Capacitor

From Electronics Wiki

A capacitor is a device with the ability to store an electric charge. Capacitance is usually measured in farads, which are usually denoted with an F. Capacitors with smaller values (<10µF) are mainly used in AC circuits due to their abilities to pass AC while removing DC voltages and to show a different impedance to different frequencies of AC. Larger capacitors are mainly used in power supply circuits to remove ripple from a DC output or in circuits that require a large charge, eg. camera flashes.

Theory

An ideal capacitor is described by this equation:

Where C is the capacitor's value in farads, Q is the capacitor's charge in coulombs, and V is the voltage over the capacitor.

Capacitors put in series are described by the equation:

Where Ct is the total capacitance of the network and C1...Cn are the values of the individual capacitors.

Capacitors put in parallel are described by the equation:

Where Ct is the total capacitance of the network and C1...Cn are the values of the individual capacitors.

The energy stored in a capacitor may be found by measuring the voltage, and is given by

where E is the energy in joules, C is the capacitance, and V is the voltage.

how to select capacitors

No capacitor is ideal; therefore there are parasitic properties to account for, when designing real circuits.

• equivalent series resistance (ESR);
• maximum charge voltage;
• maximum ripple current;
• polarized versus unpolarized?
  • electrolytic capacitors are polarized and generally have a large capacitance, and relatively large ESR;
  • ceramic capacitors are not polarized, and generally have a small capacitance

exceeding the maximum ratings may harm the device, leading to failures such as shorts, or to reduced capacitance or lifetime.