Powering A Cavity Magnetron, From A Battery

While vacuum electronic devices have largely been superseded over much of consumer electronics, there’s one place where they can still be found for now. The cavity magnetron is a power RF oscillator device in which electrons are induced to move in a circular path through a tuned cavity, inducing a high-power RF field, and it lies at the heart of a domestic microwave oven. They usually need a high-voltage mains transformer and a rectifier to work, but [Hyperspace Pirate] has managed to make a solid-state power supply to power one from a 12 volt battery. Better still, he’s put the resulting combo in a Care Bears lunchbox. Take a look at the video below the break.

The video starts with a potted history of the magnetron before looking at the circuit of a typical oven, which uses a single diode and a capacitor in a simple voltage multiplier. The capacitor value is adjusted to lower the power output, and a pretty thorough job is done of characterising the circuit.

The low-voltage supply starts with an XVS inverter to make the high voltage via another multiplier, but the interesting part comes with the magnetron’s heater. It’s designed for 50 or 60Hz household electricity, but there it’s receiving 40 kHz and has an appreciable impedance. The addition of a capacitor soon restores it to a reasonable performance.

In case you noticed that the ZVS converter might be improved upon, take a look at a flyback converter. Meanwhile, we should probably echo the safety message in the video that playing with magnetrons and their associated transformers can be a nasty way to die. Please take care out there!

47 thoughts on “Powering A Cavity Magnetron, From A Battery

  1. this is so hideously dangerous and irresponsible — If you want to blind / cook yourself / get cancer in your own lab that is one thing, but I have huge issues with making this thing portable. You might as well publish plans on how to 3d-print guns and bombs.

    1. Wait till you realize that there’s a naked fusion reactor which exposes the world to a peak of over a kilowatt of radiation per square meter EVERY DAY!

      If you think it’s hard to learn how to make a bomb or hard to acquire the supplies, you’re probably sheltered enough that you also derive your idea of right and wrong from what the law allows, and the law often allows similar or greater output on various frequency bands including this one with homebuilt equipment for amateur radio purposes.

      1. The difference is guns and bombs are more easily identifiable and are way more obvious when in use. This thing could be put in a public place and cook passers by that wont even know that damage that has been done for a long period afterward. Yes, you could also take the door off a microwave oven, but that is a lot less stealth than this thing. You could do enormous damage to someone eye sight and they would have no idea where/how it happened. I just don’t like this thing… I really don’t. I’m not saying the guy that built it is evil or dumb… I just don’t like this kind of thing being proliferated without a very lengthy and serious discussion on just how stealth and dangerous it is.

        1. When I was younger and had less sense I decided to experiment with microwaves by taking the door off a microwave oven. At switch on there was a large flash and then nothing. Subsequent investigation discovered that I had blown the street pole fuse.
          Much embarrassment explaining to an electrician how it happened. That halted any more experiments of this nature.

        2. I’ve tried a couple times to post this so I’ll summarize; cheap green lasers are worse. The invisible IR component (which is often improperly filtered out) can permanently damage vision at long ranges, and a christmas decoration with the moving dots seems like an excellent way to cover all the passersby without attracting attention.

      2. Not at 2.4GHz. Yes the nuclear fire ball in the sky is puking out 1KW per square meter, but it is not all at a frequency that heats water. Further that solar energy is at spectrums that are easily blocked by shade.. hence your cloths and sunscreen, and the buildings you inhabit. The energy density you can achieve in the near field of a magnetron far exceeds that of the sun, which is why you don’t see people turning on electric lamps by simply exposing them to the sun.

        1. It doesn’t have much skin depth, but it all heats humans. And the UV can cause cancer unlike the microwaves. I think there’s a more obvious reason people don’t turn on electric lamps by exposing them to the sun, but certainly the microwaves can do things that even intense visible light can’t – I couldn’t expect to light a neon lamp using a laser pointer.

    2. There’s nothing wrong with publishing plans for 3d printed guns or bombs. You’ve been able to make a gun for as long as there have been guns, there are plans everywhere on the net.

      It’s far more important for the public to know about bombs and what goes into making them, than it is to try to prevent information from leaking out. As a for example, knowing the information allows us to double-check whether government regulations are legit and whether reports about bombs in the news are accurate.

      It’s also useful to know what to stay away from. For example, there was an article last week about silvering glass, and did you know that the mixed components for that reaction form an explosive? Be careful how you dispose of the remaining solutions…

      Practically speaking, holding back information is both impossible and counterproductive. It’s far more useful to society to have access to correct information along with the proper safety procedures.

      Dangerous projects are simply projects that require special care. A lathe or milling machine is just about as dangerous as this project, but it’s not an issue if you take proper precautions.

    3. That’s how you troll hackaday!

      Do any of you believe that LLL actually believes it’s hard to find good (as opposed to 3d printed) bomb or gun plans?
      It’s not impossible, but would someone that stupid even come to a site like this?

      Next post is same. Use case is ‘remote testicle heater/zipper welder’.

