Learnings from building a spot welder by Bart Oostenrijk

see attached some pictures of the spot welder. I found that a long cable (like on the picture) causes too much power loss, so I made it into a very short cable instead. The inside is an old microwave oven transformer with the secondary coil consisting of only two windings, powered by a solid state relay (AC, zero-cross switching, 40A max). When you push the small push button on the handle, the transformer is engaged very shortly, and a high current will flow at a low voltage (about 4 Volt).

There will be two successive pulses: one to ‘clean up’ and preheat the metals, and one to do the actual melting of the metals. The duration of the main pulse can be set by a potmeter.

On the pictures you can see two brass bolts I used for the contacts, but they did not seem to do the work. I then changed to copper electrodes, and they seem to work better.

The SSR is controlled by an Arduino Uno,