THCAD-10 + hypertherm 45 (nonXP)

31 Aug 2017 21:13 #98312 by robertspark
robertspark created the topic: THCAD-10 + hypertherm 45 (nonXP)
Hello, this seems as good a place as any to ask this one regarding the THCAD-10 interface board with a hardware / voltage divider question.

The Hypertherm 45 (nonXP) has a 50:1 voltage divider output as standard, although if you look at the Service Manual Schematics at the back of the manual, the voltage divider output appears to be 51:1 to me (won't be the first / last time I get something wrong so I may be wrong here).

The voltage divider is a 100K resistor, and a second 2K resistor listed on the schematic with the divided voltage then directly routed to the CPC [14 pin circular plastic connector?? TYCO) machine interface output.

The Operator Manual lists the open circuit voltage as potentially 7 volts at the CPC (although the Rated Open Circuit Voltage (RAW) in the operator manual is 275 VDC on a single phase supply)

I've been told that the THCAD-10 was designed for 20:1 voltage divided output, although this just does not ring true as all of the other machine interface Hypertherm products do in fact have default 20:1 divided outputs, but their open circuit voltage is potentially 18 Volts under open circuit conditions as listed in the manuals and the THCAD is 0-10V unipolar.

So, short question, what are others doing?

Are you running a 20:1 voltage divider output, if so, why? (given the normal working range of the plasma cutter voltage is about 75 - 145V, at 20:1 this would give you 3.75 to 7.25V, but anything above 200V would be clipped?

Given the PMX 45 has a 50:1 output, and the brunt of the current (and therefore power dissipation) is taken by the provided 100K resistor, is my math right to swap out the 2K resistor, and install a 3K3 resistor in series with the 2K resistor and I should get ~ 20:1 (not quite as the resistors I have to hand are 5% tolerance), but I will do my "tuning" on the software side with a calibrated input, given I'm going to struggle to calibrate the output unless I went with say a 200V zener and rectified mains to DC, filtered it, and added zener for calibration although they also have an inherant tollerance (+/- 5%) 1N5388B, where as I would use a 3K9 resistor and a 2K multi-turn pot to tune the output to 20:1..... but I don't think I'll get better than +/- 5% as I don't have a calibrated instruments

Thanks for your time and guidance

01 Sep 2017 04:54 #98321 by rodw
rodw replied the topic: THCAD-10 + hypertherm 45 (nonXP)
Rob, the THCAD-10 can be scaled with an external resistor on the input side very simply. The instructions for calculating the resistor value are in the THCAD manual. Whilst the example is for 500 volts, it works equally as well scaling to a 20 or 25 volt range. I scaled mine on the assumption that I had a 16:1 resistor to a 20 volt scale by adding said resistor inside the plasma interconnect cable backshell.It ended up being 25:1 so my scaling resistor needs revisiting. A 1/2 W resistor is all you need.

I think I saw that the 50:1 Hypertherm at full scale was 7 volts. Even at this level, the THCAD-10's 10V native scale will still perform well based on my experience with an incorrect scale. While cutting on my machine, the cut volts is scaled to about 2 volts out of 10 volts and its still quite accurate.
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