Big Ohmic Problem ???
05 Oct 2020 08:25 #184943
by rodw
Replied by rodw on topic Big Ohmic Problem ???
When I review CBEAR's wiring schematic, the difference between what he did and what I did is that he made an additional connection the THCAD shield to -24v. I only ever connected it to an enclosure ground point. Actually to a DIN rail ground clamp. Granted my enclosure is mounted to the table but there is not a direct low impedance connection.. We always knew that the sensing circuit would see an overvoltage when it was cutting and left the Class II isolation and the THCAD's over voltage protection. In fact, we included a pin that reported this over voltage.
I also heeded PCW's earlier advice not to connect field power -24v to enclosure ground to keep the field power independent despite local advice to the contrary.
Subsequently the 24k 5W pullup resistor was added to the circuit. I have operated my table on numerous occasions without a ground clamp connected without ill effect (other than that it does not establish a valid arc). I do use good sized cables in the ohmic circuit, not just fine hookup wire.
I also heeded PCW's earlier advice not to connect field power -24v to enclosure ground to keep the field power independent despite local advice to the contrary.
Subsequently the 24k 5W pullup resistor was added to the circuit. I have operated my table on numerous occasions without a ground clamp connected without ill effect (other than that it does not establish a valid arc). I do use good sized cables in the ohmic circuit, not just fine hookup wire.
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05 Oct 2020 08:54 - 05 Oct 2020 08:54 #184945
by Aciera
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.
Replied by Aciera on topic 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.
Last edit: 05 Oct 2020 08:54 by Aciera.
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05 Oct 2020 09:14 #184948
by rodw
Because I put my faith in PCW's design. I will say I took a lot of convincing on some elements of that design.
There is still the tried and proven relay method of ohmic probing if you are not happy with the THCAD approach.
Replied by rodw on topic Big Ohmic Problem ???
I would not reccommend adding any fuses etc.
Any particular reason for this statement?
Because I put my faith in PCW's design. I will say I took a lot of convincing on some elements of that design.
There is still the tried and proven relay method of ohmic probing if you are not happy with the THCAD approach.
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05 Oct 2020 10:01 #184953
by Aciera
Replied by Aciera on topic Big Ohmic Problem ???
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.
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.
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05 Oct 2020 10:30 - 05 Oct 2020 10:33 #184957
by snowgoer540
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.
Replied by snowgoer540 on topic Big Ohmic Problem ???
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.
Last edit: 05 Oct 2020 10:33 by snowgoer540.
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05 Oct 2020 11:19 #184961
by rodw
Replied by rodw on topic Big Ohmic Problem ???
I did not really want to replace ohmic.comp with ohmic3.comp becasue I was always a bit nervous about the pullup 24 ohm resistor which was added to help with water tables. That was an application which was not envisaged when it is generally known ohmic sensing is not suitable for water tables. Maybe we pushed the envelope too far by doing the impossible with ohmic3.comp
So maybe ohmic.comp should remain the official solution and ohmic3.comp remain a non supported upgrade along with many similar such enhancements shared on the forum. Having said that, I don't mind if enhancement to the documentation and the component were made.
I thought the primer was pretty clear as I don't think it should recommend a specific power supply.
linuxcnc.org/docs/devel/html/plasma/plas..._with_a_mesa_thcad_5
Its really just another case of lack of knowledge of electronics ( I am the biggest culprit there). Clearly an Isolated power supply means an Isolation Class II power supply to some but not to others. I will say when selecting the power supply, I did do a bit of research into what that all meant..
We have become very prescriptive about plasma solutions, far more so than any other class of machine, possibly that is not a good thing as it means people are not taking responsibility for their own design.. But then there is no warranty....
So maybe ohmic.comp should remain the official solution and ohmic3.comp remain a non supported upgrade along with many similar such enhancements shared on the forum. Having said that, I don't mind if enhancement to the documentation and the component were made.
I thought the primer was pretty clear as I don't think it should recommend a specific power supply.
linuxcnc.org/docs/devel/html/plasma/plas..._with_a_mesa_thcad_5
We used a 15W Mean Well HDR-15 Ultra Slim DIN Rail Supply 24V DIN rail based isolated power supply.. This is a double insulated Isolation Class II device that will withstand any arc voltage that might be applied to the terminals.
Its really just another case of lack of knowledge of electronics ( I am the biggest culprit there). Clearly an Isolated power supply means an Isolation Class II power supply to some but not to others. I will say when selecting the power supply, I did do a bit of research into what that all meant..
We have become very prescriptive about plasma solutions, far more so than any other class of machine, possibly that is not a good thing as it means people are not taking responsibility for their own design.. But then there is no warranty....
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05 Oct 2020 13:09 #184964
by snowgoer540
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!
Replied by snowgoer540 on topic Big Ohmic Problem ???
@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!
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05 Oct 2020 14:22 - 05 Oct 2020 14:33 #184966
by Aciera
Replied by Aciera on topic Big Ohmic Problem ???
@ 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.
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.
Last edit: 05 Oct 2020 14:33 by Aciera.
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05 Oct 2020 14:57 #184969
by snowgoer540
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?
Replied by snowgoer540 on topic Big Ohmic Problem ???
@ 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?
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05 Oct 2020 15:12 #184971
by Aciera
Replied by Aciera on topic Big Ohmic Problem ???
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.
So a variety of fuses would be fine I think.
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