Editing Direct current (section)

==Various definitions==
[[File:Current rectification diagram.svg|right|250px|thumb|Types of direct current]] 

The term ''DC'' is used to refer to power systems that use only one [[electrical polarity]] of voltage or current, and to refer to the constant, zero-frequency, or slowly varying local mean value of a voltage or current.<ref>{{cite book | title = Newnes Dictionary of Electronic | author = Roger S. Amos, Geoffrey William Arnold Dummer | publisher = Newnes |edition=4th| year = 1999 | isbn = 0-7506-4331-5 |page=83| url = https://books.google.com/books?id=c4qHqtC9JkgC&q=dc+zero-frequency&pg=PA83 }}</ref> For example, the voltage across a DC [[voltage source]] is constant as is the current through a direct [[current source]]. The DC solution of an [[electric circuit]] is the solution where all voltages and currents are constant. Any [[stationary process|stationary]] voltage or current waveform can be decomposed into a sum of a [[DC component]] and a zero-mean time-varying AC component; the DC component is defined to be the expected value, or the average value of the voltage or current over all time.

Although DC stands for "direct current", DC often refers to "constant polarity". Under this definition, DC voltages can vary in time, as seen in the raw output of a rectifier or the fluctuating voice signal on a telephone line.

Some forms of DC (such as that produced by a [[voltage regulator]]) have almost no variations in [[voltage]], but may still have variations in output [[electric power|power]] and current.