Hi Sean,
Thanks for those suggestions, a lot of nuggets in there.
Cheers,
Paul

S-T Morrissey wrote:

> Re counting turns when winding a coil:
>
> I don't bother to count, but I know the approximate number.
>
> First of all, I turn the coil by hand on a short shaft under
> good light, placing each turn EXACTLY adjacent to the previous,
> and delimiting each layer with correction tape strips as I described
> a few weeks ago. (need a repeat??). It takes about 2 minutes to
> make about 100 turns. And another 10 minutes to epoxy the layer.
> A 12-layer coil takes me over 2 hours, depending on distractions
> while the epoxy cures.
>
> This lets you know the number of turns by knowing the diameter of the
> wire and the winding length. I use #36 wire from Newark (need a repost
> of that info??; they take MasterCard!). "Beldsol" enameled #36 wire has
> a diameter of 0.0055", so if I have a winding length of 0.5" with EVERY
> TURN exactly placed, I have 91 turns in that layer. Since I don't use
> a bobbin form, I step back the end of each layer by one turn, or conversely
> start with a slightly larger number in the first layer. So I use a starting
> length of 0.615" ( slightly under 5/8") to start with 111 turns; subsequent
> layers are 109, 107, 105, 103, 101, 99, 97, 95, 93, 91, and 89 turns in
> the 12th layer, for a total of 1200 turns.  The average length of a turn
> with a 5/8" initial diameter is 2.17", so there is 217 feet of wire in the
> coil, and at 414.8 ohms per 1000 ft, I should get 90 ohms, which is about
> what I measure.
>
> For your info: other wire diameters and resistances:
>
> (diameter for enamel insulation) NEWARK    roll  length, ft  cost
> #36     0.0055  414.8 ohms/1000 ft.  #36E1321  1/2lb   6400     $27.33
> #38     0.0044  659.6                #36F779   1 lb    19300     $73.49
> #40     0.0034  1049.0
> #42     0.0028  1659
> #44     0.0023  2593
>
> Actually, the exact number of turns is not important. What is important
> is to get the maximum number of turns inside the gap of the magnet, which
> cannot be done with a "scramble wound" bobbin that often results from
> using a drill. The required bobbin alone will take up an appreciable amount
> of the volume. Also a target resistance value is not necessary. The
> trade-off in my opinion is how small a wire you want to work with to get
> the maximum number of turns into a given space. #40 and up wire is probably
> not amenable to precise layering, but will give more turns and more
> resistance. But remember that the output increases only with the square
> root of the resistance. 200 ohms will produce 1.4x the signal of 100 ohms.
>
> If I use the same dimensions as above and #42  enameled wire, which is
> about half the diameter of #36, I will get twice the turns per layer,
> and twice the number of layers, for about 4800 turns. With the same mean
> length per turn, this will be 868 feet of #42, which will measure 1440
> ohms. So I get only 4 times the output (4 x 1200 turns) at 16 times the
> resistance (16 x 90 = 1440 ohms). BUT I also have 16 times the Johnson
> noise due to the resistance, and it may be difficult to damp with a
> resistor if the magnet is not strong enough.
>
> Regards,
> Sean-Thomas
>
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