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Discussion in 'General Electronics Discussion' started by davelectronic, Sep 26, 2011.

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  1. davelectronic

    davelectronic

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    Dec 13, 2010
    Hi every one. I have a question before trying this. A few threads back i was going to rewind a microwave transformer, what i want to try is 240 volts into the high voltage secondary winding to get low volts out the primary mains input, about 22 volts was surgested, as i understand it the transformers frame is part of the neutral common connection, is this correct as stated in another thread, any info very much appreciated. Dave.
     
  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Yes, in MOT's one side of the high-voltage winding is connected to the core. You'll need to de-attach this wire and connect it to neutral. Connect the core to ground.
     
  3. poor mystic

    poor mystic

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    Apr 8, 2011
    :)
    Will this be better than other 22V transformers?
     
  4. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    No, I don't think so. They supposedly have a poor coupling factor (regulation) and high losses, but it can be a cheap solution for short-term hi-umph power supplies.
     
  5. davelectronic

    davelectronic

    1,087
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    Dec 13, 2010
    Ok so had a little play, as you do with these things.
    The output from mains in the high voltage secondary worked out at 24 volts AC but, a waste of time, as the 24 volts was loaded with a 20 watt halogen lamp, 12 volt, the voltage dropped to about 6 volts.

    So after this finding, i measured a 4 amp 240 volt normal step down transformers primary, it came out at 11.5 ohms, so i measured the MOT's secondary, as this was my 240 volts input, that came out at 203 ohms, so question answered, two high a resistance in the MOT's high voltage winding to be of any use wired in that configuration.

    Its going to be remove the high voltage winding, rewind with a heavy gauge wire, aiming for about 15 volts or so.

    Ive heard, or read should i say, remove the shunts in one article, and read somewhere else leave them in, i think remove, as they will degrade the magnetic circuit, well i think they will, a useful site suggests removal before rewinding.

    Another idea to make rewind easier i saw was to cut the two weld along the EI line, but the primary is at the bottom where the welds are, so i would need to be extra careful in the removal of the primary to get the secondary off, or suffer the grief of cladding out the high voltage aperture once removed, and a fight with the heavy gauge wire.

    Ok so the efficiency is less than perfect, but to drive a 200 - 300 watt linear RF amp it could work, but good cooling, 2x 120mm axial fans full on using a thermal switch.

    About 25 - 30 amps at between 12 - 13 .8 volts will do.


    Eye opening stuff.
    Dave. :)
    PS, Thanks for your reply's, your poor coupling factor is correct.
     
    Last edited: Sep 26, 2011
  6. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Hehe, that was even worse performance than I had anticipated.. I figure the inductance & the shunts are both responsible, more than the resistance.
    Yes, I'd also try with knocking out the shunts. I think they're there to give a constant power output (more or less). Never experimented with these things though.
     
  7. davelectronic

    davelectronic

    1,087
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    Dec 13, 2010
    Hi Resqueline.
    Yep you don't know these things until you try them, me that is.
    The post and thread on the 22 volts in the configuration above forgot to mention poor everything coupling resistance every thing, a learning curve though.
    Dave. :)
     
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