# Re: Tank Capacitance: what is the limit (Q)

```Subject:       Re: Tank Capacitance: what is the limit
Date:   Mon, 28 Apr 1997 16:29:46 +1200
From:   "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Organization:  Wellington Polytechnic, NZ
To:   tesla-at-pupman-dot-com

Hi all,
A comment on Fr McGahee's post:

> While it is generally true that the primary is more than one turn of
> wire,
> as the secondary becomes LARGER in DIAMETER, it is often advantageous to
> use larger capacitors and fewer turns in the primary.

I usually run at lower frequencies and keep primary Q up by keeping
Lp large. I hear things like "lower frequency implies lower Q" a lot.
That no doubt comes from observing that Q = X/R. Let's have a closer
look at that:
Q = (2 x PI x f x L)/R

and we know that f = 1/(2 x PI x SQRT(L x C))

Let's halve f in three ways, first by making C four times larger.
That leaves L unaffected which indeed causes Q to dive in half since
f has halved.
Now lets achieve the same thing by doubling L and doubling C. Q
is now the same (as it should be because the L/C ratio is preserved.
Finally, let's quadruple L. Despite f having halved, Q is
actually doubled.

With reference to the primary, we know that "R" is a non-linear
resistance. In fact I maintain it goes down as gap current goes up.
If the voltage across the gap when it is conducting is reasonably
constant, then gap losses double as the current is doubled. Despite
gap resistance doubling as current is halved, it loses less power
because it roughly follows a VxI dissipation law.

Malcolm

> That is just what
> Tesla did on many occasions. As you change one parameter, such as the
> coil
> diameter or the size of the tank capacitor, it is sometimes necessary to
> break some of the rules that NORMALLY work quite well. Never be afraid
> to
> experiment to find out what ACTUALLY works best in THIS particular
> setup.
> The common wisdom is often all too common and not much wisdom.
>
> In reading Tesla's Colorado Springs Notes I am kept contantly chuckling
> at
> the way he did things that EVEN went against his OWN recommendations (of
> just a page or two before!) You HAVE to experiment and improvise in this
> business. What at first appears to be an absolutely minor change can
> have
> an absolutely major effect on overall circuit performance. I have
> learned
> not to just blindly accept what others say is the BEST way of doing
> anything, but to experiment on my own and try and understand WHY some
> things work and some things don't *in this particular case*.
>
> Fr. Tom McGahee

>
>

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