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Running an empty microwave oven

Discussion in 'Electronic Equipment' started by Peter Hucker, Jan 19, 2007.

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  1. Peter Hucker

    Peter Hucker Guest

    Is it really true that turning on a microwave with nothing in it will break it?

    Even more worrying - will it catch fire or explode?

    Don't they have a safety cutout? Can't it sense the Klystron overheating, or a build up of microwaves over a certain level?

    I have purchased a new microwave which has an easier to grab control. I'm concerned one of my pet parrots will switch it on! (Seriously, they do stuff like that)

    --
    http://www.petersparrots.com http://www.insanevideoclips.com http://www.petersphotos.com

    A Pakistani arrives in London City all excited. He stops the first person he meets. "Good day, Mr. British, thank you to accept me in Your nice country", but the person interrupts and says: "I am not British, I'm Chinese."
    The Pakistani continues on his way and meets another passer-by. "Thank you Mr. British for to let my family and me stay here... " Again, he's interrupted before finishing his sentence. "I no be British, I be Turk!"
    He goes a little farther and meets another person and greets him "thank you for letting me come to your beautiful country." ... "I'm not British, I'm Kosavon."
    "But," answers the Pakistani distressed, "where are the British??"
    The Kosavon looks at his watch and says ..... "Probably at work."
     
  2. Jon Elson

    Jon Elson Guest

    That would be a Magnetron tube, and yes, I thenk they all have a
    thermostat on
    the tube. This is also to prevent a fire if the cooling fan seizes up.
    But, in the
    case of no food to absorb the microwave energy, the reflected power can
    build up
    higher than normal voltages in the tube and cause permanent damage
    before the
    entire anode overheats.
    Parrots? What about kids? I suspect quite a few microwaves get "toasted"
    by kids turning them on empty. I know they can survive a few seconds of
    this abuse without noticeable damage. You could unplug it or leave the door
    open (I'm pretty sure a parrot couldn't close the door.)

    Jon
     
  3. Alison

    Alison Guest

    concerned one of my pet parrots will switch it on! (Seriously, they do
    stuff like that)
    Parrots. Cool :)

    Leave a cup of water in it. Or something like those heat up aromatherapy
    bags, the ones that have lots of little cherry stones in it.
     
  4. Peter Hucker

    Peter Hucker Guest

    13 of them. Messy. Noisy. But fun.
    That would only delay the problem. And I'd never remember to put a glass of water in after each time I use it.

    I want to know what happens if it is switched on empty. The instructions just say it will damage it. That doesn't bother me too much, it was a very cheap one. As long as it doesn't explode or catch fire when I'm out of the house.
     
  5. Peter Hucker

    Peter Hucker Guest

    So it'll just break? That's good to know. I don't want to lose a house or a parrot while out at work.
    That's a point, it must happen a lot. If they caught fire or exploded, we'd see it on the news.
    I once went for an interview at a company that makes magnetrons. As far as I know there is a thing that absorbs all returning microwaves. Wouldn't this just have a temperature sensor in it that would shut off the oven?
    The plug is behind the fridge. I used to plug it in above the worktop, but the parrots ate the plug!
    I bet it could, if it wanted to. But they are more interested in opening or breaking things. Parrots do not tidy or assemble things.

    If I leave the door open, one will climb inside and chew up the cover that's over the magnetron output (it looks to be card).
     
  6. PeterD

    PeterD Guest

    Look, the only safe thing is to unplug it. Even a glass of water won't
    provide protection if they turn it on for 20 minutes! Use a short,
    heavy duty (12 awg) extension cord, or some other similar device.

    Either that, or make up a metal shield to cover the controls...
     
