Re: The 1500t secondary myth (long)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: FutureT@xxxxxxx 

In a message dated 12/5/04 9:18:37 PM Eastern Standard Time, 
tesla@xxxxxxxxxx writes:


> > I did some tests comparing various input voltages and found only
> > a minor improvement by using for example twice the voltage.  I tried
> > using voltages up to about 50kV, but didn't see much difference.  It is
> > true however that higher voltages are theoretically more efficient,
> > and are likely to give slightly improved performance.  A lot depends
> > on the coils original design.  For example if a coil has too few
> > primary turn in use, it will tend to be lossy.  Then if a much higher
> > voltage is used, it will permit more primary turns to be used
> > (because a smaller capacitor must then be used).
>
>      Interesting that you didn't get much performance increase from higher
> primary voltages. But why must a smaller primary capacitor be used with the
> higher primary voltage? Why not a cap of the same specs and performance but
> higher voltage rating (obviously there are cap construction tradeoffs, but
> if cost was not a factor?)?


Phil,

The reason the cap must be of a smaller value at a higher voltage
it to keep the input power the same for the purpose of comparison.
I think you did say you wanted to compare the performance of
low and high voltages.  If you keep the capacitor the same size
but increase the voltage, then the coil will draw more power
if the breakrate is held constant.  You can of course keep
the same value cap and raise the voltage, but then you're
creating a larger more powerful coil which will of course give
longer sparks.  You can also keep the voltage the same and
use a larger capacitor and get longer sparks.  It all depends
on what you're trying to do.


> > Hull used a series quenching rotary.  This design has a number
> > of electrode pairs all in series.  This makes the gap both a rotary gap
> > and a multiple gap in a sense.  It's true these types of gaps quench
> > a little better.
>      I think every rotary gap design I've seen has at least *two* actual
> gaps. Just not sure what Hull was calling the magic number. Didn't he
> advocate at least one static series gap as well? If so, why?


Yes, good observation, they do tend to have at least two gaps.
Hull used 8 gaps in the series quenching rotary.  He only used
the static series gaps with the more standard type rotaries such
as he used on the Nemesis coil.  The Nemesis coil was a great
coil to see running BTW.


> > In reality the actual overall coupling of a magnifier is very similar to
> > that of a classic coil.  It's only the driver coupling that is tight.  This
> > is because the secondary and extra coil of a magnifier "work
> > together", and the coupling of the secondary and extra coil
> > combination must be used to determine the actual coupling.
>
>      I think I've got a handle on at least this aspect - you need some
> coupling, obviously, to get any energy into the secondary/resonator. But if
> it was perfectly coupled at k of 1, there wouldn't be a resonant rise, only
> what would then be the turns-ratio effects. So there has to be an
> un-coupled (from the primary) portion of the coil that can resonate. Is
> this correct?


That idea about the coil being free of the primary to permit it to
resonate higher doesn't really apply.  Those concepts are more
applicable to CW driven type coils where there's steady input
power available at the resonant frequency.  In the spark gap coils,
the energy build up is limited by conservation of energy rules.

It's true that reducing the turns by 50% only gives about a
10% reduction in sparks.


>      How is Gary (and you, John) getting such good performance? Gary has
> some interesting pressurized gap designs on his site, but are they what he
> is currently using?


Gary is presently (the last I heard anyway), using a 120 bps
sync rotary gap.  This considerably improved the performance
over that of the pressurized gaps.  The key to good performance
with NSTs is to use;  1) a 120 bps rotary gap,  2) An LTR value capacitor,
3)  A high inductance secondary and primary,  4) A suitable sized
toroid such that the toroid major diameter is 1/3 of the spark length.
That's all there is to it !


> > Hull never experimented with low breakrate 120 bps synchronous
> > rotary spark gaps.
>
>      Really? I would think that would be the obvious starting point, so I
> wonder what led them away from that.


It was not obvious in those days.  Sync gaps were rarely used in
those days, and the benefits of a low breakrate was not well known.
It fact if was much more fashionable to use high breakrates.  High
breakrates were often considered to be better.  Folks for awhile
strove for higher and higher breakrates up to 1000 bps or so.
Hull used to say that sync gaps didn't offer any advantages at
a high breakrate, and I agree with him.  Since he liked higher
breakrates, the sync feature didn't seem important to him.

John


> -Phil LaBudde