Tech In Plain Sight: Microwave Ovens

Our homes are full of technological marvels, and, as a Hackaday reader, we are betting you know the basic ideas behind a microwave oven even if you haven’t torn one apart for transformers and magnetrons. So we aren’t going to explain how the magnetron rotates water molecules to produce uniform dielectric heating. However, when we see our microwave, we think about two things: 1) this thing is one of the most dangerous things in our house and 2) what makes that little turntable flip a different direction every time you run the thing?

First, a Little History

Westinghouse Powercaster which could, among other things, toast bread in six seconds

People think that Raytheon engineer Percy Spenser, the chief of their power tube division, noticed that while working with a magnetron he found his candy bar had melted. This is, apparently, true, but Spenser wasn’t the first to notice. He was, however, the first to investigate it and legend holds that he popped popcorn and blew up an egg on a colleague’s face (this sounds like an urban legend about “egg on your face” to us). The Raytheon patent goes back to 1945.

However, cooking with radio energy was not a new idea. In 1933, Westinghouse demonstrated cooking foods with a 10 kW 60 MHz transmitter (jump to page 394). According to reports, the device could toast bread in six seconds.  The same equipment could beam power and — reportedly — exposing yourself to the field caused “artificial fever” and an experience like having a cocktail, including a hangover on overindulgence. In fact, doctors would develop radiothermy to heat parts of the body locally, but we don’t suggest spending an hour in the device.

The first Raytheon “Radarange” was nearly 6 feet tall. The 750-pound beast cost about $5,000 which is nearly $70,000 today. You also needed three kilowatts of electricity to feed it. By 1954, the cost came down to about half, along with the energy usage. As you might expect, these were commercial or niche items.

Eventually, of course, the cost and power requirements came down. They solved the problem of running the oven empty, causing damage. There were two other major problems to solve: safety and uniform heating.

Safety Dance

Like any electric appliance, a microwave oven could catch fire or cause an electric short circuit. However, if you open your washing machine and it doesn’t stop, the worst that can happen is you get wet. With the microwave, you could get a big dose of (non-ionizing) radiation! Studies show this probably isn’t as bad as you might imagine, but it is bad enough to burn you.

Of course, if your fridge can turn the light off when you close the door, why not just put an interlock on the microwave door? Microwaves do have an interlock by legal requirement. But not just any interlock: they have to meet strict requirements. In the United States, 21 CFR Part 1030.10. This states, in part:

Safety interlocks.

(i) Microwave ovens shall have a minimum of two operative safety interlocks. At least one operative safety interlock on a fully assembled microwave oven shall not be operable by any part of the human body, or any object with a straight insertable length of 10 centimeters. Such interlock must also be concealed, unless its actuation is prevented when access to the interlock is possible. Any visible actuator or device to prevent actuation of this safety interlock must not be removable without disassembly of the oven or its door. A magnetically operated interlock is considered to be concealed, or its actuation is considered to be prevented, only if a test magnet held in place on the oven by gravity or its own attraction cannot operate the safety interlock. The test magnet shall be capable …

(ii) Failure of any single mechanical or electrical component of the microwave oven shall not cause all safety interlocks to be inoperative.

(v) One (the primary) required safety interlock shall prevent microwave radiation emission in excess of the requirement of paragraph (c)(1) of this section; the other (secondary) required safety interlock shall prevent microwave radiation emission in excess of 5 milliwatts per square centimeter at any point 5 centimeters or more from the external surface of the oven. The two required safety interlocks shall be designated as primary or secondary in the service instructions for the oven.

(vi) A means of monitoring one or both of the required safety interlocks shall be provided which shall cause the oven to become inoperable and remain so until repaired if the required safety interlock(s) should fail to perform required functions as specified in this section. Interlock failures shall not disrupt the monitoring function.

Naturally, there are many ways you could meet these requirements, but most of the microwaves we’ve seen do it with three microswitches. Usually, two are normally open, and one is normally closed.

