Easiest Microwave (MOT) Salvage/Re-use Tutorial


Introduction: Easiest Microwave (MOT) Salvage/Re-use Tutorial

These videos show you how to take any broken microwave and get the transformer out of it, tear it down, and rebuild it according to whatever project you have that needs one. These form the heart of many other projects, which I cover (or will cover if not out yet) in other tutorials.

Hopefully every question you might wonder is explain in detail in the videos. I give plenty of alternative options for tools and methods.

Part 2:

Part 3:

I'm a big proponent of criticism and feedback. My first video had some audio issues. I made parts 2 and 3 several months later after feedback and I think they're much better. Please leave comments if you have any suggestions for improvement, criticism, things you thought were poorly explained, etc. I read it all and it helps me make better tutorials in the future.



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    Hi Matt, great tutorial, just wondering if
    you could clarify something for me, going through the videos and
    comments I couldn't find the answer. The winding direction for the new
    secondary should it be in the same direction as the primary, ie: say if
    the primary winds clockwise if viewed from above the secondary goes the
    same way?

    And also does it matter if the primary is on top of the secondary or vise versa?



    2 replies

    Winding direction does not matter unless you're combining multiple transformers or windings together (in which case, you want the end result to be matched so they add, rather than opposite so they subtract to net zero). Think of a sine wave. The direction you wind determines whether you start at the top, or the bottom. It alternates 120 times per second, so, unless some quirky circuit requires that precise of timing and polarity, it makes no difference. To keep my head straight, I usually try to wrap in the same direction, good habit, less confusing if you want to combine transformers later to make it more powerful.

    Order of primary or secondary doesn't matter at all. The transformer doesn't have a "top" or "bottom" and it's operation is unaffected by gravity, which is the only way "top" and "bottom" could be defined. Put it on however is convenient. I usually put the primary on first (trapped at the back of the E) because I'm more likely to change the secondary some day, or have to adjust it to fit. Operation-wise it doesn't matter.

    Thanks for that, isn't amazing when something is explained in simple logic and common sense comes into play. The only difference is that in Australia we run on 240V @ 50 Hz.

    Just as a PS to the last note. That's a fourth way of determining turns required. Measure the voltage of the secondary as I described. Unwind secondary and count turns, you could probably just use a pedometer mounted offset on the unwinding spindle if you haven't got some better turns counting mechanism.

    I'd probably use a cordless drill with a 13mm bit of dowell as a bobbin to unwind onto. Those secondarys look like they've got a lot of turns on them. (By your estimates of average primary turns, there should be 1100 odd turns on the secondary). Then divide secondary original voltage by number of turns on original secondary to give turns ratio.

    Then multiply by required voltage to give new required turns. Then work out amperage anticipated and look up a copper wire amp carrying chart to get best wire thickness. You could also do a voltage drop test with length, amperage and gauge to give heat rise if you're really feeling gung ho.

    Also means you've got a nice little spool of small gauge wire for when you build that Tesla coil you always wanted to...

    Great tutorial, just what I need to make a decent cnc servo linear power supply and get away from switched supplies (and save 2-300 bucks as well). Got a dozen MOTs from local tip shop for AUD$2 each, already out of case, including a monster that looks like it's 1500 watt at least (weighs over 8kgs).

    Wanted to test the secondary output which lead to some confusion 'til I discovered that MOTs ground one secondary lead to the core (and then the case to I suppose).

    As the MM only went up to 600vac and the secondary is probably around 2000vac I made up a resistor string of 90Mohm to boost the MM impedance from 10Mohm to 100Mohm. Had to series eight resistors in the end out of salvage as I didn't have a 90mOhm resistor to hand.

    Worked a treat. Secondary was putting out 1890vac, and the two other little secondaries (tertiaries, quaternaries?) were putting out 3vac and 9vac, probably to rectify to 5v for a logic board and 13v for the fan motor....

    I thought that maybe the meter could have handled it without the resistor seriesed with the probe, but sadly not! Fizzle, pop, smoke. One dead multi meter. Oh well...

    Hello, I've been having some trouble with my transformer. Instead of two welds, one on one side and another on the opposite side, it looks like its made of two interlocking chunks. It was salvaged from a microwave, and when I look at all these videos online on how to cut the weld apart, it doesn't match any. on the label it says OBJY2 and below that, EBJ60664602. Any and all help is appreciated.

    2 replies

    Okay, I looked that up. You are right... The welds on that model are in peculiar places in the pics I found.

