New Plasma build question

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17 Aug 2019 09:01 #142374 by rodw
Replied by rodw on topic New Plasma build question
1 & 2 you have right.
3. You would use your relay to switch field power to a 7i76e input
4. 300 V can be exceeded so make sure your electronics can handle overvoltage to about 500 volts without frying anything.

ArcOK is a set of dry contacts in the plasma cutter
. You use them to send field power from hte machine, theough the arcOK contacts and back to an input.

You need to hold about +- 2 volts from target torch voltage.

BUT:
If you are using a 7i76e, I would strongly recommend you abandon your plan to build a now obsolete second grade UP/DOWN THC and purchase a THCAD-10 From Mesa for $69 and adopt the plasmac configuration which is now the gold standard for Linuxcnc plasma control (and probably for plasma controllers in general). See linuxcnc.org/docs/devel/html/plasma/plasmac-user-guide.html

and make sure you read this guide: linuxcnc.org/docs/devel/html/plasma/plasma-cnc-primer.html

When you see Tommylight, long the champion of a configuration using the proma THC to abandon his config and adopt plasmac for all of his commercial projects, you can't ignore Plasmac.
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17 Aug 2019 10:16 #142376 by tommylight

rodw wrote:
When you see Tommylight, long the champion of a configuration using the proma THC to abandon his config and adopt plasmac for all of his commercial projects, you can't ignore Plasmac.


Well not yet completely, but for sure the 5 new ones i am trying to build will use PlasmaC.
This is the part of the year when my clients use their machines 16 to 18 hours a day every day, so till late Octomber i can not change them to PlasmaC.
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17 Aug 2019 10:28 #142377 by rodw
Replied by rodw on topic New Plasma build question
For the sake of two months, you are forgiven Tommy!
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17 Aug 2019 10:47 #142379 by CNCFred
Replied by CNCFred on topic New Plasma build question
Thank you very much for the guide!

Not according to plan I bought a cut 70p and got from Stahlwerk the connector to start the torch for free.
Question is now how to find out the arc ok signal :ohmy:

Rodw: since the cut 70p does not have a divider do you suggest to make a voltage divider myself and use the thcad-10 instead of the 300?

Thx

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17 Aug 2019 11:07 #142381 by tecno
Replied by tecno on topic New Plasma build question
In my build I have a 50A with no divider and went for THCAD300.
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17 Aug 2019 11:10 - 17 Aug 2019 11:14 #142382 by rodw
Replied by rodw on topic New Plasma build question

CNCFred wrote: Thank you very much for the guide!

Not according to plan I bought a cut 70p and got from Stahlwerk the connector to start the torch for free.
Question is now how to find out the arc ok signal :ohmy:

Rodw: since the cut 70p does not have a divider do you suggest to make a voltage divider myself and use the thcad-10 instead of the 300?

Thx

You can't really make your own ArcOK signal. I do know one guy (not using linuxcnc) who used an ACS758 current sensor to synthesise one. But Plasmac has a mode to work around this so there is no need to panic!

If you follow the plasma guide I linked to, there are some suggested rules about choosing a THCAD version. For the price of a THCAD, there is not really any reason to build your own divider. Basically, if you have room inside the case use a THCAD-300, otherwise a THCAD-10 with the resistors placed inside the plasma case to keep voltage in the non lethal range on the outside.

EDIT as tecno says, the THCAD-300 is a good option if you can mount it internally.
Last edit: 17 Aug 2019 11:14 by rodw.
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17 Aug 2019 12:08 #142387 by tommylight

rodw wrote: You can't really make your own ArcOK signal.

I did make plenty of them using an 24V electromagnet, removing the core, adding a long screw, winding 3 turns of the torch cable around it ( not easy and often has to be prolonged ), adding a rectifier and a very smal low current 12V relay.
That works in most plasma cutters, but in some that have better filtering it does not.
3 windings of the torch cabke are enough to trigger a reed relay and that works in all of them, so this is what i use now.
With THCAD that is not needed at all as you can fine tune the voltage to trigger ARCOK signal.

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17 Aug 2019 12:15 #142388 by tecno
Replied by tecno on topic New Plasma build question

EDIT as tecno says, the THCAD-300 is a good option if you can mount it internally.


And I will be bold enough to mount my THCAD300 on the outside ;)

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17 Aug 2019 15:50 #142395 by Clive S
Replied by Clive S on topic New Plasma build question
Ok I am happy for this thread to help to setup a simple Plasma . (After a few undesirables made me build one)

Update now I have started the control box and wiring etc. Its time to get the Brix box with the right version of LInuxcnc.

Is this the correct one V2.9.
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18 Aug 2019 00:24 #142412 by rodw
Replied by rodw on topic New Plasma build question
That should be fine

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