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Magnetron Fail & Empty Oven Detect

Discussion in 'Electronic Design' started by Mark rainess, Jun 5, 2007.

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  1. Mark rainess

    Mark rainess Guest

    I am building a machine that uses a microwave oven. I need a simple and
    cheap way to reliably determine 1) that the magnetron is operating, and
    2) that the oven cavity is not empty.

    My first attempt was a diode RF sensor on a board mounted outside the
    cavity with a short probe projecting inside through a hole. The food
    item being heated moves relative to the probe and causes the response to
    increase and decrease as it acts on the E field. When I see the changing
    response I know the magnetron is working and the oven is not empty. This
    works fine if the food is by itself; but, the food rests on ceramic
    which moves. The ceramic material influences the E field to a large
    degree and makes it impossible to detect the food item. I can't
    eliminate the ceramic or change to another material. It seems I need an
    entirely different approach.

    If I put two RF probes in the waveguide spaced one-quarter wavelength
    apart, I think the phase relationship between the two probes will change
    when the oven is empty.

    I want to avoid the expense of adding a secondary waveguide in order to
    make a directional coupler.

    Does anyone have an idea for a simple cheap way to detect that the oven
    is empty.

    Mark
     
  2. Maybe narrow band down to detect for the EXACT food frequency. I am an
    amatuer in RF, ask an expert about how to filter for the exact
    frequency at which food excits and re-radiates.

    Food relative to ceramics is a hard topic.
     
  3. Mark,

    Another way to detect the presence of food is to have a micro-switch
    operated by the weight of the food on the carrousell.
    The carrousell slips up and down along the driving shaft. => switch opens
    and closes. An opto interuupter may do the same.

    yet another way, use an optical transmitter - receiver and notice the beam
    being broken by the food.

    Jure Z.
     
  4. mpm

    mpm Guest

    Can't you just monitor the magnetron current?

    The response of the cavity should also change when it is not empty.
    There's an article floating around about how to measure the dielectric
    constant of an unknown material by placing it inside a cavity filter
    and measure the pre and post response. You might

    I would think that the process might apply to what you are doing.
    I will try to find the docment in my libary and email it to you... Or
    you can google it.
     

  5. Have you ever looked at a microwave oven? The HV is rectified, but
    unfiltered so the current is always changing.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  6. mpm

    mpm Guest

    Given the two things he wants to do, I do believe the approach has
    merit.
    You can check for undercurrent on either the magnetron, or the power
    supply feeding it.
    That should solve problem #1

    Note: Variability shouldn't really matter. They are either on, or
    they are off (...or they are broken!)


    As for problem #2 - You're right, maybe he needs better accuracy?

    Considering the Cavity empty / occupied, again this can be derived
    from RF response of the cavity, which might be able to be derived from
    the Magnetron current(?). Or, as already suggested, some mechanical
    or optical switch means. I think this is the trickier part..? But
    the cavity (should have) fixed and unchanging mechanical dimensions so
    it's RF response should be fairly determinable over time, even over
    temperature - at least when compared to shoving something in the
    cavity. (That sounded dirty! HA!)

    I still think measureing magnetron efficiency when loaded vs.unloaded
    is the key.

    It has been a long time since I fooled with ovens. TV transmitters
    now, that's another story. (And trust me, when those fail, they
    usually do so with a loud crack, or a goopy glycol mess, or both, --
    and never before 3AM!!.)

    ----
    I just had another weird thought, if the cavity can be made airtight,
    I wonder if you could pump air into it and measure the volume. That
    would tell you if the cavity were occupied, within limits of course.
    Might be difficult to set-up initially, but if the project would
    benefit from having a known atmosphere (pressure, Nitrogen content,
    etc..), that might make it worth the effort?

    And I think you'd want to put dual detectors a half-wave apart.?
    I have to think about that one some more......

    One more thought! - Why not just sit the whole contraption on a scale
    and weigh it before and after placing something in the cavity?? Maybe
    with one that has a digital output or something? You can even get
    pretty accurate scales these days, assuming the items being placed are
    small.

