Building an arc welder from microwave oven transformers

I’ve spent the last couple of weeks building an arc welder by winding new secondaries onto two microwave oven transformers (MOTs). The one important lesson to learn from all of this is that high power transformers are not a place where the “fuck it that’ll do” method works; I wrecked a couple of primaries so ended up having to find fresh MOTs and start again.

The first step was to find two MOTs; the bigger the better, the power will ultimately be limited by the core saturating. An arc welder is essentially a high power step down transformer; something like 20A at 240V goes in and this then gets transformed to something like 100A at 50V (minus losses). I already had the MOT cores and primaries, all I needed to do was remove the old secondaries and add new ones.

First I cut the welds holding the E pieces to the I pieces. I used an angle grinder but a hacksaw should work; the primaries were going to be re-used, so I was careful not to damage them. To help remove the coils from the cores, I softened the varnish by heating the transformers in the oven at about 200°C for about half an hour. Once the varnish had softened, I knocked the windings off using a bit of wood and a mallet; again being careful not to damage the primaries. Once the windings were off the cores I cleaned the cores up a bit, tearing out the old insulation then using a file to remove most of the varnish and smooth what was left.

Now any old wire isn’t going to take 100A so I bought rectangular section enamelled 3.81×2.54mm copper. I measured the size of the windows in the core and decided that 5 layers of 5 turns each would be the best way to fit around 25 turns on each core. Even with enamelled copper and square section, you need to give at least 20% extra to account for imperfect winding otherwise you’ll find that your lovely new coil won’t fit the core. Winding copper of this size is doable by hand, but requires a decent former, some strength and patience. I made a former from bits of MDF glued and shaved to the same width as the center leg of the core. It is longer than the core both so that the corners can be rounded and to allow better air flow around the transformer to improve cooling. The hight needs to be a bit smaller than the height of the windows minus the height of the primary as the windings will expand slightly when you slip them off the former. To make it easier to slip the winding off the former, I wrapped a layer of thin card around the former and taped it together (but not to the former) before starting winding.

The former for winding the transformer secondary

Starting the coil was a matter of poking the end of the copper through a hole in the former and bending it until it laid flat along the end of the former. The copper is rigid enough that no further retention is necessary.

The copper bent and inserted through a hole in the end plate of the former

In order to avoid bowing I had to apply a reverse bend to the copper as I laid down each pass. I had to be careful to get each pass to lay nicely in place and keep all the slightly messy beginnings of each overlap where they would be outside the core; my first few attempts with a bodged former and rushing rather than taking my time wasted both time and copper (once it’s been work-hardened once, it can’t be formed again without lots of trouble).

The secondary being wound onto the former.

Once all 5 layers were complete I applied a temporary wrapping of PVC tape to protect the enamel while I made final adjustments to the size; I squashed the coil slightly using a vice with wood to protect the coil.

The secondary, wrapped in PVC tape, being squashed in a vice

At this point I cut the excess to length, soldered on tab connectors, removed the tape and fitted the transformers together, using OHP transparency as a temporary insulator film.

Transformers just about to be assembled. Cores insulated with OHP transparency

I G-clamped the cores together, attached some 135A auto battery cable with a ground clip on one piece and a rod holder on the other. I wired the primaries in parallel and the secondaries in series, being careful to keep the outputs in phase. Initially I tried feeding it from a single 13A plug, this blew the fuse as soon as I struck an arc. After feeding each transformer from a separate 13A socket, it worked! While the welds weren’t fantastic, they were limited mainly by my terrible welding technique; there was definitely enough heat to get reasonable penetration and melt the rods. I got a little carried away and welded until the vibration damaged the insulation on one primary enough to short irreparably through the core. I tried using the remaining transformer on its own; it could just about melt the rod, but didn’t get any real penetration and was very difficult to strike an arc. It also overheated the transformer to the point of the transparency catching fire; this killed the other primary.

Luckily I managed to get a couple of transformers about a week later which had cores only slightly bigger than the originals. I expanded the secondaries slightly by putting a couple of bits of wood inside and hammering screwdrivers in between as wedges. This time one transformer had a thermal cutout in series with the primary; after looking at the burnt out transformer and confirming my hunch that the primary winding (that was being driven at something like 4 times its intended current) had been the point of ignition, I attached the cutout to the inside surface of the coil. I also wrapped the cores and the coils in high temperature kapton insulating tape to avoid short-circuit problems without causing another fire hazard.

The 2 coils and core kapton taped ready for assembly

I assembled the transformers (again holding the cores in place with G-clamps), placed them on a workmate, wired them as before and added a microwave oven fan.

The welder temporarily assembled on a workmate

I fired it up and was able to easily strike a nice hot arc; I gave it a quick test by welding together some old steel buckles I had lying about. The weld isn’t very pretty, but is nice and strong – I’ve whacked it with a hammer and it won’t break.

A couple of steel buckles welded together

Next I wanted to see how delicate a workpiece I could weld; for this I devised the idea of the Swiss Army Teapot: a teapot with an assortment of screwdrivers/allen keys/small tools attached. This quickly proved that without current limiting my welder is far too powerful to weld thin sheet steel without blowing holes.

