Slightly OT- DIY spot welder & big toroid - diyAudio
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Old 23rd February 2010, 02:30 PM   #1
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Default Slightly OT- DIY spot welder & big toroid

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?

I may be barking up the wrong tree, but at least I'm barking!
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Old 23rd February 2010, 02:39 PM   #2
pdul is offline pdul  Denmark
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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
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Old 24th February 2010, 08:51 AM   #3
Elvee is offline Elvee  Belgium
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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.
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Old 24th February 2010, 09:58 AM   #4
AndrewT is offline AndrewT  Scotland
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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)
regards Andrew T.
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Last edited by AndrewT; 24th February 2010 at 09:59 AM. Reason: 36inch gas main carrying gas past a building being demolished 600mm away!
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Old 24th February 2010, 11:41 AM   #5
djQUAN is offline djQUAN  Philippines
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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
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Old 25th February 2010, 04:08 AM   #6
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Location: Canandaigua, NY USA
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?

I may be barking up the wrong tree, but at least I'm barking!
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Old 18th March 2010, 10:22 PM   #7
Matt BH is offline Matt BH  United Kingdom
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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.
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