Mesa 7i95t connection to Thermal Dynamics 60i

Hi,

I'm converting a Crossfire Pro to LinuxCNC using a Mesa 7i95t and THCAD2.  I have the automation interface card in the cutmaster 60i so it has the torch start/stop, voltage divider, ok to move, and ohmic sensing on the CPC connector.  I'm still waiting for the THCAD2 to come in but the wiring for it seems pretty straight forward.  I've attached the wiring diagram from the service manual and I'm not quite sure if I can just wire the start/stop directly to a 7i95t output or if I need to use a relay.  If I understand the qtplasmac docs I should be able to connect the ok to move directly to an input but may need a resistor if the current draw isn't enough.  I may hook up ohmic later but would I need another THCAD2 for it as well?

Thanks,

Kevin

You don't need a relay as your card has 2 amp relays built in to the high current outputs, Use one for the torch on. Adding a relay would just add an additional failure point and add some lag into the signal.
I think your plasma has the same divider as the TD  A120 I had. In which case, use a 30:1 divider without any scaling resistors on the THCAD's 10 volt scale. If you do have a 20:1 divider (I didn't)  you could use that. set the frequency divider to 1/32 and scale the sticker frequency by the same amount when configuring qtplasmac
You do need to use a pull down resistor on the ArcOK input which is discussed in the QTP manual.

Hi Rodw,

My automation card divider is 50:1 by default but can be set to 80:1, 40:1, 30:1, 20:1, and 16.7:1. Would you still suggest the 20:1 or would the default 50:1 work just as well?

50:1 would be OK with with thcad 5 volt scale.
on 10v scale, Use 20:1 which is 200 volt full scale. That will be more accurate. It will probably go off that scale briefly when piercing but  it won't matter as we don't care about the voltage until it settles back to cut voltage of about 130 volts. The THCAD can handle the over voltage.

Is it still recommended to mount the THCAD on the outside of the plasma power unit when the power unit has a voltage divider? I was thinking it wouldn't make much difference since the start and ok to move are coming directly into the electronics box.

I have never had issues mounting the THCAD inside the control box. The plasma has a CNC interface connector so its designed for this.
You will not have issues.

I have the THCAD2 setup for 10V range and 32 divider with the plasma divider set to 20:1 and the arc voltage is displaying correctly after I set the scale and offset. The Thermal Dynamics has a 15V arc voltage before the start is initiated and it is showing correctly along with arc voltage during pierce and cut. For some reason I can't seem to get the Arc OK input to work. I have pin 12 of CPC connected to +24V, pin 14 to Mesa input 15, and -24V connected to Mesa common for the input. If I disconnect the CPC cable from the plasma cutter and short pins 12 and 14 the Mesa input goes high so the cabling should be good.

I have noticed there is a 2 second delay after start is triggered before the pilot arc lights so by setting my pierce time to (2 + pierce delay) and setting "Ignore OK" I have been able to get it to cut and the arc voltage is right around 137 that the cut table shows for the material I'm using.

The service manual for the plasma unit shows that it has an "OK to Move" LED inside the case so I may have to pull the cover and check that as well.

Any other ideas what I could be missing?

EDIT:  I disconnected pins 12 and 14 from the control and checked across them while cutting and there is no continuity so it looks like I will need to pull the case on the plasma unit and check the LED.

FINAL EDIT:  I went through all the resistance and diode checks in the service manual and verified continuity of the relay pins and the "Ok to Move" LED on the inverter board lights but the relay does not close.  So the relay is bad.  I switched to mode 0 in QTPlasmac and that works ok.  Maybe some day I'll pull the inverter board and see if I can just replace the relay but the Mesa/LinuxCNC side is working fine.

The pierce delay starts from the beginning of arcOK so the delay turning the torch on makes no difference.
The internal relay will need a minimum current of 100 mA and the Mesa board generally can't deliver this. You need to add a pull down resistor as discussed in the qtplasmac docs.

100 mA is a typical value for 5V, at say 24V the current is much less for most contacts (closer to 10 mA)
A small capacitor across the contacts can provide the wetting current without a large constant load.

If the relay does not show closed at 24V, its likely faulty...