Tesla coil spark gap question

[I14R10]

I am somewhat embarrassed about asking this since I built a Tesla coil years ago, but I did not understand this basic thing that I'm going to ask.



Let's say that positive voltage is on the top side (in the schematics) of the capacitor. It begins charging. Spark gap is not firing yet. When the capacitor is full then the spark gap fires and keep on firing until capacitor is empty. At least this is what everybody says.

My question is why doesn't spark gap fire immediately when we activate Tesla coil? Why does the capacitor need to be fully charged before it can fire? The capacitor will not be charged at larger voltage than power source, so why is that?


And even more confusing


The same question. Why doesn't the spark gap fire all the time, shorting power supply?

 

[BradtheRad]

Falstad's animated simulator has a Tesla coil in the circuit library. (Your computer needs to have Java installed.)

www.falstad.com/circuit

Choose Circuits >> Misc Devices >> Spark Gap >> Tesla Coil.

As far as I can tell, the coil fires 60 (or else 120) times per second.

(1) The capacitor charges during a rising mains waveform.

(2) When it charges to 10kV, the spark gap fires.

(3) The right half of the circuit becomes the scene of action as LC resonance causes many firings of the spark gap, at high frequency.

(4) The entire process occupies 6 mSec. At this point the mains waveform has not even reached peak V.

(5) When mains crosses zero, the above process repeats at the opposite polarity (I think).

 

[I14R10]

So what happens when we look at the time interval between first firing of the spark gap and until mains reach its peak voltage? Where does the current from power supply go? It was charging the capacitor before spark gap fired, but where does it go during the spark gap firing?

 

[BradtheRad]

If there is sufficient time, perhaps the capacitor charges up again, and starts another spark event while the mains sine amplitude is high.

However the Falstad simulation does not show that happening. Instead the capacitor wanders up, then back down as mains approaches zero. The behavior becomes difficult to follow because 3 or more frequencies are acting in the circuit. It is not always clear what conditions create a spark event.

- - - Updated - - -

When opening the simulation, it is set up with prior conditions that cause a spark event right away. It is easy to watch what happens. And so I thought I understood its operation. But then I tried to adjust component and transformer values, in an effort to make spark events happen more frequently. I did not succeed. I have found there is a lot going on to make a Tesla coil work.

 

[I14R10]

Yeah. Tesla either experimented a lot before he made working Tesla coil, or he was really smart so he figured all that in his head.

 

[betwixt]

The spark gap behaves like an open circuit until flash-over voltage is reached. The ionization in the spark behaves like a short circuit, making the capacitor discharge rapidly across the primary. The output voltage is dependant on the rate of change of primary current and the spark gives a very rapid current jump.

Brian.

 

[c_mitra]

Qualitatively it is very simple.

In your top figure, The capacitor starts charging at a rate determined by the time constant. Because the R is rather small, the voltage across the capacitor is following the sine curve and when it reaches the breakdown voltage, the spark takes place and acts like a short.

As soon as the spark takes place, it becomes a dead short and we have the LC circuit complete. As the capacitor is already charged, high frequency oscillations will take place (depending on the LC time constant) and this will induce very high voltage on the secondary of the tesla coil. This will be a RF frequency (at the secondary of the tesla coil).

About the same considerations for the second figure: The LC cannot discharge via the secondary of the power transformer because it is acting as the voltage source. However, as soon as the voltage reaches the maximum for the breakdown of the spark gap, again the same LC oscillations will take place. These two circuits are identical for all practical purposes.

About the question: why the sparking is not continuous? Because the voltage is sinusoidal and as soon as the voltage comes close to the value (becomes less than) required for the spark to sustain, the spark is extinguished.

A common neon indicator lamp also flickers are the main frequency (100-120 Hz)- and so are the common fluorescent lamps (not the CFL- they have an internal HF oscillator). This is a feature of all discharge lamps that run on AC. When we use high frequency AC, the voltage comes up before the discharge has time to extinguish- they appear continuous.

 

[FvM]

About the same considerations for the second figure: The LC cannot discharge via the secondary of the power transformer because it is acting as the voltage source.

It does not act as a voltage source. Transformer leakage inductance is an important parameter to understand the operation of this circuit. Considering that transformer leakage inductance is much higher than the primary inductance of the tesla transformer, first and second figure behave almost identical in the high frequency region.

 

[c_mitra]

I agree that my description of the events above are technically wrong. I was just trying to simplify beyond the limit of simplification. I should have used a proper model.