A teapot with tools welded on, holes have been blown at some weld sites

As it stands, it is a pile of components attached to a workmate with no control past which plug sockets I turn on. This is obviously not a finished product, but is a perfectly functional welder. The to-do list currently stands at:

  • Weld the cores together; this will reduce core vibration and make the transformers easier to handle.
  • Seal the transformers with varnish; this will improve the insulation and cut down on the vibration of windings.
  • Adjustable current limiter; I’m thinking saturable core inductor at this stage, built from another MOT core.
  • Case; it can’t live clamped to a workmate for ever.
  • Single 13A feed; once I have a current limiter, it should be possible to run at lower powers from a single 13A plug. I could have the option to add a kettle lead to give extra power for higher power operation.
  • Enforced duty cycle; currently the welder can be used until the thermal cutout cuts power to one primary when it reaches 160°C. It would be nice to have an enforced duty cycle with an LED readout to display how close the limit it is and how long it’ll need to cool.

Read the follow up “Adding a current limiter to my MOT welder”

32 thoughts on “Building an arc welder from microwave oven transformers

  1. panos

    oooo it is a great welder…!!!!i want to make it…can i use 8awg wire and how much power it will give me???(ampers)english is not my native language…..

    1. Mike Post author

      8awg wire has about the same cross section of copper as what I used; are you talking about using plastic insulated cable? If there is lots of space taken up by insulation then there will not be space for as many turns, this will lower the voltage and make it more difficult to strike an arc.

      I found that the proper enamelled copper was approximately the same cost per meter as insulated cable, have you tried looking for enamelled copper?

  2. panos

    no i havn’t look for enameled copper… much power does it give???(ampers)i insist on th power because i want 100ampers at least…….

    1. Mike Post author

      I haven’t directly measured the current(Amps) but the transformers managed to blow 10A fuse wire with 240 turns; this should mean they are capable of more than 100A with 25 turns. It has plenty of heat to weld with 1/8″ rods; this supports the idea that there’s more than 100A.

      What is your mains supply? If it is different from the UK 240V 13A you might need to alter the design slightly.

      1. jason seven

        You are deffinately pulling about 150+ amps to weld with1/8 6010. If they are 7018 rods your still up there in the same range.and, thats a dialed temp., if your burning real hot your probably around ,or above 200amps.

    1. Mike Post author

      This design should work ok on Greek mains; the only problem is that the plugs can be put in either way round. If you aren’t careful you can end up with the phase of the transformers half a wave out; this will not cause any damage but the welder won’t work properly, if this happens just flip one of the plugs.

  3. Peter

    have you installed a current control yet and if so can you tell me how you made it.i made a mot welder using three mot’s and find that it is way too hot for thinner materials and would like to be able to have some control over the output. thanks Peter

    1. Mike Post author

      Unfortunately my welding projects have had to be put on hold for a while after the fire audit of my university room (the idea of bedroom welding seemed to go down badly for some reason); how are your mots arranged? Inductive limiting is the best way to go, the 2 obvious ways to change an inductor are to change the geometry of the core and saturate the core; both have advantages and disadvantages, I’m still not sure which is the easiest solution until I can start trying things out. I’m happy to advise on the theory if you want to give it a go.

      1. Peter

        Hi Mike, thanks for your reply. first thing i should tell you is i am in Canada and the supply is 120 volt dont know if that makes any difference and also i am an older ex motor mechanic so dont have that much idea of electrical terminations so if you could possibly describe what to do in simpler terms i’d like to have a go at changing things around to see how it works. i have tried it with two mot’s but it just does’nt want to know. .


        1. Mike Post author

          The 120V supply shouldn’t cause a problem if you use 120V MOTs, how much current can you draw from a socket? You may need to use multiple sockets (or maybe even multiple sockets on different circuits) as with half the voltage you’ll be drawing twice the current to get the same power.

          What have you tried so far? Is the problem that 3 is too hot but 2 struggle to strike an arc?

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    1. Mike Post author

      I know I’m not going to get a decent joint welding stainless steel with mild rods, but I have seen stainless rods for sale; I was under the impression that these would weld stainless? The stainless bits here were just what was lying around and more of a proof of concept than anything else.

    2. Casey

      That’s not true in the least bit. That’s the whole reason they make stainless rods. I’ll bet you think you can’t stick weld aluminum either, don’t you?

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  6. kaluya moses

    Hi Mike,
    Am so much interested with this wonderful work your doing .
    On my side i would like help me to build a home welding machine for my own use without using M O Ts but using a slightly bigger transformer of about 20A because i have one presently and I have little knowledge on transformer winding.
    (1) what is the gauge of copper wire/number of turns needed, and the current/ voltage for the primary coil?

    (2) What is the size /gauge of copper wire and the number of turns required plus voltage output for the secondary coil
    (3) How are the coils connected to the torch and what other things to be connected to the system?.

