[joekneale]

Jody has a few videos that talk about the benefit of having an on/off switch for scratch start tig applications:

https://www.youtube.com/watch?v=CA3w9Wq5mh0

It basically lets you retain shielding after the arc goes out because you can turn off power without having to "snap out" of the weld. Pretty good idea - have a DC stick welder I am converting to DC scratch start tig and am sold on the idea of having on/off control for the tig torch.

I wanted to do something similar but thought back to a hobby electronics project I did that used relays. Relays are basically on/off switches like the knife switch Jody has set-up that can be controlled by another switch. Ie, you mount a small radio shack on/off button on the tig torch and wire that to the relay which shuts juice off at the stinger.

I did some research and found that for high amperage/high voltage applications the term isn't "relays" but "contactors". i did some research on this forum before posting and found a few interesting posts:

http://forum.weldingtipsandtricks.com/v ... tor#p50268
http://forum.weldingtipsandtricks.com/v ... tor#p11069

I started looking up contractors that could handle 120vdc at 100a and what I found is that those units are several hundred dollars. I began questioning the assumption that I needed to be at 120 volts and found from this forum that when people have found ways to measure the output of stick welders at the stinger that it is usually something like 30vdc @ 80 amps or 20vdc @ 50amps. I know little about electronics so its been tough for me to feel confident buying something off eBay and just trying something out. A few questions for the knowledgeable folks out there:

1. What would be a safe volts / amps rating for an 80amp dc stick machine that plugs into a normal wall outlet?
2. Is there a way to measure the actual output of my machine to know for sure what it is putting out at the stinger?
3. Does anyone have experience or tips on where to buy a relay / contractor suitable for this application?

[joekneale]

After thinking more about this, there is no way to get more watts (amps x volts) at the stinger than there is at the wall outlet?

If that's true, I have the welder hooked up to a 120v ac outlet with a 30amp breaker, so 120x30=3,600.

I found a contactor / relay used in automotive applications, it's rated at 500a continuous at 12vdc or 6,000 watts:

http://m.ebay.com/itm/New-500A-AMP-DC-1 ... nav=SEARCH

This sounds like it can move the right amount of total electricity, but I'm concerned the low volt rating per it's intended use (automotive) will lead to it being burned up? Can anyone with some electronics experience weigh in here?

[Poland308]

It's the amps you need to worry about. Contractors in the 75-100 amp range from an HVAC supply house should do what you want. But even one rated for 100 or more amps will arc up the contact area very fast. This will cause a bad connection in your weld circuit, make the arc unsteady, overheat the contractor and melt it down or burn up the wires attached to it.
There is a technique were you start to pull out slow at the end of your weld this allows the puddle to cool some making it less likely to crater. A lot of pipe work is done from scratch start machines with out any extra controls it just takes practice.

[Arno]

Yup.. As mentioned above the issue with running a contactor in this kind of high(er) amp setup is that every time you open up the connection the current doens't like that and will want to continue, so it tends to arc across the contacts for a while as it opens. This tends to wear down the contacts pretty rapidly and they become flaky as far as good conductivity goes.

Can be (mostly) fixed with some additional circuitry if you google for 'spark supression' with relays and contactors, but it will make things a bit more complicated. The contactor still will wear over time, so it's more or less a 'consumable' that may need some periodic replacement.

The beauty of the knife style 'switch' Jody used in some of his videos is that it has such a huge contact area that's also sliding across each other as the switch is operated that it will always have clean contact and take a long time to wear out. And even then it's cheap

An alternative path to take would be to go to the source and open up the welder itself, try to get some schematics, and see if you can find a low voltage, low current location in the machine that drives the inverter and use a relay on that. That would need to be a much more simple relay and could be hooked to a torch-switch.

Bye, Arno.

[PeterM]

All switches and contacts have a current and voltage rating. The voltage rating has to do with how far apart things are spaced in the switch or contact, when in the open position. High voltage will arc across openings. Switches with a higher voltage rating have greater spacing between parts. If you use something at a higher voltage than it was designed for there is the possibility for it to turn on even when in the off position, a dangerous situation. The 120RMS Volts actually has a peak value of 169Volts. So you will be applying voltage 14 times the rating of the switch.
As Gary Larson would say "Trouble Brewing"

[MinnesotaDave]

Just buy a contactor for a mig or tig machine - they open and close all the time while welding.

Some use 120v AC, some use DC.

They are available on Ebay pretty cheap.