Re: Another capacitive transformer TC

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

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:
> 
> Original poster: "Jolyon Vater Cox by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
> 
> Antonio,
> Measuring C3 without sophisticated equipment would appear to be
> problematic -would it be best to design as a directly-coupled
> transformerless TC and then reconfigure it as a capacitive transfomer (as I
> have done);

When building my capacitive transformer coil I never had to measure
this capacitance. I just tuned C2 and C3 simultaneously by looking at
the voltage waveform over C1 while operating the circuit at low power
(with a "ping" circuit made with a 555 timer in place of the spark gap).
The element values listed in the web page were obtained by designing
a network that fits the observed experimental waveforms (the mode)
and the known elements C1, L1, and L2. The elements calculated for
the directly coupled coil can be always used. The tuning relation
for the directly coupled system is L1*C1=(L1+L2)*C2, and for the
capacitive transformer version is L1*(C1+C2')=L2*(C2'+C3).
With practical values, C2~=(C2'+C3), and any distribution of the
terminal/L2 capacitance between C2' and C3 results in a working
system (but with different numbers of cycles for energy transfer). 

> I have a meter capable of reading capacitance
> down to 1pF -would this be of any use?

It's difficult to measure C2 and C3, because they involve the self-
capacitance of L2. In an approximate way, C2 and C3 can be measured
with a capacitance meter between the influence ring, the terminal,
and the ground, with the elements in their usual positions and
insulated from everything. The capacitance of the meter leads must
be discounted (discount the reading of the meter with the leads in
the position where the measurement will be made, but disconnected).
The self-capacitance of L2 would be then split in two and added to 
the measured values. The result will be a bit larger than what 
actually appears. You can also measure the self-capacitance of
a coil, approximately: suspend it high above anything else, 
connected only to the meter, and measure its capacitance to ground. 
The self-capacitance is one half of what you measure. 
 
> I have experimented with changing the elevation of the ring with respect to
> the coil L2
> Lowering ring had little obvious effect on spark length or corona production
> at the terminal
> -although this may have been obscured by low power of the HV supply
> (rectified
> ignition coil).
> Raising the height of the induction ring resulted in mild corona appearing
> in the middle
> of the ring where the metal foil abuts the plastic "spark-guard", which
> developed into
> actual arcing from L2 into the encircling space when the topload was
> removed.
> Apparently, no functional damage to L2 or puncture of of the spark-guard
> occured and
> normal operation resumed when the topload was replaced.
> 
> This suggests that spacing between the ring and L2 should be increased in
> proportion to the elevation of the ring to prevent corona or arcing-
> but doesn't the bigger separation increase the parasitic capacitance C3 by
> allowing more field lines to escape capture by the ring
> -or does perhaps the higher voltage that can be premitted between L2 and the
> ring without arcing allow the same amount of energy  (or more)to be stored
> in C2 despite the fact that its capacitance is smaller?

Ok, but if you increase C2 the voltage gain of the system decreases.
No free lunch.
 
> Wouldn't an inverted-cone -shaped induction ring be more effective in
> maximising C2 and minimising C3 than a flat ring?

Possibly yes. The ideal would be to eliminate C3, but keeping C2 as 
small as possible to have large voltage gain. A cage around the 
terminal would be a curious possibility.
 
> Also does it not suggest that the terminal size be increased proportionately
> to the elevation of the ring -again to prevent arcing or corona?

This would also reduce the voltage gain. Ok if you can add more primary
capacitance to compensate.
 
> Incidentally, I have seen a similar effect with regard to the primary tap on
> my  inductively-coupled
> TC, where reducing the number of turns to the tapping point causes
> ultimately  causes
> primary-to-secondary insulation to breakdown with the appearence of
>   "racing" sparks
> (?) ; the problem disappears when the terminal size is increased.

The voltage gain decreased.

I am making available a program that designs the directly coupled
coil and the capacitive transformer version (calculates the elements).
See at the end of the page about the capacitive transformer coil:

http://www.coe.ufrj.br/~acmq/tesla/mres4ct.html

Antonio Carlos M. de Queiroz