Inputs & outputs & relays ohmic

Phill

I suggest 890K to 1.2M in series with both the work piece and the electrode inside the case and the THCad in a small metal box outside the case close to it or on it. The current limited input nodes are pretty high impedance so are susceptible to EMI. Go with the divide by 32 option as suggested by PCW else where for increased noise immunity.

I think any of the Hall effect modules that are fast enough should work fine for sensing main arc establishment. Get what is available with easy power supply and output voltage. I think the convention is to sense on the work piece lead. Of coarse your Plasmac does have arc ok sensing using the arc voltage so this will be a good test.

John

So I was finishing the wiring on the control cabinet and relized the 8ch relay module I got is not what I thought it was. Yes it works with my sinking 7i76ed but the other side of the relays only have 1 common. I was thinking it had one common for each relay. I actually think it has 1 common for each set of 4 relays. So now what do I do, I was planning on using one of thoes relays to be the torch start. I though maybe I could use one of the extra opto22’s to switch the torch but I’m not familiar enough with them to fully know how thy work. I have 3 ODC24 optos that say 60v @3amps output and 24v logic. Any ideas on how to get around my predicament?

Wait a sec are these just optically isolated SSR’s that have a 24v trigger voltage and switch up to 60v through pins 1&2?

I bet your Opto rack has active low inputs which should match your 7i76ED card. Use a volt meter to get the polarity correct and those DC output modules will switch your torch on, get the polarity wrong and the module protection diodes may switch it on continuously until you shut off the plasma unit!

John

Yeah that’s how it’s set up. Also in wiring up the IDC24 I could not get any logic voltage output. I tested it and found it has a dead short between the colector and emitter. So now I’m trying to figure out how to do this with a ODC24 and relay block.

better to use a ULN28xx

Just did an arc test and the pilot arc sits right at 135v. I then started cutting and would rase the torch to increase arc length to see how high I could get the volts befor the arc terminates. I was never able to hit 200v. At about 1/2”-3/4” off the steel I hit 197v and then it kicked back to pilot arc. So I’m going to set my resistors to 200v max. That’s 1,900,000 ohms but if I rounded to 2m that would be 210v max.

Sounds great. I thought that would be a good setting for a 30 amp machine. Just remember the world won't end and your THCAD will not die if you exceed that 200v full scale reading as it can handle significant overvoltage. Whilst the output will be clipped, it will only be on the start when your THC is disabled so you are not using the arc voltage to control anything.

So how are you using an IDC24 opto22 as the ohmic signal generator? From all my research the IDC24 is a negitive logic opto22. This means the logic output pin is allways high till it’s triggured. I can’t quite wrap my head around it. Only way I can see it working is if you removed the pull up resistors from the G4PB4 board and did not connect the ground to it either. What’s up?