PlasmaC Guide Voltage Divider Question for Mesa THCAD-10

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23 Apr 2020 15:28 - 23 Apr 2020 15:28 #165268 by dvn4life1972
Hello everyone. New to the forum and absolutely new to building any sort of CNC machine. I'm a welder and fabricator by trade, so the actual build portion has gone relatively easy, but the electronics aren't (and I fully acknowledge my ignorance there).

My question is this: in the PlasmaC guide it states that typically for voltage dividing purposes, just run a 1 megOhm resistor from raw arc + to ve+ on THCAD-10 board, and the same for the - side. Mathematically doesn't that only reduce the input voltage by half (max raw voltage 210 per PlasmaC guide/2 = 105V)? I see all over the place where it is stated that this will not harm the THCAD-10, but will it work for the THC?

For background info the plasma I'm using is a Hypertherm 1000 g3, which has no voltage divider native to the machine.

Many thanks for the wealth of knowledge on the forum here. This is the first of many questions I will have, as Linuxcnc has me baffled...but I need to get everything ready to plug in first.

Pat
Last edit: 23 Apr 2020 15:28 by dvn4life1972. Reason: Typo'd

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23 Apr 2020 15:57 #165269 by tommylight
Welcome.
To be blunt, make it work and do not worry about those things, all in good time.
As for the math regarding THCAD, even i leave that to the master who goes by the nic PCW who is the owner of Mesa, He is by far the most helping person on earth and he is around here a lot.
As you may have noticed from reading this forum, we have built many plasma machines, especially in the last two years, but as usual, most of those who get the machine working do not bother to come back here and show what they are doing with it, but some do.
One thing to remember with almost any plasma cutter, the place where to get the wires for arc voltage to avoid having high voltages messing things up. Usually, the output from the rectifier is the best place for avoiding such issues.
And just in case, Mesa makes also the THCAD300, so it does not need resistors.
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23 Apr 2020 16:11 #165270 by dvn4life1972
Thanks for the reply Tommy!

If I'm not mistaken, PCW is the one who authored the guide I was referring to...I'll double check, but in any case if it was him that should be good enough and I'll run with it. I already have the THCAD-10 board, so I don't want to be in the situation where I need to obtain the other one (resistors are dirt-cheap).

I'm totally willing to share any part of my build. I wouldn't think it would be very helpful to others at this stage, as I am learning from scratch about mostly everything involved (except the basic fabrication, i.e. fit-up and welding the table frame, etc).

I'm using some openbuilds v-slot but have it fixed to a heavy steel square-tube frame to lend a lot of extra rigidity. I have no idea how effective or ineffective it will be yet.

Pat

Pat
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23 Apr 2020 16:29 #165271 by tommylight
I think PhillC and RodW wrote most of PlasmaC guide, of course with the help of PCW and some others. i will leave it to Rod to mention that it is the most comprehensive guide by far, and he is right.
THCAD will do perfectly.
As for rigidity, there are two parts of a plasma cutting machine ( i always build them separately ), the machine part that has the rails and moving stuff that needs no particular rigidity and can be made form anything from 40X40mm metal tube for small ones, or a bit thicker ALU, and the table part that needs to be able to withstand countless drops of material onto it without bending, hold a lot of water for water tables and uses thin slats as to not interfere with the plasma stream on the underside while cutting. This part i always make as rigid and heavy as possible, heavy to prevent it from moving while setting sheets on it.

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23 Apr 2020 18:18 - 23 Apr 2020 18:19 #165283 by rodw
That's Phils handiwork in the Plasmac user manual.

The value of the resistors is based on a formula in the THCAD manual on Mesa's web site. It does not matter if the resistance is on one leg or two but its safest to split the required value equally on each leg to reduce the current to non lethal values.

There is more information in the Plasma Primer which I put together which is more of a constructors guide than a software manual.
linuxcnc.org/docs/devel/html/plasma/plas..._with_the_mesa_thcad
The primer is intended to give you the background you need to build a plasma machine and gives some info on choosing the correct thcad based on a forum discussion which included input from PCW. Persoanlly, I think the THCAD-10 is the way to go becasue you have total control
Last edit: 23 Apr 2020 18:19 by rodw.
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23 Apr 2020 18:44 #165286 by dvn4life1972
Here is the portion I'm referring to that has me confused...

THCAD-10

If connecting to a plasma CNC port then the divider ratio is selected from the plasma machine.

If connecting to the plasma machines full arc voltage then a common setup is a 1 megOhm resistance from arc negative to THCAD negative and a 1 megOhm resistance from arc positive to THCAD positive. This gives a full scale reading of 210V. The THCAD can handle over-voltages up to 500V so this is not a problem.

To get the divider ratio use (R1 + R2 + 100000) / 100000 so for the above it would be (1000000 + 1000000 + 100000) / 100000 giving a ratio of 21.

...

This would cut the voltage in half iirc (two equal resistors give half of input voltage). I thought it needed to be 0 to 10 volts at the card.

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23 Apr 2020 19:23 #165292 by tommylight


This would cut the voltage in half iirc (two equal resistors give half of input voltage). I thought it needed to be 0 to 10 volts at the card.

In this case that cuts the voltage by 21 fold.
And the "two equal resistors give half of input voltage" is very wrong. Resistance is calculated with regard to voltage that has to be dropped through it and the current passing through it. From those two values the power dissipated by the resistor can also be calculated.
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23 Apr 2020 19:25 #165293 by tommylight
Just for clarity, two equal resistors wired in parallel give half the resistance! Notice wired in parallel, not on different wires.
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23 Apr 2020 19:26 #165294 by dvn4life1972
Thanks! As I said before, I absolutely know I'm ignorant about these things. I saw somewhere else online that statement about the half output voltage thing.
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23 Apr 2020 19:41 #165297 by tommylight
You are welcomed, always.
And no worries abut that, i was pretty sure you read that on the net, frankly i am getting tired of how much bad info is on the net, and this is the second time i am mentioning bad info today.
Another info on the resistance, just in case, putting two resistors together in parallel will half the resistance but it will double the voltage IF the current passing through them stays the same, but it never does, it will also go up.
Again, please do not worry about such things, we are here to help, and we do it gladly, so you are welcomed to ask anything, even stuff not related to CNC on the general discussion thread.

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