Re: eddy current with secondary coil

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

Original poster: Ed Phillips <evp-at-pacbell-dot-net> 

Tesla list wrote:
 >
 > Original poster: Dave Lewis <hvdave-at-earthlink-dot-net>
 >
 > In my view, eddy current effects are a good reason to place your torroid
 > a few inches above the last turn on the secondary.   A conductive sheet
 > right over the top of the secondary looks like a shorted turn that is
 > magnetically coupled.  Putting some air gap between the last secondary
 > turn and the torroid loosens the coupling up so that its not an issue.
 >
 > One way to visualize how much space you need is to imagine the magnetic
 > flux density within the center of the secodary as more or less evenly
 > distributed along the cross section.   Given that the area is pi*D where
 > D is the secondary diameter, you'd like at least that much area for the
 > flux lines to exit the top of your secondary and bypass the torroid
 > without compressing.  That would be a vertical spacing of at lease D/4.
 > Thats derived by equating the area of the secondary pi/4*D^2 to the area
 > below the torroid and last turn pi*D*X.
 >
 > Dave Lewis

	I and others here have made careful measurements of the inductance of a
secondary with and without a toroid slapped up against the end.  For a
coil of 3" x 15" I find the change in inductance is less than 0.1%
which leads me to conclude it can't have any effect on secondary
losses.
Ed