    1. Mike Post author

      Do you have any more details on the transformer you’re planning to use? You say it’s good for 20A, at what voltage? If it’s capable of enough power to weld with (a few kW) then it will almost certainly already have a mains primary that you can use.

      I wound the secondaries with 0.1″x1/8″ solid rectangular enamelled copper; there are 24 turns on each core connected in series. This gives something like ~100-130A @ ~48Vac

      The rod holder I bought, it comes with a screw terminal that accepts thick, stranded cable; I used car battery cable rated for 135A. The only other thing needed is the ground clip, it’s a big spring clip with a screw terminal to connect the cable.

  7. kaluya moses

    Thanks Mike,
    The detail of the transformer am planning to use it was for a battery charger, it had
    240v primary coil, and 48v center- tape (24v-0 – 24v ) unfortunately, both coils were damaged by power. Now i have it plain and was written on 20A so please help on this.
    secondly, I have got a smaller transformer,rating 10A 240v primary coil, and 12v -0- 12v secondary coil.
    Therefore,pleas advise me on each transformer in order to be successfully.
    and there is only one part i did not understand is where the ground cable is connected between the two coils (primary and secondary) plus live cable.

  8. kaluya moses

    Hi Mike,
    This is moses eagerly waiting for your response to my request.
    so please ! i love tohear from you.
    Best regards
    kaluya moses.

    1. antoine

      salut je pourrais peut ètre t’aider mais il faudrait que tu envoie une photo de ton transformateur

  9. Dave

    I would like to make a magnetic particle inspection power supply. These devices use high amperage transformers just like the welders. However, it is important that you be able to control the output current. I would like to be able to control it from about 5 to 100 amps. What is the best way to do this?

    I am a metallurgist by trade and have limited electrical experience. If you could keep it simple I would appreciate it.

    1. Mike Post author

      Hi Dave,

      Your timing is good, you left your comment while I was in the middle of building an adjustable current limiter for the welder; you can see my write up here:

      I don’t have a current clamp to put any exact numbers on things, but working on what rods I can weld with I suspect that I’m able to control somewhere around the 20-150A range. I don’t know the details of your application, but it looks like something similar might work with a little bit of tinkering.

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  12. Anonymous

    ok im sorry but I just have to comment if anybody else hasn’t already said this. Testing your weld strength by hitting it with a hammer will do you about as much good as asking wiley coyote for rocket building advice. BEND TEST it or don’t call it a weld at all. be careful what you attempt to weld with a homemade welding machine

    1. Mike Post author

      I should have been a bit more clear- when I say “whacked it with a hammer” I mean “held the bit below the weld in a vice then whacked the bit above the weld with a hammer”. This does apply a bending stress to the weld, but is obviously many miles from a proper testing procedure. I agree that welding anything important with this welder that could hurt people if it failed is not a great idea.

  13. Suusi M-B

    Nice idea. Some technical stuff for you to consider. For transformers power in = power out less iron and copper losses. To all intents and purposes 10% regulation from no load to max load.

    The Transformer will saturate if you draw more that 10% above the rating.

    Power = Voltage X Current.

    So if we have a 650 watt microwave transformer then at 50 volts we can draw 650/50 = approx 13 amps for a mains current in of 3 amps. For a 800 watt transformer 50 volts @ 16 Amps and 1000 watt 50Volts @ 20 Amps.

    So I think that you are correct about the transformer being in saturation when welding. The voltage required to strike an arc is higher than that is used to sustain it. In this an arc welder is little different than a neon bulb.

    So if you were to use a current relay to switch between the striking voltage to the welding voltage. Because the welding voltage being lower than the striking voltage, you can draw a higher current!

  14. Jovan

    I have been working on my own MOT arc welder for the past few weeks. It has been MUCH more preparation than I expected! You really need to plan out how to split open the cores and wind the secondaries. I used 10 gauge solid enamel coated wire for my secondary coil, but I had a hell of a time trying to wind it. I tried making a jig out of wood, but I used a softer wood and it started to split it. In summary, I had to wind one secondary on the transformer itself after cutting the inner corners off of the middle core post (this transformer was like a dwarf to the other). I had some problems with nicking the wire and cutting the enamel off with a few windings, but I sealed it with polyurethane, checking with a multimeter for any shorts.

    Last night I tried testing it out and got a few sparks, but I realized the big transformer was sparking… another short! I have to split that one open and make sure it’s well insulated with polyurethane and cured, so it’ll be a few days before it’s really working like I want it, I’m sure.

    I was just wondering about the efficiency of the iron core itself. Do you have to have the core totally put back together as in the ” I ” sealing the ” E ” piece back together, for efficiency purposes, or does it not make a significant difference to have a transformer or two open topped? Also, since I know it is necessary to run two separate 115V lines (one for each core), does it matter if there is a 20′ extension cord going to one?

    1. Anonymous

      Sticking 10A down the secondary designed for about 0.5A and 100A down the primary designed for about 4A is a quick route to setting the transformers on fire. You can’t go throwing welding currents down any old bit of wire.


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