Re: The 1500t secondary myth (long)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Dr. Resonance" <resonance@xxxxxxxxxx>


Very interesting information.  I never thought much about the impedance
factor.  Now, I'll have to go back and do some math work to see if I can
optimize my designs even more.

I usually keep adding capacitance until I can tune large coils at 4-8
primary turns.

The lower freq coils definitely seem to have long sparks (plasma formation)
probably due to pumping more total energy into the sec with it's large
toroid.

Dr. Resonance

>
> If everything else is the same, the lower frequencies also tend to need a
> higher inductance primary coil which tends to lower primary currents which
> reduces primary losses significantly.  Higher inductance primary systems
> tend to have lower losses in the coil, wire, gap, etc.
>
>
> >By this line of argument I wouldn't expect massive differences between
> >higher and lower frequency coils. And indeed in practice this seems to be
> >the case. There does seem to be a sweet spot around the 1000-1500 turn
range
> >but it's nothing drastic.
>
> DR R. also tends to make very nice coils with low loss primary
> systems.  His results may not be completely true for those of use whose
> coils are not as well built.  But the 1200 - 1500 turn idea seems to be a
> nice area to work in.  You can't really go too wrong with it.
>
>
>
> >The new theory that I have been working on (with a lot of input from
Terry,
> >Malcolm Watts, and others) suggests that the sweet spot is not any
> >particular number of turns, or even any particular inductance. Rather,
it's
> >when the resonator has a characteristic impedance (Zo) of around 36,000
> >ohms. Zo is a function of both secondary inductance and secondary/toroid
> >capacitance:
> >
> >Zo=sqrt(L/C) or Zo=2*pi*f*L or Zo=1/(2*pi*f*C) where f is your coil's
> >resonant frequency
> >
> >To check my theory, I have calculated Zo for various coils with good
> >performance documented on the net, and they all come out around 40 to
50k.
> >See my earlier posts for details. John Freau's TT-42, which is still one
of
> >the most efficient coils ever, has Zo=44k. John found that adding a
bigger
> >toroid to his TT-42 (which would lower the Zo towards 36k) increased the
> >spark length.
> >And, Richie Burnett found that a coil with Zo=22k performed poorly, but
> >changing to a resonator with Zo=~50k gave much bigger sparks with the
same
> >bang energy.
> >
> >Hence I think there is fairly good evidence for my theory. If anyone
knows
> >of any good coils with Zo greatly different from 36k, please let us know.
>
> That Zo range does seem to be a winner!  We don't know enough about
> streamer dynamics to know "why", but secondary impedance matching should
be
> "real important" according to the known data and models.  With just simple
> static models, playing with loading and impedance has dramatic
> effects.  You say 36K is the sweet spot, I'll carve that number into my
> desk ;-))
>
>
>
> >So my recipe for an optimised coil is-
> >
> >1) Decide what length of sparks you want
>
> Using john Freau's formula, you can come up with that based on input power
> too which is usually a good starting point given what type of NST one
found ;-)
>
>
> >2) Work at around 100 bps
>
> 120 in USA  ;-)
>
>
> >3) Use Freau's efficiency equation to get the required bang energy
>
> You usually have to start with the power source one finds and then get the
> best streamers out of that.
>
>
> >4) Size the toroid so it will just break out at this bang energy
>
> I tend to like to over size them.  I have had many too small, but none
that
> were "too" big.  You can always force breakout of the toroid were too big
> and too smooth, which never seems to be a problem.
>
>
> >5) Choose the secondary height big enough to avoid flashovers and racing
> >arcs (ie more than 1 meter of height per megavolt- you can get a more
> >accurate figure from Paul Nicholson's TSSP voltage gradient plots)
>
> Yes!  Don't make it too small or too short.  You just can't "fix" that.
>
>
> >6) Now choose the secondary diameter and number of turns so that Zo is
36k.
> >If this is inconvenient (maybe it gives too high an operating frequency
for
> >your *SSTC) you can increase the inductance provided that the "Q-limited
> >streamer length" works out bigger than your target streamer length. I
think
> >I already posted how to calculate this.
>
> CW coils may have different Zo matches than tube, disruptive, etc.
>
> http://hot-streamer.com/TeslaCoils/MyCoils/CWCoil/CWImpedance.txt
>
>
> >7) Finally choose the primary and tank capacitor to match your power
supply
> >voltage/current and secondary frequency, and deliver the required bang
> >energy. The procedure for this is well known already (at least for
spark-gap
> >coils and OLTCs)
>
> One will have to match the primary cap for the LTR NST match.  But higher
> inductance primaries reduce losses if that can be worked in too.  However,
> in the case of the DRSSTC, very low loss primary systems might not be good
> as Dan found.  A little resistance there can dramatically reduce IGBT
> currents without hurting performance much.
>
>
> >PS. For what it's worth, I favour using a "fat" secondary like Dr.
Resonance
> >does. A fat secondary is better at "catching" the magnetic field from the
> >primary than a thin one, so you can achieve high coupling without getting
> >the primary dangerously close. And it makes the coil look mean. It also
has
> >a higher unloaded Q than a long skinny one, but in the light of the new
> >theory I don't think this is much of an issue (as the loaded Q of our
> >optimal coil is only 6)
>
> I also like larger diameter secondaries as long as they "look" right.  I
> note that most coils all have pretty much the same general proportions
that
> have been carved out over the years.  Making things that look "odd"
> compared to other coils is usually not good unless one knows exactly what
> they are doing.
>
> Cheers,
>
>          Terry
>
>
> >Steve C.
>
>
>