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Powering a TC with a stereo amp
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From: Harri Suomalainen [SMTP:haba-at-cc.hut.fi]
Sent: Wednesday, June 03, 1998 3:16 PM
To: tesla-at-pupman-dot-com
Subject: Re: Powering a TC with a stereo amp
>"tuning" process consists of making the primary circuit resonant
>frequency match that of the secondary circuit. This process is
>complicated (!) by the fact that the two tuned circuits are linked and
>interact quite a bit. Part of the tuning is finding the optimum amount
>of coupling between the primary and secondary.
That is *not* the case with CW-driven coils. With those ones there
is one definate optinum where you drive it at resonant frequency
and with maxinum coupling.
>Overcouple, and all the
>energy of your excitation impulse goes into the secondary, and the
>primary never really "rings".
And that's exactly what should be done with cw coils. We want the
power there to get good results.
>Under couple, and the energy stays in the
>primary circuit and is eventually dissipated as heat.
Yap. No sparks preferred at primary. :)
> This is
>complicated by the fact that the secondary tuning is not constant,
>because the sparks created by the high voltage have a significant
>capacitance, and cause the resonant frequency to change, which in turn,
>causes the coupling to change.
That might be so. However, I've observed with cw coils that there is
almost constant hiss instead of breakdown every now and then. This
will keep the capasitance more or less stable. I've had no problems
with changing capasitance. I'm sure this might be quite another
story at high power levels.
However, objects do change things a bit. I was a bit confuced when
I saw the first time the corona stopped when I got closer to take
it with a metal rod. So, I had to actually mistune it and then it was
just perfect when I was close :)
>If, instead, you drive the primary with a source (either a voltage or
>current), you don't have to worry about the primary resonance, because
>there is none.
That's the point. I have a hard time finding any audio amp anything but
a voltage/current source!
>And, you can have a coupling of 1 (if you could get it),
>because you aren't worried about the interaction of two tuned circuits
>anymore. The more power you pump into the primary, the more power comes
>out the secondary.
That is obtainable. You can also drive the signal right to the end of the
coil (with no primary at all). It works. However, usually you need too
high voltage for that and step it up with a transformer before "end
feeding"
the coil. With a step-up transformer made of ferrites very high coupling
is easily obtained.
>A tube coil uses this principle. The tube is essentially directly
Yap. With solid state systems you are usually better off with lower
frequency ranges (ie. not in a few MHz range). At low frequency like
100-200kHz ferrite transformers become more feasible than the
traditional primary/secundary setup. That kind of setup would be
more like an air-core transformer not suitable for such a low freq.
>The real problem is efficiency. It turns out that a spark gap is a
>pretty efficient switching device in terms of power lost in the gap
>compared to the power being switched. A tube coil, operating Class C, is
>going to be 60-70% efficient. A Class AB stereo amplifier is going to
>be, at best, 50% efficient. A spark gap scheme could easily have an
>efficiency of 80-90% (although it has other problems, like exponential
>decay, etc.)
And a resonant topology SMPS could approach 90-95% with reasonable
effort. However, peak power would still be lot less than with spark-gap
types. Do not expect the same bang with tube/solid state coils.
--
Harri Suomalainen mailto:haba-at-cc.hut.fi
We have phone numbers, why'd we need IP-numbers? - a person in a bus