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Coil Winding

Discussion in 'Electronic Basics' started by Dave.H, Mar 22, 2008.

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  1. Dave.H

    Dave.H Guest

    I'm winding a coil for a home built regen radio, and I need to know
    how much wire I need. The coil form is a 2 inch mailing tube. Main
    winding is meant to be litz wire, but I'm using 28 gauge magnet wire,
    and is 66 turns. The second winding is meant to be 8 turns of 30 gauge
    wire, but I'm using 28 gauge magnet wire again. Third winding is 20
    turns of 30 gauge, but again I'm using 28 gauge.

    Radio schematic (at bottom of page)
  2. Bob

    Bob Guest

    Surely you where taught basic mathematics in school, such
    as the circumference of a circle?

    Pi times the diameter gives the circumference. Multiply by
    the number of turns and add a few percent because it's not
    in perfect circles.

  3. Take a piece of string, surely you have some of that around, and make one
    winding on the coil form. Then measure that, and multiply by the number of
    turns. Add a bit extra for leads and "just in case".

  4. Every dimensional change you make to the design will require
    a compensating turns change. The trick is to either make
    one as directed, measure its inductive properties, and than
    adjust the turns count on your modification to have about
    the same measured properties, or make calculations or an
    educated guess of how much you have to change the turns
    count to compensate for the dimensional changes you have
    made. At the very least, you should have an idea which way
    the turns count much change to be a correction in the right

    If your mailing tube has the same outside diameter as the
    author's, that is one variable eliminated.
    The author says his Litz wire is the same size as 28 AWG, so
    this coil should have about the same length as his 66 turn
    one. Its inductance will be just a little less, because the
    solid wire repels the flux from inside some of the wire
    cross section with eddy current, but this is a tiny part of
    the total flux, so maybe a single additional turn will
    compensate for that. The effect is proportional to
    frequency, so there is no count that will work exactly like
    the Litz coil would. Your coil will also have a lower Q
    because it is absorbing a little of the RF energy into eddy
    current losses.

    The second winding is meant to be 8 turns of 30 gauge
    Your winding will be quite a bit longer than the author's,
    so will have lower inductance (less well coupled turns),
    but, again, an extra turn might compensate, or put it just a
    bit closer to the tuned winding so that the mid point of
    this winding is about the same distance to the midpoint of
    the tuned coil as is the case with the author's design. It
    won't take much of a correction, and the author may not have
    done it the best way possible, so your changes might even
    make it better. Too many variables. You might have to make
    more than one and compare them in experimental trials.
    Again, your coil will have more axial length than the
    author's, but it is not tuned, nor part of a feedback gain
    ratio, so its inductance is not critical. I'm guessing this
    one doesn't matter so much. It also might work better if
    this coil were closer or further from the tuned one. I
    doubt if the author optimized it in any way. It is probably
    just something that worked.
  5. Bob Eld

    Bob Eld Guest

    Fifty feet. Why don't you purchase a 1/4 lb spool of 36 gauge magnet wire.
    Parallel seven strands to make the 28 gauge litz wire and parallel four
    strands to make the 30 gauge litz wire. In litz wire, the strands are
    insulated from each other and connected together only at the ends.

    To make the 28 ga.stretch out the seven strands about forty feet. Use nails
    in boards at each end to keep the strands from tangling. With an electric
    drill twist up the strands from one end into the litz bundle. Keep the
    bundle loose, do not twist too tightly. Use this forty foot length to wind
    the 66 turn winding. Tin the ends together into a single group for

    Repeat with about 18 feet of four strands to make the 30ga. Litz wire.

    Note, you can use more strands of thinner wire to make the litz bundles, but
    thinner wire is more expensive and harder to handle. For example, you could
    use 17 strands of 40 gauge wire to make the 28 ga. bundle but that's a bit
    harder and not necessary for your application.

  6. winding 1 = 2*3.1416*66/12 = ?feet
    winding 2 = 2*3.1416*20/12 = ?feet

    so you need to do this simple math. Add them together, then add a couple
    feet for the fly leads and a little extra, just in case there is some
    variation in the tube's diameter.

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  7. whit3rd

    whit3rd Guest

    The form size and turns count gives you a length; but magnet
    wire is sold by the pound, so using 28 gauge wire you look
    up... it's half a pound per thousand feet.

    The minimum purchase quantity is about a thousand feet.
    That should be enough.
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