Continuous output magnetron possible?

Discussion in 'The Projects Forum' started by kunkle11, Mar 5, 2016.

  1. kunkle11

    Thread Starter New Member

    Mar 5, 2016
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    Hi all. First post here. I have very limited electronics knowledge so far. Working on plasma research project. Main issue is the pulsed nature of my magnetron to ionize plasma. I already have a 700W magnetron from an oven running satisfactorily- getting plenty of output into the vacuum chamber. If possible, I need to convert the pulsed microwave output to continuous.
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,010
    3,783
    Let me put my mind reading cap on and visualize the schematic from the model number of the microwave you are thinking about. Just a minute, concentrate on that model number really hard, I can see it better that way...
     
  3. ian field

    Distinguished Member

    Oct 27, 2012
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    Most likely the bottom line is dissipation.

    Pulse operation is usually at a much higher power level than you can get away with continuous.
     
  4. kunkle11

    Thread Starter New Member

    Mar 5, 2016
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    Off to a really bad start here. Hit some wrong button and it posted when I'd barely written anything, and then it dumped my entire post when I tried to edit it because I took longer than 10 min. Now I don't see any way to edit it. Sorry. Let me try again:
    Main issue is the pulsed nature of my magnetron to ionize plasma. I already have a 700W magnetron from an oven running satisfactorily- getting plenty of output into the vacuum chamber. If possible, I need to convert the pulsed microwave output to continuous.
    Don't know the model of the oven anymore- it's long gone. FWIW, the transformer is iron core with a doubler circuit using a .75uf 2100V cap.
    After exhausting google and searching this forum, here's what I came up with. Convert wall outlet to continuous 120V DC with a full wave bridge rectifier, then a 680uf cap in parallel. I also need continuous 120V DC for some tungsten emitters, so If a different cap would work better- let me know.
    A reply to an old post on this forum suggested taking 2 MOTs and wiring their secondary output in series to get from 2k to 4kV. I always thought this was a no-no, but no one rebuked that reply either. So, can I take the continuous output 120V DC from the above circuit and feed that into 2 MOTs in series, then to the magnetron? Seems to easy to be true. Thanks for any help.
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    The output of a full-wave rectifier with a smoothing cap will be about 170 VDC without any load. What it will be with a load will depend on your cap and the load. The average voltage will drop and the ripple will increase.
     
  6. kunkle11

    Thread Starter New Member

    Mar 5, 2016
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    As far as the emitters circuit goes, they will use about 150W. Any suggestions to keep the ripple at minimum at that rate? Thanks.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    You need to convert that into an average current draw from the DC supply (150 W draw does not translate directly to current if, for no other reason, the efficiencies involved).

    Can your circuit handle 170 V DC?

    If you want 120 VDC, then you would probably be well-served by regulating the voltage to 120 VDC. Doing so would also drastically reduce the ripple voltage.
     
  8. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,034
    1,643
    Do you think you can run DC through a AC transformer? o_O

    More than likely the pulsing is coming from the simple half wave charge pump type voltage multiplier circuit that is use to power the magnetron. Adding a few more diodes and a capacitor or two more would convert that to a full wave charge pump greatly reducing the overall voltage fluctuations that the magnetron is seeing now.
     
  9. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    First, good luck. That tube was not designed to run in CW mode. But since it was cheap, lets see what it can do.

    Be careful, the voltages and RF power outputs are damaging if not deadly.

    You will have to run the tube at a reduced voltage. What that voltage is, I don't know. Start low and slowly increase.

    What equipment do you have to detect and measure the RF output of the tube?? If you do not have equipment to make these measurements, stop until you do have the equipment.

    You will need to measure the power supply current as you increase voltage. Do this safely.

    If it does start to oscillate, the frequency will probably not be at the normal pulsed frequency.

    This is not a casual experiment. It is dangerous. If there is a crack in the waveguide it will paint you with RF energy. Your eyes are the most easily damaged.
     
  10. Ramussons

    Active Member

    May 3, 2013
    557
    92
    As far as I can remember, the working principle of a magnetron is Pulsed Mode. I would be very thankful if someone here can explain how a magnetron can operate in a continous mode :confused:
     
  11. andrewmm

    Active Member

    Feb 25, 2011
    30
    6
    wow,

    Please be careful.
    Worked for years in radar design works,

    these things kill.
    the electricity is the least of your worry, the RF is the thing. Ensure your lab has RF radiation detectors, that work. Also make certain you have plenty of kill switches around, that others know about and can always be easily hit...

    and don't allow any distractions, that friend that pops in with the coffee and distracts you as you erradiate your eyes, is a lousy excuse to go blind. ... ( seen it ... )
    The health and safety paper work is amazing.

    ok back to the questions.

    Magnetrons can be run continuous or pulsed,
    As mentioned above its thermal dissipation that can catch you out.

    CW magnatorns of more than a few watts tend to have water cooling for a reason.
    They are about 60 % efficient, depending upon the load VSWR... so if you have a 700 watt output, your magnatron is going to dissipate about 500 watts . Pulsed at about 10 % , thats 50 watts, if you up it to 100 % thats a fair amount of dissipation.

    Wiring outputs of transformer in series, quiet normal,
    You can get many transformers that have multiple secondary windings, and depending how you connect them depends what voltage out you get. Using two separate transformers, primaries wired in parallel and secondary in series is just the same, if you get the phases correct. Get it wrong and you will know about it.

    I assume you know about coronal discharges, and HV cables over regular cables. If you have to ask. STOP NOW. 2 Kv at 10 mA will kill at a distance, even through plastic 'insulators' . Please remember that HV capacitors have a hidded kick, you discharge them, and leave alone and a few moments later they have a charge back across them. HV capacitors that can be accessed , like on experimental gear should have a fail safe dump circuit across them.
     
    nsaspook likes this.
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