    I love low-tech sometimes!! Good luck -mpm
     
  7. Just an idea, but the actual power consumed by the oven will probably go up
    when it contains food or other heatable substances. You would need to
    measure true power, as the VA may not change, but the phase angle might (if
    you measure the AC input). The DC magnetron current should also change, but
    it might be hard to measure.

    Paul
     

  8. I have run and maintained several TV transmitters. One Gates 250 W on
    Ch 8, One RCA TTU-25B, at 25 KW, and a Comark/Thales 130 KW visual,
    including changing the coolant on the EEV Klystrons and flushing the
    system with Tyglos. I made bypass adapters to keep the Tyglos out of
    the Klystrons, as well. The only on air failure of a transmitter I've
    seen was when the local PBS station reluctantly let me and an army buddy
    take the nickel tour. Neeley was having trouble not laughing in the
    guy's face, because the place was a nightmare. Just as he was bragging
    about how reliable his transmitter was, a bypass capacitor on a 4CX250
    driver exploded, taking the socket with it. He turned to me and
    snarled, "You damn jinx!" My buddy tore into him, telling him that he
    had watched as I rebuilt a AFRTS TV station with a couple hand tools and
    almost no spare parts. ;-)


    See, the old engineers knew what they were doing by being off the air
    for four hours every night. You can't do PM while you're on the air,
    but I did make some repairs to that Gates transmitter while on the air.
    I was standing on the plate transformer, adjusting the drive coupling
    with a fiberglass rod while the other engineer took the readings. ALl
    the interlocks were in place, but there were none on the end cap.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  9. Meat Plow

    Meat Plow Guest

    You could talk all night about this stuff and I would listen to every word.
    One of the few things these days that still fascinate me. Got any good
    links to high power transmitters, tv stuff etc?

    --
    #1 Offishul Ruiner of Usenet, March 2007
    #1 Usenet Asshole, March 2007
    #1 Bartlo Pset, March 13-24 2007
    #10 Most hated Usenetizen of all time
    #8 AUK Hate Machine Cog
    Pierre Salinger Memorial Hook, Line & Sinker, June 2004
    COOSN-266-06-25794
     
  10. Harris Broadcast had a lot of data on their new solid state TV and radio
    transmitters, but I can't find it anymore. It is such a small market
    that everyone is afraid to let out too much info. They don't make TV
    transmitters the size of that old RCA gear. It took 4' * 36' of floor
    space for the nine interconnected 4' *4' cabinets. The ONLY
    semiconductor was in the Bird watt meter at the drier stage. The TTU-25
    series used water cooled power tetrodes, designed for that transmitter.
    They had a pair of 1.5 V 1000A filaments that had to be balanced to .01
    V differential. The "adjustable resistors" were long copper buss bars
    that had studs embedded in them to stretch the copper to raise the
    resistance. The tubes had 7000VDC on the plates, so there was 7000 VDC
    across the water in the cooling system. Water purity was very critical,
    because if it becasme conductive, electrolysis would build up oxyegen
    and hydrogen to the pint it would blow the cooling system apart.

    http://hawkins.pair.com/wlw.shtml has pictures of some radio broadcast
    transmitters, including the experimental 500 KW AM transmitter WLW used
    on 700 KHz.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  11. mpm

    mpm Guest

    Oh yes they do!
    I'll have to send you some photos of the TV-68 NEC transmitter that
    used to be in Orange City before it was moved. I think the beam
    supply cabinet was the size of your RCA!!

    Actually, it was a few feet short of 36. But it was still a beast!

    I know the forklift wouldn't budge it....

    How much white wire was in the RCA?
     
  12. LED and photodiode aimed just above the platform. If there's food on it,
    it'll break the light beam. You should be able to drill two tiny holes
    in the walls to keep the components outside the oven cavity.
     
  13. The Ch 68 transmitter that they never got to work right? That was
    given to a college, when they traded channel allocations for Ch 18, and
    they replaced the NEC with another brand. I was there when they were
    trying to get the bias right on the finals, which should have been done
    before it was shipped.



