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Re: Big Primaries, Small Caps

In a message dated 9/3/00 11:37:12 AM Pacific Daylight Time, tesla-at-pupman-dot-com 

> Original poster: "Gregory R. Hunter" <ghunter-at-accucomm-dot-net> 
>  Dear List,
>  I'm curious about Tesla coils with large primaries and small tank caps.
>  John Freau and others have articulated numerous times over the years that
>  coils so designed have lower gap losses and longer sparks relative to coils
>  using big tank caps and few primary turns.  I'm curious as to why this is
>  so.  It seems to me that the bigger bang size delivered by a larger cap
>  would thump the secondary harder, yielding longer sparks.


I must clarify my position here:

I do not advocate the use of small primary caps (except
on a very high voltage coil).  I advocate the use of a *large* bang
size, with a low bps, along with a large primary inductance
(usually many turns).  You are correct that a large bang size
gives the best results, and this is the method I use.  A large
bang size can be achieved either with a large cap, low voltage,
or using a small cap with high voltage.  It's actually better to
use a high voltage with small caps, since this will permit the
primary surge impedance to be much higher..... making the
gap losses much lower.  Most folks do not have access to super
high voltage transformers however, so I normally speak more about
the large cap with large primary combo (which demands a large
secondary L for tuning).  The goal is to keep the primary surge
impedance high which keeps the gap losses low, since the gap
losses are proportional to the gap current.

The difference with what I'm advocating (vs. what is normally done),
is that I recommend the use of a lot of turns of thinner wire in the 
secondary which permits *both* a large primary capacitor and a lot
of primary turns (high L) to be used.  This is for the typical voltages
that are used.  If you have access to a super HV xfrmer, then you
can use a smaller cap, with many primary turns, and keep the
bang size large and the gap losses low that way.

Normally, when folks use a large capacitor, they use a small 
primary L, which lets them use fewer turns of a thicker secondary
wire.  This approach will reduce the secondary losses a little, but
will greatly increase the gap losses, and give them overall shorter
and weaker sparks.

I have stated in the past that a secondary that is narrow will work
about the same as one that is wide, but I may have made a mistake
there.  I'm now seeing evidence that a wider secondary is better up
to a point.  But even using the wider secondary, a lot of turns is
still needed for best results, at least on the small coils I'm working
with.  I've obtained 42" sparks using about 550 watts with a wide (6.5")
secondary with 1600 turns, but a 4.2" wide secondary may need
650 watts to give the same spark length.  It is possible that a wide
secondary with fewer turns (1000?) will give about the same spark
length as a narrow coil with 1600 turns.  But.... if the wide secondary
with 1000 turns, is re-wound with 1600 turns.... it will be even better.

This is all large cap, large primary Z, large bang work at low bps
(around 120) for best efficiency.

One could supply the 1000 turn wide secondary coil with a higher
voltage transformer, and use a smaller cap, and equal the efficiency
of the wide 1600 turn secondary..    But, one could then install the
1600 turn wide secondary and most likely obtain *even longer sparks*.

There have been folks in the past who have advocated the use of a
small primary cap with a small bang size, along with a high bps, but
I am not in that camp.  My tests (at least at low powers) shows
a very clear and dramatic benefit to using a large bang at a low

I am now beginning work on a new TC setup that will use about
46kV peak in the primary for even lower losses and greater

John Freau

>  Can someone in the know provide an explanation?  I'm not an engineer, but I
>  am an experienced comm/nav technician, so I can grasp pretty deep concepts.
>   However, a side trip into Calculus land will lose me pretty quickly.
>  Best Regards,
>  Gregory R. Hunter