Direct Current or DC is a type of flow of electric charge which is only in one direction.
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Direct current (DC) is a flow of electrical charge carriers that always takes place in the same direction. The current need not always have the same magnitude, but if it is to be defined as dc, the direction of the charge carrier flow must never reverse. This contrasts with alternating current which varies the direction of flow.
Sources of direct current include power supplies, electrochemical cells and batteries, and photovoltaic cells and panels. The intensity, or amplitude, of a direct current might fluctuate with time, and this fluctuation might be periodic. In some such cases the dc has an ac component superimposed on it. An example of this is the output of a photovoltaic cell that receives a modulated light communications signal. A source of dc is sometimes called a dc generator.
Batteries and various other sources of dc produce a constant voltage. This is called pure dc and can be represented by a straight, horizontal line on a graph of voltage versus time. The peak and effective values are the same. The peak to peak value is zero because the instantaneous amplitude never changes. In some instances the value of a dc voltage pulsates or oscillates rapidly with time, in a manner similar to the changes in an ac wave. The unfiltered output of a half wave or a full wave rectifier, for example, is pulsating dc.