The two normally open microswitches prevent the magnetron from receiving power when the door is open. One of them actually breaks the power to the tube. The other is used as a digital input to the control board. Closing the door actuates the switches and allows power to flow. You would think the switches would be in each leg of the magnetron, but it isn’t that simple. If the switch connected to the board fails, the light, fan, and turntable will operate when the door is open. If you ever open the door and your turntable starts spinning, it is probably one of the normally open switches shorted.

So why is there a normally closed switch? That shorts out the power to the magnetron. So even if, somehow, both normally open switches fail, the normally closed one will short the power and blow a fuse. If it fails, you assume one of the other two switches will still cause a failure if the door opens. A very smart appliance repairman explains it in detail in the video below. Watch it all the way through to get a good tip about checking the transformer power without a lot of trouble.

Our pro tip: buy your microswitches from the usual places, not the appliance part places. You’ll be able to replace all three switches for way less than one switch will cost from the parts house. Just make sure they are exactly the same switch. Obviously the normally open and closed part is important, but the mounting holes, actuator, and the voltage/current ratings need to match, too.


The other issue is getting things to heat evenly. The radio energy will have standing waves, which can cause cool places. Some older microwaves have a mode stirrer to reflect microwaves in the oven. But Sharp started using turntables around 1964, and that’s what most modern microwave ovens use. Have you ever noticed that, usually, the turntable will spin one way until you turn it off. When you turn it on again, it will usually — but not always — turn in the other direction.

We were always fascinated about how that might work internally. It turns out the real answer is anticlimactic. Microwave ovens are price-sensitive. The cheaper you can make them, the more you can sell and the more profit you make on each one.

As a result, the motor is almost certainly a synchronous AC motor. The natural direction the motor spins will depend on where it was in the cycle when it stopped. In many cases, the drive gears in the motor will require a little torque to start, and this will cause the motor to change direction. Clock motors, for example, have a spring arrangement, so if it starts in the wrong direction, it gets pushed into the correct direction. The microwave doesn’t have that.

If you’ve taken apart a microwave, you might say that the turntable doesn’t have a drive train, just a motor. But the motor has a surprising number of gears in it, as you can see in the video below.

In Plain Sight

You probably use your microwave every day, but you probably never thought about it having a triple interlock switch and a gear train. These parts aren’t as sexy as the high-voltage transformer or the magnetron, but they are no less interesting. The deep theory of why it all works is pretty interesting, too.

Delightful banner image: Cover of Short Wave Craft magazine.

31 thoughts on “Tech In Plain Sight: Microwave Ovens

  1. Interesting. I’ve got a microwave which failed such that it’s on (light, turntable, and magnetron) whenever the door is closed. Opening the door turns it off.

    I’m guessing a SSR on the control board which switches everything on failed closed. Thankfully the other interlocks still work.

    Obviously no good for food, but sitting in the garage because I’m sure it must be good for something…

    1. Have wound oddball sets of voltage windings once removing HV winding and keeping mains winding, four microswitches from two microwaves are in my home built CNC right now, have a multifunction selector switch around here somewhere waiting for a use

    2. Microwaves usually like using relays; magnetron is on one relay with lights & tray on another. Sounds like it welded itself shut or something. Amusing failure, easy to fix if you could be bothered or just disable as you don’t really need the light or tray to spin.

      As an aside, the door interlocks are booby trapped to blow the fuse.

  2. My dad and I were both technicians for decades. It was my own microwave threw both of us for a loop. It just refused to work. Display, touchpad… nothing worked. We took the back off, plugged it in and everything worked. Couldn’t find anything wrong. Put it back together… wouldn’t work. Finally, for some reason, I put a piece of metal over the controller board to act as a shield. I don’t remember now why I tried that, but it worked! Has been for over a decade. I don’t know what was causing it, but we checked for radiation leaks and couldn’t find anything. I wouldn’t have done that for a customer, but… :shrug:

  3. We had a Toshiba microwave that had *four* interlocks. One logic level that told the micro to open the power relay, two in series to open line power to the transformer. The last one was a crowbar: If the others had not turned off power to the transformer it would short the power line, presumably blowing the line fuse.

