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Re: [TCML] Primary selfC

Hi Gary,
I'll await Paul's reply also. I personally just looked at the ribbon coil as "similar" to a rolled capacitor given it's geometry both in conductor and shape. It may or may not be correct to look at it this way. The value surprised me, but the numbers came out as they did. My initial gut instinct told me the capacitance would be very low, the resonant frequency would be in Mhz, etc.. (like anyone else), but then I started thinking about basic variable air plate capacitors. This is similar. I may need to do a math check or have someone else check my math, but the numbers showed a somewhat significant capacitance. I'm not sure what to think of it either.
However, I am in the process of building a ribbon primary (cut the standoffs this week using 3/8" thick Garolite). Maybe this next week I can test it myself (assuming my honey-do list is short). 

Regards,
Bart

Lau, Gary wrote:

Hi Bart,

Until Paul can speak to my earlier question, I think you're subscribing to the same false intuition that I and many, many others apparently do.

For a secondary coil (where we are more accustomed to discussing Cself), the value of Cself is not affected by the number or turns, the spacing of the turns, the diameter of the wire, or the dielectric between the turns.  All that matters is the geometry of the overall cylinder that contains the coil.  Try it with your favorite simulator!  No doubt the same is true for a primary coil as well.  This is why Paul was suggesting modeling the primary as a free-standing disk - the shape of the flat spiral primary.

I whole-heartedly agree - it sure does seem that the relationship between turns would be a dominant factor in Cself, but apparently it is not.

Regards, Gary Lau
MA, USA


-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
Behalf Of bartb
Sent: Thursday, July 24, 2008 9:59 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Primary selfC

Got to thinking,

Didn't rolled caps have 2 metallic sheets rolled up? If so, then 1/2 the
value, so 3.2nF and 940kHz. Chris, you can verify by measuring Fres and
back calculating C from L.

Bart

bartb wrote:

Hi Paul, Chris,

It might be more accurate to look at the total length of the ribbon
and width to determine area and treat it as rolled capacitor separated
by 1mm, using air as the dielectric, and determine C as one would with
any flat plate capacitor, but in this case, as a "rolled capacitor".

C = 0.2248*k*A*(N-1)/(d*x)  (in pF)
   where
    k = Dielectric constant (1.0006 for air)
    A = Effective plate area in square inches (length x width)
    N = Number of conductive plates (3 for a rolled cap)
    d = individual dielectric film thickness (.03937" in this case)
    x = number of stacked sheets of dielectric between plates (1 in
this case)

I think Chris is looking at about 6.4nF for the ribbon primary. So
he's probably correct to ask the question given the very low tank
capacitance. Primary C may be much higher than expected (a good deal
of area at about 557 square inches). Should be able to verify by
inserting a signal to the primary as one would with a secondary and
looking in the range of 470kHz assuming inductance is about 18uH.

Regards,
Bart

Paul Nicholson wrote:

Chris Swinson wrote:


Does anyone know how to work out , or know of an online
calculator for crunching primary Self Capacitance ?

Does JavaTC not give the Cdc of the primary?  If not,
use JavaTC or FANTC to work out the self capacitance of a
disk.

Then, divide the Cdc by two to get an approximate effective
capacitance.
--
Paul Nicholson
--

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