    The exciter was a TTU-1 transmitter, in three cabinets. The 25 KW
    Visual and 12.5 KW Aural amplifiers, controls and power supplies filled
    six more cabinets. Actually, I forgot that there were two, two foot
    wide cabinets for the the sections of reinforced rubber hose used in the
    cooling system, so it was 40 feet long. That job was around 1990, and
    I've forgot a few details. Since this was in the Florida panhandle, I
    set the cabinets up on 6" * 6" landscape timbers just in case the area
    flooded during a hurricane. They were less than a mile from the gulf,
    and a couple feet above sea level:

    Try moving a full size wave solder machine sometime. It took two to
    lift it and get it into the building.


    It took three 28 foot U-haul trucks because of the weight. One load
    was less than half full, and I was less than 25 pounds from it being
    overweight.

    None when I moved it. It was all 14 gauge stranded rubber covered,
    with a fabric jacket. It was so old that I had to replace the entire
    inter cabinet harness, one wire at a time. Just moving the old harness
    made you sick to the pit of your stomach, knowing that more rubber had
    turned to dust, and the chance of another exciter going up in flames,
    like the original.

    The only local electrical supply house had three colors is stock, so
    the new harness was Brown, blue and white, and made in expandable mesh
    tubing. It took three full days to make, label and install that harness.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  14. JackShephard

    JackShephard Guest

    It's a moving platform retard.

    You need your bones breaking
    I have worked for years on all aspects of microwaves.
    Your retarded idea is dangerous to the consumer and
    voids the warranty.

    Keep your retarded foolhardy ideas to yourself
     
  15. JackShephard

    JackShephard Guest


    Another forgery by Lamey The ForgeTard.
     
  16. mpm

    mpm Guest

    The same. I guess it's still in service for another year or so?
    I would not want to be guy converting that to DTV, I can assure you!!

    But as for things shipping from the factory "correct", well I can
    recall (2) cases where that just didn't happen:

    The first was WCIX TV-6 after hurricane Andrew (1992). 1750' tower
    comes crashing down, hits the building, water in the building, floods
    the transmitter. (Not that could get it out of the crushed building
    anyway..) You get the idea.

    By pure luck, we were just about to ship a 40kW Larcan solid state to
    Charlotte for the DTV tests.
    It was fully tested and ready to go, and was already tuned to CH-6!
    (What amazing luck that the Charlotte DTV test were taking place on
    Ch-6!). So we offered it for sale, and eventually CBS took it off our
    hands.

    But when it arrived, one of the blowers was installed backwards
    (possibly by the local engineers, though nobody can say for sure...),
    and for the next year or so, they "mysteriously" kept blowing finals
    in the PA drawers. It was on low-power on a temporary tower for a
    couple years while the replacement tower was being erected (which we
    also provided!), and this got fixed / noticed when they added the
    extra PA cabinets. But this one you can almost forgive, because they
    had a transmitter in a week vs. being off the air for ...??? (You
    know, there aren't too many high power CH-6's laying around..)

    The next one I won't give call signs or antenna manufacturer names (to
    protect the "guilty" - hey, we all make mistakes...)

    Eastern United States. Multiple Combined FM's (all Class-C's) all
    operating from a common panel antenna.
    One night, antenna goes up in flames! Everyone off the air.
    Everyone pissed. And of course, it's well past midnight! Swept the
    line - major problem at top elbow complex.
    Mobilize crew. Bring elbow complex to ground.

    Guess what was Inside one of the elbows? A pack of washers!! (Yes!
    That's right boys and girls. These were the very same washers that
    were supposed to tighten up the flanges so water wouldn't get into the
    line in the first place.) I guess the original tower crew couldn't
    figure that out?!

    Needless to say, it was a little hard to hide what went wrong when all
    the station engineers looking on......
    And of course, it pretty much resolved the nagging question of why the
    line had so much trouble holding pressure!
    In all my travels, I had never seen a miter-reinforced 6" copper elbow
    get that bad. (I think it almost melted!, it was definitely
    warped.) That's a lot of RF.

    Ah, those were the days. Did I mention I don't really miss 'em?
     
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