    Ironically, what finally killed the thing, after 30+ years of service, was one of the interlock switches overheating and taking out enough of the surrounding wiring that it was just time to get a new one.

    The commercial-grade high-end Panasonic that replaced it is now 5 years old and has been opened twice already for minor repairs. They really don’t build them like they used to.

    And FWIW, neither had turntables. They are a stupid notion, unnecessary and existing to sell to folks who are wowed by the feature.

    1. The crowbar door switch is called the monitor switch, and yeah it’ll blow the fuse.

      The turntable is to stop water getting superheated and exploding in your face, the vibration caused nucleation point to form or something and so the water boils. It’s a safety feature.

      Commercial ovens skip the spinning plate because they want the the dish handles to be accessible (ie in the same spot as they put them in) rather than have to waste time waiting for things to spin around. (If your cook’s assistant gets a face full of superheated steam, well, they’re easy to replace.)

      Some ovens keep the tray spinning to accomplish the same thing (aka boomerang turntable), which really only needs an extra microswitch to achieve. My oven does that.

      1. Funny. The only time I’ve had water get superheated and explode on me was in a turntable-equipped microwave oven. We were trying to make pasta without a hotplate or stovetop available. Pulled the Pyrex pot of water out of the microwave, and as soon as the first speck of pasta hit the water it promptly erupted, ejected most of itself out of the pot, and soaked the carpet.

        I have tried to replicate it with my turntable-free microwave ovens, and have not been able to get quite as spectacular an eruption.

        So I call BS on the idea that the role of the turntable is to prevent superheating. Rather, it’s sold as a visible feature that purports to get more uniformly-heated food. Something a mode-stirrer can do more neatly, with more effective use of interior space, and without the nastiness of a separate parts or the inconvenience of the handle not being where you left it.

        1. Turntable was both for safety & even heating, presumably it was cheaper than a stirrer. “Ooh shiny new gimmick” was a bonus. Was a lot of panic about safety back in the day (hence the interlocks as well). The even heating was partially safety rather than quality as well, stopping mouth burns from accidentally biting into a “hot spot” and all that.

          For superheated water try your Pyrex (nice & smooth) with distilled water – no minerals or aeration like from the tap. Stove top to improve the odds.

  4. My first experience with the inside of a microwave was as a young teen. I found one on a curb, and like a good little hacker and biked home, got a screwdriver, and helped myself to little bits that would fit in my pockets. Mostly microswitches and thermal cutouts.

    The next time, as an older teen, it was the day before Thanksgiving (US holiday), and Mom was trying to prepare something for the big day, and the fan/light were only running with the door open, not closed. Mom was *slightly stressed*, we had family visiting and she was counting on the microwave as support/prep for some things I’ve forgotten the details of. So off came the cover and out came the multimeter and eventually we figured out one of the interlocks was bad. I dug out my bin of switches, and pulled out the ancient ones I salvaged from that first microwave, found the appropriate one, and in it went. Holiday saved. (And yes, the switch was rated appropriately too). That microwave was a beast that lasted forever. We weren’t even the original owners. But eventually it died and it wasn’t a simple switch. I wasn’t around to dig into it and it got replaced.

    The next one must have had a slightly mis-adjusted mechanism for triggering the interlocks, because it physically destroyed a specific one of them every nine months or so. The plunger would stop springing back up and the switch would remain in the normal state. The door was also difficult to open and close, the whole microwave just wanted to slide around instead. Eventually it failed with an intermittent “magnetron always on when the door is closed” despite all the interlocks checking out. I think one of the relays on the control board was iffy. But it was far faster to get a new one than replacement parts, and the people who were making the decision chose to do that.

