Rick's b.log - entry 2022/11/19

Winter is coming

Sorry about the F in fog.

Since then, it's been a warmish week (about 14°C, and maybe 8°C at night) with a lot of rain and wind as storm mumble-whatever passed through. The storm has gone, and it's only just made a double digit temperature. But last night? Well, I was up at half three (Saturday work) and the grass was crunchy and the temperature was zero-point-something.

So... winter is coming.

A microwave oven

I stripped down and cleaned the tabletop, took the toaster apart and cleaned it - it's ridiculous how many little nooks and crannies bread bits can fall into. There's a removable tray at the bottom, but most of the crumbs were elsewhere.

I also stripped the microwave, in case its fan was clogged with dust or whatever. Actually, the mechanics of the microwave are in good shape. Which means it's a bit of a shame, given that the inside is rusting. Well, a certain person no longer around did used to have a go at me for leaving the door open. But I was, at least, aware that if something was cooked for a long time in the microwave (as opposed to just reheating it), the thing would be soaked inside, the walls and door literally running water and the bit under the glass dish, a puddle. Closing the door on a soggy oven only exacerbates the problem.
This isn't unusual, the microwaves at work are similar. It's part of why there's such a powerful fan inside. It isn't to blow the waves around. The fan probably makes it take longer to heat stuff. It is, in point of fact, to draw in cool dry air from the outside, blow it across the magnetron to stop it from overheating, then push the slightly warmer dry air through the microwave itself in order to push out the steam from what's being cooked.
Of course, living in a cold damp house, not ideal for pushing through a microwave as you really want dry air. But I don't have a choice there. Well, not unless I win the lottery and get some sort of heat exchanger gizmo fitted...........but then I'm used to it being cold and damp, to the point where work, the changing rooms heated to the current legal maximum of 19°C, seems somewhat unpleasantly warm. I mean, FFS, just put a thicker jumper on.

Anyway, the microwave is a retro design red thing with electronic timer and a bunch of intelligent preset programmes that I've never ever used. It's not an LG, but it has an LG magnetron inside.

Inside a microwave oven.

On the right is the power conditioning to prevent interference going back into the mains. Below that, a big fan.
The big lump is the magnetron. This isn't terribly interesting to look at. The internal principle is, however, quite interesting. Instead of taking high frequency pulses, which would be difficult to create and manage with sufficient intensity to cook food, the magnetron's oscillation is created by having resonant cavities drilled into a solid piece of metal (usually copper). The typical arrangement is to have a large hole in the middle, with smaller holes around it. The smaller holes are connected to the large hole using cutouts.
Try to imagine the barrel of a six shooter pistol. It's a little like that, the holes for the bullets are the cavities.
The entire block is the anode.

The large hole in the middle is for the cathode, a smaller tube that is electrically isolated from the rest, and runs through the large hole. It is negatively charged by high voltage DC. A transformer will step the mains up to something like 4kV. A powerful diode will clip that so only half of the waveform is available. And a big-ass capacitor, like, say, that large silver tube to the lower right of the magnetron, will dump stored power into the magnetron in the time when the AC swings the other way (and is clipped by the diode). Hence, a slightly wobbly but passable DC supply for the magnetron.
With the powerful magnet around the anode, and the cutouts, an oscillation will begin (for reasons way too complex to get into here). Since it is designed to oscillate at around 2.4GHz (yup, just like Bluetooth and WiFi), some of the oscillating RF energy is extracted, either with a coupling loop (like a little aerial inside) or simply punching a hole in one of the cavities for the energy to escape. This passes into a wave guide, then through that weird plastic-cardboard-like thing on the side of the oven, and then into your food.

The frequency of this style of magnetron is not precisely controllable. It depends upon load impedence, supply current, and temperature. This is why some days it might trash WiFi in half the house, and other days not have any effect.

This style of magnetron is self-oscillating. All it needs to get going is power. But it does take a short while to get up to oscillation speed. This is why microwaves tend to make a low pitched buzz as the magnetron starts up. You most typically notice this when using a lower power setting.
The magnetron cannot have its power output altered, so the way lower power modes (such as defrost) work is by simply turning the magnetron on and off for set durations. 30% power means it'll be on for just under a third of the time, and off for just over two thirds of the time. When it's on, it'll be on 100%. Averaged out, it's 30% power.

The oscillation is a bit like the noise you can create when you blow into a saxophone, the magnetron is naturally resonant. Only instead of sound waves from air, it's making radio waves from electric fields. The wavelength of a 2.4GHz signal is 12½cm. But why 2.4GHz? Why have a microwave crash into WiFi crashing into Bluetooth crashing into DECT telephones crashing into video senders crashing into... you get the idea, it's a heck of a lot of stuff all hanging on the same basic frequency.
The reason is because this bit of the electromagnetic spectrum has been set aside as a "free for all" part of bandwidth. If you want to transmit into the FM radio band (roughly 88-108MHz depending upon country), you need a licence. If you want to transmit on CB radio (~27MHz), you need a licence. But thanks to microwave ovens, which spew out RF radiation at as close to 2.45GHz as possible, it was necessary to block off a chunk of the RF spectrum (remember I said the frequency can't be precisely controlled). Since this was an open unlicensable band, everybody else jumped on it. And the annoying interference, like Netflix bbbuuuffffffeeerrriiinnnggg when you turn on your Bluetooth headphones, it's thanks to microwaves.
But while this answers why everything else uses 2.4GHz, it doesn't answer the question of why 2.4GHz.
Well, the way microwaves cook food is something called Dielectric Heating. That is to say, the radio energy blasting out of the magnetron excites the molecules in water and fat, while passing right through stuff like glass and plastic. The excited molecules jiggle. Jiggling makes heat. So radio energy is converted to heat. And as you ought to be able to work out, the age-old idea of microwaves cooking from the inside outwards is complete bollocks.

