The equation for the capacitance of a plate capacitor is...
C = E0 * Er * A / D
C = Capacitance in Farads
E0 = Permittivity of free space = 8.854 pF / m
Er = Relative permittivity of dielectric
A = Area of plates in square meters
D = The distance between the plates in meters
To increase capacitance, you can do any of the following...
- Increase the area (use more foil)
- Decrease the distance (use a thinner insulator)
- Use a dielectric material with higher relative permittivity (Er)
- Use a wet dielectric
Theoretical capacitance of a 100mm x 200mm capacitor made from various materials.
- Seran Wrap (polyethylene): E0 x 2.3 100mm x 200mm / 12.7um = 32.1nF
- Wax paper: E0 x 2.5 100mm x 200mm / 25um = 17.7nF
- 20GSM Printer paper: E0 x 2.3 x 100mm x 200mm / 100um = 4.1nF
Its typically not possible (especially with a homemade capacitor) to make perfect contact between the foil and a solid dielectric material. This will have the effect of increasing D as well as reducing the effective Er. So, the real capacitance will probably be less than calculated above.
USE A WET DIELECTRIC
Using a wet dielectric allows the dielectric to make contact with the aluminum foil without any gaps due to surface roughness. Also, there are many readily available non-conductive liquids that have very high Er values.
Glycerin is a good choice. It has Er = 47-68, its non-toxic, it doesn't evaporate, and its readily available in places like Walmart in either the hand lotion or pharmacy sections. To use it, impregnate a thin piece of paper with the glycerin and then place the paper between your foil as usual.
- 20GSM Printer paper + glycerine: E0 x 47 x 100mm x 200mm / 100um = 83nF
- 10GSM Tissue paper + glycerine: E0 x 47 x 100mm x 200mm / 50um = 166nF
- 10GSM Tissue paper + glycerine: E0 x 47 x 100mm x 1m / 50um = 832nF
- 10GSM Tissue paper + glycerine: E0 x 47 x 100mm x 10m / 50um = 8.32uF
So, if you use glycerine impregnated tissue paper and make the foil longer (>1m) you can get values approaching 1uF or more.