Re: Fixing my primary mistake (was: I Think I Messd up!)

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Patrick,

>
>At 10:59 PM 12/3/2001 -0600, you wrote: 
>Someone suggested I use an additional coil in series with my primary to 
>increase the inductance. I've only ever seen one 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 a "tuning coil"

It is just an added coil in series with the "primary".  These extra tuning
coils are sometimes used on real fancy looking coils that do not have the
visible primary tapped.  The primary is just a coil going straight in and
out of the base.  Tuning is done on a hidden coil inside the base.  You can
even have the tuning coil under the primary and rotate it for tuning.
Depending on the angle, it will add or subtract inductance from the
primary.  Thus, you just have a big tuning knob rather than taps.  This
does require more room and some thought.  You could also "squeeze" or
"stretch" the coil for tuning since the inductance varies with the coil's
length.  Some people use coils that have a wiper on them that slides around
the coil for tuning but the high current usually burns up the sliding
contact.  If one wants a no tap tuner.  The squeezing/stretching coil thing
is probably the best using well insulated wire.  One does have to get the
thing designed just right so it will always be within the needed tuning
range.  Adjustable coils like this are rarely used but they do work fine.

> 
>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).

You can make the coil anyway you like.  The equations for inductance are at:

http://home.earthlink-dot-net/~electronxlc/formulas.html

Pick the equation for the coil you have.  One could almost guess at the
dimensions but I think you already got bit once from the guessing stuff ;-))

> 
>Lastly, how does such at 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?

The extra coil probably will not couple energy to the secondary.  That is
no problem.  You present primary will do that fine.  The extra coil is just
changing the overall inductance and thus lowering the primary system
frequency to match your secondary.  Adding inductors in series simply adds
their inductance.  So two 10uH coils in series is 20uH.  You can actually
put the extra coil "anywhere" in the primary circuit.  Next to the gap,
between the gap and the primary,...  The placement in the circuit does not
matter as long as it is in a safe location given the voltage on it.

Cheers,

	Terry

> 
>Thanks,
>Patrick.
>