An old battery charger, re-purposed as the charging supply for the welder is rewired and functional. With thanks to some correspondence from Bob Van Valzah, I went with the dual half-wave arrangement using the center tap to get one diode-drop more output voltage, but replaced the “ugly” old stud rectifiers with half a potted bridge. I also went with a 1 ohm series resistor to limit the initial current surge, since the cap looks like a short circuit at the start of charging. Thanks to Bob also for pointing out an error in the schematic of the full-wave bridge diagram in my email 🙁
The supply charges the cap to 13V in about 3 seconds, and to 16V in about 11 seconds. That should be fine in practical use. The quickest I can picture being ready for a second weld – repositioning the electrodes for the second of 2 pairs of spots on the same strip on the same end of the same cell – would be a few seconds.
The red/white pair is now connected so a dry contact across them will fire the SCR. (One wire to the gate and the other through 100 ohms to + on the cap. That avoids the battery many others use to trigger the SCR.) And it’s all mounted to a board which may soon have a handle hole at the balance point with edges nicely rounded off by Andrew’s router. I’m considering mounting an LED voltmeter that came on top of one of the caps to the board. I’m not sure about that one yet, as it costs a continuous 30-90 mA.
One remaining step is making a foot switch to shut off the supply and fire the SCR. It will have two switches mechanically staged so the one that shuts off the supply will always close before the one that fires the SCR. The other remaining step is making the solid state relay to shut off power to the charger.