# help designing capacitors

Discussion in 'General Electronics Chat' started by lokeycmos, May 13, 2012.

1. ### lokeycmos Thread Starter Active Member

Apr 3, 2009
432
7
i cant afford to buy capacitors for my tesla coil, so im going to try my hand at making my own from sheets of glass and tinfoil. i have several sheets of 8.5"x11"glass. i only need it to be 8.6nF. what im wondering is if there are some formulas that will help me at least get a ball park figure based on area of the glass and area of the tin foil? any help would be appreciated! BTW, ive already tried the salt water bucket cap, it actually shattered while testing it.

2. ### kubeek AAC Fanatic!

Sep 20, 2005
4,691
806
C=ε.A/d that is permitivity times area divided by distance. Assuming you have 4mm glass with ε=5 one cap will have 668pF.
Dont forget about creepage distance around the edge of the glass sheet, you need to either calculate it for your voltage or use the cap submerged in oil or something.
http://www.daycounter.com/Calculators/Plate-Capacitor-Calculator.phtml

3. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Capacitance meters are pretty common, so you could always multilayer and measure. As a guess you will want some very thin glass, as this will increase capacitance (and decrease voltage rating).

4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Your equation is OK if you use the actual permittivity of the glass, but your example used ε=5, which is relative permittivity. The equation is
C= K*Eo*A/D, where Eo= 8.854x10-12
The calculator includes the permittivity of vacuum, ε0= 8.85 × 10−12, and K, which is the dielectric constant (relative permittivity)..