Current Limiter For A MOT Welder


[Mike Worth] wanted the option to run his Microwave Oven Transformer welding rig at less that full power. After being inspired by some of the other MOT hacks we’ve featured he figured there must be a lot of ways to do this. But his searches on the topic didn’t turn up anything. So he just designed and built his own adjustable current limiter for the welder.

At the beginning of his write-up he details what we would call a bootstrap procedure for the welder. Go back and check out his original build post to see that he had been holding the framework for the cores together using clamps. To make the setup more robust he needed to weld them, but this is the only welder he has access to. So he taped some wood shielding over the coils and fired it up.

The current limiter itself is built from a third MOT. Adjustment is made to the cores by changing out the E and I shaped pieces. This allows for current limiting without altering the windings. [Mike] holds it all in place with a couple of bicycle wheel quick connect skewers.

It just goes to show that you should never get rid of a microwave without pulling the transformer. Even if you don’t need a welder wouldn’t you love a high-voltage bug zapper?

14 thoughts on “Current Limiter For A MOT Welder

    1. Genius! The old ideas are the best sometimes, especially with high-power stuff.

      How would it work in series? I was thinking of saturating the core with variable amounts of DC.

    2. I did consider this, however there are a few things that concerned me; I suspect the cores are already being partially driven into saturation (iron has got expensive recently, microwave manafacturers are stingy) which might complicate things, and as far as I can tell I’d be disappating a notable amount of power into clamping diodes (unless I’ve misunderstood something about how the circuit would work).

      These things combined with the fact that I have loads of MOTs that I don’t know what to do with (the limiter was entirely built from things I had lying about) meant that I went with the mechanical method. I’d love to see a saturatable core one to see how it compares, if you know of anyone who’s built one.

    1. I tried using 3 transformers for a fractal wood burner. I also used triple shielded 8ga stranded wire for the leads. It worked for 10 seconds (until the breaker blew, but I could feel the current passing from the concrete, through my thick rubber boots. The wood I used for burning was only a foot long. The leads were maybe 5 inches apart and it arced the entire distance. I was messing with waaayyyy too much power for the small amount of knowledge I had. Luckily I was not injured or killed.

  1. I shocked myself with a MOT once, not fun. I learned the importance of wearing shoes and gloves that day and using pliars with the rubber handles. Safety is rarely a priority.

    1. That is the completely wrong approach. You should never even try to touch high current high voltage equipment, you turn the power off, make the adjustments you want, step back and turn the power back on. Manipulating live high voltage high current systems is a last resort, and might just be the last resort you ever try.

      1. Ordinarily yes, what you wrote is correct. The issue with a microwave oven is that there is also a fat capacitor in there connected to the transformer with a potentially lethal charge on it even when disconnected from the mains. There is usually a bleeder resistor connected across the capacitor to slowly dissipate the charge. The problem arises if you don’t wait long enough or if the bleeder resistor is missing or damaged.

        Microwave ovens are dangerous if you don’t know what you’re doing. Take suitable safety precautions from both the high voltages inside and also the most likely sharp metal edges inside. Don’t do anything daft like running the microwave oven with the safety interlock bypassed either.

    1. yes, you are right. welding with MOT requires changing out one of the coils to a 2-3 turn large gauge wire. But you do need to be able to control the current. Too much current and you can melt your base metal away before it can be welded together.

  2. Looks as tho he is running the mots backwards, running 110V through the HV windings and using what were the primaries as low voltage high amp secondaries. no 1Kv+ HV here

    1. I do believe that it is wiser to keep projects simple. A transformer converts AC to AC. It is both practical and less time consuming to apply source AC to the secondary winding to yield a lower voltage/higher current on the primary winding. Using two identical HV transformers with the source AC wired in parallel and maintained at the same phase to the HV secondary allows more options for the welder. One option created is the allowance to select the level of current produced simply through using one transformer or using both. This is a valuable feature if the user is a more seasoned welder through craft or industry. The best part of this design is that it is both scalable depending on desired application and the parts can simply be disconnected and reused in the future. I personally prefer to keep repurposed parts intact and unmodified so that I can repurpose them multiple times. The primitive method of carelessly hacking a transformer apart and creating a health and safety hazard through making a winding from material not made for such purpose is ill advised.

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