Capacitor

A capacitor (or cap) stores energy in an electric field. There are a variety of types but all contain at least two electrical conductors separated by a dielectric. Basically the type of capacitor is named after the dielectric. Capacitors along with resistors and inductors are passive components.

Farads
The unit for capacitance is the Farad (F). In practice typical capacitor values range from a few pF (picofarad, 10-12 F), through nF (nanofarad, 10-9 F) to a few hundred μF (microfarads, 10-6 F). In a linear power supply the smoothing caps will be some tens thousand microfarads.

Conversion between pF, nF and µF
The same capacitor's value might be given in any of pF, nF or µF, e.g. 100nF is the same as 0.1µF. It is necesary to be able to convert easily between these units. For example the bill of materials (BOM) might give the value in one unit while an electronic component distributor may list it in another.

Voltage
The working voltage of a cap is the maximium voltage that it's rated for. As a rule of thumb choose at least the next available value above double the voltage the capacitor is likely to encounter. So for a 15 V DC supply select at least 35 V caps. Low value caps will be available with a working voltage of 50 or 100 V. For the same capacitance higher voltage caps are larger.

Circuit principles

 * $$\frac{1}{C_\mathrm{total}} = \frac{1}{C_1} + \frac{1}{C_2} + \cdots + \frac{1}{C_n}$$.

The total capacitance of capacitors in series is equal to the reciprocal of the sum of the reciprocal of their individual capacitances:


 * $$C_\mathrm{total} = C_1 + C_2 + \cdots + C_n$$.

The total capacitance of capacitors in parallel is equal to the sum of their individual capacitances: