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Discussion in 'Electronic Design' started by buzz, May 3, 2004.

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  1. buzz

    buzz Guest

    How is the interlock on a typical microwave oven implemented?

    Recently, an 8-year-old oven became homicidal and would turn ON when the
    door was opened and OFF when the door was closed. At the time, the oven
    had just finished a cooking cycle and turned itself off; when the door
    was opened it started up again. My hand (near the open door) got warm,
    and I could tell from the loud hum that the oven was in cook mode (it
    wasn't just a fan running). Pressing the "clear" button had no effect.

    I unplugged the oven for about 10 seconds, but the problem persisted;
    whenever the door was opened, the oven would start cooking. After the
    oven sat unplugged for a week, the problem went away and the oven seems
    OK. The whole time, the light worked properly: off when door closed and
    not cooking, on when cooking.

    I would have thought there would be a "dumb" interlock based on a door
    switch, perhaps backed up by software also. But this appears to be a
    software-only interlock.

    Just curious.
  2. Sal Brisindi

    Sal Brisindi Guest

    If I were you I would get rid of that microwave oven, not worth the risk.

    Sal Brisindi
  3. Winston

    Winston Guest

    Thanks for posting that, Buzz. Safety stuff is important.

    Sal makes a good point, but forgot to mention the simple adjustments to
    make to the oven's software, first.

    1) Unplug the power cord.
    2) Snip the cord off where it exits the oven.
    3) Place the oven on a tarp in the back yard.
    4) Don your face shield, earplugs and gloves.
    5) Carefully remove the door. (You may use an angle grinder or torch)
    6) Cut the door into two pieces of approximately equal mass.
    (You may use a fire axe.)
    7) Gently tap the exterior in several places using a long handled five
    pound maul. (Overhead swings, with as much force as you can muster.)
    You may stop tapping when the cubic area occupied by the oven is reduced
    to 70% of its original value.
    8) Retrieve the various pieces of oven that attempt to exit the area.
    If you have tapped the exterior properly, exterior parts will be found
    as much as twenty feet away.
    9) Toss one of the door pieces in the trash, along with the power cord.
    10) Over the next few weeks, place two random pieces of oven per week into
    the trash until it is all gone.

    This will put the proper values into those registers and cause the
    oven to stop endangering you and your visitors.

  4. Ken Taylor

    Ken Taylor Guest

    While both responses are correct, the manufacturer *needs* to know that this
    can happen. That is **really** bad design. It should indeed be a dumb-ass
    interlock so that this can't happen. I'm appalled - what brand is it? I
    don't ever want to see one in my vicinity!

  5. Dan Mills

    Dan Mills Guest

    Ken Taylor wrote:

    I thought the standard way to do this was with 2 microswitches, one to cut
    the power when the door was opened and the other to short circuit the
    primary of the transformer under the same conditions (obviously mounted to
    ensure the correct sequence of operation).....

    Why the hell didn't whoever made the silly thing do what every other
    microwave manufacturer has been doing for years (I know, a small signal
    switch and use the same power switching device used for the duty cycle
    control is cheaper!).

    BTW: the correct fix for this unit surely involves thermite?

    Regards, Dan.
  6. Ken Taylor

    Ken Taylor Guest

    Agreed, but that only solves the immediate problem. What about all the other
    units out there?

  7. Tim Shoppa

    Tim Shoppa Guest

    I would claim that the majority of "broken" microwave ovens in the
    US are because of failed door interlock switches.

    The switches are indeed designed to fail into the "safe" position but
    they fail the other way too. (especially the ones on the door latch
    that trigger the light and fan.) The thing we rely on for safety is that they
    don't fail simultaneously in the "dangerous" direction, but with hundreds
    of millions of microwave ovens made over the years I'm sure it has happened.

    I work in an industry that prides itself in vital relays that are
    (supposedly) more reliable than those used to arm nuclear weapons, and
    there are always freak failures if you look hard enough at a system that's
    been operating for a good chunk of a century. Most of these failures
    get investigated and written up by the FRA and/or DOT, and the root cause
    is usually much more complex than a simple failed component or two.

  8. John Miller

    John Miller Guest

    Why has nobody yet pointed out that the actual make and model of the
    microwave have not been mentioned? That'd put the circuit configuration to
    rest in about 30 seconds.
  9. Ben Bradley

    Ben Bradley Guest

    I pointed this out indirectly by asking for it to be posted, and I
    think someone else asked too. The OP's posting address is obviously
    fake, OTOH google shows a few recent legit-looking posts from that
    address, posted through
    Of course, even the most legit poster could decide to have fun by
    posting a "troll"...
  10. Ken Taylor

    Ken Taylor Guest

    My suspicion too - even a clown in a Krusty-burger hat would feel some
    indignation if their hand got warm when the microwave oven did funnies.
    Still, if he posts more detail....

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