Even though I am mostly using off-the-shelf modules (with some modifications) in this project, maybe it would be also a good idea to have an option to build this spot welder with custom-made, purpose-built PCB. For now I created a simplified schematic which should demonstrate how this device operates. I re-created the relay module using discrete components. However, the step-up converter module is shown as a "black box" for now. I will need to think about a suitable IC which could be used here. Current converter can charge the capacitor bank in around 4 seconds with 12V/3A input, which is a pretty good result. I thought about using MC34063 as a replacement, but it would be at least 4 times slower. And everybody knows it's ancient, but it's also very cheap and easy to be implemented. There are also more modern chips, which are better, but more difficult to implement and more expensive. I will need to evaluate all possible options and decide on something.