Slightly OT- DIY spot welder & big toroid

[Conrad Hoffman]

I want to build a small spot welder for battery terminations, thermocouples and thin stainless steel. The usual route is to find a big transformer, unwind the outer secondary (secondary has to be outside for this to work!) and wrap it with about 4 turns of #4 wire. I have a large toroid (feels like about an 8 pounder) that would make this trivially easy. Don't terminate the secondary, just add the #4 wire. My question is, is there anything unique about toroids that would make this a bad choice for the application?

Thanks,
Conrad

[pdul]

Just be shure to have a well dimensioned overload protection on the primary/mains side or you can end up with a serious bonfire, if the electrode
sticks during your welding

[Elvee]

Microwave ovens transformers are nice for that purpose. The original secondary can be easily hacked away to make room for some turns, and they include a magnetic shunt that limits the short circuit current.

[AndrewT]

I saw a field worker spot welding a strain gauge to a cast iron pipe that needed testing.

He used an off the shelf gizmo that was expensive looking, but really all it contained were timers and voltage setting and a big bank of capacitors.

They must have been high discharge current rated (=expensive)

[djQUAN]

was gonna make something like this but is too busy and couldn't find a suitable SCR at junk stores.

DIY Battery Tab Welder - Capacitive Discharge Type - RC Groups

[Conrad Hoffman]

Thanks- I didn't know that MOTs had short circuit protection, and the battery welding info was interesting. I've used Ewald welders in the past, and they're fairly simple. A variac controls a rectified voltage (0-400 or so) to charge a big electrolytic. When the unit fires, the cap is discharged through the primary of a big transformer with a 1-2 turn secondary. The old ones used a mercury relay, but I assume the newer ones probably use a big SCR.

Does anybody have a list of what can be welded to what? Can nichrome be welded to copper, or is an intermediate needed, like nickel or something?

Thanks,
Conrad

[Matt.B.H.]

We used to make thermocouples now and again at my last job. We would just twist the two metals together, they just have to be in contact and then silver solder them together. Not the best but it works really well, I have seen some copper and nichrome ones that are spot welded so it is possible.

Cheers Matt.