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i've been googling my way through a problem and now i am stuck and haven't found an answer.

So here is what I am trying to do. i have 24vdc connected to the positive side of the coil on a relay and the negative side of the coil connected to the positive side which is connected to the tungsten on a tig welder. when the tungsten touches the part (which is grounded), the coil energizes and the contacts close. All of this happens while the welder is on but not welding. In theory this should work.

The problem i am having though is that when the welder is on, touching both the tungsten and the part energized the relay. but when the welder is off, only the negative side energizes the coil as expected. the negative on the welder is connected to the same potential as the negative of the 24vdc supply.

the strange thing is that when the welder is on and the negative end of the coil touches the tungsten, there seems to be continuity between the tungsten and the part even though they are not physically connected.

I am sorry if this not the right audience to ask but i have tried many solution with no success. I even went to my circuits professor and he couldn't figure it out.

Any help is appreciated.

ps: I have tried both mechanical and solid state relay

Schematic

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    \$\begingroup\$ This is the right audience but your description is confusing. The positive is connected to the negative is connected to the positive is... huh?? You will get helpful answers to your questions here if you can provide a drawing of the schematic. We are electrical engineers here, that is our lifeblood. :) \$\endgroup\$ – Joel Wigton May 2 '16 at 3:34
  • \$\begingroup\$ sorry for the confusion. i have added a rough drawing. \$\endgroup\$ – melmatvar May 2 '16 at 3:59
  • \$\begingroup\$ Any chance that the positive feed or negative feed is common grounded? \$\endgroup\$ – Sparky256 May 2 '16 at 4:00
  • \$\begingroup\$ That diagram is most confusing. It implies that the welding current itself drives the circuit. This diagram could not be right. Please submit a more complete diagram, separate the AC power source from the welding cables. \$\endgroup\$ – Sparky256 May 2 '16 at 4:04
  • \$\begingroup\$ Maybe you can find a second grader with a box of crayons to draw the schematic more neatly for you. \$\endgroup\$ – Olin Lathrop May 2 '16 at 10:55
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Welder monitor circuit.

It sounds as though you are describing the circuit in Figure 1 and the problem is that the relay is switching on tungsten contact with the part when the welder power is turned off. (You only want it to switch when it's capable of welding.)

I suspect that there is a path to ground through the welder. This is shown as R1 in Figure 1.

schematic

simulate this circuit

Figure 2. A possible solution. (D1 is shown for reference only.)

If this were a low current application the addition of D1 would solve your problem by preventing the relay coil current finding the path to GND through the welder. Since welding currents are very high this solution probably is impractical.

A simpler way is to, somehow, monitor the welder power signal. I've shown an additional contact on the power switch but it could be a mains relay across the welder power input. Use a contact of the power monitor in series with the relay coil to prevent it energising if the power is off.

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  • \$\begingroup\$ Sorry for the poor schematics. @transistor: I have tried your suggestion of placing a diode. \$\endgroup\$ – melmatvar May 3 '16 at 1:49
  • \$\begingroup\$ I said, "Since welding currents are very high this solution probably is impractical." What sort of diode did you use and what happened? \$\endgroup\$ – Transistor May 3 '16 at 5:16

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