Re: Picofarad Capacitor Standard

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

Original poster: Bart Anderson <classi6-at-classictesla-dot-com> 

John,

I have a 1nF BMMC (baby mmc) array measuring 1.04nF which you are welcome 
to use for your research. I'm not sure this value is low enough for you, 
but I think you would "feel" a little more comfortable knowing the calc's 
match the measurement within tolerance. It's simply a 4x4 array using high 
voltage poly-pro film/foil construction (Cornell Dubilier DPPM12D1K). 
Single cap value is also 1nF. I built this for a tiny 0.375" x 3" 
secondary. The array is mounted on a 3 x 6 pcb w/ hv 18 awg leads (roughly 
20" length). The array is designed for 5000V, so that could be a limiting 
factor.

If something like a 20nF cap would suit you, I can send you a 60kV CSI 
prof. pulse cap. Dimensions are roughly 3" W x 16.5"L. This cap has a 
0.375" brass lug at each end. The cap has an arrow pointing to one end 
(mount with this end up [oil distribution]).

Your welcome to use either for the toroid research. If interested, just let 
me know and I'll ship out.

Take care,
Bart

Tesla list wrote:

>Original poster: "John H. Couture" <couturejh-at-mgte-dot-com>
>
>Paul N, Antonio, All -
>
>I have attempted to make a homemade capacitor with a reasonably exact
>capacitance value at the low picofarad level for toroid research. This
>capacitor consists of two flat metal plates at a certain distance from one
>another. The main problem of this type of capacitor at low pf capacities
>would be the edge effects. I have tested the capacitor and developed three
>curves as shown below to find where they intersect when varying the distance
>between the plates. I believe the point of intersection of the curves is
>where the edge effects are eliminated? In other words the plates at .25
>inches giving 40 pf would be the real world capacitance of the two plates
>with the elimination of edge effects.
>
>The theoretical equation for two metal plates at a distance d apart in air
>is
>
>        Picofarads = 8.85 (area sq meters)/(d meters)
>
>        1 inch     = 39.37 inches
>        1 sq meter = 39.37^2 = 1550 sq inches
>
>        Picofards = (39.37/1550) sq inches/inches
>            "     = .225 sq inches/inches
>            10 pf = .225 sq inches/1
>        Area sq inches = 10/.225 = 44.4 (6"x7.42")
>
>   pf1 - Theoretical capacity    y = 10 X^(-1)
>   pf2 - Beckman DM25L           y = 21.3 X^(-0.45)
>   pf3 - Wavetek DM27XT          y = 15.6 X^(-623)
>
>The equations above are power regression equations obtained from Excel. Note
>that the 3 curves intersect near the d = 0.250 inches point. You can make a
>graph that illustrates this condition by entering the values into your Excel
>program and using the Chart Wizard.
>
>       d inches   pf1    pf2     Pf3
>        0.125      80     56      55
>        0.250      40     40      38
>        0.500      20     28      25
>        0.750      13.3   23      19
>        1.000      10     21      15
>        1.500       6.7   19      12
>
>The values for pf2 and pf3 were adjusted for leads, etc. The two meters are
>rated +/- 2%+4 for the 2nf range. The results above show that the meters
>only agree with each other and with the theoretical capacitor at the point
>of intersection
>(d = .25"). This is also the point where the capacitor edge effects are
>eliminated?
>
>I would be interested in all comments.
>
>John Couture
>
>------------------------------------
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