Wiring 7i96 and THCAD

The 12 & 14 pins are the transfer or arcOK signal. 3 & 4 are the torch start signal pins.

It's my understanding that the output pairs are each an in and out of a mosfet, or they are switches. But if y'all still say I need a relay I'll certainly use one.

The 12 & 14 is wrong though for sure. At the very least I didn't wire up common.

ISOLATED INPUT CHARACTERISTICS
The isolated inputs use opto-isolators with a 4.7K input series resistance. This results in an approximate current draw of 5 mA at 24V. The inputs will operate with +-3Vto +-36V signals relative to input common. Isolated inputs are relatively slow and not suited for signals faster than about 5 KHz. For sinking type inputs, the input common pin is grounded and a +3 to +36V signal is applied to the input pins to activate the input. For sourcing type inputs, the input common is connected to +5 to +36V and the input pins are grounded to activate an input.

ISOLATED OUTPUT CHARACTERISTICS
The 6 isolated outputs use full floating MOSFET switches ( a DC Solid State Relay or SSR ) and can be used just like a switch or relay contact. Maximum voltage is 36 VDC and maximum load current is 2A. Inductive loads must have a fly back diode. The output polarity must be observed (reversed outputs will be stuck-on). Note: The 7I96 outputs are not short circuit protected so a current limited power supply or a 2A to 5A fuse should be used in the power source that supplies the outputs.

Also, PCW, you mentioned if the polarity on the output pins were reversed that the fet would get stuck on. Would reversing the polarity do any permanent damage?

Finally, for testing with the THCAD can I just hook up some AA batteries straight to it's -IN +IN terminals? Or would I need a current limiting resistor?

You do no need a relay for torch start, the floating MOSFET outputs are a SSR
(Solid State Relay)

And yes, you need to tie the input common to 24V common

( This also means that Limit/home switches will be wired with one side to +24V
and the other to the isolated input )

Oops, you've replied while I was editing my last post. (I can't tell you guys how much I appreciate the responsiveness on this forum)

You mentioned if the polarity on the output pins were reversed that the fet would get stuck on. Would reversing the polarity do any permanent damage?

Finally, for testing with the THCAD can I just hook up some AA batteries straight to it's -IN +IN terminals? Or would I need a current limiting resistor?

[edit] Now that I think about it I guess it would be more convenient to setup a voltage divider circuit with a pot for testing purposes.

If the polarity is reversed, the MOSFET behaves as a forward biased diode.
This can damage the MOSFET if you have a high current load connected ( say 1/2 A )
since the diode will have a much higher voltage drop than the MOSFET so the MOSFET can overheat.
This is not an issue in your circuit since you only have a few mA load.

The THCAD 10 has a continuous input voltage rating of 500V, you are not going to hurt
it by connecting its input terminals to a battery (or even 120VAC)

You really only need a 9 volt test to confirm you have stuff set up right and you are seeing the correct frequencies on the encoder count at 0 volts and 9 volts (90% of full scale). Just view the encoder count in halsow. You won't need current limiting resistors for that test. I just use the Mesa calibration data on the back of the THCAD to set up the voltage values for plasmac. I always figured Peter would have much more accurate calibration methods than I have!

Indeed. Though I thought it would be fun to put a potentiometer on the thcad input circuit in a voltage divider and watch what happens in plasmac since I'm not able to wire it into the plasma cutter right now.

I strongly suggest that you use a relay for your arc start signal to preserve the galvanic isolation between your electronics and plasma cutter. While with the correct polarity connection the 7i96 out should work ok I certainly would not build my machine that way.

John

John,
And because the input pins use optoisolators I don't need further protection there?

The 7I96 outputs _are_ galvanically isolated from the rest of the 7I96 circuitry
(and each other)

No it is because the ArcOK signal from your plasma is a dry contact (relay) closure the supply for it comes from your side of the system.

PCW

Thanks for the clarification on 7i96 outputs. As a general practice I think I will still be inclined to recommend using a relay for arc starting to be doubly safe. Most people can't repair a surface mount board at home.

John