    Two possibilities:

    1 - the welds are in weird places but the transformer us a normal not. Cut where the E and I join and follow the tutorial as-is.

    2 - the transformer is "interleaved", which means the E and I pieces plates aren't each in a big 100-piece stack that are the welded after, they're alternating every single layer. This is the better way to make a transformer, but harder, and it's almost impossible to disassemble (and then reassemble) without damaging it. In this case you'll have to cut the secondary off and thread your new secondary as per the tutorial "hard way".

    Take some pictures in good light (bathroom), post them on Imgur and link the album here, I'll try to help more.

    If it was a book I would say it not only contained information I needed it was also an easy enjoyable read. Thank you!

    So how many wraps would i need on both P and S for 120v in and 24v @50+amps and size of wire

    5 replies

    Which part of the tutorial are you stuck on?

    I don't mean to be rude, but, I took the time and effort to create a tutorial so that people like you could learn, and then answer your own questions, not just ask me every time you need an answer. I think it explains exactly how to answer this question, so, let me know where you got stuck and I will do my best to assist further.

    I watched vids again and think I got the amps figured out please help if I'm wrong. If i want 24v @50amps on Secondary (=1200Watt) and using 120v on primary amps will be around 10amps(1200watt). So I would need to find approximately 12 pound MOT

    I know you said voltage ratio= turn ratio and MOT's are at the bottom limit of the P wraps. Does wire size effect that? Would less wraps with larger wire increase Power? Or would adding more wraps on primary and secondary increas power?

    That leads me to wire size on both primary and secondary, mainly secondary if I find a MOT with correct power rating(1200Watt). If I am pulling 50amps @ 24 what wire size do I need on secondary so I don't burn them up.

    As for "not just ask me every time you need an answer." I'm lazy why not have someone else do the work for me :P I would appericate your help though .


    If you increase both wire size and number of turns on the primary it will be able to transfer more power to the secondary without overheating. Power = Voltage * Current, and in order for a coil to work at a higher voltage it needs more turns, while in order for a coil to work at a higher current, you need thicker wire. Therefore more powerful transformers will take up more space and cost more. That is why substation transformers are multiple feet across, they are made to handle a hundred thousand watts or more. If you are serious about designing a transformer you should have a good understanding of Ohm's Law and circuit design theory. You have to be good at math to work with electronics. You also should have some common sense so you don't electrocute yourself, always turn off the power before working on a circuit. Only specially trained people can work on live circuits safely, and even they are taking risks by working with live circuits

    1200 watts is correct.

    No amount of turns or any ratio effects power. Adding more turns increases voltage but lowers amps, and power is Volts x Amps, so it always stays the same. The limitation is magnetic, set by the size of the core. The only thing that affects the max power of a core is the frequency of power, which is very complicated and expensive to change (building a fancy inverter power supply or using a "Variable Frequency Driver [VFD]"), and even then you have to use a core made out of special materials, so forget I mentioned it.

    For wire size, pick the largest you can, to fit ~24 turns in. An MOT is already pre-engineered, so you know whatever was figured out originally was something that would work. Wire size and max amps scale with size, so, by simply picking the thickest you can, you know it'll be about right.

    Thinner wire will work, but will tend to overheat and melt. So that effects duty cycle. Use the thickest you can.

    I am rewatching your videos and I believe I am answering some of my own questions (please correct me or compliment me)

    1. Fundamentally it doesn't matter if your two MOTs are from different wattage microwaves. You can use the "winding magic" to make a MOT from a 900W microwave, and a MOT from a 1100W microwave to produce ~equivalent voltage.

    However, it would be to my benefit to use the largest MOTs available because the more physical space you have the larger the chances are to use thicker/more reliable wire or improve cooling.

    2. Still no clue.. any help on this would be awesome


    4. Where would you recommend to get scrap aluminum? I want to melt/pour/cast aluminum into molds, but having a hard time thinking of where to find it for free (besides beer/soda cans.. I am already starting a collection)

    4 replies

    1 - Correct.

    This is in the script of the followup video I haven't filmed yet.

    In short, you have 2 choices: A - Match voltages each to your target voltage, join in parallel, and have the currents add up. Or, B - Match wire thicknesses (current), join in series, and have the voltages add up.

    In Case A you would wrap, I dunno, 30V each, and use different wire sizes for whatever ~30 turns would yield. The voltages have to match really, really closely, as they'll fight each other to balance if not. Sometimes you have to tweak the primaries by adding turns just to get the two secondaries to match.