    Amusingly enough, the best choice was the same model. It’s interesting that the door on this one is so much easier to open and close. I suspect that we wont be going through interlock switches like we did on the old one.

    Either way, I’ve got a whole donor microwave in storage, and a compliment of spare relays now. Which probably means the thing wont break for a good long time. (I did remove the power cord from the donor, lest someone think it works and discover that the magnetron just runs forever by way of it catching fire or something)

  5. When Percy Spencer retired in 1970 he and his wife moved away and my parents bought his house. It had unusual features (like a motorized bbq/rotisserie in the kitchen, some high tech intercom, some good tools in the basement etc all of which my EE dad repaired) but to my dismay no microwave. We did eventually get one (Amana — yep, Raytheon). My mum made us leave the kitchen when it was operating — unsurprisingly she understood 1/r2. Occasionally the Raytheon lab where he worked was open for tours.

    I built my first circuits in that house.

  6. > However, if you open your washing machine and it doesn’t stop, the worst that can happen is you get wet.

    A spinning washing machine barrel can be almost as dangerous as a lathe if you manage to stick your arm in during the spin cycle. GIve it some respect.

    1. One day I forgot to remove chuck key from my cheap chinese lathe with all the /annoying/ safety interlocks bypassed. Luckily it hit me in the thigh so it was just painful. Had I been less lucky I could get hit in the head or even worse – in the balls.

      The next day I took it apart to rebuild and bring back safeties and covers.

    2. True. It’s been some time ago, but there was a story in the news of a boy that got his arm caught in a running washing machine and it was taken off at the shoulder. Thankfully, a follow-up story a week later reported that it had been “successfully reattached”.

  7. “With the microwave, you could get a big dose of (non-ionizing) radiation! Studies show this probably isn’t as bad as you might imagine, but it is bad enough to burn you.”
    … and damage your maculas and give you cataracts. Microwaves don’t care about what they are cooking.

    1. In the 70’s, I toured the Litton Factory here in Minnesota / USA. I noted that none of the many ovens being tested had any doors! The engineer said that you “might” feel some warmth if you stood closely in front of one for an hour or two. They were testing turntables. Each table had a few small neon lamps taped to it. Interesting to see them go out when encountering the low nodes of the standing waves as they rotated.

  8. ” With the microwave, you could get a big dose of (non-ionizing) radiation!”

    It’s not big. The maximum a typical microwave puts out is a kilowatt of RF power. When confined in a cavity, sure, it can heat a liter of water at the rate of a quarter degree per second.

    However, open that door and let the power escape, and it quickly becomes trivial: a half meter away the power density is less than you get walking outdoors in the sunshine.

    It’s even less than that: incident sunlight is concentrated in the first millimeter of skin, but microwave power takes a couple of centimeters to get absorbed, diluting its power density by a factor of 20.

    Not to say you can’t get hurt by doing something dumb, but the dangers of microwave energy are overstated in the extreme. The blame lies squarely on microwave anti-zealots Ralph Nader and Paul Brodeur.

      1. It’s true that corneas are a bit more sensitive to RF energy than, say, brain tissue, because they are rather poorly served by blood flow to cool them.

        But you missed the point: the power density just a short distance away is very low, less than that commonly found in nature, and it *won’t* heat very much.

        Heck, you get more thermal radiation hitting your body from your kitchen walls than you will get standing a meter away from a kilowatt microwave source. Don’t believe me? Do the Stefan-Boltzmann arithmetic yourself and see.

  9. I worked at a factory that had about 1MW of microwave heating capacity. The EEs went to a class on microwave engineering and on the first day the instructors had a tabletop microwave with the door off and all the interlocks defeated. The instructor turned it on, stuck his hand in it, and told the students “here’s everything you need to know about microwave safety: when your hand gets warm, take it out”.

      1. I wonder if anyone who expects radio waves to cause cancer knows that their body emits radio waves (and infrared) naturally due to its temperature. Or that unless they live in a dark cave, they expose themself to way more visible light than radio on a daily basis.

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