  7. Jon Elson

    Jon Elson Guest

    Well, if it has the safety thermostat on the anode, then it will
    just shut down, probably forever. I have no idea whether the
    safety thermostat is mandated by law or required by testing
    agencies like UL, and whether the really cheap ones from China
    have them. Without the safety stat, I don't know what would happen.
    Well, I think there have been some fires from them, but probably
    not a lot.
    Not in an oven. Maybe in a radar transmitter, they have an
    expensive part called a circulator that splits waves going in
    different directions, and shunts returning waves to a terminating
    resistor. Nothing like that in a microwave oven, believe me.

    Jon
     
  8. colin

    colin Guest

    or a build up of microwaves over a certain level?
    concerned one of my pet parrots will switch it on! (Seriously, they do
    stuff like that)

    When I designed some of the early mcu controllers for them eons ago
    we used to give them a heck of a lot of abuse such
    as running them empty to try and get the mcu to crash,
    (wich with a piggy back eprom mcu was all too easy)
    ofc always a microwave leakage meter nearby,
    the most spectacular thing was to melt the glass tray,
    the turntable mechanism was the first thing to melt usually.

    If theres food inside it will eventually catch fire and the flames will come
    out of the back,
    this was a serious liability worry at the time I assume theyve added some
    safty things since I worked on them.

    Some have water vapour detectors wich sense the peak and stop cooking, I
    assume they will stop if nothing is detected.

    Colin =^.^=
     
  9. Peter Hucker

    Peter Hucker Guest

    This is a £27 Tesco Value microwave - probably only has legal safety requirements.

    I've already turned it on momentarily with nothing in it, as I'm used to an electronic one. If my food has gone cold due to a phonecall etc, I tend to shove it in and just turn the timer round to get it to go, then open the door after 20 seconds. The electronic ones cancelled the timer when you opened the door, but this one turns back on when you shut the door.
     
  10. Peter Hucker

    Peter Hucker Guest

    Nowhere is safe to plug it in without them chewing the plug. The plug has to be out of sight, which means I can't get to it to turn it off.
    I bit of a hassle, and I'm useless at that kind of thing. And too lazy to do so unless I'm sure it would explode. Maybe I should buy another one and try it in the garden....
     
  11. colin

    colin Guest

     
  12. Geoff

    Geoff Guest

    They will never reach the button, from inside.

    CDs go really well in microwaves !

    geoff
     
  13. Warren Weber

    Warren Weber Guest

    Parrots are smart. One will go inside, next one could shut the door and turn
    it on. Baked parrot for supper?
     
  14. Microwave ovens don't care what you put inside. They do not have
    feedback loops. You can cook a raisin or a melon or nothing at all.
     
  15. g

    g Guest

    This isn't completely correct. A magnetron running into a perfectly
    matched (lossy) load will see no reflected power. One operating into a
    perfectly reactive load, an open or short for example, will see a large
    reflected signal which will add to the forward wave. The result will be
    higher voltage/current at the tube and a different operating point.

    This is in fact a sort of feedback. It might be possible for the
    tube or PS to fail due to excessive peak voltage or peak current. The
    changed operating point might also cause more heat dissipation in the
    tube or PS.

    g
     
  16. Jon Elson

    Jon Elson Guest

    Assuming a true, perfect short at the end of the waveguide, then the
    only dissipating
    element left is the filament/cathode of the magnetron. Since there is
    no real waveguide,
    and the oven compartment is far from a perfect short, energy will be
    dissipated in everything,
    metal, plastic, glass, RF seals, etc.

    Jon
     
  17. Peter Hucker

    Peter Hucker Guest

     
  18. Peter Hucker

    Peter Hucker Guest

    As far as I know they're vegetarian.
     
  19. Peter Hucker

    Peter Hucker Guest

    But where do you think the power ends up if it is not absorbed?
     
  20. Peter Hucker wrote:

    (Seriously,

    Aha. My Cats like to turn on my Vaccuum cleaner and I get shocked, in
    the first moments. :)

    (Seriously, I have left the Vacuum cleaner connected to the wall-socket
    many times, indeed. But it was never on, when I came back home)
     
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