This is entry #2 of my 52 Project 2012: Foundational Electronic Components Crash Course series.
What is a capacitor?
A capacitor another very basic type of component. A capacitor is a device used to store energy, kind of like a battery but on a much, much smaller scale.
How does a capacitor work?
Capacitors have two terminals, like resistors. But instead of impeding the flow of energy like resistors do, capacitors are built to store energy, and—when necessary— to release that energy. Although this sounds fundamentally similar to the way a battery works, there is an important difference: batteries can generate their own charge between the two terminals through a chemical reaction, while capacitors can only store and release charge fed in from other sources.
A capacitor is made up of two thin metal plates separated by a non-conductive substance called a dielectric . The proximity and size of the metal plates type of dielectric used control how much charge the capacitor stores. Different kinds of dielectric also exhibit unique electrical properties in different circumstances, depending on the type and/or frequency (in the case of AC) of current running through the circuit.
A capacitor gathers charge until the stored charge reaches the voltage being used to charge it. Electrons flow through the circuit at a slower and slower rate until the charge is reached, at which time no more current flows. A capacitor connected straight across a battery will not hurt either the battery or the capacitor, because as soon as the capacitor is charged, the current stops flowing, and the battery will not use any more power.
In face, it is very common to see capacitors connected like this, from a higher voltage source straight to ground, or the negative terminal on a battery. Since capacitors take some time to charge, this smooths out the effects of power-on and power-off. The quick charge/discharge ability can also filter noise out of a power line, or to help in the conversion process from AC to DC.
Capacitance is measured in farads, or more typically in microfarads, nanofarads, and picofarads. These smaller units are used because the full farad is a much larger capacitance rating than most common parts use—by a whole lot. 1 farad is defined as the capacitance to store one coulomb of charge at 1 volt. (A coulomb is defined as the quantity of electrons carrying 1 amp of current for one second, which works out to be 6.25 * 1018 electrons.) Common small capacitors have ratings at least five or six orders of magnitude smaller than this; 0.1 microfarads, 22 picofarads, etc.
Capacitors also have a maximum voltage rating, measured simply in volts; charging the capacitor with a voltage greater than this may damage it.
Finally, capacitors have a tolerance rating, specified as a percentage. The tolerance indicates by how much the actual performance may differ from the rated performance.
For a description of this 52 Project series and links to the all existing entries, go to the 52 Project 2012: Foundational Electronic Components Crash Course page.