This is the first of many questions I have about the project I'm working on which is a partially homebuilt welder using parts from two different welders plus some extra sourced parts. The 240 Vac primary power will be controlled by phase angle through two scr's. I would like to isolate the firing circuit from line and load power through a transformer for saftey reasons. My first question; is it better to have the isolation transformer in series with, or in parallel to the welding transformer? If the former is better, then the primary of the isolation transformer will have to carry the full load of the welding operations, and be wound with 6ga wire. If the later is better, I was thinking, the size of wire for the primary of the isolation transformer could be smaller being that the firing circuit won't require as much power. Some other considerations that I have are a couple time delay circuits as well as some relays that will also need to be powered and rectified, but only in the 9 to 12 Vdc range. Would it be better to have one secondary winding with a 1:1 turns ratio for the scr firing circuit, and another secondary with a different turns ratio to power the other circuits, and take care of everything with one transformer? Or would having a separate transformer to power the time delay circuits be a better option? I would be thankful for any help, and maybe some suggestions for the transformer specifics of either option.
