Tech in Plain Sight: Microwave Ovens [Hackaday]

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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

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.

Turntable

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.