Re: Primary/Secondary Frequency Difference?

["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>

Jolyon Vater Cox wrote:

>  figures for the primary do not appear to agree
> with the figures for the secondary...

> secondary resonance for 10 7/8" tall 3" dia 440 turn secondary
> should not be more than 700kHz, even without a toroid.

Will be up to 1200 kHz, if the coil is in a spacious environment,
and is 1" above a ground plane.

> Are calculated figures likely to be significantly inaccurate, or are
> small construction errors likely to result in gross deviations from
> calculated values
> ... particularly with regard to the primary capacitance and the
> primary and secondary inductances.

No matter how accurately you calculate things out before hand, its
usually a good idea to reckon on 5% or 10% error in things like
frequencies, currents and voltages, and a factor of two or so in
impedances.  Wherever possible, try to build in lots of room for
adjustment of things like primary tuning and coupling. 

Your coil's effective capacitance is only around 6pF, so it will be
quite sensitive to stray capacitances, especially at the top of the
coil, for example if you have an inch or two of wire hanging off the
top as a discharge terminal that could add 5% or so to your C.

Recipies for calculating C usually don't take account of the coil
former's dielectric properties.  For a small coil like yours, that
could drop the frequency by another 5% or 10%.

Nearby objects will all add their own bit to the total C. Consider
the 1200 kHz as an upper limit, since all these extra bits of C
will bring it down maybe 10% or so from that.  You'll probably want
to make a primary that will tune from 1200kHz down to a lot lower,
so that you can experiment with toploads. For example, if you put a
4" sphere atop the coil, the frequency could drop to perhaps 950 kHz
or less, and a big toroid could take it quite a bit lower than that.
 
> I understand that 24SWG wire (0.022" dia) is a bit thin for a
> primary but could this account for apparently wide differences
> between the calculated primary and secondary resonant frequencies?

There no discrepancy to account for.  Your 5.5 turns suggest the
secondary is resonating at around 1100 kHz, which sounds quite 
reasonable.

No problem with the thin wire.  The inductance is mainly determined
by the path the wire takes around the coil, not the thickness of the
wire.  Use the thin wire while you experiment with different primaries.
It doesn't even have to be all that neat and tidy, it'll still be in
the right ballpark.  When you have something that suits - make a tidy
thick wire version of it.

> Also, is it likely that the coronal sheet around the saltwater
> capacitor is increasing the plate surface area and hence increasing
> capacitance significantly -making the primary resonate at the
> frequency of the secondary with fewer turns?

Interesting idea.  I'll bet it does add to the primary C, but probably
not much.  If it increased the C by 10% it would only lower the 
frequency by 1%.  You just have to build a primary with plenty of
adjustment range to allow room for lots of experimentation. Remember
that the available adjustment range of the primary determines the range
of toploads you can accomodate, the range of priamry caps you can
use, and the range of coupling coefficients.  So give yourself plenty
of room to maneuvre.

Oh, and have a dabble with programs such as E-Tesla, 

 http://hot-streamer-dot-com/TeslaCoils/Programs/E-Tesla6.zip

for estimating the secondary frequency.
--
Paul Nicholson
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