Re: Any ideas on this proposition?

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Original poster: "R.E.Burnett by way of Terry Fritz <twftesla-at-uswest-dot-net>" <R.E.Burnett-at-newcastle.ac.uk>


Hi Alex and all,

> This is an old message, but I suddenly had a thought - wouldn't a
> solid-state driven magnifier coil be a good proposition - ie the 'driver'
> coil driven by a full-bridge bunch of MOSFETs? The low impedance of the
> driver coil would seem a good match (1) at the primary side for the
> h-bridge (2) at the secondary side for the third resonator's base.

This is essentially what Gary Johnson has done,  but he uses a high
voltage inverter to apply a +/- 1500 volt squarewave directly to the
base of a free resonator.  His resonators are quite large so they have a
high Q factor and low base impedance.  Around 30 Ohms I think ???

In my case I use a more compact resonator which has a lower Q and its
input impedance is higher.  Around 450 Ohms on a dry day.
My SSTC requires very tight coupling between the primary and secondary
windings due to the relatively high base impedance of the small
resonator and the low operating voltage of the inverter.  The tight
coupling is required to reflect a low impedance back to the inverter and
get lots of power flowing ;-) The reflected impedance drops as one 
increases the coupling,  and the sparks grow steadily until eventually
there is a flashover due to poor clearance.

The problem is that the resonator impedance actually rises as you drive it
harder and the spark length grows.

At first it seems that a magnifier approach would be beneficial because
it performs the necessary impedance transformation.  However doing the
calculations shows that the driver coil requires a large number of turns
and now needs incredibly tight coupling with the primary to acheive a high
overall system coupling.  I don't think you would actually gain anything
from the magnifier approach because the overall coupling would be lower
since you have split the resonator,  and are now only exciting one half.

I am going to concentrate my efforts into decreasing the base impedance of
the resonator before breakout,  then I can use a looser primary coupling.

I think the magnifier approach may be more applicable in the case of tube
coils.  The impedance matching seems easier and does not demand such a
tight coupling because the primary circuit operates at a higher voltage.

> Howver what I can't think is whether to discard the primary
> caps and just drive the primary untuned, or another idea is to drive the
> primary with an MMC in series with it, thus pumping the max. current into
> the load - which is then seen as a series-tuned circuit, low Z at resonance
> and developing really high voltages as the primary sees things (ie as a
> parallel circuit).

Introducing a capacitor into the primary side makes the behaviour quite
complex.  Since both primary and secondary are now resonant then frequency
splitting takes place,  and energy cycles between the two tank
circuits.  Essentially the coil now has two seperate resonant
frequencies.  I'm not sure what implications this would have for the
inverter.

Note,  this is not the same as the capacitor which appears in series with
the primary in my SSTC.  This capacitor does not resonate with the primary
winding,  it is merely for DC blocking purposes.

I hope this is of some help,

						Cheers,

						-Richie Burnett,