Tank Circuit Design
Subject: Tank Circuit Design
From: richard.quick-at-slug-dot-org (Richard Quick)
Date: Mon, 2 Oct 1995 08:13:00 GMT
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Quoting Scott Myers <102505.61-at-compuserve-dot-com>:
> if I go with a .5 mF capacitor, I get less turns. If I go
> with .1 mF, even less turns.
> There seems to be a tradeoff made with capacitor size. Smaller
> capacitor, more turns and more coupling. But what is lost by going
> with a smaller capacitor?
Nothing is lost really. You would think there is a tradeoff, but in
reality there is not. A larger capacitor will produce a bigger pulse
(more amps), but the smaller primary applies this larger pulse with
> Can't I go with greater conductor spacing to increase coupling if I
> use a larger capacitor?
At this point I will have to refer back to Mark Barton's original state-
ment, which I agree with 100%, that the higher the inductance in the
primary coil, (within reason) the higher the tank circuit Q. So even
if you were able to produce a more dispersed field flux by spacing the
turns widely, you can't escape the fact that a larger primary coil with
more inductance ends up being more efficient.
There are very good reasons for increases in tank circuit Q when
using larger primaries. One important reason has to do with the fact
that the tank circuit has self-inductance. The capacitors have in-
ductance, the gaps have inductance, and the wiring has inductance.
Most of this inductance is what is referred to as "off-axis" inductance.
Off-axis inductance produces magnetic fields which do not couple energy
into the secondary coil.
Lets say you have a primary made from a short conductor length and this
coil has an inductance of 5 micro-Henries. Lets say the rest of tank
circuit has a self-inductance of 2.5 micro-Henries. In this example 50%
of the magnetic field produced by the tank circuit is off-axis and cannot
really couple in the secondary coil.
Now let's say that you replace the small 5 uH primary with a very large
primary that has 16 turns available. Suppose you tune in at 14 turns
with a .025 microfarad cap. At 14 or so turns it is likely this primary
coil will have over 50 micro-Henries of inductance. With the rest of the
tank circuit inductance remaining at 2.5 micro-Henries, the amount of off-
axis inductance has dropped to 5% from 50% in the example above. This is
a very significant difference.
In order to say that your tank circuit and coupling are efficient, you
should have at least 90% of the tank circuit inductance in the primary
coil. If you have less than 80% of the tank circuit inductance in the
primary coil then you are robbing yourself. One easy way to keep 90% of
the tank circuit inductance in the primary is to use a smaller cap and
a larger primary coil. Also, the percentage ratio of inductance keeps
rising as larger and larger toroids are placed on the secondary coil and
more and more turns are used on the primary to regain the system tune.
... If all else fails... Throw another megavolt across it!
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