Showstopper Plasma Cutter retrofit

increasing the ohmic threshold to 23.5 seems to have solved this issue for me.
After the nozzle got wet from the watertable the ohmic voltage was showing at above 22volts.

SteveatSteelCut wrote:

increasing the ohmic threshold to 23.5 seems to have solved this issue for me.
After the nozzle got wet from the watertable the ohmic voltage was showing at above 22volts.

Great news. All it took was a little tuning. Perhaps the settings that worked for me were not appropriate for a default setting. Also, many may have forgotten that the reason why the pulse torch feature was added to Plasmac was to get air flow running before the first pierce to keep water off the material on the initial probe. This was added for islander261 who adopted plasmac during the early development days. Islander261 also contributed the original ohmic relay circuit detailed here linuxcnc.org/docs/stable/html/plasma/pla...r.html#ohmic-sensing

@snowgoer, thanks for the refresher. Its been a long time since we did all the experimentation on THCAD ohmic. There are many people using it sucesfully including James in New Zealand, Joey in Ireland, Santiago in Portugal  to name just a few recent users. I can't help but think making the same change Steve did would have solved your problem. There is enough data on the forum that proves the voltage caused by a water induced short is lower than full contact. Your video shows instant failure when probing starts so the threshold is clearly too low. We need to set the threshold high enough to be above the wet voltage. 

 

In regards to Ohmic triggering, did anyone try to add a resistor in parallel to THCAD input?
That should raise the current required to trigger, and might improve usability a lot.
A 220 ohm 5W should be a nice value to try, it raises the reuqired current to roughly 120mA, instead of uA. Should be more than enough to eliminate water induced triggering.
470 ohm will be under 50mA, if that proves to be problematic on dirty and rusted sheets, although i doubt it.

Tommy, We use a 24k 5W resistor as suggested by PCW a long time ago.
You need to calibrate the adjustable voltage on the power supply to be 24 volts.
And you need to monitor the ohmic voltage when configuring it and ensure the ohmic threshold is above the off voltage when the torch is wet.
Initially, Steve had this set too low even though it worked for me.
I was going to ask Steve to try a threshold of 24.9 volts to see how reliable it is as an experiment. I have always seen a full 24 volts on contact.
There are so many advantages of using a THCAD for Ohmic sensing, it would be good if a few like minded people could experiment further.
 

I do not have any torch with shield, so i can not test although i do have 2 of THCAD for now.
I have to go pick up a Hypertherm 3070 as i do have it's torch and cables in my shop, but the transformer was to big to fit in a Skoda SuperB!

You need an F truck