Welder cut-off switch

For a mechanical engineer with some electrical tendencies an arc welder is somethine that one simply cannot resist monkeying with. There's just not that many things around the house that (intentionally) crank out plasma.

One of the dangers of working with welders (besides the carcinogenic UV, the flesh sizzling heat, the lung wrecking smoke, and the chance of burning your garage down) is that the O/C voltage of the electode is around 80VAC on the older units. The CSA standard for "intrinsically safe" electronics is 24V max, meaning that the resistance of your (dry) skin alone is enough to protect you from this voltage. 80VAC is comfortably enough to fry you even with dry skin.

Some of the newer welders have low voltage start circuits which keep the O/C electrode voltage in the 20-30V range, making them safer in this regard, but since the old Lincoln Electric unit I use has been around longer than I have, it isn't so equipped.

I'm an electromechanically inclined fellow, as I mentionned, and often approach this sort of an issue in terms of "What scrap auto parts can I use to address this problem?". Well, conveniently enough the main starter relays that come with most cars fulfill this need nicely. The ones I've found to work best are the insanely oversized beasts installed in early 80's Fords. Here's a picture of just the type:

This relay is designed to switch 800A in your car, with an O/C voltage of only 13V, so it's built with a very low on-state resistance, and can handle currents larger than your welder can put out. I get these babies from old junkers at the local yard for cheap.

Before it can be used in our application, it will need some changes made to it:

1. Grind or drill out the four rivets that hold the metal plate onto the bottom. There is a cork or rubber insulator gasket in the lid, don't hurt it. There's springs in there, which might make a mad dash for freedom. Be careful not to loose them.
2. Observe, and be impressed by, the ridiculously oversized copper cookie used as a conductor, and shudder at the heavily gouged out boltheads. Compare this to the wimpy copper spring work in a modern relay, and you'll know why I like these old monsters.
3. Undo the two little nuts from the small screws at the side, and the two large nuts from the power leads.
4. Extract the armature, the small screws, the big screws, and the coil.
5. There's a wiggly strip of spring copper connecting one of the small screws to the armature. Disconnect it from the armature (snip) and discard it. You'll need it's little screw as one of the coil terminals.
6. Drill a number of cooling holes in the black bakelite body, and in the coil shield. This relay isn't meant to operate at high duty cycles, so it'll need cooling to work. There is a thermal cutoff inside, so it won't fry if you don't drill enough holes. It'll just cut out on you annoyingly and unexpectedly.
7. Reinstall the coil into the mangled shield. Make sure to get the orientation right.
8. Strip insulation off the coil wire leads and loop them under the heads of the little screws, so that these become the new coil terminals. On some relays one of the coil wires is already welded to the screw, which saves half the work as long as you don't break it during dissasembly.
9. Reinstall the two big bolts, with their smooth (unabused) side facing upwards.
10. There's a brass cup on the end of the armature, which controls how far away the cookie is from the bolt heads in the non-activated position. You'll need to grind this cap down to reduce the chance of arcing inside the relay (don't worry, copper doesn't hold an arc well, try it and see some time). Grind it down to about half it's original height.
11. Reinstall the armature, and poke it a few times to make sure it cycles up and down smoothly.
12. close up the case, and the relay is done!

Now you need a 12V power supply that can source a couple of amps, and a switch you can mount on the electrode holder. Attach the switch (microwave door switches work well if you can find a small one) to the electrode holder in a comfortable trigger position. Epoxy putty works well to hold it in place. I'd set a loop of stiff wire in the putty to act as a guard on the switch. This helps to prevent it triggering when you set the holder down.

Mount the relay onto the welder body (or a convenient place nearby). Connect the work lead from the welder to one of the relay's high current lugs, and a then use the other as the new work lead source. The electrode leader would work just as well electrically, but it tends to get yanked on more.

The rest of the wiring is simple. 12V drive from the power supply is controlled by the switch on the electrode holder. When you press the trigger switch the relay activates (with a loud, very solid sounding *CHUNK*), and your circuit is live. Let go of the switch and the relay releases (with another *CHUNK*) and the circuit is once again dead.

Besides the safey aspect, there's five other handy things that come from this relay:

• Because the circuit is dead until you press the trigger, you don't have to worry about accidentally starting an arc as you move the electrode close to the work. This reduces the blind groping you need to do once you put your mask down, because the electrode can rest right on the work until you're ready to arc.
• When working on light guage materials you can start and interupt the arc in short bursts to keep the heating down, without trying to keep an arc live at 30A.
• Not that this ever happens to me, but if current isn't well matched to electrode size and your electrode gets stuck, you simply let go of the trigger switch. No more panicky arm flailling yanks.
• No splatter all over the place when breaking the arc. Again, it stops when you let go of the switch.
• Use of a big solid CLUNK-CLUNK relay means the you KNOW it's working. You hear it turn on, and you hear it turn off. No uncertainty there.

Handy thing, all in all. BTW, if you still have problems with the coil overheating, try putting a couple of automotive turn signal bulbs (parallel) in series with the relay. This works well because the bulbs start with a low impedance for the innitial armature grab, and then rise when hot to keep a lower holding current.

Alternatively, use a 9V power source to run the relay instead of a 12V power source. This should cut the heat generated in the windings in half. 9V is still within the range that one should design automotive electronics to work in, so if the fellows who designed this relay did their job well she should catch and hold at this reduced voltage. Using the lower voltage is a neat solution, as it allows the power supply to be smaller.

Here's some pictures of what the installed setup looks like. Note the light bulbs used as current limiters, and the XT power supply driving the whole mess.

20030506 I've since been told that RV battery selector relays would be a perfect choice for this application, as they can handle high surges and steady currents, and are designed to run a 100% duty cycle. No more chopping holes in the relay casing!

20030929 I just spent a while trying to find any of those relays. It seems that a line of latching relays has largely replaced them. I found something off a marine site, which has very macho ratings and slick case, but isn't going to be cheap. The altered starter relay may not be such a bad deal.

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