10v on IN + and -?

sorry for my stupid questions, but this is my first project with linuc cnc and mesa components, also my english is not the best and i dont realy know how to config everything in this HAL thingys, but i read as much as i can in this forum.

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05 Apr 2020 14:32 #162774 by Uthayne
You also need to supply +5V to the THCAD on the low voltage side (it's marked). The other connections are correct. Pay careful attention to the 10V signal you're connecting to IN + and -. It doesn't have to be exactly 10v (a 9v battery will do, but may hover ~9-9.5v), but you need to use that voltage your calibration calculation. Use linear interpolation/extrapolation to calculate the frequency output at 10v. You'll know you're in the ballpark if you get similar values to the original sticker on the -300.

Then you can use this calculator to set the scale and offset values for plasmac. jscalc.io/calc/NTr5QDX6WgMThBVb

The Fluke 179 says it can measure up to 100 kHz signals which won't work with the card while in F/1 mode. You'll have to change it to F/32 to get the signal slow enough to what your meter can read.
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