CNC Plasma cutting table

**Sgbarker** · Nov 20, 2019, 11:29 AM

Thanks Jon... not sure why I was having trouble with the link. Any suggestions for future posts?

**Sgbarker** · Nov 21, 2019, 08:46 PM

This video show how I did the Torch height controller interface to my Hypertherm powermax 900.

DIY CNC plasma cutter part 3

**Sgbarker** · Nov 22, 2019, 02:43 PM

I had some questions about how I did electrical and electronic isolation. So I put together a short video to explain how I did it. Hope it helps!

**jdurand** · Nov 22, 2019, 04:58 PM

Obviously in your case it isn't a problem, but when using multiple switch mode power supplies like that you can run into issues with inrush current. That's like the starting current on a motor. Power supplies usually limit that current to 50 amps for 1-2 cycles of the power main so wire heating isn't as much an issue as the power switch blowing up.

A place I worked at was testing lots of production units with those power supplies. They were plugging them into a 6 outlet power strip on the test bench one at a time and that always worked. For some reason a person (NOT me) turned off the switch on the power strip and then back on with all 6 devices plugged in. The power strip blew up in his hand.

Don't do that!

**Sgbarker** · Nov 22, 2019, 05:45 PM

That’s interesting, I haven’t had any issues at all. I’m using Nema 23 stepper motors so they don’t draw a lot current even at start up, I think it’s because the motor drivers limit the current anyway. I’ll have to keep an eye on it... thanks

**jdurand** · Nov 22, 2019, 05:50 PM

Originally Posted by Sgbarker

That’s interesting, I haven’t had any issues at all. I’m using Nema 23 stepper motors so they don’t draw a lot current even at start up, I think it’s because the motor drivers limit the current anyway. I’ll have to keep an eye on it... thanks

Load on the supply isn't critical, the surge is just getting the power supplies started up before they even start producing an output.

I ran into the problem on a smaller scale on some medical equipment I designed mumble decades ago, I had one switching supply powering another. The startup current of the second one would trip out the first one. I had to devise a soft-start for it. Now most supplies have fold-back current limits on the output, they reduce the voltage to keep the output wattage in check but since they still supply some it lets things get going. Solves most startup problems like I had.