here may be an answer to having both AC and DC capabilities
the conversion was less than $50
Thanks for posting that. I am familiar with rectifying AC to get DC. That was one of my questions in my first post - the stick machines that have DC capability - do they simply use rectifiers like that or is there any filtering to smooth out the DC?
If you take an AC signal and run it through a rectifier you will not have clean DC on the output. The power out of the rectifier will pulse at 30hz with a 60hz AC input - the opposite cycle of the voltage will be reversed and stacked with the initial cycle. So your 60hz signal is still there, twice, on top of each other = 30hz pulsed DC. It is DC because the voltage is all either above or below 0. It does not cycle polarities - that is what the rectifier is to correct.
Back in the day, inductors were used to act as some filtering to smooth out the ripple in the rectified DC post-transformers. It would seem to me, in today's day, and even back to the 80's or so, that capacitors would be a better solution to filtering the rectified DC to smooth it out. That is not overly complicated - you just add a capacitor, or series of capacitors, of appropriate values across the positive and negative leads of the rectifier.
What I don't know is if that filtering would make a significant difference in weld quality. The inner-electronics-geek in me says the DC must be filtered. But I don't know how you would go about capacitance values that would keep the voltage stable under arc. Mind you, pulsed DC (purely rectified) is NOT clean/stable DC.
I'll dig a bit and see if I can find some schematics for some of the AC/DC stick machines and see what they show. If a DC version of the tombstone welder is just a rectifier and a switch (no filtering) then I might as well just buy an AC machine and do the mod. We'll see.