Now, there are other frequencies that could cook food. Not all frequencies, as exciting water and fat molecules works best with certain specific frequencies. 915MHz, 2.4GHz, 5.8GHz, 24GHz... but only one frequency was reasonably capable of doing the job and reasonably cheap to mass produce. That's the 2.4GHz magnetron.

Oh, and just so you know, metal is highly excited by microwaves. To the point where an old AOL CD-ROM (remember those?) will go from cold to on fire in a matter of seconds. This is why it is recommended to never put metal into the microwave.
If you forget and stick in a bowl of porridge with a spoon, it's not the end of the world so long as the spoon doesn't touch the inside of the microwave (which is earthed and at the same potential as the magnetron's anode, so it'll arc across) and you don't grab the spoon afterwards because it'll be bloody hot. Ask me how I know... ☺

It is also imperative to run a microwave only when there is something inside of it. If it is running empty, some of the emitted radiation will be absorbed by the oven itself (the door, the turntable, the walls) but a fair amount will find it's way back into the magnetron (the Standing Wave Ratio or SWR that people who used CB radios might remember). Think of the radiation as bouncing around inside the thing. Now when some of that gets back into the magnetron, it'll collide with the energy trying to escape and cause heat. Eventually, unless you have a really crappy device, a thermal switch will kill the power. However there is a risk that too much heat may have warped the cavities slightly, leading to a device that may not be as efficient as before.

The magnetron itself is a fairly efficient device. Around 1100W of input power will result in around 800W of microwave power (but note the actual consumption will be higher because of the fan, light, controller...).

If you want to know more about the design and operation of a magnetron, you can read the patent from 1941, but it gets real complicated real quickly.

By the way, don't take a magnetron apart, or attempt to use it outside of the oven. The reason for not using it out of the oven (or spending too long looking through the door) is that your eyes do not have blood vessels to cool the lens. So it's quite possible that stray RF energy will not only heat parts of the lens of your eye, but you won't even be aware of it. At least, not until the cataracts start many years later.
As for taking the thing apart, it's basically three parts. Some fairly powerful magnets, loads of bits of weird shaped metal, and a little ceramic tube holding the cathode and insulating from the anode. If the ceramic tube breaks, you're looking at particles of such lovely stuff as Thorium or Berylium wafting around the air. Modern microwaves don't tend to use these substances as they are highly carcinogenic, but older ones will be full of nasties.
A bit like older transistors, then... like in my younger days taking apart Gallium arsenide transistors to see what was inside. Erk!

On the left, since my model is a fancy electronic gizmo, there's a control board instead of a simple mechanical timer. Actually, there are two boards. The yellow one you can see holds a tiny power convertor to make the ~5V for the ciruit, as well as a relay for turning the magnetron on and off, and sensing for the door closed switch.
The green board behind is the low voltage computer board with a cold cathode fluorescent display, programme buttons, and some sort of microcontroller. Unfortunately the microcontroller was mounted on the front facing side of the board, and it wasn't possible to remove this without risking breaking the rotating knobs (they aren't just slide-on). The microcontroller was a fairly large thing, not unlike an eight bit CPU such as the Z80 and 6502. So I'm going to take a pure guess and suggest that it's probably one of the many incarnations of "system on a chip" based around a clone of the 8051.

I didn't want to break this as not only do I use it, but it's also the only digital microwave I have come across where you can spin the control knob and the time will count up.
The digital microwave at work is infuriating. You have to turn the knob painfully slowly for it to register. Spin it too fast, you can hear a gentle click as it moves from one position to the next, but nothing happens on the display. Some deft fingerwork to keep the knob spinning and maybe every two thirds of a second it will register... unless you spin too quickly in which case it just won't see it. If I owned a microwave like that, I probably would have allowed my pickaxe to have a word with it.

But, there you go. I cleaned my microwave, and you got to look inside...

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Zerosquare, 20th November 2022, 04:38

Another good reason not to take your microwave apart, unless you really know what's your doing: that big high-voltage capacitor can deliver a potentially lethal zap.    (Minor correction: the display is vacuum fluorescent, not cold-cathode fluorescent, as it includes a heating filament.)

Matthew Butterworth, 20th November 2022, 20:20

Are you still alive?

Rick, 20th November 2022, 20:31

Having gotten up at half three yesterday for a 5am start at work, then not sleeping much last night (no idea why)...    ..."possibly not" would be my response.

J.G.Harston, 21st November 2022, 20:38

I'm clearing my Mum's house as she's moving into a care home. Opening another cupboard and finding yet more decades-old food is just one of the merry joys involved. At least the freezer contents have been edible.

Anon, 23rd November 2022, 16:51

Rick - didn't you once dismantle a microwave (when you were still living in the UK) and attempt to make a 'death ray' with it? If memory serves I remember you talking about it back in the days of Fidonet and being too much of a chicken (or should I say rather too keen on not microwaving your testes) to actually try powering it on?

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