I believe one of these has been posted before, but not both.
From HackADay.com, here are links to what's being termed a "Junkyard" DIY TIG welder, and a "Commercial-Grade" DIY TIG welder.
The welder has a 180 amp capacity and has most (if not all) of the features of a modern welder. Instead of using a microprocessor, Dave Barrett decided to use discrete TTL and CMOS logic. That decision should make the design a lot easier to troubleshoot. Schematics and board layouts are on his site.