Primary Design

 * Original msg to: I_hopley-at-wintermute.co.uk
 * Carbons sent to: usa-tesla-at-usa-dot-net

Quoting Ian Hopley <i_hopley-at-WINTERMUTE.CO.UK>:

> I read in some notes written by Jerry Gore of TCBA that the 
> best approach to primary design was first to determine the HV 
> transformer's impedence then match the capacitance so that its
> impedence is equal at the line frequency, i.e. maximum power 
> transfer. This seems to make sense to me but i dont recall any
> other mention in other postings so I was wondering about the 
> relavance of it. Would there be any gain in working out the 
> primary details this way?

Please accept my apologies for getting this out at such a late
date. I am only now finding "free" time since I moved, and I am
still not settled, nor firing as of yet...

Your question is a good one, and deserving of some expansion.

This school of design theory is stating that the best Tesla coils
are achieved by matching the primary capacitor to the transformer
and then working out the rest of the coil parameters from this
starting point. But really, does this make sense? Let's say you
start out with a simple neon sign transformer or two. You match
your capacitance according to your calculation, then design the
rest of the coil system. Three months later you decide you want
to pour in more input power, so you double up on your neons...
and the whole coil design goes out the window? Humm... 

Matching the power supply to the capacitor is important, your
coil won't peak out unless you have a pretty decent match, but I
would not agree that this is a good starting point. The xfmr/cap
match is something that you want to check from time to time as
the design process advances, but I don't think you want design
the entire coil around it. Following this school of thought
limits the system flexibility.

I typically start my design process by determining how big I want
my coil to be and how much power I intend to process. For most
beginner/intermediate coilers operating in the range from several
hundred watts to several kilowatts, I recommend starting with a
six inch diameter coil form. Now if you follow my "rule of thumb"
guidelines (experimentally derived and repeatedly proven) for the
construction of 1/4 wave Tesla resonators [Tesla secondary coils]
you will end up in the ballpark of around 280 - 290 kHz as the
resonate frequency of the bare coil. My "rules of thumb" demand
the resonator be topped with a substantial toroid discharge
terminal, usually sufficient to reduce the resonant frequency of
the bare coil by around 25% (for starters). You are going to be
operating in the 200 - 220 kHz range. A "20 x 4" (inches) or so 
toroid should be adequate for the task, though you can start out
at lower power with a smaller discharger.

Now you need to design your primary. It needs to be big and it
needs to have lots of turns. Plan on allowing fifteen or so full
turns for tuning and power level flexibility. I like copper
tubing, someone else recently posted about the availability of
strap: use what you can get cheap unless cost is not an object
(or you are a perfectionist). Corona loss from the edges of
copper strap primaries can be reduced with strips of electrical
tape, beads of epoxy, or polyurethane varnish applied with a
small paint brush to the sharp edges.

I still get lots of questions regarding the angle on inclination
or type of primary: flat pancake, "saucer" (inverted conical
section) deep or shallow, or vertical helix. There are lots of
projects underway, and each project is going to have different
primary requirements that depend on the anticipated input power.

If you have one of the new capacitors just shipped by Scott Myers
then you are looking at an eight inch coil most likely with a
flat pancake or very shallow saucer primary. These caps are built
for shock excitation and the peak powers will be high. A primary
that couples too closely will cause problems (that are cured by
simply rasing the secondary coil up some). If you are looking at
a six inch diameter secondary coil powered by neon or similar
transformers and homemade or adapted capacitors, you will need
closer coupling to run efficiently at your lower powers. The hard
line here is that you can always loosen the coupling up by rasing
the secondary coil in relation to the primary, but coils that
couple to loosely may need to be re-worked to regain the
performance lost in the design. I try to set the primary/
secondary coils in relation so that the first turn of the primary
is on the same level with the first turn of the secondary. The
inside turn of the primary should be about three inches greater
in diameter than the outside diameter of the secondary; this
allows a 1-1/2 inch space between the two coils at the base of
the secondary to route your RF ground strap and insulate against
flashovers between coils.

The beauty of this method is the amount of flexibility that is
built into the system. To operate at various power levels you can
change the toroid and re-tune the primary against a fixed
capacitor value. Rather than having a single transformer or power
supply, I generally keep a few power supplies kicking around with
different voltage ratings. This way you can step or scale your
power levels up and down for various experiments/demonstrations
and be able to tune and peak the coil perfectly with only a few
minor adjustments. 

Richard Quick

... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12