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thod9992 years ago

I made one from an old microwave transformer a few years ago. Works well, 4V about 100A i'd say.

Some background. FYI this is super dangerous and basically shouldn't be done by anyone. Ever.

Microwave transformers massively up the voltage (i.e. secondary:primary coil ratio is a high number) so we actually change where we apply 240VAC to what was the secondary coil.

Then we replace the primary windings with some super thick earthing wire (about 6mm2 - ish) I only did 4 windings as i wanted 4V, i.e. wanted about a 60:1 (primary:secondary) ratio

Then put a 240V switch on the primary coil side (what was originally the secondary) and used some plywood to hold it together and some copper pipe for the welding surfaces.

I was pretty scared to turn this bad boy on originally but it worked a treat.

Wouldn't advise it following my lead but i had a good time making it


Chris, Sydney

raytoyama3 years ago

Hello Mr. Lopez. I am interested in building a welder like yours. May I know what kind of transformer you used. Regards from Japan, Ray

RUAN1115 years ago
Here is a guy that built a really nice battery tab welder
I actually built a homemade spot welder, a few years back, intended for attaching tabs to batteries. The performance wasn't all that great, and you kind of needed three hands to use it (two to hold the electrodes in place, and a third to press the trigger button). Basically it was small capacitor bank, discharged through a spark gap switch. It was similar to some of these "coil gun" circuits the kids are building these days, except the load was the resistive spot where two pieces of metal came in contact.

BTW, I think the trick to how spot welders work is that the spot where you want the weld to go has to be the largest resistance in a series circuit of different sized resistances. In a series circuit, every element shares the same current, I, and the element with the largest resistance, R, will dissipate the most power, I2*R.

Let me see if I can find that old thing, blow the dust off from it, and take some pictures, and draw you a circuit diagram...

While you're waiting for me to find that thing, if I still have it... take a look the page linked below. They've got some good videos of hot, explicit, battery spot-welding action!
Attached are some pictures of my homemade capacitor-discharge spot-welder.  It is crude, and it doesn't work very well.  But I think there is the possibility of improving it.  Anyway this can maybe help give you some ideas.

Links to larger resolution versions of the pictures attached below:
framistan6 years ago
If you are just a hobbyist I suggest just soldering the tabs together. You might have to use a knife or sandpaper to "rough-up" the surface first. this makes soldering easier. I repaired hundreds of battery packs this way. Sometimes if a tab is missing, just solder a copper wire to it. Dont use wire that is too thick or you will not fit the battery back into the pack. Dont use wire that is too thin, or it will not carry the amperes needed. If you are starting a battery repair business, then it would be best to purchase the tab welder and do it right. do not ever solder those tiny WATCH batteries. they EXPLODE.
lemonie6 years ago

Difficult - this is the sort of device you're best buying.
Otherwise if you had the means and ability you wouldn't be asking the question.

Yes, Google for it.