Carl,If we assume that corona as composed of small streamers and "The appearance of streamers will increase the capacitance". Then surely the corona will increase the capacitance. I also think that as a finite element toroid appears to have roughly the same capacitance as a similarly sized traditional one the field must account for the extra charge handling capacity. As I said earlier I suspect this is a small effect and possibly very small on a regular toroid. But on a finite element toroid or one designed with bumps maybe the effect is larger..
I totally agree regards the breakdown voltage of a smaller radius, and I don't think irregular toroids will be the next great improvement in tesla coiling, but I'd like to prove that there was some change in capacitance and of what scale it was..
RE: Streamer loading IIRC Steve Connor and Terry Friz did some tests a while ago regarding the capacitance and loading of streamers, have a search in the TCML archives or on 4hv for the details, Terrys rule of thumb for streamer loading was 220k + (1pF/foot).
Cheers
Derek
On 07/04/2011 15:50, Carl Noggle wrote:
Hey---The capacitance of any object is the effect of the electric field between the object and the rest of the universe. If we visualize the EF of a toroid, the nature of the surface will be seen to have little influence on the whole EF. A rough surface will cause the toroid to light up with corona at a lower voltage, since the EF writ small at any part of the toroid is what determines the breakdown voltage at that point or area.Since the breakdoown field for air is about 25 or 30 kV/cm (depending on altitude), a smooth sphere will have a breakdown voltage of 30kV times the radius. A toroid will have a breakdown voltage of about twice that of the sphere if the "tire" radius is the same as the sphere. That is, a clean, smooth toroid with a 5cm "tire" radius will have a breakdown voltage of 30 x 2 x 5, or 300 kV. Any small imperfection will reduce the breakdown voltage, usually drastically, as the local EF at the point is much higher than the EF at the smooth surface of the toroid. Therefore a corona will form at that point at a lower voltage. However, this effect will not change the toroid capacitance.The appearance of streamers will increase the capacitance. The plasma channels of these streamers probably have relative high resistivity, so this adds energy dissipation in the sparks (which is why they tend to rise). The advantage of this is that it lowers the Q of the secondary tuned circuit, so that the tuning of the coil becomes less critical. This is why TCs usually seem to work pretty well over a 10% or 20% primary tuning range.Has anybody actually measured this change in Q, or does anybody know the resisitivity of the spark channels? These things would be very interesting to know. The sparks in the main switch gap have very low resistivity, and like lightning their resistivity may be lower than copper or silver, so the primary Q should be quite high.---CarlPhil,Dave, All,My gut feeling is that the effect is quite small, especially with smooth top loads (spheres/toroids). Because of this, any breakout loading will overwhelm the additional field created capacitance.As it happens, the coil that I am building at the moment is a classic SSTC with a non-smooth toroid design. As it utilises base current feedback, I'm hoping that running the coil with no visible breakout, I will see a resonant frequency change as the bus voltage is increased, indicating a change in the capacitance of the top-load. This should prove or disprove the theory.Also as Tesla coils are an AC system, it would also suggest that the capacitance of the top load would vary over the period of the driving frequency, with zero crossing having minimum capacitance and peak voltage points having more. This should be seen in the shape of the field waveform measured in open air near the coil. A comparison to a similar sized but more conventional topload would help to eliminate any driver/coil related waveform shape irregularities.Cheers Derek On 06/04/2011 23:14, Phil Tuck wrote:Derek, et alIf it did then people would have noticed an anomaly in the tuning figures by now I think. We know that best performance generally occurs when the primary is set to resonant lower than the secondary because of the streamer loading and this normally is by an amount of around 1.5pF per foot. However, if what you say were right, then would we not have noticed additional factors at play on the bigger coils that could not be accounted for purely by streamerloading alone? Regards Phil Tuck www.hvtesla.com -----Original Message-----From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On BehalfOf Derek, Extreme Electronics Sent: 06 April 2011 05:49 To: Tesla Coil Mailing List Subject: Re: [TCML] Tesla's toploadThis does raise an interesting question, does the capacitance of the topload increase as the voltage on the topload increases because the field around the topload adds to the effective external dimensions of the topload, soincreasing capacitance ? Derek On 06/04/2011 01:48, bartb wrote:I dunno, it would be interesting to model: say of sphere's with diameters equal to a corresponding toroid minor diameter, but with the spheres spaced apart so that 1/2 of the diameter of each sphere intersects the other creating large bumps of sorts. Unfortunately not something Javatc can do as the topload sphere's would have to be offset from center (not something it can do now). Possibly Paul or Antonio's software could do a quick and dirty comparison. Take care, Bart_______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla_______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla_______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
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