Big Ohmic Problem ???

I would not reccommend adding any fuses etc.

Any particular reason for this statement?


Just to clarify, the moment you run any one of the outputs of your double insulated powersupply along anything connected to ground earth with single insulated wires your double insulation is gone. The moment you your wiring has an insulation fault to a cabinet or piece of machinery and you forget the ground clamp (which you apparently do sometimes) your wiring can catch fire because I doubt your sense line can handle the current your plasma box is able to put out.

Damn I know I wasn't going to go on about this. Just bugs me.

Rod,
Look, I don't criticize your design. I think it's awesome what you guys came up with. And even better on you for sharing it with the community.
Now I don't know if PCW runs a plasma table in the basement or garage of his family home but I'm sure he is aware of the hazards.
My point is that there are a lot of people who have a lot less experience running these things in their homes. Just look at all the questions about what "floating" means.
One user already had his wiring go up in flames another user was already halfway there. You can wire this up the wrong way and never notice because you have a properly ground referenced table or you never forget the clamp. But forget it once and you have a fire.
I know some people think it's every users responsibility to check everything over themselves and I agree to that but I also have kids and I believe that the older members of the group should look after the younger ones.
To come out and actively speak out against trying to use a 50 cent safety device after all we heard is rather a bit of a mind boggler to me.

Aciera wrote:

Rod,
Look, I don't criticize your design. I think it's awesome what you guys came up with. And even better on you for sharing it with the community.
Now I don't know if PCW runs a plasma table in the basement or garage of his family home but I'm sure he is aware of the hazards.
My point is that there are a lot of people who have a lot less experience running these things in their homes. Just look at all the questions about what "floating" means.
One user already had his wiring go up in flames another user was already halfway there. You can wire this up the wrong way and never notice because you have a properly ground referenced table or you never forget the clamp. But forget it once and you have a fire.
I know some people think it's every users responsibility to check everything over themselves and I agree to that but I also have kids and I believe that the older members of the group should look after the younger ones.
To come out and actively speak out against trying to use a 50 cent safety device after all we heard is rather a bit of a mind boggler to me.

I have to agree here. I have no problem admitting my ignorance and saying that I never even knew it was a possibility. I thought "isolated power supply" meant "dont use the power supply you use to power the inputs/outputs" not "dont create a non floating ground nightmare that could cause a fire". Looking back, it surely is stupid, and I can see now that adding a wire from -24 to shield definitely gives a path to ground through the ohmic wires.

I would rather read posts that say "I blew ANOTHER Ohmic Fuse" than "Today my table caught fire and nearly burnt my garage/house/shop/etc. down"... or worse.

I'm lucky that I didn't catch my table on fire. REALLY lucky. Cody is lucky his fire wasn't bigger. REALLY lucky.

I'm just going to say it: The ohmic documentation is a bit lacking, and the whole thing at this point feels like a pieced together, hodgepodge afterthought. "Sweet it works, let's package it up and ship it out", with no refinement. There's an outdated schematic in the plasma primer, with mentions to an outdated component. The current is "Ohmic3.comp", while the packaged with LinuxCNC version is "Ohmic.comp". The only way you'd know that is to thumb through all of the forum posts. Lastly, there's no mention of safety. Just "we used a double isolated power supply". The word floating isn't even mentioned, much less what happens if you mess the whole thing up.

Aciera wrote:

@snowgoer
I think 500mA should be plenty.

Would this be suitable:

www.mouser.com/ProductDetail/Schurter/80...4L92YmMJxzV1xQ%3D%3D

That's a 500VDC 500mA fuse. I just want to be sure I use the right thing

Thanks all for the help!

@ snowgoer Seems fine to me. Give it a try and report back.

Rod,
I don't mean to lessen any of your achievments. You did FINE. You created something really great. It works!

But there are some issues with this when used by less experienced users that's all. So why not just try to add a couple of fuses to the sense lines and maybe it will be a safer solution for all of us.

Aciera wrote:

@ snowgoer Seems fine to me. Give it a try and report back.

Rod,
I don't mean to lessen any of your achievments. You did FINE. You created something really great. It works!

But there are some issues with this when used by less experienced users that's all. So why not just try to add a couple of fuses to the sense lines and maybe it will be a safer solution for all of us.

is 500mA on the high end of what would would be expected as a fault in the circuit? Or is on the conservative side?

The reason I'm asking is if I'm going to place an order, I might as well add some variety in case the 500mA blows instantly under "normal" use... but maybe that isnt possible?

Well the moment you have a short and that plasma current is coming down that sense line pretty much everything is going to blow. I haven't done the math as to how much sense current there is during normal operation. No need to split hairs. You could even use a 5 Amp fuse if your wiring is of decent size to handle that.
So a variety of fuses would be fine I think.