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Subject: coils and wire size
From: S-T Morrissey sean@...........
Date: Thu, 13 Jan 2000 21:22:16 -0600 (CST)

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.



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Larry Cochrane <cochrane@..............>