RE: caps

Hi Tom,

>I wonder if anyone can give me their opinion as to whether cap size is
>influenced by power through the system?I understand that the L/C of the
>tank circuit for tuning is all important but with the support of high
>inductance pri coils the capacitance is getting small (<.01uF). Is this
>advisable or is there a rule to follow?
>For example, a 15 turn pri by my calc is around 200uH.If my sec is to
>resonate at 200KHz then my cap is around .005uF!Have I missed something?

No, I don't believe you have missed a thing.  Cap size is influenced in that the
more capacitance you have, the more current capability you need from the
transformer.  It is better to have more transformer than you need than to have
more capacitance that it can provide current for.

Is there a rule to follow on maximum inductance / minimum capacitance?  I'm not
sure.  But I think common sense should prevail here.  You obviously don't want a
40 turn primary with a tiny capacitor, or vice-versa.  It would be nice to get
the primary to be at least 2/3 the diameter as the secondary is long.  You just
want to be in that ballpark.  Too many turns of a small conductor primary, and
you get more resistance, less surface area and a lower Q.  Too few turns of a
large conductor primary and your coupling suffers.  Is there a perfect mix of
capacitance and inductance in a Tesla tank circuit?  Probably not.  It is a
world of trade-offs.  Just don't get too far on one end of the spectrum and I
think you'll be fine.

As far as using a .005 uF cap, I use one exactly of this rating now.  It is in a
small coil system and works well.  However, I am resonating it at about 450-500
kHz.  At 200 kHz, .005 uF does seem a little small to me.

To resonate at 200 kHz with a .005 uF capacitor, my calculations tell me to use
about 127 uH of inductance.  I double checked this with a program and it gave
the same results.  With a 200 uH inductor, my calcs say it will take about .0032
uF to resonate at 200 kHz.  You might want to double check your calculations.


Scott Myers