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Winding layered coils

Discussion in 'Electronic Design' started by Adam Hamilton, Jun 9, 2007.

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  1. Hi

    I was always taught that when winding a second layer onto the coil that you
    bring the wire back to that start and then wind the second layer otherwise
    the magnetic field would be cancelled. I can't see that it would cancel the
    magnetic field because the magnetic field lines of the second coiled layer
    would still be pointing in the same direction as the first coiled layer.

    Thanks

    Adam Hamilton
     
  2. JackShephard

    JackShephard Guest

    As long as your turns are all in the same direction, they ALL count.
     
  3. Eeyore

    Eeyore Guest

    Of course not.

    If you're winding clockwise at the start, then all turns in that direction
    count.

    There are other reasons for using layers though.

    Graham
     
  4. Winfield

    Winfield Guest

    Correct. The only benefit I see from doing it as Adam
    was taught would be that the maximum voltage difference
    between the winding layers would be the voltage developed
    on a single layer, rather than twice that, like the
    conventional winding scheme. This may be an issue for high-
    voltage coils - I've made coils for up to 25kV, but have
    found it more practical to add a layer of Kapton tape,
    rather than suffer the lump in the winding for the return
    wire. Also, a simple halving of the maximum layer-layer
    voltage is often insufficient, and tape is preferred. In
    fact, sometimes even tape can't easily handle the voltages
    encountered, and I've found a banked winding approach to
    be effective. I see from Terman's 1943 Radio Engineer's
    Handbook that this method was known from way back when.
    A friend makes coils to 50kV and he uses a type of banked
    winding approach, with each coil portion wound in its own
    section of a custom-machined bobbin.
     
  5. default

    default Guest

    Out of curiosity who teaches that?

    You keep the turns going in the same direction and the current flow
    and it doesn't matter where you start or finish.

    Two layers can be wound with starts on one side of the form and
    finishes on the other - both wound in the same direction and the two
    starts and two finishes connected together for more current handling .
    .. . maybe that was the idea they were getting across?
     
  6. Thankyou Jack.

    Looking at my original post, I kinda left out the actual question which was
    can I wind the first layer and then wind the second layer starting from the
    end of the first. Your answer did in fact answer my question.

    Adam Hamilton
     
  7. JackShephard

    JackShephard Guest

    Divide breakdown voltage of the mag wire by your volts per turn, and
    come up with a maximum turns per layer number. I would actually cut that
    number back by at least 15%.

    We used to wind 800 turn secondaries, and the max turns per layer we
    would use was around 60.

    Another important thing in multi-layer secondary winding is to refrain
    from "scatter winding", which means do not wind a layer chaotically. One
    should wind evenly across the face of the bobbin (for more than a few
    reasons), or previous layer, then place the transformer tape over that
    layer. It should appear flat. Do NOT stretch the tape. Also, keep the
    start of the layer turns and the finish INBOARD on the winding face at
    least half a mm. This keeps the start or finish turn of one layer
    becoming proximal to an underlying layer or one above it.

    Yes. There is no reason to reverse turns direction on any winding,
    unless you want to degrade performance, and that should never be a goal.
    On a scatter wind where all turns are on the same layer, the final turn
    is many volts away from the first, so a breach can occur between them.
    This is the whole reason a good xfmr shop winds high turns count
    secondaries (or primaries) on multiple layers, each of which are
    segregated by insulator media.
    There is no lump if the method used is right. The tape is needed to
    keep turns away from each other. One winds across and back until the max
    turns per layer of the designer are met, and then a turn of tape is added
    AS the wire continues turning about the bobbin. Then the next layer's
    turns get stacked right on the underlying layer, and there was no
    cross-over or bump of any kind. Turns can start at the "bottom" or "top"
    of a bobbin segment. It doesn't matter. The only thing that does is the
    direction of the turns.
    Tape doesn't seal off layer edges and should not be relied upon to do
    so. It only separates layer faces. Keep turns inboard a bit to increase
    creepage distances, and always vacuum impregnate an HV design in varnish
    or at the very least, potting compound. That is, of course, unless it is
    an oil bath design.
    If it is too high, a segmented bobbin is required. I posted a picture in
    a.b.s.e a couple weeks back that one troll jumped all over, but it does
    perfectly show a proper EHV xfmr (miniature low power) that has about 5
    bobbin segments, each with nearly 300 turns, which is pushing the limits
    of the mag wire. I'll post it again.
    Yes. We had many customs made, and another easy way to do it is to
    have G10 or FR4 bobbin parts made and epoxy them up yourself.

    Start with off the shelf G10/FR4 tube, and get discs made for the bobbin
    segment separators out of non-clad flat sheet stock It can be milled
    just like a PCB design, and popped out from little perfs.

    We even took an off the shelf bobbin, cut off one face (end), and added
    discs to it, and then replaced the end we cut off. It really does work
    well. I actually have some pics on one of these old hard drives here
    somewhere.
     
  8. Tim Wescott

    Tim Wescott Guest

    That sounds like a pi-wound coil for a radio, except there the goal is
    to reduce capacitance rather than prevent spark over. Reducing the
    voltage from one wire to the next should reduce both capacitance and
    spark over, however.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  9. Bill Beaty

    Bill Beaty Guest


    With *toroid* inductors, if you wind a single layer, you've
    accidentally created a 1-turn loop going around the donut. If you
    instead keep going and wind a second layer back along the first, this
    1-turn loop is cancelled out. When winding toroids, it's a good idea
    to use an even number of layers, and go back and forth. For high
    power high frequency toroid transformers, you DON'T want a 1-turn loop
    which spews b-field out into the world. For small toroid inductors,
    you don't want a 1-turn loop that couples signals from outside.


    (((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
    William J. Beaty http://staff.washington.edu/wbeaty/
    beaty chem.washington.edu Research Engineer
    billb eskimo.com UW Chem Dept, Bagley Hall RM74
    206-543-6195 Box 351700, Seattle, WA 98195-1700
     
  10. My electronics teacher at college said this and searching the internet also
    showed the wire being brougt back to the beginning before winding the next
    layer. I asked this question because when I followed the magnetic field
    lines by winding the second layer from the end, they still pointed in the
    same direction. I thought that there maybe some advantage to bringing back
    the wire to the start.

    Thanks for all your help everyone.

    Adam Hamilton
     
  11. Rich Grise

    Rich Grise Guest

    ....

    Whoever told you this is an idiot.

    Cheers!
    Rich
     
  12. Rich Grise

    Rich Grise Guest

    The advantage is to the professor, who has tenure, and teaches idiotic
    crap to the students so that they won't threaten his position when they
    grow up.

    Cheers!
    Rich
     
  13. JackShephard

    JackShephard Guest

    You're such a fucking idiot.
     
  14. YD

    YD Guest

    Late at night, by candle light, "Adam Hamilton"
    Just take apart any old relay coil or something like it and see for
    yourself. You do have it right, it's the direction of the current in
    all the layers that set up the field, not the starting point in each
    of them.

    - YD.
     
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