New Plasma build question

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22 Jun 2019 16:15 #137598 by pl7i92
Replied by pl7i92 on topic New Plasma build question
the tourch we use on 5 Plasmas here is a P80
cheep very good spare parts and nice to handle
there are 2 versions for connecting to the standard plasma cables
in length up to 25M

very good nice stuff
the nozzle hold for about 2500meter cut at 45A on 3mm sheets (most straight cuts)
at 1.5USD

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22 Jun 2019 23:29 #137617 by tommylight
If you do not mind messing with resistors and making a divider, do get the 10V.
Also if you plan on getting another plasma in the future ( no matter what type ) get the 10V.
If you plan on using that same one till it dies ( it will ), get the 300, but that would bring us back to point 2 of this discussion.
Peraonally i got the 300, add two wires and be done with it. I have no problem building a THC for 15 euros and a lot of time, but time is much more expencive, and i have also goten very fond of Mesa cards.
After dealing with all sorts of electronics and the usual " that is not possible, this has to be exactly this way, you can flash the firmwarw but you need a 2000$ device and there is no ready firmware so you have to make your own, etc etc etc ",
-Mesa cards are a blessing, and PCW is awesome.
So let me use this opportunity to say, PCW, thank you for everything.
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23 Jun 2019 07:25 #137640 by Clive S
Replied by Clive S on topic New Plasma build question
Thanks Tommy.
But I don't have a clear answer to:

But I this point I am not sure where to get the voltage from in the machine ie is it straight from the torch output terminals on the machine ie +ve and -ve and route those to a metal box with the THC in it and fitted to the case of the plasma machine.

Or do I take the cables via two resistors one in each leg to minimise the current to a few mA something like 15k-20K


I know this cut 40 will die:( , so according to what you have said the THC 10 is the way to go:)

Could you please draw a simple sketch on how the THC 10 is connected to the machine .

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23 Jun 2019 20:21 #137667 by tommylight
It will look ugly !!!
Back in a few....

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23 Jun 2019 21:34 #137675 by tommylight
Here it is, a rough outline, PCW should add the values for the THCAD10, i did not want to mess something in the calculations. PCW, thank you.



Note: R4 can be removed, it is there for fine adjustment just in case you want to have the exact voltage shown in the Linuxcnc GUI. That would require measuring the voltage at the plasma terminals, something i never recommend due to it's inherent danger.
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23 Jun 2019 22:08 #137679 by Clive S
Replied by Clive S on topic New Plasma build question
Thanks Tommy
So would you put these resistors as close to the plasma terminals or in the metal box holding the THC screwed to the case of the cut40 ?

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23 Jun 2019 22:33 #137680 by PCW
Replied by PCW on topic New Plasma build question
I would at the minimum put R1 and R2 (and perhaps R3) at the plasma supply itself

If you use a THCAD10, I would probably just use R1 and R2 and not use R3 and R4

If R1 and R2 are 1M then full scale would be 210V
the wire carrying the signals from R1,R2 to the THCAD should be shielded
with the shield connected to plasma+ = table ground at the plasma end and left
open at the THCAD end
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24 Jun 2019 07:35 #137716 by Clive S
Replied by Clive S on topic New Plasma build question

PCW wrote: I would at the minimum put R1 and R2 (and perhaps R3) at the plasma supply itself

If you use a THCAD10, I would probably just use R1 and R2 and not use R3 and R4

If R1 and R2 are 1M then full scale would be 210V
the wire carrying the signals from R1,R2 to the THCAD should be shielded
with the shield connected to plasma+ = table ground at the plasma end and left
open at the THCAD end


Thank you Peter for the clear explanation. So 1/2 watt should be more than enough but 1 watt would be more sturdy physically .

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25 Jun 2019 00:52 #137767 by tommylight
I agree with Peter, leave out the R3 and R4.
That was just a general purpose voltage divider i use often, and since i can not always find the right values, i add the Pot there for fine tuning.
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22 Jul 2019 16:46 - 22 Jul 2019 16:54 #140200 by Clive S
Replied by Clive S on topic New Plasma build question
Ok the table is nearly built. I am waiting for a couple of pulleys. So in the meantime I am try to sort out the crappy cut 40 for the THC .

Question: I have notice that the terminal that goes to the work piece +ve is not connected to mains earth is this correct? BUT it would be when being used as the table will be connect to mains earth.

Edit : also on reading the THC manual it shows the W1 mode as down for unipolar BUT the board shows it as up. Which is correct?

Also using scratch start with 2 x 1meg R's one from each terminal what would be the divide Freq. ie f/32 I will be using a Mesa 7i96
Last edit: 22 Jul 2019 16:54 by Clive S.

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