Re: self-C comments
Tesla List wrote:
> >From richard.craven-at-mkbbs.co.ukTue Aug 27 22:58:08 1996
> Date: Tue, 27 Aug 96 19:31 +0000
> From: richard.craven-at-mkbbs.co.uk
> To: TESLA-at-pupman-dot-com
> Subject: self-C comments
> Hello Group,
> My measurements and calculations show that for most TCs the value of
> capacitance of the secondary terminal itself lies between Cself/2 and
> Cself of the secondary coil in isolation. By this I mean that for a coil
> with a calculated self-C of say 30pF the toroid will typically be 15-
> 30pF in size.
> The secondary coil capacitance is dominated by its area per turn (i.e.
> larger diameter coils have higher self-C). Some TCs were built which
> have conical secondaries: typically the perpendicular is 1.5 times the
> base diameter. These constructions would have a large inductance per
> unit height at the bottom of the coil which would provide good coupling
> (because the flux linkages per unit turn would be high). The top of the
> secondary would have a lower self-capacitance because the area of the
> turns is smaller.
Note: Only the surface area of the terminal (toroid) determines its
capacity! I find the opposite of your result! The Cself is always a
small fraction of the terminal capacity. In my current system the C self
is 11pf, and C terminal is 50pf. (Measured, not calculated). General
statements like the above can usually be blown away pretty easily by
more bizarre constructions. (Like mine.)
> Bearing in mind erudite comments from Richard Hull (basically all the
> strays due to buildings floors and the like), has anyone any comments on
> conical constructions? I am not planning on building one, but it is an
> area of some interest.
I have built two conical coils. They work best as magnifier drivers, but
are the not the ideal driver construction method.
> Some may wonder why the capacitance of a secondary is unaffected by the
> dielectric coil form upon which the coil is wound. If we remember that
> the electric field of a TC extends outwards from the coil towards
> ground, it can be seen that the dielectric in which this field exists is
> air. If we put polyethylene inside the coil form, the electric field to
> ground is hardly modified. If we coat the wire we are using with a high
> permittivity dielectric, the percentage volume occupied compared with
> the air is also tiny so again the self-C is virtually unaffected ( I
> would guess we are looking at changes of a fraction of 1% and anyway
> these are swamped by extraneous C effects).
> The capacitance between turns may be affected by the addition of a
> dielectric coating on our wire, but if you consider that we might have,
> say, 500 turns in series, we then end up with 500 tiny capacitances in
> series which still adds up to a tiny total interturn capacitance. Even
> if we fill the gap between turns with a high dielectric, the total
> effect is still very small compared with the capacitance to ground.
> All of the expressions for capacitance of coils (from Terman, Langford
> Smith, Radiotron ITT reference books) express the capacitance to ground
> in terms of the coil height and radius (or diameter or area). The
> expressions are identical to those for isolated cylinders to ground
To measure C self base feed a resonator with a signal generator of 50ohm
impedance or less. Use two ultra bright leds back to back in parallel
connected as a unit in series to the base of the coil from the
generator's hot lead.
Tune for max brightness (resonance) read the frequency. (Fr) now measure
(never calculate) the coil's inductance (Lcoil). use the equation:
C self= .0253/((Fr^2)(Lcoil))
All values are in hertz and henrys and farads. Complete details on our
Cterminal can be found by fitting the terminal and redoing the above and
recalculating the Cself+Cterminal (for the combo). Now subtratct Cself
obtained in the first pass and you have Cterminal. Note these are static
values, but are real and are the results of real measurements. The only
value which changes dramatically with firing is C terminal.
> Again, as Richard Hull mentioned when this subject came up before, the
> self-C of a coil can be obtained in isolation but this won't have much
> relevance to actual operating conditions. Malcolm Watts found that even
> the Q of his coils shifted when he measured in different rooms; Robert
> Golka's big coil got pulled by the effects of a new environment when it
> was moved to a different building.
> I have to go and lie down in a dark room now; my head's hurting
> Richard Craven
> CMPQwk #1.42 UNREGISTERED EVALUATION COPY