Re: Equivalent lumped inductance and toroidal coils

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Bart Anderson <tesla111-at-sbcglobal-dot-net> 

Hi Paul,

I think the approximation should read:
Ldc~= mu * N^2 * rt^2/(2*r1)

I ran this with low and high turns and with a few different tube dimensions 
against:
Ldc = mu * N2 * (r1 - sqrt(r12 - rt2)) - mu * r1 * N * (1.2 - ln(pm/rc) + H)

The two equations followed about (+/-)1mH difference. I let the inner and 
outer pitch values be calc'd from the turns, conductor, and tube dimensions 
by using the inner and outer tube lengths. As long as there are no inner 
conductor overlaps or tube overlaps, the approximation followed as 
expected. I notice error increases with spacing between adjacent turns and 
when tube width is small compared to overall diameter, however, low 
inductance values didn't seem to cause much of an affect as long as 
dimensional ratios were kept in bounds.

As for the Ldc toroidal formula, the results look good. Your right, time 
for measurement and verification. If I get the itch to wind a toroidal 
coil, is there an approximate dimension desired (24 awg). Also, is aircore 
preferred over say a foam core?

Take care,
Bart


Tesla list wrote:

>Original poster: Paul Nicholson <paul-at-abelian.demon.co.uk>
>Paul wrote:
>
>but the approximate formula for use when rt << r1 given by the
>same handbook,
>
>  Ldc ~= mu * N^2 * rt/(2*r1)
>
>seems to come out with rubbish. Maybe there's a typo error in
>the handbook - it wouldn't be the first for their inductance formulas.