THCAD and Arc Voltages

Well, I'll attempt to keep this short and sweet. I did a lot of cutting recently and I keep having tapered cuts. It occured to me that I never double checked my THCAD with a known voltage to ensure that what is being displayed on PlasmaC represents reality. So, I did some testing and I got interesting results.

I should note that I am running a THCAD-300 with a Hypertherm PMX-65, and the voltage divider is currently set to 50:1 (default).

I guess before I get to the data, I should say that I had a THCAD-300 because I had an everlast previously and I measured raw arc voltage. I am not sure the error with such a wide range of voltage reading ability. I realize that most cutting voltage for the PMX is at most 150v, meaning at 50:1, the THCAD is reading up to 3 volts. It seems a very small percentage of the scale it is capable of, and therefore maybe is some of my problem. The other problem I have is that even at idle, my THCAD seems to fluctuate ~5 volts (on the high side).

At any rate, did some experimenting today. I left the frequency at 1/32. I hooked up a known voltage from a PC power supply, and monitored it with a Fluke multi meter. The entire time it read 5.249v steadily. I'm not going to try to type out the data in a table here, so I embedded a screen shot from Excel below:



As you can see, as the divider amount went up, so did the difference between the high fluctuations and the low fluctuations.

I guess I have a few options, I can either get a THCAD-5, and run 50:1, or I can grab the raw arc voltage from the machine and use the THCAD-300. Hypertherm does have a service bulletin for this.

At any rate, I wonder how to quiet down the THCAD as well. Why doesnt my voltage reading show a steady 0 when the torch is off, and why am I having a hard time getting it to show a steady 5.249v?

The last thing I noticed is that the plasmac.arc-voltage-out in the HAL Watch does not ever match what is being shown in Plasmac, when Plasmac was showing 6v, the pin was showing 4.98v, etc. Am I watching the wrong pin? Or is there something else going on here?

Sorry for being long winded. Please let me know if you need more data, or I am being a knucklehead

PCW wrote:

If you run a 300 V full scale THCAD from a 5V source you are throwing away
almost all the resolution. The THCAD has about 10 bits of full scale resolution
at a 1 ms sample rate (about 1/3 of a volt for 300V full scale) and this can be
increased by low pass filtering but if you only use 1/60 of the full scale range
you only have about 4 bits of basic resolution at 1 KHz, that is only 16 discrete
voltage readings...

This is a bit like trying to read 2.5V on the 1000V range of your voltmeter.
Note that you can modify the THCAD-300 for 5V full scale by replacing all 10
through hole resistors with 4.99K 1%

I sort of figured that was the case.

So what is better...
1. Read the raw arc voltage with the THCAD-300?
2. Read the 50:1 Arc voltage with a THCAD-5?
3. Read the 20:1 Arc voltage with a THCAD-10?

Also, is it even possible to make a THCAD-300 a THCAD-10?

Edit: Thank you for the explanation!

PCW wrote:

Yes you can make any input range you like by changing the through hole resistors.

All 300K = 300V full scale
all 4.99K = 5V full scale
all 10K = 10V full scale

These should be 1% or better resistors

Thank you sir!

Any recommendations on which scaling and thcad pairing makes the most sense? Is 1:1 always the best option? Or does it not really matter?

PCW wrote:

I would probably arrange things so you get about 200V full scale
So your 20-1 divider and a THCAD10 make sense
or a 50-1 divider and a THCAD5

Last question (maybe)... can you explain how the frequency changes things? Should I be shooting for 1/1? Or is 1/32 most appropriate?