Secondary Considerations :)

From:  Thomas McGahee [SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent:  Wednesday, January 28, 1998 3:02 PM
To:  tesla-at-pupman-dot-com
Cc:  cfed-at-dynasty-dot-net
Subject:  Secondary Considerations :)

This is in response to a private post, but I thought it might
hold some interest for other members of the Tesla list as well.
Fr. Tom McGahee
> From: Chris Federico <cfed-at-dynasty-dot-net>
> To: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
> Subject: Re: Two choices
> Date: Tuesday, January 27, 1998 8:59 PM
> Fr. Tom,
> While I'm not very technically astute as to the behavior of 
> electrons, and considering that my common sense has been somewhat 
> faulty regarding TC operation, it did strike me, while reading your 
> post, that wire length would necessarily be an important factor to 
> consider in building a coil.  As you stated, at the very least, sec' 
> coil height (with the diameter factored in) is a representation of 
> wire length.
> But I digress. . .
> > But since they are not 'loaded' they do not really affect the rest
> > of the circuit very much
> Funny you should mention "loads" regarding the primary coil.  In my 
> varying ponderances of how to best achieve resonance in my TC, I was 
> considering the fact that the sec's top load size  is inversely 
> proportional to its resonant frequency.  However, I have been unable, 
> thusfar, to successfully extrapolate that fact to the problem of 
> lowering the primary resonant freq.  
> I seem to be stuck at a lack of understanding as to how exactly the 
> top-load effects the res. freq., and whether or not it would be 
> possible to add another load to the primary circuit other than the 
> self-inductance of the windings, and the tank caps.  
> This line of thought has also led me to question how self-inductance 
> operates, and wether or not it is possible to layer the primary 
> windings in some manner (e.g. two 15" coils instead of one 30").  
> And, again, not being all that smart about electrons and the fields 
> they can create, I don't know if such a scheme would be beneficial, 
> or detrimental. . .that "common sense" thing failing me again:).
> Any light you can shed on these questions will be greatly appreciated 
> (even if it's just to say "That's silly!!, now go build that new 
> primary!!" :)
> Thank You,  Chris

The Tesla coil will 'resonate' when the resonant frequency of the
primary exactly matches the resonant frequency of the secondary.

You can 'tune' the primary by changing the main cap value and by
changing the primary inductance (tapping the primary). Less known
and less used is the fact that you can ALSO have another inductance
in series with the primary. Tesla used this method quite often,
and it is clearly shown in the Colorado Spring Notes. The problem
with adding this 'off axis' inductance is that while it does in fact
change the resonant frequency, it also represents a power loss as
the resultant magnetic field is NOT coupled to the secondary. It
is useful strictly as a TUNING aid. 

The primary can be constructed in MANY ways. The flat spiral primary
is one of the simplest and most elegant styles. Solonoid styles and
rising spirals are also used. Sometimes it is the visual aesthetics
that will determine which kind is 'best' for a given coiler. 

In general, inductance will be increased with a) increasing NUMBER of 
turns   b) Decreasing space between turns   c) increasing Diameter
of turns.  With a solonoid type primary the diameter and spacing
are held constant. Doubling the number of turns doubles the inductance.
With a spiral type primary the diameter is constantly growing, so
with each tap OUTWARD you get an increase in inductance that is
greater with each succeeding turn. This is usually good news.
In general larger inductances result in less problems with quenching,
so many coilers like to use primaries with 13 or 15 turns.
All other things being equal, it is better to use many turns
rather than just a few turns. For one thing, tuning is less
'critical'. This is not to say that you couldn't make a very decent
Tesla coil with just a one-turn primary. It has been done. But
in the sizes and frequencies and power levels that most coilers
deal with, more primary turns is generally better. Experiment!!!
One genuine experiment is worth all the cogitating in the world.
Think. But then check out your thinking with the facts by actually

Many of us here on the Tesla list have at one time or another done
something that everbody else told us was not a good idea, only
to discover that sometimes they were right, and sometimes they were
wrong. Heck, some of us experiment with things that we know won't
work very well, simply because we want to understand exactly
WHY they don't work so well. The real joy is not in having the 
finished Tesla coil hurling sparks around. It is the wonderful
anticipation we experience as we build and experiment and tinker
and learn and apply that new knowledge and come up with an even
better design and then build *that* and throw the switch and
cross our fingers and hope that we have got it right and we can 
squeeze another inch or two out of our beloved bolt hurlers.
If I couldn't improve any one of my designs in any way, I think I
would probably be very disappointed and move on to something else.
I'm sure I am not alone in feeling this way.

