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.

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.

For the sake of two months, you are forgiven Tommy!

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

In my build I have a 50A with no divider and went for THCAD300.

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.

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.

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

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.

That should be fine