[rahilm]

First off, let me start by thanking this community for all the help I've gotten.

Anyway, I'm setting up an ohmic sensor for the CNC plasma, and I've run into some trouble. I've shown how my isolation circuit is set up in the attachment below. (I got this from another post on Mach support forums)

As it currently stands I can open and close the first relay fine, but the second one won't close when the torch shield touches the metal (which of course is attached to the ground clip). Now the way the circuit is set up, this makes some sense to me, since the plasma ground is not connected to the ground on the 12v circuit the relay is running from. Is this correct? And if so, is it safe to connect the plasma ground to the ground on the rest of the electronics?

Thanks in advance.

[Torchhead]

I'm confused. If those are conventional relays the way you have it drawn it won't do anything. Technically you could do it with just one relay. Supply the + 24VDC to one side of the relay coil. tie the other side of the coil to the tip of the torch. Tie the negative side of the 24 V power supply to the table. Then the tip hits the metal it (should) tun on the relay and the contacts go back (isolated) to you input.

DO NOT tie the plasma table ground to circuit ground unless you are 100% confident that that ground is at the same potential as your safety ground on the controller/PC othersie you will fry the PC. Make sure you put a diode in line with the relay coil or reverse voltage when the torch is running could damage the relay coil. They way it i s drawn I assume the other relay is to enable the ohmic touch only when you wan a touch off but it eats an output.

TOMcaudle
www.CandCNC.com

[Torchhead]

Looking at it again.....it should work IF the primary relay is ON and you have a floating 24VDC supply (like a wall plug) and tie the negative side to the table (plate) side.

you stil want to keep separate grounds for you table and controls to block noise and nasty spikes. The independent supply does that for you. the setup won't be very sensitive. You will need to have a very solid connection between the torch and the metal. The whole current of the relay has to flow through the connection. If the tip is dirty or has slag or the metal is dirty or oily it may not pass enough current to turn on the relay. Make sure you have backup it for some reason it misses and tries to jam you torch down until something breaks.

[rahilm]

Looking at it again.....it should work IF the primary relay is ON and you have a floating 24VDC supply (like a wall plug) and tie the negative side to the table (plate) side.

you stil want to keep separate grounds for you table and controls to block noise and nasty spikes. The independent supply does that for you. the setup won't be very sensitive. You will need to have a very solid connection between the torch and the metal. The whole current of the relay has to flow through the connection. If the tip is dirty or has slag or the metal is dirty or oily it may not pass enough current to turn on the relay. Make sure you have backup it for some reason it misses and tries to jam you torch down until something breaks.

Yes, this is what I'm doing. The two relays are there just incase of a voltage spike in which the voltage jumps the contacts in the relay, so only the second relay could be fried and not the BOB. I decided to just hook up the ground to the ground on my power supply (a separate wall wart), and so far it all seems to work all right.

Thanks for the help