I needed to have a spot welder, for making packs and building powerwall from 18650 li-ion batteries. It is also useful in other ways, for example welding thick steel plates or repairing some broken stuff.
I was following this instructable: https://www.instructables.com/id/How-to-Make-a-Spo...
But i put there PCB with timing circuit. This allows me to have "pre-weld" feature and easily adjust the time of the "main" weld.
Step 1: Salvaging the Transformer
I've salvaged the 1100VA transformer from old microwave oven, then i cut the secondary coil as is in referenced instructable. For secondary wire i used 70m2 copper and made a two loops (not possible to do more in my case).
Step 2: Electronic Switching
For switching and timing i have used my own design, check it out:
There is just ATtiny microprocesor, optocoupler and triac for switching.
Source codes you can download as zip below. (Eclipse project, C code, compiled with AVR GCC)
You will need some kind of AVR programmer to upload firmware, i have made myself nice programmer, usbasp https://www.fischl.de/usbasp/
This solution was the easiest thing for me, you can use arduino or for example 555 timer.
On the image you can see the black isolated thing, that is just 5V cell phone adapter put out of his plastic case.
There is a switch on the end of arms, that's a start button.
There is two leds, green for power-on signalization and the red one for active welding signalization.
Step 3: Arms
I have put pieces of plastic material like in the picture to have more smoother and robust arms support.
If i want to use opposite welding, i just replace the straight copper electrode with the bent version.
Step 4: Results
I have used it on spotwelding of li-ion pack for screwdriver and to weld some test pieces of 1mm steel. I am very satisfied with the power and welds. I think the pre-weld feature also helps a lot.