Tutorial Series Shows You Everything You Need To Salvage Transformers From Microwaves


Transformers certainly have a tendency to increase the cost of any project, especially if you need a large transformer to get the job done. Microwave ovens are great sources of free transformers, though they are not always in the shape required for your next build.

[Matt] put together three great tutorial videos covering the basics of salvaging Microwave Oven Transformers (MOTs), that anyone new to the process should watch before giving it a go. The first video covers MOT removal and disassembly, which is a time consuming yet easy process providing you follow [Matt’s] pointers.

The second video delves into transformer theory, and discusses how to achieve optimal performance when rebuilding an MOT or hand wrapping coils to fit your project specs. The third video in the series follows [Matt] as he rebuilds one of the salvaged transformers, documenting his pitfalls and successes along the way.

If you haven’t given much thought to salvaging MOTs, we definitely recommend taking a bit of time to watch the video series in full – it’s definitely worth it.

You can see the first video in the series after the jump – the rest can be found via the YouTube link above.


22 thoughts on “Tutorial Series Shows You Everything You Need To Salvage Transformers From Microwaves

  1. LOL, I`v always taken off the secondary coil the hard way, but I have some lovely High Current PSUs using these MOTs.
    one of my faves, although it took a lot of trial and error working it out, is a 6.3VAC source for Vacuum tube (Valves) heaters, I can pull a good 50A out of it using Earthing Wire as the secondary.
    apparently Copper Pipe can be used too, for Spot Welding projects, I have one half built here, I just need the copper electrodes.
    it IS worth putting the Overheat thermal cutout bi-metal sensor onto the primary though, as they do get hot if not properly used.

    1. If you’re happy with the size of the primary, use a chop saw or angle grinder in a stand to cut the secondary, then punch out the remainder.

      Take a piece of insulated wire and wind it through where the secondary was, power it up and measure the voltage (AC) on your new secondary. That’ll give you volts per turn, so if you read 11v then each turn is 1.1v.

      Need 5v? Well, that’s 5 turns to give 5.5v, now find the thickest wire that’ll fit in there 5 times and you’re done.

      Spot welders are low voltage, so you may only need a couple of turns.

      You can parallel thin wire, so group 3 wires together and wind them around 5 times – easier than trying to stuff heavy gauge wire in there.

      Pull the magnetron apart as well, there’s 2 big donut-shaped magnets in there.

      1. Hrm. Tony appears to not have watched any of the tutorials, ’cause the first thing I mention is that the chisel method is not the one to use. And then 2nd video covers the ~1 turn per volt.

        In any case, for a spot welder, use a 1-turn secondary. MIG tips are expensive and inadequate. Find a dead starter motor (any mechanic shop probably has one laying around waiting for the metal junkers) and grind/unscrew the copper connectors. They’ll be 1/2″ sometimes. They look just like bolts. Then grind those to a tip.

        I’ve got the 2nd video scripted, but not filmed.

      2. Either you did watch the video either or you didn’t understand what I mean – I assume the latter.

        I don’t recall mentioning using a chisel…

        You use the angle grinder to cut the exposed bits of the copper secondary, leaving you with two squares of copper in the iron. You then take a punch (long thin round thingy) and use it to knock those out.

        And where I an, MIG tips are as cheap as chips. And plain copper doesn’t last very long in a spot welder, which is why real tips are some weird alloy that I’ve forgotten the name of.

      3. “You use the angle grinder to cut the exposed bits of the copper secondary, leaving you with two squares of copper in the iron. You then take a punch (long thin round thingy) and use it to knock those out.”

        Yeah, that’s the difficult way. I show that in the first video, chisel, hacksaw, grinder, whatever you use to cut it apart and then punch the insides out. Disassembling the core instead is 10x as fast and 10x as easy.

        And, it’s also necessary if you don’t want to spend $20 on wire for the new secondary, because you need the wire on the old secondary to create the new one.

        I’m not sure if we just have a difference of opinion on the best method, or if you didn’t actually watch any of the 3 videos…

    1. Yep! That’s one of the easiest projects.

      It’ll overheat after about a half hour unless you keep a fan on it constantly. Keep the shunts in if you don’t add extra turns to both.