Now for the secondary.

The secondary coil has an inductance that a)increases with more turns,
b)increases with decreasing spacing between windings, and c)increases
with diameter.

For a given Tesla coil these factors are pretty well fixed in place
when you wind the coil. However, people like John Freau and others
have experiemnted with coils in which the secondary consists of 
sections that can be stacked together. This allows you to experiment
with different lengths without having to wind a complete new coil.
(Note that the avid experimenter will always find interesting ways
to do as many experiments as possible with the least amount of
un-neccesary work. They are characterized as being people with lots
of strange parts stashed away, and bits and pieces of previous
experiments littering every available space. Some are neater than
others, but whether their junk is neatly stashed or not, they always
have an interesting assortment of it.)

Others have experimented with secondary windings that are non-linear
and found that such coils have different characteristics depending
on which end becomes the base! Terry Fritz comes to mind. He build 
a special winder using stepper motors so that he could wind truly 
weird coils. And then he shared them with Dr. Rzeszotarsky, who
did some more experiments with them and shared the results with
all of us on this list. There are still some surprises out there
for all of us. Finding them is half the fun!

Every secondary has a Cself (Self Capacitance) that makes it want to
resonate at a particular frequency. You have to match the primary
resonant frequency to this frequency to get Tesla coil operation.
Adding a topload increases the capacitance aspect of the secondary
resonator system, thus lowering the resonant frequency. 

When you lower the frequency of the secondary resonance system, you 
must match the frequency of the primary resonance system to it
or you don't really have a Tesla coil. 

Now, add to this the fact that the maximum output voltage attainable
is a function of the ratio of the inductances (or the inverse ratio
of the capacitances), and you can see that we are involved in an
intricate Dance in which changing *one* component can have several
different *effects*. The key is to try to keep the system always 
'in tune' while making changes, and to maximize the good results
while minimizing the bad results.

For example, if you made a larger diameter torroid but with thinner
diameter heating duct, then you might increase capacitance while
actually decreasing the breakout voltage point. Making the heating
duct diameter bigger and the overall diameter smaller might actually
increase breakout voltage while reducing capacitance. That is why
in experimenting it is often useful to vary only *one* parameter at
a time if at all possible. Thus, using the same diameter heating
duct but with a greater size 'circle' would increase the capacitance
while keeping the breakout point about the same. I say 'about the same'
because the e-field shape *is* influenced by the relationship of
the relative size of the heating duct and the circular diameter. It
is also affected by other subtle things such as the height above the
top of the secondary, the height above the floor, the *kind* of
floor you have, the very geometry of the room it is in,
the temperature, the humidity, and a thousand and one other variables.

Sometimes we are lucky (or have done our homework right), and manage 
to hit on just the right combination. We have found the 'sweet spot',
that peculiar serendipity that makes our coil perform splendidly.
Making a seemingly *innocuous* change in the mix of parts can
sometimes have a tremendous influence on its behavior. Sometimes it
can be absolutely maddening as you try to find out what the heck
went wrong. And the 'better' your system, the higher its 'Q', the
more finicky it is about such little changes.

No one can give us a recipe for the perfect Tesla coil. All we
can do is try to understand as many of the nuances as possible, and
apply this knowledge as best we can. But always be prepared for the
totally unexpected. It is from these moments, perhaps, that we learn
the most of all. It is humbling to realize that we don't know
everything. But it is great to know that every day we have the 
wonderful and exciting opportunity to learn something new. I would
not have it any other way. The hunt is often much more exciting
than actually capturing the little fox. It is nice to know that
tomorrow when I wake up I will have something worth doing, 
something still to be discovered and investigated and delighted in,
another Tesla list post to answer, another question to ponder,
another experiment to be performed. Ah! I love it!!!

Hope this helps,
Fr. Tom McGahee