Last year I built myself a welder using microwave oven transformers; I used it for a variety of things, but it was far from finished. Recently I’ve done some work to finish off the main transformers and to add a current limiting feature.
The first thing to do was welding the main transformer cores together, this reduces vibration and allows more convenient mounting. Ideally I would have had access to a second welder, with which I could have welded the cores while they were not plugged in. Unfortunately I didn’t, so I had to settle for a slightly precarious method which consisted of gaffer taping wood over the windings and welding them using themselves. I sanded a patch of varnish off each core and clamped them one at a time in a vice, to which I had connected the ground clamp to complete the circuit.
I didn’t run into any problems caused by the cores being in use while being welded; they’re a bit ugly because I’m shit at welding, but work fine. Once they were welded together, it was a trivial matter to bolt them down to a board.
Previously it lacked any way to control the current; I frequently found that it was too powerful, It would burn 1/8″ rods and weld steel down to about 3mm, but anything smaller it would just blast holes and generally make an awful mess of. To sort this, I built a current using a third MOT, this time I’ll show how I stripped the transformer.
The first step is to cut the seam holding one side of the core together with a cutting disc in an angle grinder; once it has been ground out, a sharp upwards tap with a hammer will break any remaining weld or varnish and open the join up slightly.
I then baked it in the oven at around 200°C for about half an hour, this was to help soften the varnish. After taking it back to the garage, I used a piece of wood and a mallet to alternately hit each end of the primary; after a few minutes it slipped fully off the core. Unfortunately in the process the paper wrapping remained stuck and pulled a few turns free of the main winding.
Luckily there was no damage to the insulation and I was able to carefully bend them back into position and wrap the whole thing in kapton tape to both hold them in place and protect the winding against future damage.
I was not so careful with the secondary (I’ve already got more lying around than I know what to do with), I walloped it out and in the process damaged the former and possibly the insulation in one or two places.
Without going into too many details, an inductor essentially acts like a resistor to AC currents (although an ideal one doesn’t dissipate any energy as heat); my plan was to put it in series with the primary of one of the transformers to limit the current going in and therefore reduce the current coming out. Using an MOT as an inductor is simple, the only tricky part is making it adjustable. Generally inductors consist of a coil and a core; changing the number of windings on the coil would have been tricky and would probably have resulted in bad connections and ruined insulation, but changing the core was fairly straight forward. The E and I sections placed together form a flux circuit; the aspect that matters here is that changing the cross section of this flux circuit will alter the total inductance.
It is easy to change this cross section by partially sliding the E piece off the top of the I piece; to support the core while this happens, I bent and welded a couple of bits of steel into guards and added some wood to support the E piece.
The core needed securely clamping together, but in such a way as to allow easy adjustment. I made a retaining bar to apply pressure from a piece of ~3mm steel (the handle I cut off an old gas bottle in another project) and added a pair of quick release bike axles to hold it down. They were a bit too long, so I used a die to extend their threads further up the rod then trimmed them down to size; I also reused the original nuts as hand adjustable lock nuts.
I cleaned up the wiring with a couple of kettle lead sockets and added a switch to enable/disable the current limiter; Ideally it wouldn’t have had the middle “off” position, but it was the only 15A 250V switch I had to hand.
I installed some neat-ish wiring, leaving some slack in the current limiter wires to allow movement without any strain. I have not yet cut the board down to size as I intend to add a fan and/or carry handle at some point, but I have not planned out where these will attach.
The full circuit diagram is as follows.
The two mains inputs are not connected directly together for several reasons, but if they must be plugged into the same phase for the welder to work properly. At full power they must go via separate 13A fuses, but when the current is limited to the lower end of the available range it is possible to use a single extension lead without blowing the fuse.
While I still don’t have a current clamp to measure the currents, it certainly drops it down to a more sensible range for smaller work. A partially completed version of it allowed me to weld a ~1mm steel box, and a setting slightly up from the lowest current allowed me to weld together some steel rods I had lying about that are a shade over 2mm diameter (that’s a 3.2mm welding rod for scale).
Again, my abilities are a bit dubious, but the limiter clearly works (without it these rods would just have been destroyed).