"Increasing the electrode diameter will give you more welding current as well although you may be limited by your transformer winding and leads."
Thanks, I may try that! I may also add another transformer in parallel. I'm welding steel sheet. Probably need to clean it better as well.
Appreciate your ideas.
I don't know what specific design you used to make yours but in looking at some of the designs on line, it appears that there is a wide range of secondary coils used. My Miller is a 1.5 kva unit and the secondary is a U shaped piece of copper strap with the tongs connected to the legs of the U. The secret with spot welding is to build up heat very quickly and quit. My time settings for welding are typically between 1 and 5 seconds.
The Miller has a 1.6 volt secondary, implying a 75:1 turns ratio. It also puts out over 4,500 amps with 12" tongs. The resistance of the secondary is about .1 milliohms so closed circuit voltage will be somewhere around .5 volts. This would imply a power output of around 2.5 kw. The duty cycle of the welder is 50% so it kind of fits.
when designing a welder, you want just enough voltage to overcome the secondary IR drop. Fewer turns means less resistance and more current. It makes sense to run a honkin'
big secondary, single turn, out to big tongs and concentrating the output power in the work instead of losing it as heat in your windings.
I see that copper pipe is used for the tongs on some of the home brews. The resistance is governed by the cross sectional area of your conductor. 5/8" copper bar has a weight of 1.19 lb./ft. Schedule K hard drawn copper water pipe has a weight of .27 lbs/ ft. This means that the resistance will be about four times as much as for the 5/8" solid bar. If I were building a spot welder, I would use the largest size tubing practical and probably fill the interior with a length of large gauge copper wire to decrease resistance further. Better yet, I would order some copper bar from McMaster. My tongs are bent at a right angle and the ends threaded to take the welding tip. If you are running a copper bolt through the pipe to form the welding electrodes, you want to make sure that you have solid low resistance connections. The objective is to decrease the circuit resistance to a minimum. The currents that you are dealing with are an order of magnitude greater than arc welding currents and the voltages an order of magnitude lower.
Good luck wit sorting it out.
Bob
