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Modifying MOT to get 40KV

Discussion in 'General Electronics Discussion' started by Lash432, Dec 23, 2016.

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  1. Lash432

    Lash432

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    Dec 23, 2016
    Hello,
    I am building a negative-hot power supply to output 40KV DC at 20 milliamps. In order to achieve this I was planning on simply getting a high voltage transformer and connecting to a variac (to vary the voltage) , and then using diodes to get the DC.

    Now, my idea right now is to modify a microwave oven transformer by taking out both coils and rewinding it as a core-type transformer (instead of the shell type MOT's seem to be). I would wind the transformer in a ratio of 10:10,000 and of course, to prevent arcing, submerge the transformer in mineral oil and pull a vacuum on it. The reason I thought of modifying the MOT is because the core is already used to 60Hz and they have KVA ratings high enough to handle the power.

    What do you think of this? Could it work?

    Also, another question, in order to make it "negative hot" all I need to do is connect the positive lead of the secondary to ground correct?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    How much work have you done previously with EHT voltages (in years)?
     
    Bluejets likes this.
  3. Lash432

    Lash432

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    Dec 23, 2016
    Less than a year. The only high voltage I have previous experience with is wiring for Helium-Neon laser tubes. The voltages dealt with there were usually about 6KV. I will admit, I am inexperienced, yet eager to learn, however I want to also make sure I am not just going to waste my time and money, but also be safe, so I am asking here.
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Hmm. 40 kV at 20 mA is 800 watts. What are you going to do with all that power? Why not just use the MOT "as is" since it already has a high voltage winding? In combination with a variable transformer at the input and a voltage doubler at the output, you should be able to get your 40 kV DC without any trouble. Of course, in a microwave oven, the magnetron acts as a diode, and another high-voltage silicon diode serves in a half-wave voltage doubler circuit.
     
  5. Lash432

    Lash432

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    Dec 23, 2016

    I plan on using it for experiments with plasmas in a high vaccum chamber. Also possibly some IEC stuff in the future.

    Also, I am not sure if the transformer would be able to supply that voltage AND amperage...
     
  6. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Most microwave transformers will only provide a few thousand volts (instead of the higher voltage you need) unmodified, but the key is to rectify the AC voltage with DC voltage multipliers to get the DC high enough for plasma work. It is easy to build Cockroft-Walton (C-W) voltage multipliers, and the current capability will mainly only be limited by the capacitors values you use.

    This seems to me to be a much easier approach than re-winding the MOT. But if you have deep pockets, you can purchase commercial HV DC supplies from a company that uses kilohertz-driven "fly-back transformers" driving C-W multiplier stages... typically six to ten stages... that deliver fifteen to fifty or so milliamperes of current, depending on voltage and power rating. Glassman HV used to drive a few dozen 2N3055-type NPN power transistors in parallel to get the power they needed, but this is very old technology. Their newer units use different (more modern) transistors, and add lots of protective bells and whistles, but the basic technology hasn't changed.

    You can "easily" build a basic 40 kV 20 mA power supply with negative output polarity yourself for a few hundred bux using the MOT "as is". Biggest problem is finding inexpensive HV capacitors on the surplus market... they need to be in the microfarad range for use at 60 Hz (instead of C-W multipliers driven at kilohertz frequencies using a few thousand picofarads). You also need some sort of arc protection, else you will be replacing HV diodes every day. Running a C-W multiplier at 60 Hz also means there will be a LOT of stored energy, so you may want to discard the MOT and go with a hefty fly-back running at 60 kHz or so.
     
  7. Hypatia's Protege

    Hypatia's Protege

    8
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    Jun 10, 2015
    Although theoretically feasible, your proposal is fraught with many difficulties and even more contingencies!o_O -- If you insist upon line operation, I advise procurement of a small radiography transformer (as from an old dental or 'Porta-Ray' head)...

    That said; Please be advised that an EHT PSU meeting the cited requirements may readily be implemented via a TV LOPT (AKA 'flyback transformer') driven by a resonant Royer/Mazilli power oscillator...

    The answer, In the case of P/S isolated line operated transformers, is an unqualified yes!:)

    In the case of LOPTs featuring internal rectification (i.e. DSTs etc...), said approach is complicated by said units' end-point-grounding scheme -- The most direct 'workaround' being use of a unit designed for/anticipative of 'external' rectification -- in which case one may select the polarity via rectifier orientation or per cascade polarity...

    FWIW (Re: LOPT topologies) - You may (via dint of no small measure of patience:oops:) find the contents of THIS THREAD (with emphasis upon THIS POST) useful --- Please note, however, that said resources (as of this writing) represent a 'work in progress'...

    Best regards And good luck!
    HP:)
     
    Last edited: Dec 24, 2016
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    The main problem as I see it is that @Lash432 may not have any significant experience with EHT.

    And at these power levels, he may not last long enough to get it.
     
  9. davenn

    davenn Moderator

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    Sep 5, 2009

    agreed

    thread closed
     
    hevans1944 likes this.
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