Re: Off-axis tuning coil? (was: I Think I Messed up!)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

The resonant frequency is determined by the total inductance and
capacitance. An off axis inductor adds inductance (lowering the Fres) but
doesn't change the coupling between primary and secondary. Also, some of
the primary voltage will be developed across the second inductor, but, this
isn't all that important.  Current is what makes magnetic field, and
they're in series, so the current is the same in both inductors.

If you have the axis of the added coil the same direction as the axis of
the primary coil, then the field from the added coil will either add or
subtract from the field from the primary (depending on the direction of the
turns and how you've wired it.

The added inductor can be either a flat spiral or a standard helical (i.e.
a coil looking thing)

For size and formulae.. You presumably know what the resonant frequency of
your secondary is.  You also know what the inductance of your existing
primary, and the value of the capacitance is.

The basic thing you need to know is how much added inductance you need:


Ls*Cs = Lp*Cp
Ls*Cs = (Lp + La) * Cp

rearranging:  La = (Ls * Cs)/Cp = Lp


For formulae, you can use the standard things...

L = r^2 * N^2 / (9*r + 10 *L) , r = radius (inches), L=length (inches), L in uH

for what it's worth, a 3" diameter, 8 inch long coil with 15 turns is 0.36 uH

A pancake 6" to the center, with a 4" hole in the middle, and turns spaced
1/2"apart is around 30 uH...


Tesla list wrote:
> 
> Original poster: "Patrick Leonard by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <transactoid-at-rogers-dot-com>
> 
> Someone suggested I use an additional coil in series with my primary to
> increase the inductance. I've only ever seen one or two other coil like this.
> It had maybe only 4 or 5 turns, and used an additional coil of maybe 7 or 8
> turns from which a wire was used to tap the secondary. I think the builder
> called it an off-axis "tuning coil".  One of the coil's on
> <http://www.hvguy-dot-com>www.hvguy-dot-com also has such a "tuning coil".
> 
> How would I calculate the dimensions, or is this something I can just kind of
> guess at? Also, is it wound flat spiral like the primary or cylindrically?
> (that's how the above tuning coil was made).
> 
> Lastly, how does such an addition work? >From what I understand, the flat
turns
> of the primary generate a magnetic field which induces a current flow in the
> secondary. I don't see how a seperate coil, located somewhere else, not
in the
> "plane" of the secondary transfers any energy over?? Am I mis-understanding
> inductance?
> 
> Thanks,
> Patrick.