tesla coil

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hi, i examined the scheme of a tesla coil (a simple one ) and there is a spark gap which creates a resonance frequency(as far as i could get from it)
now i wonder how a spark gap creates a resonance frequency? yes spark gap will create a high frequency instantenous current but what is needed there is a specific one isnt it? pls give me some theory of it.. i guess the maximum frequency of the surge current will be more than resonance frequency?
 
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I only have a crude understanding of this, but:

The spark gap does not 'create' the resonant frequency; instead the spark gap is acting as a switch to close the circuit between an inductor and a capacitor. The LC circuit has a natural resonant frequency, and the spark is simply a relatively low value loss in that circuit.

The L in the LC circuit is actually the primary of a transformer, and the secondary of that transformer is _another_ LC circuit, this one consisting of the big coil and the capacitance between the air terminal and the earth.

-Jon
 
how a resonant freq. of about 100-200 khz can be achieved with an utility electric? it is the spark gap what makes this high freq. resonance occur with sparking instantly i think, secondly u say a "capacitance" but as far as i know there is a capacitor fixed for creating resonant frequency, u say if i am not wrong as if there is no a component but the natural capacitance works here....
 
1) There are probably a zillion different ways to do a 'tesla coil'. So a specific description of one way may not directly apply to another setup.

2) Any rapid switching event (including an arc) will create a tremendous range of frequencies.

3) The basic Tesla coil circuits that I have seen start with a step-up transformer operating at mains frequency. The output of this transformer is in the 1-10KV range, at mains frequency. The output of the secondary just powers the high frequency stuff; the low frequency AC doesn't really change the operation.

4) In the basic circuits that I've looked at, the transformer output charges a capacitor. When the capacitor voltage is high enough, it triggers the gap to spark over. This gap is simply acting as a switch, and closes a circuit to let the capacitor discharge into an inductor. Once you have a capacitor discharging into an inductor you have a resonant circuit. In the basic circuits that I've looked at, the inductor is a very few turns of wire that also acts as the primary for a loosely coupled high frequency air core transformer.

5) I've also seen systems where the big coil is 'end fed' rather than transformer coupled. These simply require a high frequency source at the 'ground' end; so I am sure that some sort of spark gap right there would produce some sort of voltage amplification...but it seems to me that you want some capacitance discharging through that gap.

6) To further this discussion, you should probably post a schematic diagram of the particular circuit that you are considering.

-Jon
 
The most crude explanation of a spark gap for a tesla coil is that of an impulse generator. When the gap is breached it creates an impulse that contains "all frequency components" (that's what an impulse is). While inefficient, it contains whatever frequencies are required to resonate in the primary/secondary of the coil. The reality is that much better performance is obtained by tuning the "spark gap" behavior to that of the primary/secondary resonant frequencies.
 
100225-2213 EST

Consider a real world LC circuit. This will also include R. Consider a parallel LRC circuit. Initially separate the capacitor from the R and L with a switch.

Charge the capacitor to some voltage like 100 V. Close the switch and monitor the voltage across the parallel LRC circuit. The resulting waveform will be a damped cosine waveform with an exponential decay of the amplitude of the oscillation. The larger the parallel resistance the slower the decay. But in the real world there always will be decay.

Parallel this network with a negative resistance equal to the positive resistance of the losses of the LRC circuit and the result is a constant amplitude sine or cosine oscillation.

A DC arc discharge over certain current ranges exhibits negative resistance characteristics. Thus, an arc discharge coupled to an LRC circuit can produce a continuous sine wave output.

See pages 141, 152, and related pages in "Differential Equations", Rainville, MacMillan, 1949.

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