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Coil Geometry and making our own spools...

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  • Coil Geometry and making our own spools...

    I'm trying to make an SSG with a 9 filar coil consisting of 8, 18awg windings and 1, 20awg winding. I stretched all the wires out to 130' this morning and litzed 'em. It soon became obvious that there was too much wire to fit on the normal 3 1/2" spools we use for these so a question arose in my mind. What is better geometry wise? Taller with the same diameter, wider with the same height, or some ratio of increase to both dimensions? Also, how about the ratio of coil core diameter to coil wire diameter? Any takers on these questions? I cut two of the regular spools and made a longer spool because I have plenty of room to raise my bike wheel to accommodate the height increase. Not so much room width-wise, although I could certainly widen it if need be. Just trying to save some effort on the frame modifications.

    Also, since it appears a feller will have to make his own spools as different configurations are attempted, what are some favorite materials some of y'all have used to make your own spools?


    Last edited by Woody; 02-08-2013, 04:59 PM.
    "It's not a mutiny if the commander is leading it!" - Wally Schirra, Commander Apollo 7

  • #2
    Woody, I make my spools from acrylic sheets. I use a hole saw about 100mm diameter and 25 mm poly pipe as the core.
    Acrylic glue doesn't work well, but liquid nails does. But it takes a week to cure properly!
    A suggestion would be to use at least 6mm acrylic sheet so you have more gluing surface for the pipe. Sadly the consequence of using hole saws for the 25 mm cut for the tube is that it can be a little sloppy. A drill would be better but very expensive in that size.
    The height of the tube I use is also 100mm. I fit a 9 filar all of #18 gauge on this spool....just.


    • #3
      James I have used your procedure for making spools but I use locktite 401 instant glue on the plastics George


      • #4
        Hello Woody,
        good idea to start this thread.
        I think the geometry of the spool is very important but also the resistance of the wire we use.
        Your AWG18 has lower restistance than AWG20, the transistors have to drive more current and you must
        probably lower the base resistors, (as i did when i used AWG18 in the past).
        Now about the geometry of the spools. With every new layer you turn, you need a longer part of wire,
        so the more layers, the less number of windings. You see it is better to make the spool longer, because the
        strengs of the produced magnetic flux depends on the number of turns and the the current through the wire.

        I never use a ready made plastic spool. I use a poly tube 25mm like James does, and i cut out octagons of
        poly plates 3-4mm thick. Using first a 10mm normal drill for the center and then a conical drill (5-35mm) to
        wide the hole to 25mm. Before i glue the plates to the ends of the poly tube, i use a hot air gun and heat up
        one end of the tube to get it softer for the following step when i stick in the cone drill to widen the end to
        increase the outer diameter (26-27mm). After cooling down, i stick the tube through the holes of both
        octagon disks. Now i do the same to the other end of the tube to widen it. The disks can never leave the
        tube now and are easy to be positioned and glued exactly to the ends of the tube. There is no need to wait
        for the glue is hardened completely because the discs cannot flip over the ends of the spool.
        Thats it!
        hopefully this a useful practice for you too



        • #5
          Yes good topic indeed. I wonder if someone has ever experimented with different coil dimensions with the same wires to see if geometry affects its behavior.