DC Voltage and Current Sources

DC voltage source

The term DC is used to refer to power systems that use only one polarity of voltage or current, and to refer to the constant, zero-frequency, or slowly varying local mean value of a voltage or current. For example, the voltage across a DC voltage source isconstant as is the current through a DC current source.

Practical DC voltage source

A practical voltage source can be considered to consist of an ideal source in series with a resistance.This is 'internal resistance' of the source.In case of ac voltage source it is internal impedance.