Re: flat coil ?

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

Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

Chris Swinson wrote:

> I just finished the winding of the flat coil...
> The winding dia is 45". I counted 316 turns'ish...
> inductance is 44mH.

With 45" and 316 turns, I get around 40mH, rather than 44mH, so
something is not quite right with your count or dimensions. You'll
need to make sure that your outer diameter is given accurately, and
as Bart says:

> It would be good to know the inside diameter as well.

And perhaps you could also measure the DC resistance of the winding?

For the rim-grounded center-hot configuration, the Fres is now
around 160 kHz.  Voltage and current distributions are in 

 http://www.abelian.demon.co.uk/tmp/cs1r.vi.gif

in which 0% position is at the rim and 100% position is the center.
The highest stressed part of the coil, for both current and voltage,
is around 10% to 15% of the way in from the rim.  The point marked
'inflection' is the place where the voltage gradient is highest.

Chris wrote:

> Would I expect the flat coil to work better than a tube
> wound coil ?

Not necessarily.  One feature of the flat coils that stands out,
especially for the center-hot arrangement, is that the effective 
inductance at resonance is quite a bit higher than the DC
inductance, whereas for solenoid coils the effective inductance is
usually a little lower than the DC inductance. So you get a little
extra inductance for free, so to speak.  In your case, Ldc is around
40mH and Les is around 50mH.  This might mean that you could get a
little higher efficiency from the primary circuit, but I'm not
convinced that the difference would be noticeable.  Set against
this possible benefit is the extra difficulty of winding the flat
coil, and the possible problems of premature breakdown in the outer
25% of the winding, and between the central 'topload' and the coil.

My guess is that you may end up with a more efficient coil, but one
which will take less maximum power, than the same wire wound into
a solenoid.  With this in mind, the flat secondary may find its
niche as a CW secondary or tertiary coil, in which the high Les and
high transfer impedance can be exploited in the presence of modest
peak powers.  There is a dearth of measurements on this type of coil,
hence our interest in your efforts.
--
Paul Nicholson
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