New Project with Mesa 7I96 and THCAD-05

Not sure if I can get to the power supply at this point as it is below the shelf that has the 7I76E and THCADs.
 
 

phillc54 wrote:

Sorry Tom, I forgot to post this yesterday. It needs a bit of work but should give you the general idea of what I did.

 

Please do not insert any components between the THCAD, the 24V DC power supply and the ohmic probe (like the relay shown). That is something PCW was very clear about.
The THCAD circuit is deigned to be exposed to arc voltage and the use of appropriately rated components good for overvoltage of 500 volts (power supply and THCAD) makes this safe. Any relay there is likely to fail badly....

The whole intent was to remove any components from the torch side and provide a method of adjusting sensitivity.

Stick to the relay circuit or the published THCAD circuit. Do NOT mix and match!

phillc54 wrote:

rodw wrote:

Please do not insert any components between the THCAD, the 24V DC power supply and the ohmic probe (like the relay shown). That is something PCW was very clear about.
The THCAD circuit is deigned to be exposed to arc voltage and the use of appropriately rated components good for overvoltage of 500 volts (power supply and THCAD) makes this safe. Any relay there is likely to fail badly....
If you read the text you would see that I have no interest in the ohmic probe being active while not probing so I will isolate it if I choose to.

You would also see that the relay is rated plenty high enough for 500V as are the diodes.

There is basically no difference to the published circuit that John uses succesfully other than the relay and the THCAD.

Of course I understand you are not using a THCAD from what you wrote. Its fine for you to do what you want.
I was going to proceed the same way until I got this advice from PCW at Mesa:

No relays or diode should be needed when using a THCAD for OHMIC sensing
(basically its impedance protected)

We should not be publishing things that differ from Mesa's view.

phillc54 wrote:

rodw wrote:

No relays or diode should be needed when using a THCAD for OHMIC sensing
(basically its impedance protected)


I agree with that statement, however it does not say "relays or diodes should not be used".

No but this does

I would not isolate the input but rather just turn off the power supply when not
probing if you are concerned about electrolytic corrosion

Respectfully, I spent several weeks working through this with Peter Wallace from Mesa. The intention is clear as conveyed in the circuit diagrams published.

Its not as straight forward as that. We are measuring voltage with the THCAD, not just a switch closing. Its very important that the power supply is outputting 24.0 volts and the recommended power supply has an adjustment screw to allow this. You need to calibrate that with a multimeter.
If you add 2 diodes into the circuit,  you would see a voltage drop of around 1.2 volts. (typically there is a 0.6v voltage drop across a diode). You would need to recalibrate the power supply and possibly the THCAD scaling resistor to account for this. You would also likely have to adjust the thresholds in the hal file.

The voltage should fall to instantly zero when the probe breaks contact. If it doesn't, then the only possible cause is some sort of short in the nozzle that allows voltage to flow where it shouldn't. If thats not resolved, probing is likely to be problematic.

If the voltage is too close to the threshold, you can change the threshold in hal. Ideally, you would allow adjustment of this from within your config if it was a major issue.

There is nothing in the circuit like a capacitor to store electrical potential so the only way I could see that voltage could slowly increase over time is if there was some kind of drift in the power supply (or a short in the torch).

Don't forget that Ohmic3 (not used in Toms Config here) was deigned to do the impossible and that was to  allow ohmic sensing on a water table. Thats long been acknowledged as problematic and many industry sources recommend NOT using it on water tables. The effectiveness of Hypersensing is dependent on a lot of environmental factors some of which have been mentioned above. 

I have long bypassed the optional mains relay as I found there were some issues in my setup including the long time discharging capacitors on power off.

Whilst mileage for others may vary, I have frequently done cutting jobs with over 1500 pierces on a water table with no probing issues. I feel the performance is improved over the earlier relay circuit I used.

My concern is that there are many circuit diagrams on the forum here and some of them of dubious quality have been used by many people with the mistaken belief that they are the gold standard. There are versions of the THCAD and relay circuits in the official Linuxcnc documents, its best we defer to them.

In terms of design, Ohmic sensing does not require a probe-enable signal because the additional isolation is not required. But to remain compatible with Plasmac, it does accept this signal and will not turn on a probe signal unless its present. A much cleaner design could be achieved if the probe-enable was removed from the ohmic component. 

The THCAD circuit is probably more costly than the relay circuit but it requires fewer components and arguably more reliable. If correctly implemented, there is no risk of fires in the wiring. Properly configured it performs flawlessly where other systems don't. I can understand Phill added a relay when testing it becasue he had the relay in his control box. In a new build, the temptation to add these components should be ignored, this is just adding more expense with unnecessary components which do nothing other than to reduce MTBF.