Manekinen has come up with this home-brew spot welder that’s powered by a guitar amp transformer. It produces 2.6V at 1kA and can make clean spot welds between two pieces of steel up to .75 mm thick.
The device is enclosed in an old computer power supply box and has safeguards built into the electronics. A limit switch gives a delay before the welder turns on so the user can get his/her pieces in place. Also, a potentiometer varies the time of each weld between one and four seconds depending on the material being used. A BT138 Triac closes the primary circuit but Manekinen suggests using something more rugged for the job. There are more detailed instructions on the project at his site if you want to try it yourself.
[via Hacked Gadgets]