Recently I followed an instructables on building my own MOT spot welder, and the build went great. I am getting power from a normal home outlet, which is 120V and 15A, and I am getting out around 2 volts, so around 900A for the output of my second winding. I used a large gauged stranded wire for the second winding, and at the end of it, tied so smaller gauge solid wire that is around 2 mm thick as the soldering tips. I started out using it and it was going alright with decent and clean welds, and an occasional spark, with the nickel plate burned with a hole through by one of the copper tips. Now, everytime I try and weld with it at least one of the tips makes a spark, the surrounding weld has a black burn mark, and the nickel strip is burned straight through making a hole, so nothing ends up sticking to the battery well. I have tried reshaping the tips I am using and re-positioning them in so many different ways, but cant get a clean weld anymore. I have also tried changing the pulse time from 10-30 ms with the arduino im using as a timer, but shorter or longer pulses does not prevent the sparks and burns from happening. Any one have any idea why this could be happening and if I just got lucky with the weld I did on roughly 30 batteries?