THCAD selection including ohmic sensing

islander261 wrote: Well it may not be quite that simple depending on ones actual hardware. In most case the arc voltage sensing (THC) is sensed between the torch electrode and the work piece. For ohmic sensing usually you want to measure the potential between the torch shield and the work piece.

John

That makes sense. But it may be worth testing. I know on my machine the hypersensing circuit sees the Arc Voltage while cutting as a full scale reading on the THCAD but that is only 24.5 volts. Maybe there is some leakage. The component I created does have an output for the voltage output so it would be easy to see in halshow or halscope while cutting if the THCAD was scaled for the arc voltage.

In any case, there would be no need to switch circuits. Just connect both the shield and the arc voltage to the same THCAD terminal as we can depend on the isolation to keep things safe. We will know from the is probing signal what is happening (cutting or probing)

EDIT: And when cutting, an AC relay on the is probing pin can turn off the mains power to the isolated ohmic power supply so there is no voltage applied to the shield while cutting.

Rod

I think you will find that your power supply will see an error condition with the shield connected to the electrode while cutting. Also I think the plasma arc will wander to the shield instead of the electrode while cutting. One of the firmware features on our vintage TD power supplies while using the automation torch is to check for the shield to contact the work piece and throw an error (#85). I defeated this feature on my machine by adding extra circuitry to control the solenoid in the torch.

John

Maybe. But we are not talking about connections at the plasma cutter, we are talking about making connections to the divided arc signal from the Plasma machine's voltage divider. Some of the better quality machines (eg. HT and TD) may have some isolation there.

Placing a relay on the wire between the THCAD and the shield may be required but that then defeats the whole simple design principle of the Hypersensing circuit which was to have no additional components in the sensing circuit.

If you use a THCAD and an isolated power supply for Ohmic sensing, there _is_ no connection between the shield sense and the plasma voltage except the common work sense point. Adding a relay makes no sense

PCW wrote: If you use a THCAD and an isolated power supply for Ohmic sensing, there _is_ no connection between the shield sense and the plasma voltage except the common work sense point. Adding a relay makes no sense

Peter, I am looking to do this hypersensing on my table. I have been holding off on ordering a second THCAD, as I am unsure if firmware exists to do this with a 5i25 and a 7i76. Is that out there and I just missed it?

Thanks,
Greg

FWIW I was thinking of trying something like this: