Rods "Spaceship" Scratch built Plasma Cutter build

Bevins

One of the Ethernet cards is a much more conservative design path for a plasma cutter.

The isolated power supply for probing is only used for the probing circuit it is not the same as your field power for the rest of your I/O. When used with good galvanic isolation, opto module, relay(s), you never have conductive connection between your control electronics and the table chassis or work piece.

John

Rod,
Thanks for the information and circuit drawing. I've been playing around with some ideas myself and have attached what I plan to use for the ohmic sensing circuit. Interestingly, I found some single chip SSRs ($2.16 each) that I think will work well for this project. I also have a circuit for ohmic sensing that will allow you to adjust the sensitivity so that less than optimal metal could be detected (maybe overkill and not needed). I'm using an optocoupler to send the final signal back to the control board as it has a much quicker response time (10uS) than the SSRs (0.3 - 1.0 mS). It's also probably overkill for the probing speeds that are used.

John,
Thanks for the information about the estop circuitry.

Steven

docwelch wrote: Rod,
Thanks for the information and circuit drawing. I've been playing around with some ideas myself and have attached what I plan to use for the ohmic sensing circuit. Interestingly, I found some single chip SSRs ($2.16 each) that I think will work well for this project. I also have a circuit for ohmic sensing that will allow you to adjust the sensitivity so that less than optimal metal could be detected (maybe overkill and not needed). I'm using an optocoupler to send the final signal back to the control board as it has a much quicker response time (10uS) than the SSRs (0.3 - 1.0 mS). It's also probably overkill for the probing speeds that are used.

John,
Thanks for the information about the estop circuitry.

Steven

What is the POT for? Some kind of sensitivity circuit depending on the ohmic input?

Nevermind, I just reread your post.... lol

bevins wrote: Great work all on this plasmac. I cant wait to get to that point. Rodw, can you give me part numbers on your ohmic sensor?

Bevins, the RS Components part number for the relay was on the drawing. I used a Meanwell power supply that had a high isolation rating. The Power supply is totally seperate from field power and field power ground is not connected to the cabinet/table frame

John, thanks I'll look into it. I doubt its my wiring. The power supply has a plastic mounting clip so its not earthed to the rail. It will more likely be the power supply or dodgy wiring in a very old industrial shed. It was not insignificant at around 150 V AC

Steven, the relays I used are 50 uSec, way faster than the Servo thread requires. I would not over complicate things as its a proven design of Johns. I think some commercial tables do actually make it more complicated than what it needs to be as some have some sort of sensitivity adjustment. The other advantage of plasmac stemming from John's wisdom and experience is the test Ohmic sensor button. If the ohmic sense light comes on when you press this button (test before running a job), then you have a short inside the torch between shield and tip (happened once to me).

Rod

Please have your electrical buddy help you find the source of this soon. I can think of several causes all of which are serious personnel/fire safety issues. I can give a bunch of guesses from here but this really needs someone there to check it out, consider hiring a licensed electrician if you buddy can't help, it is that serious.

John