Some of you more experienced guys will probably laugh when I describe this project, but here goes..... A few weeks ago I got the idea to build my own arc welder, as I haveseveral old lawn mowers that needed welding and I was too cheap to either take them to a pro or buy a decent welder. Also, I have a fair knowledge of electronics and I've always wanted to learn how to weld. I found some plans on the net describing a homemade unit, made from 8 microwave oven transformers (MOT's), and using a high-power SCR module to adjust the power output. I have plenty (literally dozens) of MOT's to work with, but I wanted to keep the weight under 80 lbs so that the unit could be somewhat portable. So, I decided to use three MOT's instead of 8. For those of you unfamiliar with MOT's, these transformers weigh anywhere from 8 to 16 lbs, the primary has about 150-200 turns of ~18AWG and the secondary has several thousand turns of ~28AWG. With an input of 115VAC, the output is typically around 2000-2300V. I selected three large transformers, and with the use of some power tools I managed to get them apart, remove the secondary windings, and wind new secondaries. Initially, I wound the secondaries from #4 AWG cable - there was only enough space for about 4 windings on each transformer. The total voltage output of all three transformers (in series), with no load, was about 12 volts. It's hard to estimate the current potential but it was probably around 100 amps or more. I decided to give the machine a try at this point (before adding the cooling fans/thermal protection/other gadgets) so I wouldn't have to backtrack too far if there was a problem. By now I had obtained a proper ground clamp, electrode holder, welding mask ($5 at a flea market) and a box of 5/64" rods. I took the rig outside and plugged it in via what appeared to be a heavy extension cord. For my first try, I decided to attempt to cut a thin (less than 1mm thick) piece of steel pipe. I tried to strike an arc many times, using a scraping motion, but to no avail. It would not sustain an arc, and the rod would just keep sticking to the pipe, sometimes melting the rod. At this point I assumed that the voltage output was not sufficient to create an arc, and I set about making new secondary windings. Since I couldn't fit anymore #4 AWG on the transformer cores, I removed it and rewound the secondaries with #10 (solid) wire. Yeah I know it's rather thin, but this unit is only intended for light to medium-duty use, and it will have forced-air cooling and thermal overload protection when done. Anyway, I wound about 8 windings on each transformer, for a total voltage output of about 24 volts. I tried it again, and this time it almost managed to start an arc. So I added a few more transformer windings (voltage now up to about 35V) and this time I was finally able to strike an arc. Quite thrilled that I had succeeded so far, I continued cutting up the pieces of steel and trying to get the hang of this new hobby - this was the first time I'd ever welded, after all! But remember that extension cord I mentioned earlier? It was a 100' retractable cord, and I had only pulled out about 10 feet to reach outside the house. The rest was left rolled up on the spool, next to the wall where it was plugged in. A few minutes later, the welder dies and when the fuse on the machine checks good, I decide to check the extension cord. I guess I shouldn't have been surprised to see smoke rolling off the coiled cord. I took it outside, and in an attempt to keep it from melting together, pulled it all out and allowed it to cool. Closer inspection of the wire revealed it was very light - only 16AWG, but I suspect that wasn't the only problem..... Once I located a heavier extension cord, I fired up the welder again and to my surprise, it was much hotter than before and would actually blow the breaker box in the house whenever I struck an arc. Based on my electronics knowledge, I suspect the first extension cord, in its coiled state, may have acted as an inductive ballast, limiting the current available to my welder. Would I be right? So that's how this project has gone so far. Closer inspection of my house wiring revealed that two entire floors, in addition to the outside plug I was using, are all wired on the same 15-amp breaker. I'll probably have an electrician install a 20 or 25-amp breaker, as well as perhaps bring the wiring up to date. At this point I'm not sure whether my transformer design is OK or not. Which brings me to my last question - what effect do current and voltage have on the arc? My uneducated guess is that the voltage is needed to sustain an arc, and the current is needed to heat the rod. I still have a lot to learn about this hobby, but if I can at least get this machine to function reliably that will be a good start. Thanks for any advice.