    4. Okay, y’all have your fun, but this is easily the sketchiest thing I’ve ever seen. Just as sketchy as when those two Russian guys made a “microwave gun” and were pointing it at things and zapping them on YouTube (nope, not giving a link – search for it yourself and may the energy gods have mercy on your soul).
      But I do have to hand it to Hyperspace Pirate (who has also done some other pretty cool things): normally when I see a metal stud sticking out the side of a lunchbox, I know better than to ever, EVER plug it in. But now even that isn’t safe enough!
      I mean, I have a 1200W inverter, and an 800W (500W output) microwave oven, so I could get the same job done with a lot less work, but re-engineering the whole power supply is a whole other level. I would like to point out, LouLouLou, that his power supply can’t supply anywhere near the power that your average microwave oven puts out. So just chill a few degrees.

      1. You must not have seen the x-ray gun some people made on youtube – no shielding, lethal voltages, just a pile of loose components held together by melting hot glue if I remember right. Pretty high flux, and tons of backscatter off of all the nearby objects, surely.

    5. as another individual who plays with dangerous things, I struggle with this exact conundrum.

      I make really cool unique art that’s exceedingly dangerous, IF the individuals in question aren’t careful.

      I am reticent to be as transparent with how I made the tools I use as I don’t want to be irresponsible and give someone else a wild itch to try something that will result in them hurting themselves.

      at the same time, this level of immediate negative response isn’t helping anyone… and it certainly isn’t bringing more awesome into the world.

    6. Its really not so bad. First up – there was mention of lowering the power output. Secondly – inverse square law is your friend. Running a magnetron like this free radiating at full power isnt really much risk unless you happen to be holding the box. At 5 metres away the energy density is lower than the established safety limit mandated globally. I have tested this. Your mobile phone at full blast is not meeting that same emission limit. So if you see someone with a box like this, walk 5 paces away and watch and you will be fine, as long as you dont try to call someone and tell them about it.

    1. Apparently entertainment, by producing radio waves able to light up nearby tubes much like a tesla coil without the exposed high voltage. Seems to be a fraction of the power of a regular microwave oven, with better safety than most of the dumb things people do on the internet all the time with microwaves and their transformers.

      It’s very obviously much less useful as a weapon than the high voltage that it contains, so if you still don’t like it, please close the tab and read something else!

    2. Don’t you just love it when you have people, if something is ‘perceived’ as to dangerous (by them), they want to ban it, hide it, make a law against it…. And they are very vocal about it :rolleyes: too even when it doesn’t affect them. I suspect they probably don’t have any knifes in the house, and all the wall corners are rounded off because you might hurt yourself :) . Need to lighten up :) .

      Anyway, neat interesting project. Understanding what is going on and applying your knowledge for a ‘fun’ project is rewarding … I like it.

      “I’d be fine with all of that” … I agree.

    1. 30 years ago, I proposed bypassing the door switches and mounting a microwave oven on a tower. Using a rope on the door, it could be used to transmit Morse Code.

        1. It is noisy, but not that noisy.
          However, many microwaves have significant energy out-pf-band, the last one I checked had the largest spike on 2466MHz, above 2450MHz by a good distance.
          So to use them for amateur radio they need to be phase locked to keep them inband.
          Unfortunately a current regulator for for a few hundred mA at 5kV is not trivial nor small.
          Still have a stick of 1500V BJT’s for one, somewhere.

    1. I did some not-quite-by-the-book EME bounce when I was in the USAF, and had access to a 2 MW 5GHz pulsed RADAR system. I also had a copy of the ITT Electronics Engineering Handbook, which among other things, had a formula with which you could determine how much power you needed to transmit to get a signal to a particular destination, with a whole bunch of parameters. Accounting for the total path length of HALF A MILLION miles, my 2 MW was actually enough, with a few dB to spare! How could I NOT try it? So try it I did (with the not-really-authorized permission of the guy in operations that day) and what I saw was a representation of lunar craters on my ‘scope. But trust me, a 1 kW transmitter made from a microwave oven maggie is not going to do it unless you’ve got some MAJOR antenna gain and a pretty decent receiver.

        1. Radars don’t exactly have the lowest possible noise figire in the frontend.
          And their transmissions are not exactly narrowband.
          So needing far more power than CW or JT65 makes sense.

          Reminds me of the story from IRC of a dude radar imaging the moon surface by chirping a 11kW 10GHz klystron.
          So how was the chirp generated?
          By hitting the free running klystron with a hammer.

  2. Okay, so this is low power,
    back in the mid-1970s, I thought it would be neat to have a small 12 volt powered microwave oven in my vehicle(s), to heat up a sandwich while on the road.
    This was before they were commonplace in RVs .

  3. Back during one of those proxy wars in eastern europe 90’s or later the US sent expensive anti radar set missiles to the theater and the belligerents used ovens with the door interlock wired out and brought down many missiles wasting them. That cost ratio was out of sight!

    1. I remember reading about Syrian rebels doing the exact same thing much more recently, apparently the microwaves running with their doors open skyward look a lot like SAM sites to fighter jet radar systems.

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