Bert's comments on self-C



TL>Many coilers are now using significantly larger toroids to get higher
TL>discharge currents and "hotter" performance. I've found, through
TL>indirect measurements/calculations, that the self capacitance of my
TL>10" x 31" secondary seems to be about 16pF, and the "effective"
TL>toroid capacitance (8" x 32") seems to be about 26pF. The estimated
TL>"free space" toriod capacitance would be about 35pF. 

This is a point that I hadn't fully appreciated; I knew that the toroid 
size was increasing but I hadn't put that into context viz. the size of 
the secondaries themselves.


TL>However Malcolm has shown that lossier coil form
TL>dielectric materials DO have significant impact on reducing the
TL>measured "Q" of the secondary coil (with no secondary discharge).
TL>It's not exactly clear why this should be if we use your argument
TL>above... However, the losses due to output discharges have a MUCH
TL>greater impact on lowering effective Q. 

I think Malcolm had a coilform that had several layers of polythene 
sheet wrapped around it (the base form was a cardboard or similar tube, 
I think). I wonder if the coil was being shunted by ionised air trapped 
in the layers and this was accounting for the apparently low Q.

TL>> The capacitance between turns may be affected by the addition of a
TL>> dielectric coating on our wire, but if you consider that we might
TL>> have, say, 500 turns in series, we then end up with 500 tiny
TL>> capacitances in series which still adds up to a tiny total
TL>> interturn capacitance. Even if we fill the gap between turns with a
TL>> high dielectric, the total effect is still very small compared with
TL>the capacitance to ground. Right! There is a slight decrease in the
TL>characteristic impedance of the coil. The protective benefits of a
TL>coating usually make this worthwhile, particularly for 2-coil
TL>> systems. 

As you say, the mechanical advantages are worth having.


TL>> All of this ignores the electric field coupling that exists between
TL>> primary and secondary; when the gap is non-conducting the electric
TL>> field to ground will be high and will be largely symmetrical about
TL>> the plane of the spiral . This infers that the secondary must
TL>> experience a field due to the primary and I wonder what pulling
TL>> this might exhibit on the secondary.


TL>Hmmm... I'm not sure I understand this. The effective coupling
TL>capacitance and the electrostatic field between the secondary and the
TL>primary should be largely unaffected by whether the gap is firing or

Yes, I re-read my words and didn't follow my argument either. I think I 
mean that the e-field is big when the gap is non-conducting compared 
with the H-field i.e. I am referring to the e-field being the dominant 
coupler when the gap is open-cct.

The "floating" voltage on the primary coil when the gap is
TL>extinguished will be a limited by the HV transformer's output
TL>capacitance to ground, presence of any bypass capacitors in the
TL>safety circuit to ground, and other stray capacitance in the primary
TL>circuit. Since a 2-bushing pig secondary is not grounded like the
TL>center tap on a neon, you might see a somewhat greater voltage
TL>excursion capacitively coupled from the secondary discharge terminal
TL>to the TC primary in a pig-driven system. However, in all cases, this
TL>excursion should not be greater than the breakdown voltage of the
TL>safety gap to RF ground (say 15 - 25 KV). Since this is normally MUCH
TL>less than the voltage across the secondary:primary coupling
TL>capacitance, the primary should look pretty much like "ground" to the
TL>secondary irrespective of the whether the gap is firing or not. 

I agree with all of this: the primary of the TC is, if you like, the 
reference ground plane for the secondary to work against, in the fashion 
of a Marconi quarterwave.


TL>> Suffice it to say that for data on 11 toroids, C/d =0.79. This was
TL>> based on calcs from textbooks, quoted data from manufacturers and
TL>> averaged calcs from my own derivations that were within 10% of
TL>> other sources. Most toroids that I have looked at have had a
TL>> major/minor ratio of 4. 

TL>> If you build a toroid, measure its big diameter in inches. This is
TL>> the sort of capacitance it will have in operation. When added to
TL>> the self-C of the secondary,it will ring with the self-L of the
TL>> secondary at around the right frequency. You then need to tune for
TL>> maximum output in situ. 


TL>This sounds feasible! It would also be interesting to compare
TL>breakout currents (streamers to air) verus hot discharges to ground.
TL>I suspect that the maximum discharge currents coming off the
TL>secondary could be quite large. At the maximum voltage point,
TL>virtually all of the system energy is in the large electrostatic
TL>field surrounding the toroid and secondary coil. If we now discharge
TL>this fully charged capacitor to ground, we've removed all of the
TL>energy in a very short time as opposed to the relatively long "ring
TL>down" time associated with only streamer discharges. They certainly
TL>look more evil!

TL>Anyone care to estimate what the magnitude of these current peaks
TL>might be??  10A?  100A??  More???   :^)

I think we might source out of a few tens of pF at several hundreds of 
kV which gives us a stored energy of, for 50pF at 1MV, 25J.  Equating 
against the magnetic energy stored might give us pulses whose peak 
currents are several tens of amperes I expect. This give us a V*I 
product of several tens of MW.

Cheers Bert

Richard Craven