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Reduce power of a microwave oven?

Discussion in 'Electronic Repair' started by mike, Jan 18, 2011.

  1. mike

    mike Guest

    When I ask a question on line, I go to great lengths to describe the issue
    in detail to keep the discussion from running off in all directions.
    It rarely works, but I keep hoping that people will actually read the
    info before shooting from the hip.

    With this, and every other low-end non-inverter microwave oven I've ever
    encountered, the power is adjusted by pulsing the magnetron in bursts
    of approximately 15 seconds followed by an off-time required to get the
    average power you programmed.
    The RELEVANT number is the 15 second MINIMUM on-time. Doesn't matter
    what buttons you push, you can't get an on-time less than 15 seconds unless
    you program a single burst that's shorter.
    If 15 seconds is long enough to boil the liquid in part of the payload,
    defrosting can make a mess of things.

    In this case, the "defrost" button has some algorithm that they don't
    disclose
    and I've been too lazy to reverse-engineer, but the minimum on-time
    is still about 15 seconds.
    It's a fundamental property of the design.
     
  2. Lab1

    Lab1 Guest

    Reminds me of this interesting video showing how microwave energy is
    distributed inside a standard microwave...
     
  3. mike

    mike Guest

    Well, that's what I was asking about.
    I don't think there's anything you can do in the primary circuit.
    The magnetron filament runs off the same transformer as the high voltage.
    If you reduce the average input, you also reduce the average filament
    voltage/temperature.
    There are two basic issues that I can think of.
    1) It takes time for the filament to come up to temperature.
    2) The more you thermal-cycle the filament, the shorter the life.
    The 15-seconds is a compromise that mostly works.

    If you want to pulse the anode, you have to have a separate transformer for
    the filament to keep it up to temperature while you pulse the anode.
    Conceptually trivial, but it adds to the cost.

    As I mentioned in the original posting, you probably can change the value
    of the secondary cap, but that takes a high-voltage switch.
    I'm interested to learn if anybody's done that. Or if there's
    any theoretical basis for concluding that it's a bad idea.
     
  4. mike

    mike Guest

    It's worse than that.
    A triac can fix the relay issue. The problem is the thermal
    time constant of the magnetron filament.
     
  5. mike

    mike Guest

    Thanks for repeating one of my suggested options.
    Can you be a little more theoretical?
    One reference suggested that the cap is actually
    sized to resonate with the transformer. That would make
    the change of cap value much more sensitive than just a power
    ratio.
    Relevant input?
    Thanks, mike
    Simple matter of price ;-)
     
  6. Ineed, the patent I referred to shows the filament secondary as part of the
    main transformer. But the system presumably works.
    I can't imagine a separate filament transformer being /that/ expensive.
     
  7. I guess I'm showing my ignorance here, as I don't know much about
    magnetrons, but why can't you just let the filament burn while you cycle
    the HV, like you'd do with any other ordinary tube? I mean, with other
    tubes, it's OK to apply power to the heater without any anode voltage,
    right?


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
  8. mike

    mike Guest

    Yes, that's what I had in mind. All you need is a switch that can
    handle that much volts safely...and maybe some means to make sure
    that switching a charged cap doesn't explode the diode or the switch.

    Some months ago, I had to replace the diode in my old one.
    Hooked a resistor to a couple of clip leads and hooked them to the cap.
    I never did find any of the resistor pieces. May have gotten swept up
    when I cleaned up the puddle I made. ;-)
    It's all relative. Paid $80 for mine. I did visit a few stores and the
    inverter ones seemed to be almost 3x that and they don't go on sale
    often. Knowing what I know now, I might have gone with the inverter.
    Spilt milk now...
     
  9. Gee, ya think?

    Another incredibly useful 1-line post from the shoot-from-the-hip master ...


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
  10. PeterD

    PeterD Guest

    The filament is a minor current, the main HV voltage is the issue. To
    cycle off the transformer rapidly with a relay would likely result in
    a relay lifetime of a few weeks at best.

    I don't believe there ever was an issue with the filaments.

    I agree, a triac could possibly be used, though you are talking about
    swtiching as much as 1500 watts, inductive (because of the
    transformer). Might be too expensive to find a triac that will take
    that kind of abuse and survive!
     
  11. PeterD

    PeterD Guest

    Neat trick since the filament emitts teh electrons...
     
  12. Well, I have to admit that it (maximum resistor voltage ratings) doesn't
    often occur to me, simply because I don't work with HV. But the person
    Mr. Terrell responded to above was certainly aware of why his resistor
    got blown to smithereens. Hence my annoyance.


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
  13. PeterD

    PeterD Guest

    Absolutely they do, but I suspect he also exceeded the power rating of
    that resistor by a tiny bit as well... <g>
     
  14. Well, point taken.

    While we're on the subject, where would one get HV resistors, anyhow?
    Any US suppliers?


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
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