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more on air core chokes



Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Whipping out NBS Circular 74, I found the following equation (p256) for
long multiple layer coils.

L = L0 - 0.01257 * N^2 * a * c/ B * (0.693 +Ba)
where L0 calculated as below
n = number of turns
a =radius of coil center of winding cross section
b = length of coil
c = radial depth of winding (i.e. distance from center of first layer to
center of outer layer)
Ba is a correction factor depending on b/c and ranges from 0-at-b/c=1, to
0.1200-at- b/c=2, to .2292 at b/c=5, to .2844 -at-10, to .3099 -at- 20

L0 (the single layer coil formula) is calculated by equation 153, (p 252)

L0 = 0.03948 * a^2 * n^2 / b * K

where a = radius of coil, b = length of coil, K is a function of 2a/b, and
tends to 1 for long coils, 
For 2a/b = 1, K=.6884

dimensions are cm, L is in microhenries

The example given is of some interest, since it is for an inductor of 12.9
mH, a convenient value.

400 turns, 20 cm diameter form, 10 turns per cm (i.e. 40 cm long coil). 

I note that bare AWG #10 wire is 0.10 inches in diameter, so you couldn't
wind a single layer coil this tightly.  However, 400 turns * roughly 2 ft
per turn is 800 ft.


Iron cores would greatly reduce the size and number of turns required for a
given inductance.

I suspect you probably want something like 0.1 H maximum inductance... This
would take 1200 turns or so, which is quite a pile of wire (2400 ft),
especially for AWG10.