Re: Resonator base impedance

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

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

R.E.Burnett <R.E.Burnett-at-newcastle.ac.uk> wrote:

> Anyhow, I am going to wind a new coil which will be short, fat, and
> use less turns of much thicker wire in order to get the same
> inductance. Hopefully this will lower the base impedance. I will let
> you know the result.

I've been looking closely at secondary operation for a while now, but
still cannot offer a suggestion as to the best secondary design for
either impulsed coils or CW. 

For CW operation, clearly the Q factor is very important, as it
determines how much input power is necessary to spin the secondary up
to a given stored energy. Also, and this applies to impulsed coils
too, is the requirement to minimise as far as possible the coil's
capacitance, as this determines the voltage achievable for a given
stored energy. Thus Q and C are perhaps equally important for a CW
secondary, but I'm afraid that a design formula which will spit out
the optimum secondary design for an available supply power and
frequency is still at the top of my christmas present list. 

If I were to suggest that thick wire, with a modest h/d ratio, say
around 3, were in any sense the best to use, it would be purely wish-
ful thinking, since that describes my own secondary coils! However I
have measured a Q of over 700 on a good day. The equivalent energy
storage capacitance for these 360 turn 0.6 m x 0.8 m, 150kHz coils is
around 30pF.

> I am using a primary coil to match to my solid state driver,
> although the primary-secondary coupling needs to be very tight to
> get good power transfer at the moment.

Yes. I admit to abandoning attempts to find a satisfactory coupling
through a direct primary winding. I now drive the bases via a matching
transformer which is removed from the field of the tesla coils.

This doesn't eliminate the need for the tight coupling, but it does
ease the breakdown and protection requirements, and it also allows the
secondaries to be more easily raised high off the ground in order to
minimise the energy storage capacitance.

> I must decrease the impedance of the bare resonator, so that I can
> reduce coupling, otherwise insulating becomes impossible :-(  

Fully agree with you there. Its a big step-up in impedance from the
couple of ohms required by a driver, to the hundred or so seen at the
coil base. Anything you can do to raise Q will be of benefit in this
respect. I chose the bipolar configuration mainly to eliminate ground
current losses to give a substantial increase of Q. Those with bins
full of dead silicon will probably not agree if I suggest that the
driver design is not as hard as solving the coupling problems,
especially if they're not concerned with handling the load impedance
presented off-resonance - perhaps accounting for some of the dead
silicon :).

PS, if you've been monitoring this list, you'll know to steer clear
of sonotube for CW coils.

Cheers,
--
Paul Nicholson,
Manchester, UK.
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