Arc OK intermitent issues on torch fire

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02 Apr 2021 15:13 #204513 by txtrone

I will say, I don’t understand anyone recommending to not use a fuse. That just blows my mind. Cheap insurance. Worst case you size it too small and need to get a bigger one. Best case you don’t set your wiring on fire and burn down your dwelling.

I do sometimes feel bad for new people trying to wade through all of this. But I guess that’s part of the learning process. A lot of us were figuring it out together as we went along. Still are, honestly.


heh ... I am sure it causes much re-hashing of the same subject with the same people (with opposite opinions) when you are trying to give advice to someone new. I do need to look at some 'in-system' fusing.

I have one last question before I land all of these wires and see how far I get trying to fire up the torch, you will probably tell me to RTFM ... but here it goes. I understand connecting the shielded torch tip to the -IN on the THCAD5 but where on the table do you connect the -24V from the isolated power supply?

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02 Apr 2021 15:24 #204515 by txtrone

That was for THC use, for Ohmic sensing, there is no connection to the CPC port at all


d'oh! How does this look?

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02 Apr 2021 15:48 #204517 by PCW
That looks right though its missing the 24K 5W resistor from the torch sense pin to
the 24V+ (to set the sense current to 1 mA).

Also if the table and torch sense lines are long (THCAD and 24V PS not in plasma box) I would fuse them at the plasma source end.
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02 Apr 2021 16:13 #204523 by txtrone

That looks right though its missing the 24K 5W resistor from the torch sense pin to
the 24V+ (to set the sense current to 1 mA).

Also if the table and torch sense lines are long (THCAD and 24V PS not in plasma box) I would fuse them at the plasma source end.


I added the 24K 5W. Does that look better?

The last part is confusing me. I understand the torch and sense lines, what do you mean though by "THCAD and 24V PS not in plasma box"? Do you mean the THCAD5 and the isolated PSU? If so, does that mean those two devices need to be outside of the main 'control' box where the 7i76e is located?

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02 Apr 2021 16:40 - 02 Apr 2021 16:40 #204529 by snowgoer540

I have one last question before I land all of these wires and see how far I get trying to fire up the torch, you will probably tell me to RTFM ... but here it goes. I understand connecting the shielded torch tip to the -IN on the THCAD5 but where on the table do you connect the -24V from the isolated power supply?


I welded a weld nut onto the bottom side of my water pan. I cut my work lead from the power source and then added two large ring terminals, one connected to the weld nut, with the ohmic wire you speak of, then the other ring terminal which goes to the work lead clamp. Then I can connect the clamp to the work piece when I cut. So effectively that wire is hooked to the table, and the workpiece at the same time.

This way, if I forget to hook up the work lead, it is always connected to the water pan at least. When CBEAR's table was wired incorrectly, he had forgotten to hook the work lead to the material, and since it was wired incorrectly, it provided a path back to ground and caused his wiring to catch on fire when it fired the torch.
Last edit: 02 Apr 2021 16:40 by snowgoer540.
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02 Apr 2021 17:02 #204533 by txtrone

I have one last question before I land all of these wires and see how far I get trying to fire up the torch, you will probably tell me to RTFM ... but here it goes. I understand connecting the shielded torch tip to the -IN on the THCAD5 but where on the table do you connect the -24V from the isolated power supply?


I welded a weld nut onto the bottom side of my water pan. I cut my work lead from the power source and then added two large ring terminals, one connected to the weld nut, with the ohmic wire you speak of, then the other ring terminal which goes to the work lead clamp. Then I can connect the clamp to the work piece when I cut. So effectively that wire is hooked to the table, and the workpiece at the same time.

This way, if I forget to hook up the work lead, it is always connected to the water pan at least. When CBEAR's table was wired incorrectly, he had forgotten to hook the work lead to the material, and since it was wired incorrectly, it provided a path back to ground and caused his wiring to catch on fire when it fired the torch.


Cool idea! I have a similiar permanent ground lug on my table where I have an isolated earth ground connected. I am wondering if I should disconnect that and only use the ground from my mains. Anyhow, I could 'bond' the sense cable and the power source lead at that location... and then run another lead from that point to the clamp (I use a magnet) that goes on the work piece which is being cut.

I just looked at my bin of resistors. I have 22K and 25K (both 5W) not sure how critical the 24K value is in this equation.

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02 Apr 2021 17:03 #204534 by snowgoer540
Either one will work fine
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02 Apr 2021 17:13 #204535 by txtrone

Either one will work fine


Thanks a ton! I am still confused about exactly where PCW was suggesting I fuse the isolated 24+ and torch sense cables a few post back... but I have plenty to do in the meantime. What size fuses did you use?

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02 Apr 2021 23:59 #204583 by txtrone

Also if the table and torch sense lines are long (THCAD and 24V PS not in plasma box) I would fuse them at the plasma source end.


PCW - As I was wiring the THC and Ohmic circuits this afternoon I was still puzzled by this post. Do you mean the fuses should be in the sense lines directly next to the THCAD5 and another directly next to the 24- post on the isolated PS? Or just the opposite of that, a fuse on the gantry near the torch and another on the table near the connection point of the sense?

Also, what size/type fuses would you recommend using?

Thanks again!

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03 Apr 2021 00:05 #204584 by PCW
The fuses are to protect the sense wires from fault currents so they
should be as close to plasma machine as possible. 1A should be fine
(It should be rate to interrupt the plasma voltage and current)
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