Welder I built. 

(click images to see larger views)

Welder I designed for soldering everything from dial feet to gold jewelry.  I actually built this and use it daily.  The only modification I made that is different from the schematics above is that I omitted the rheostat because of cost and size.  The unit puts out 3.2 - 16 volts AC 8-16 amps.  The transformer used is  dual tap 3.2/6.4 v 8 amp AC, the switches were all purchased from Radio Shack, I made the output terminal block housing on my milling machine out of Teflon, the housing holds the five micro switches to complete the volt meter and indicator circuits as well as brass tubes for the banana jacks of the output leads.  The unit could be made much more simply by omitting all the lights and meters.  The the circuit above allows transformer taps to be used in series, single and parallel and allows two voltage ranges to be outputted to the jacks, if the rheostat is used the input voltage could be varied so the unit could be fined tuned very closely.  If anyone is interested in making this let me know.  BTW, Yes the drawings below are copyrighted and can only be used for single personal use.  The design, schematics, drawings, contacts and welder were all 100% created by Denis Carignan of Carignan Watch Company, Inc.

Welder front view

Negative contact top view

Negative contact fingers open

Negative contact bottom view

In use

Zoom of arc

Drawing of front layout

Drawing of component layout

Wiring diagram

The design, schematics, drawings, contacts and welder above are copyrighted and can only be used for single personal use.  The design, schematics, drawings, contacts and welder were all 100% created by Denis Carignan of Carignan Watch Company, Inc.