Re: Stacked Primary

["Malcolm Watts" <M.J.Watts-at-massey.ac.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

hi Steve,
          I've used stacked spirals for a few years.

On 22 Jul 00, at 23:01, Tesla list wrote:

> Original poster: "Steve Young" <youngs-at-konnections-dot-com> 
> 
> I am thinking about building a stacked flat spiral primary to cut down the
> diameter some.  I need a sanity check and some ideas on mutual inductance
> effects.
>  
> A "normal" flat spiral primary for my design would have an inside diameter of
> 7.2 inches, be wound with 1/4 inch tubing spaced 1/4 inch apart, and be about
> 12.4 turns which ends up 19.1 inch diameter.  According to Wintesla, the
> inductance will be .0563 mH.
>  
> Now, if I have two flat spirals in series, each half the above
inductance, each
> coil will be 8.9 turns with outside diameter of 15.6".  This is better
because
> it is more compact.
>  
> If the two coils are stacked, with 3/8" acrylic in between for insulation and
> support,  Then the total inductance will go up by some factor due to mutual
> inductance.  This means I could reduce the turns by some factor to end up
with
> the same inductance.
>  
> Can someone calculate the approximate number of turns my stacked primary will
> need to have about 0.56 mH?  
>  
> I will probably make a temporary one with #14 wire first to measure the
actual
> inductance, before I build one from tubing.
>  
> Thanks,
> --Steve

The simplest approach is to design a spiral for about 30% of 
the total inductance needed and then build a second identical
spiral and stack them with a spacing between spiral layers of 
about 4 times the conductor diameter which allows room for 
insulation.
     I prefer to spiral one inwards and the other back 
outwards and tap inwards from the outer edge of the lower 
spiral and usually use a sheet of acrylic to separate the 
layers.

Regards,
malcolm