Practical Voltage Source

An ideal voltage source means a device that offer a terminal voltage which doesn’t depend on the current flow through it. But such ideal sources never exist practically. Suppose a battery would produce 12V when no load is connected or no current flows through it, then the battery produce a less voltage than 12V when a load current flows through it. These sources are called as practical voltage sources. However, as long as small power or currents drawn from the load, these practical sources represent the ideal voltage sources.

             V-I characteristics of practical voltage source

Therefore, the real model of device (practical voltage source) is obtained by an ideal voltage source with a series resistor. This series resistor resembles the voltage drop in the device when the current flows through it. The series resistor is called as the internal resistance of the voltage source. This does not mean that we can find such arrangement inside every practical source device but merely represents the reduction in terminal voltage on an account for an increase of load current flow.

                    Practical voltage source with internal resistance