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Re: [TCML] DRSSTC, driving system, and a strange problem.



Em 31/01/2014 07:44, Udo Lenz escreveu:

Running the coil as you do, i.e. the driving frequency close to the secondary resonance, probably is the most effective way of transferring energy from the
bridge through the primary into the secondary.
I can see this operating the system with an incandescent lamp in series with the power line (that I use in limited-power first tests to avoid disasters). Varying the driver frequency I get longer streamers and less light in the lamp with the
same adjustment.
Capacitive arc loading will lower the secondary fres, though, and move it away from the driving frequency. That causes a more inefficient power transfer to the secondary. Inefficent in the sense, that more primary current is needed to transfer the same
amount of power. The secondary will then draw less power from the primary
so that more power is fed into the primary tank than going out of it. This will cause
the primary current to rise.
Agree.

Most DRSSTCs aren't run this way. Usually the primary tank is tuned quite a bit lower than the secondary and since they often use zero current switching on the primary,
the running frequency will be about primary fres.
Between the resonances there is a zero-current switching point too, that would be the frequency for zcs even with heavy load, were not for the capacitive loading. So far I am insisting on no feedback, operating the system with waveforms similar to a regular Tesla coil, starting with a rise without much loading that the tuning used makes as fast as possible, and then keeping the burst at the same frequency. This would be the ideal, were not for the extra capacitive loading of streamers.
I want to see by how much they detune the system.
Since the runnig frequency is much less than the secondary fres, primary current will rise for some time almost without beats until the arc breaks out. This will then lower secondary fres, which moves its frequency closer to the running frequency. The power transfer to the secondary will be enhanced, which will grow the arc and in turn lower secondary fres more. Basically this is positive feedback loop leading to a rapid discharge of primary energy into the secondary and then to the arc. During this surge the power delivered to the arc can be
much larger than the power the bridge supplies.
The concept seems reasonable.

I believe it is generally preferable to use longer bursts. The power delivered by a bridge is proportional to the primary current I, while the losses in the transistors are proportional to I^2. So a longer burst with less current will stress the fets less than a shorter one of the same total energy. On the other hand, an arc of longer duration will consume more energy
without necessarily getting longer.
The scheme above, though, allows to make the arc time, i.e. during the postive feedback surge, to be made much shorter than the the burst time. This resembles the operation of an SGTC: The primary is slowly charged up but instead of a gap triggering the arc,
the breakout of the arc triggers the discharge of the primary tank.

For long bursts feedback is probably necessary. In my no-feedback coil I see that the current rises above the designed limit of the first current beat when streamers appear. A combination of resistive
and capacitive loading (detuning), probably.
My idea of a "perfect DRSSTC" would be one using a large primary inductance, which could store a lot of energy for a given max primary current and a corresponding long burst time to charge it up. That would reduce stress on the transistors. Most of this is
unexplored territory.

Larger primary inductance reduces the primary current, but reduces the maximum output voltage too. Probably, once a certain voltage is achieved bursts length can control the streamer length. Again the question that I never saw properly addressed of how much voltage you need at the output
of a Tesla coil.

Antonio Carlos M. de Queiroz

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