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Dex, the way you phrase this it sounds more like an assumption about a spark being like a wire than a measurement. How would you measure the actual secondary resonant frequency during operation? Not that this can't be done, I've posted about this some time ago, but it requires measuring primary and secondary currents and phases inbetween. Very complicated.
The Elektrum data does indeed show not very much capacitance. I'd guess the top capacitance to be about 100pF. The current going into the arc is about 10% of that going into the top, so arc capacitance would be at most 10pF.
Burst length of the Elektrum is relatively short and the operating frequency much lower, so it needs a much higher voltage to grow long sparks. High voltage and low arc current amount to low arc capacitance. The Elektrum arc is not anywhere near its equilibrium state. I think, that if you could sustain the million volts during the burst for say 1 ms, you'd get large capacitances and much longer arcs.
Registered Member #2566
Joined: Wed Dec 23 2009, 05:52PM
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Uspring wrote ...
The Elektrum data does indeed show not very much capacitance. I'd guess the top capacitance to be about 100pF. The current going into the arc is about 10% of that going into the top, so arc capacitance would be at most 10pF.
so,you really think that capacity of a gigantic 30' long sgtc spark is comparable with capacity of a 3' long drsstc spark? or even smaler? lol..
I think the main reason is that DRSSTCs make streamers that are bigger compared to the secondary length, than any SGTC ever did while it was being measured.
I believe, that in DRSSTCs and particularly QCW ones, secondary sizes can be small due to the relatively low voltages they need for long arcs. Low voltages can produce long arcs, given enough burst length. Probably also high frequencies reduce voltage requirements.
Dex wrote:
so,you really think that capacity of a gigantic 30' long sgtc spark is comparable with capacity of a 3' long drsstc spark? or even smaler?
You need to distinguish between the whole capacitance of e.g. a 30' wire and that actually seen by the coil. In a simple model an arc looks like a resistor/capacitor series circuit. The resistor will shield much of the capacitance. A large resistor will make the arc look mostly like a resistive load. And resistances will be large for short bursts, since the arc does not have enough time to accumulate heat and become very conductive. The capacitances I was talking about were thought to be directly parallel to the top load. I chose to specify them in this way, since that value is the relevant one for detuning.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
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Udo, Wonderful measurements! Especially because you get very similar R/C values to what ive been plugging into my DRSSTC models to look at tuning and driving methods. I arrived at my numbers by doing steady-state measurements of primary current, secondary current and secondary voltage and adjusting the streamer lumped RC to get the model to settle out at the same amplitudes for all these measurements. My QCW arc models had to get resistance down to ~20k ohms and series capacitance of 20pF for a 6 foot spark (with maybe 1 or 2 branches in it). "Transient" arcs modeled well with 80k+15p for about 40" max arc length even but they are highly branched sparks.
I'm glad to hear, that you could confirm these measurements. Can you tell us, at what frequencies the QCW and transient measurements were made? For settled arcs, I believe very much in Conners hungry streamer model, which states that R is approximately 1/(2*pi*f*C), i.e. 45 degrees phase shift between arc voltage and current. My measurements indicate a somewhat larger C during arc growth.
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Isn't the secondary arc current the same current that is flowing in the RF ground of the secondary?
If you look at the path of current, from RF ground, to secondary coil, to toroid, to break-out, neglecting any stray losses, it should be the same current, right?
I was always under the understanding that the RF ground current would be the same as through a grounded current strike from the topload.
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The current that goes into the bottom of the secondary is (to a first approximation) the same as what comes out of the top into the toroid. That current splits two ways into conduction current in the streamer root, and displacement current in the toroid-to-ground capacitance. The whole point of Uspring's experiment is to measure these two currents separately.
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Steve Conner wrote ...
The current that goes into the bottom of the secondary is (to a first approximation) the same as what comes out of the top into the toroid. That current splits two ways into conduction current in the streamer root, and displacement current in the toroid-to-ground capacitance. The whole point of Uspring's experiment is to measure these two currents separately.
And i'm guessing the displacement current may not be current that is actually visible? Just leakage due to the stray capacitance between toroid and ground?
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
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And i'm guessing the displacement current may not be current that is actually visible? Just leakage due to the stray capacitance between toroid and ground?
Visible? You mean, plasma?
Its important to point out where this secondary base current is measured, and that would be between the bottom winding of the secondary coil and your RF earth connection. This current will reflect the circuit current of the secondary. If you arent producing sparks, then nearly all of this base current is simply charging the coils capacitance (MOST of the capacitance is from toroid to ground, only a small fraction of it is internal to the winding itself). Also, considering the impedance of a toroid at Fres, it will take many amps of charging current for most TC designs to get the right voltage out. The toroid current is probably higher than the average streamer current in most tesla coils. It might seem like a waste, but that is a requirement for resonance and high Q.
The current that goes into the bottom of the secondary is (to a first approximation) the same as what comes out of the top into the toroid. That current splits two ways into conduction current in the streamer root, and displacement current in the toroid-to-ground capacitance. The whole point of Uspring's experiment is to measure these two currents separately.
Exactly, and thanks to Greg Leyh for that idea. Trying to get an estimate of secondary voltage by measuring secondary base current has 2 problems. One is, that this current does not come out fully at the top to charge the toroid, since the secondary winding has a capacitance of its own. The other is, that a significant part of the current coming out of the top doesn't charge the toroid but goes into the arc. I remember a discussion on pupman set off by a measurement of Steve Ward, where he could not reconcile the secondary base current with the top voltages, which he obtained differently. I believe the reasons for that are just these problems.
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An arc measurement
