OK, I decided to make things right this time. The FETs arrived, and their summed lead cross section exceeds that of the 300A ANL fuses that I also ordered. Lets hope that the cross section of all the bond wires is not too far from that. At least the fuses should be okay for a 1kA pulse of up to 0.5 secs.
The next problem was to find a way to connect the FETs to the AWG8 wiring. A PCB trace would certainly not be enough, and I don't want to spend money for PCBs with solid copper inlays.
The idea now is to use solid brass bus bars (8 x 12 mm) very close to the transistors, which will be able to carry the current, and hopefully also provide enough heat transfer to keep the solder from melting.
The first one is finished, that took again quite some time. But I started this, and I will succeed


I also started designing a decent controller with the following specs:
- pulse energy control instead of time control, this will hopefully provide more consistent welds
- ANL / MEGA fuse holder
- 1kA switch for max 0.5sec
- 10 - 24V power supply is a separate unit, I will be using LiPos but capacitors could also be connected
- STM32 micro
- 1x8 LCD with backlight
- beeper
- input for foot pedal switch
- potentiometer for quick adjustment of pulse energy (I don't really like these 'modern' HMIs with push buttons)
- size approx 100 x 100mm
- most important: as low cost as possible, I estimate something around 50-70€
I would be happy to receive comments or feature requests. I want to make this available for everyone once it is stable, maybe run a small batch at Elecrow.
EDIT: circuit diagram outdated, see update here
https://www.eevblog.com/forum/projects/guesses-on-what-i-am-attempting-here/msg1240839/#msg1240839