The idea behind resistance soldering is to stuff a tremendous amount of current through a relatively low-resistance joint, thus producing enough heat to melt the solder and bond the parts. Because power dissipation varies as the square of the current, more current is much better!
Commercial resistance soldering units have relatively small transformers with small windings that impose a low duty cycle: you must let the transformer cool off between joints. A 50% duty cycle seems about the norm for hobbyist-grade units; you can obviously pay more to get more.
Given that the transformer is free, getting one that can supply a kilowatt for half an hour without breaking a sweat seemed like a Good Thing. It hasn’t warmed up appreciably during any of the relatively short projects I’ve used it for… although the electrdode holder can get pretty toasty.
Find a discarded microwave oven: the bigger, the better. Our ancient Sears ‘waver went casters-up at an opportune moment; IIRC, the magnetron filament failed.
I harvested the transformer, line fuse, triac, snubber, and a handful of other odds and ends. The magentron shell became a really nice desk tchotchke.
Transformers really don’t care, at least to a good first approximation, whether the secondary is producing a huge voltage at a low current (which is what ‘waver transformers do for a living) or a piddly voltage at a huge current (which is what we want). The core flux is proportional to the ampere-turns in the primary (a kilowatt is about 8 amps at 120 V) and we can rewind the secondary to get what we want.
This transformer had separate primary and secondary windings, which made life easy. The secondary is on top, the primary below. The primary has about 120 turns of stout wire, giving a turns-per-volt of about 1; that seems to be common with relatively cheap transformers. Remember that number…
With the transformer in hand, apply your least-favorite wood chisel to the secondary winding: chop the entire winding out. Pay very careful attention to not damaging the primary winding. You’ll probably find a few turns of relatively heavy wire for the magentron filament; chop that out, too.
After a bit of beating, the secondary should come out in two hunks that you can toss in your copper recycling box. It’s pretty much solid metal:
This transformer had a pair of flux shunts between the primary and secondary winding that stabilize the output power. We don’t care about that, so pry them out; they’re the small laminated steel blocks just above the primary winding. I also removed the cardboard liner around the core opening. The primary terminals are 1/4-inch quick-disconnect tabs, aimed straight at you so they’re hard to see.
The primary has about 1 turn per volt, so the secondary will, too. You can use nice floppy 4 AWG silicone insulated wire, but I went with four parallel strands of 10 AWG wire stripped from a length of house wiring to get the same cross-sectional area. Five turns produces a 5 volt secondary, which may be a little high; it seems commercial units run at about 3-4 V.
I terminated the secondary in heavy copper crimp lugs, but, not having the proper crimper, I made an open-top clamp to support the sides of the lug. Applying a punch to the top did a satisfactory job of making the lug one with the wire. I also soldered the joint, less for electrical goodness than simply excluding oxygen and improving the mechanical rigidity.
Caution: you must use a crimped joint, because the whole point of this exercise is to put enough heat into a soldered joint to melt the solder. Think about this: if you have two soldered joints in series with the one you’re trying to make, what could possibly go wrong?
I put some thin cardboard around the opening to prevent insulation scuffs, but, frankly, that’s not needed: this wire has really tough insulation and can take care of itself.
And then it looks like this…
The components are what you’ll need to measure & plot the B-H curve, as described there. For what it’s worth, here are three curves with 20, 60, and 120 VAC on the primary.
The core is pretty much saturated at 120 VAC, which is a simple form of power regulation: small voltage changes won’t make much difference in the power output. The peak flux density is about 20 kG, out there on the limbs of that hysteresis curve.
Next: put a triac in the primary circuit…
[Update: Herewith, the oven schematic. The transformer core is bonded to one end of the secondary winding. The other end is capacitively coupled to the halfwave rectifier that drives the magnetron. Note that the three-turn filament winding is attached to the hot side and is floating at 4 kV off ground.
None of that matters here, because you're chopping those windings out and replacing them with five turns of husky wire.
Don't you wish all consumer electronics came with schematics?