    In Case B it's not as big of a deal, you just coil and go. Maybe the big one fits 16 turns and the small one 14. Together they make 30. Also, if you're a little high or a little low you it doesn't matter, just pack turns in.

    2 - Phase is tricky. Trial and error is easiest. Basically, you've got a 50/50 chance of getting it right on the first try. If you get it wrong, you pick one of the primaries and swap the wires. With 2 separate transformers you're okay to do trial and error.

    Connect your big transformers and measure the output of the secondaries. If you measure almost zero volts on the output, your voltages subtracted rather than added. Flip the wiring on ONE (only one) of the primaries. If you measure the right amount of volts you're set.

    Think of it like two guys pushing a car. If one of you is pushing on the front bumper, and the other on the rear... the car is going nowhere unless one of you is stronger, in which case it's still only barely moving. You need to both be pushing from the same side, so one of you needs to switch. If both of you switch positions, it's still wrong.

    2a - Correct. Power generation is 3 phase and is each phase 120' out of synch. Also correct that home circuits are 180' out of phase. Here's how:

    Residential wiring in the US is "split phase". What that means is, when the 3 phases from the power plant make it to a community, they take each of those those phases and give them to a different block. So each block gets only 1 phase. So residential is 1 phase wiring. Into your house comes 1 phase at 240 volts. This is on a center tapped transformer, so, one side of your panel is -120v to 0, and the other side is 0 to +120v. These are 180 degrees out of phase. Most plugs in your house are all 120v, and it doesn't matter which it is because it's relative to itself. Some appliances like your stove and clothes dryer use 240V, which means they ignore the middle tap and use wires from -120v and +120v, which is the full 240v difference.

    Which matters if you're running your welder from 240v, or, if you intend to run them from 2 separate breakers so you can get a full 15amps out of one, and a full 15amps out of another.

    But for the questions you're asking, also phase matters even if power wasn't that complicated and you only had a single 120v ac signal. That's because if you're hooking up two things to it, you can still hook them up backwards to each other. Both would work on their own, (like pushing a car from the back or the front), but the polarity of a coil determines whether you start at the peak or valley of the AC cycle.

    3 - I do my best.

    4 - If your area has deposits for empty beverages, don't melt them down. They're worth like, 50x as much as empties. If you area does not have them, go ahead, but pop cans are awful, there's about 50% waste when you melt.

    A good source of scrap aluminum is old hard drives. Just bash out the platters and electronics, the rest is cast aluminum (which means it's a good alloy to cast again). Save the big neodymium magnets! Broken equipment works well too. Most air compressors are aluminum, they're junk when they get cracked. CPU heatsinks are aluminum (or copper) too, but they're expensive and if you need heatsinks for anything (like your welder, or to epoxy to your transformer to help it keep cool) then don't, metaphorically, whittle an ornate coffee table into a walking stick. Just grab a stick.

    Broken aluminum bike parts are a good source too. Cargo racks, frames, seat posts, handlebars, rims. If they don't hold a magnet (steel), they're probably aluminum. Call up a bike shop and ask them to set aside their aluminum scrap for you. Every major city has some non-profit community bike place that turned donated junk bikes into working bikes, find yours and ask them, they'll for sure be tossing away junk frames. Bike parts break all the time.

    Car parts are often aluminum too, but I'm not a car person.

    Alternatively, aluminum is only $1/lb as scrap, go to a scrapyard and ask to buy some from their lot.

    awesome sauce..

    Okay yeah the first part you explained about the phase within a 2 MOT welder was pretty clear and straightforward (that idea I understood even before, but thank you for califying)

    The whole 3 phase to 3 different blocks was completely knew knowledge and enlightened me. Thanks!

    Unfortunately I don't live in a large city, but I do live on an military base which throws stuff away all the time. So I'll start looking around, maybe i'll take a trip to my local dump and see if I can't stand the stink long enough to salvage some stuff.

    Headed to the big city this weekend, hopefully to buy a second 1100 watt microwave for $25.. i know i know I could find it free, but meh I don't have time to look ha

    Again thanks. I am sure I'll be back again with questions. And thanks for the quick response.

    Ugh. $25 each for a microwave you're turning into scrap? You can *buy* a crappy stick welder for like, $50. You're whittling furniture into walking sticks.

    forgive the typos and mispelling.. wow.. I guess I am so used to swype on my phone, when I hop on a computer I just type whatever the hell I want and expect my computer to figure out what I am saying! LOL

    Here are the list of corrections:

    thank you for clairifying*

    completely new* knowledge