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Re: [TCML] Telsa coils + capacitor bank = ?



Hi All,

Aaron wrote:

I 99% sure Bill Wysock and/or Dr. R. did a similar experiment years ago.

Perhaps one of those folks will chime in?

Cheers,
Aaron, N7OE

GL: Not sure who else has performed this misdeed, but I originally tried it in Graz, Austria in 1992, then once again in Austin TX in 1997:
http://www.lod.org/Projects/120L50K/austin01.jpg
Lots of fun, but no decent instrumentation in either case!
Now we're trying to resurrect some old dusty HV equipment, and attempt to more carefully characterize how initial conditions affect the runaway current development.


Bert wrote:
[snip]

>> Josh Bailey wrote:

> Here are two series' of photos (frames from an MJPEG capture

running at about 30 fps):
> > http://vandervecken.com/images/capbanktest1/
> http://vandervecken.com/images/capbanktest2/
> > Q1. What conditions must exist for the capacitor bank to

> discharge? We noticed that sometimes the graphite was struck
> multiple times before it discharged. We also noticed that

> sometimes the bank was not fully discharged (still had 3-4kV on it).


>> In general, the brightness of a spark is a proportional to the amount of current passing through it. The base of air discharges from a Tesla Coil (TC) is the brightest and hottest part since the sum of all the branching discharge currents pass through the main channel. As you move away from the terminal, the discharges begin branching, with each branch carrying a portion of the total current. As the current flowing through more distant branches becomes progressively lower, the channel diameters become smaller, and the discharges become more purplish and dimmer. The tips of the discharges transition to a bluish glow (the streamer-corona region) where the gas temperature is barely above room temperature.

>> Unconstrained sparks and arcs in air have a negative resistance characteristic: the more current you force through them, the hotter their cores become, and the lower their electrical resistance. Because the more distant channels are cooler and smaller in diameter, these remote channels are considerably poorer electrical conductors.

>> When a Tesla Coil is operating near the limit of its spark length, you may see sparks sporadically/weakly connect the TC terminal to ground without the characteristic brightening that we usually associate with TC ground strikes. In this case, although the spark has indeed bridged the gap, the combination of high resistance from the discharge path, and the fact that we are close to being max'ed out on TC terminal voltage, prevents any major increases in channel current - even though we've actually bridged the gap.


GL: Thanks for the great description, Bert. The early misfires make more sense now. I have a tendency to view any visible electrical discharge as a wire at these voltages, but your point is now obvious that the tendrils of the arc must be presenting relatively high resistance if the TC terminal voltage is maxed out, as in the case of a barely bridging arc. Clearly the relatively low energy storage bank voltage won't do so much in such a case.


[snip]
>> During each oscillation, the arc current passes through zero, temporarily extinguishing the arc. Usually it is quickly reignited by the high voltage from the capacitor bank. As the energy in the system decreases, so does the arc current, causing the arc resistance and overall arc voltage drop to rise. The voltage drop across the arc is also proportional to its length. As the bank energy decreases, a point is eventually reached where the arc can no longer be reignited, and the arc goes out for good.

>> Once the arc finally extinguishes, residual energy will still be left in the capacitor bank. This is typical behavior for an LC and spark gap system. Because of the relatively long arc involved in Greg's system, the stranded bank voltage is proportionally higher. You may also see significant variation in residual bank voltage if the Tesla coil continued to operate during the bank discharging process, since it would assist in reigniting the arc channel.


GL: Makes sense. The rather slow [70Hz] resonant frequency of the secondary and energy storage bank might also be assisting the ion recombination?


>> Bert

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