On Tue, 9 Mar 1999 08:47:13 -0600 Lawrence Lile writes: >On the HV side, a relay would be simpler than Triacs, though. Just >how fast >can a 50 amp contactor operate? Probably pretty slow. I'd be >surprised if >such a contactor could close and open in less than 0.2 seconds. >That's WAY >too long for welding thin sheet >metal. Besides, I can probably lean on somebody for samples of a >triac! I think the one in our shop pre-charges a capacitor to a certain voltage (adjustable by the operator). There is a meter on the front calibrated in watt-seconds, though obviously it is just measuring voltage squared. When the button is pressed, it discharges the capacitor into the weld through a contactor. Thus the speed of the contactor is not important, as the amount of charge in the capacitor approximately determines the energy delivered to the workpiece. An inductor in series with the contactor would probably be a good idea to keep the current from rising too fast. I've never seen the inside of the machine, which is a testimonial to its reliability. It's for small work. The maximum energy is 100 W-s. For bigger welds, a step-down transformer-based machine like you describe seems practical. Definitely use a triac to switch the primary on and off. If you want to optimize it, don't switch at the zero-crossing point, it actually works better to fire the triac about 40-60 degrees into the cycle. This keeps the transformer from saturating on the first half-cycle. ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866]