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Re: Big Primaries, Small Caps

Hi Greg, 

Tesla list wrote: 
> 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.

With a static cap value (regardless of value), more turns on the primary
reduces losses because more turns equates to a higher impedance (due to the
added inductance). The higher impedance reduces the current. Reduced current
(at gap conduction) reduces I^2R losses. Reduced losses equates to better
efficiency. Better efficiency equates to longer spark lengths for the power

With larger cap values (regardless of pirmary turns): due to the increased
joules, there will be higher losses at the gap due to the increased current.
The inductance at the primary will either reduce or increase the losses for the
cap energy depending on how much inductance is used, but still, a larger cap
value will probably have higher losses than a smaller cap value used. I've run
20nF, 40nF, and 60nF with the same coil. Spark length did not increase
linearly. It did increase with increased cap energy, but at a fraction of the
spark length, and with less efficiency. 

A couple years ago, I added 12" of length to my secondary (was 34"L, now 45"L).
This caused more turns for the secondary and more turns on the primary. I
gained about 15" of spark length simply due to this change. Still, there are
gap losses. I later built a G10/tungsten RSG. This gap was much better than the
previous versions "because" I gained approximately 10" of spark! The gap was
more efficient and it was very clear to me that the gap itself made a
tremendous difference regardless of cap size or primary turns. To this day, I
realize that gap losses are significant and more primary turns helps reduce
these losses. 

But still, there are losses and the losses can increase with cap size. I think
it is possible to build a coil that can run a large cap size and be efficient,
but the coil must be designed to reduce losses at the gap. This means gap
design, primary turns high, low Fo, properely sized toroid, etc.. 

John Freau and Malcolm Watts are two coilers that have helped identify "why" my
coil reacted to the changes with the increased turns on the secondary. Ed
Wingate believed in pure tungsten electrodes and G10 for a rotary gap which I
built and I was amazed at the efficiency of the gap compared to the copper
electrodes or spinning pipes (remember that one?) I tried previous to this. The
previous gaps were poorly designed. Now, if I used copper electrodes with the
G-10 I would probably run just as efficient as I am now, but the tungsten life
is far superior. My previous copper electrode RSG used an alluminum flywheel

I've also run variable RSG speed (DC RSG), non-sync, and sync gaps. They all
have their good and bad points with the non-sync being the worse (IMHO). I use
a sync gap currently simply due to the ability to keep the gap at a steady
state for whatever test I am running at the time (it helps remove a variable).
I really liked the DC gap because it's easy to find a break rate that the
system enjoys, but for testing purposes, it throws in a factor that prevents
comparisons between setups. 

Take care, (sorry to ramble) 
> 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