Introduction: Spot Welder

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:

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

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.

Update 2020: When building powerwall from 18650's, the power of this spotwelder was not enough to properly weld nickel. Could be due to heating of the copper after several welds or just because resistance of the contact vary. I had accident, when one 18650 starts to fire, because i have used too long weld time, so it get too hot. Now i do welding only in garage, to avoid risk of burning my house.

Solution to this, was to connect this spotwelder to line voltage (400V) instead of phase voltage (230V). This has improved weld current really remarkably and the transformer is able to sustain that. Now the time weld is really quick and the cell doesn't get hot.