ArcOK when not built-in

You might find that the plasma cutter has a neat one internally. Picture shows tight coil of copper with a reed switch glued in on the ground side. Ideal really...

I didn't end up using it but expect would be highly reliable

phillc54 wrote: I am pretty sure they the type that Clive used here:

linuxcnc.org/docs/devel/html/plasma/qtpl....html#qt_reed-arc-ok

Yes to confirm I used a glass one from an alarm contact fitted inside a bit of plastic tube with about 3 turns of the ground clamp cable as shown in the pic

Clive - so did you just use the alarm part as is or did you extract the reed switch from it?

I have a couple of bare glass reed switches that i will need to mount on some perf board to be useable. Easy to do and easy to add some screw terminals. But perhaps not as clean as just using pre built reed side of an alarm switch with screw terminals all ready to go.

Cheers.

-solder two wires to reed
-get a pen and cut a length a bit longer than reed body
-insert reed in
-wind 3 turns of the ground or the torch lead
-use zip ties to secure the windings in place.
The worst thing in all of that is the wire, it is thick and hard to wind, most times to short so lengthening is needed. Takes about 15 to 30 minutes of time.

What reed switch is suitable? any link?

So far any reed i used just worked, be it the glass ones or plastic ones. Usually 20-25mm in length, no idea what model or type.