      The tricky part is finding a matching core. There’s only a half-dozen standard sizes, so, you’ll find one eventually. Of course, then you won’t have a primary for the 2nd MOT, so that core might not have any use.

      Another option for an isolation transformer is to just take two MOTs as-is, with similar output voltages and connect their secondaries to each other. Then you have 120V –> 2000V –> 2000V –> 120V. It’s a bit sloppier, but takes zero effort and zero tools. 5 minute isolation transformer.

  2. Great set of videos-very informative. The title is a bit misleading however. Salvaging implies just removing. What he is showing is heavily modifying for another purpose. If anything, he is salvaging the transformer core. Once again, great videos. Eagerly awaiting part four.

  3. Nice!

    I grew up with my father teaching me little things like this, but never about how the transformer actually ‘transformed’ the voltage.

    Pretty nice how the math works out too, and I don’t want any part of math to be honest :P

  4. A plastic-handle flat blade screwdriver as a chisel, a 1/4-inch extension as a punch, and a chisel as a prybar. All rightee…. >:|

    Also, why rely solely on the transformer to heat itself? Why not speed the process by letting it sit in a pot of boiling water? Or perhaps a propane torch to the iron (gently and WELL before it reddens, of course!)? Just seems safer and more efficient than having a live, open transformer wasting electricity for half an hour.

    The varnish remaining on the E-I core will soften with lacquer thinner, but beware: lacquer thinner will also soften the windings’ enamel, so keep it away from those! Voice of experience: arcing between windings due to the enamel being thinned or taken off is NOT cool, but the noise is!

    1. Because the idea is an iron core and not an iron oxide core, boiling water may not be a good idea. I was thinking maybe just throwing it in the oven at a suitable temp (when my wife isn’t looking).

    2. …why rely solely on the transformer to heat itself?

      Read back the list of tools used, and there’s your answer.

      Anyone can do the self-heating method. An oven on low temperature should work, but may exceed the temperature rating of some parts (plastic insulation on wires etc). With the self-heating method those parts will be air cooled, in the oven they won’t.

    3. As to the wrong-tool for the job? Anyone with the right tools (you) knows to use them instead. Anyone without them probably only has a screwdriver, so I want to show them what can be done without all the perfect tools. It’s why I tend to show 2 or 3 methods for everything, so no one gives up if something doesn’t work.

      Why rely on the self-heat?

      No reason. It’s lethally dangerous if you screw up and slow. Use an oven instead, like I said. I just included it ’cause I found it really creative. Works 3x as fast if you knock the shunts out first btw (high leakage inductance).

      It’s not “wasting electricity”. There’s no “waste”, since 100% of the electricity used, goes into heat, which is what you want. An oven is far more wasteful. Also, for 30 minutes, it will waste $0.05 of power max.

      Don’t use a blowtorch. Don’t put it in water. Don’t use lacquer thinner. Any of the above will ruin the project, too hard to control heat/liquid. Just use an oven.

    1. Correct, BUT.. Beryllium Oxide is only present in the tips of the magnetron CRT, and even then is safe to touch unless you grind it into powder and then inhale it. It will be either pink or white and looks like clay. Safe to handle and cut off above or below (mates to steel or copper), just don’t go grinding into the clay part.

      Unless you’re after the 1/2 pound of copper in the magnetron (underneath the heatsink fins) or the heavy tungsten filament (22 gauge tungsten coil), there’s no reason to even strip the magnetron down that far.

      The spot welder video might show it, ’cause you could feasibly use the 1/8″ thick copper wall of the magnetron, coiled up a bit, as an electrode tip.

  5. Matt in your videos which are really good may I suggest include some form of info which can be used to track you down ie website, email address or just the name you postunder on Youtube. This allows someone to watch a video then can look up details to find you again.

    1. Yeah, I’ve “made” probably 4 or 5 “coming soon” websites for hobby stuff over the years. I suck at design… and adding content. I didn’t expect these to blow up like this until I’d figured out what I’d like to do with it.

      Videos are good to watch once.. but then for reference text + pics is way better.

      Anyway, fortuitously, great minds think alike, I just registered a domain name 3 minutes ago. Now to actually build a site…

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