A two terminal device which can maintain fixed voltage is called a voltage source. In case of constant voltage source the output voltage is independent of the load resistance. The output current also does not affect the output voltage of the constant voltage source. Battery, generators acts as very good voltage source. So the constant voltage source can be defined as the voltage source which maintains same voltage level across the terminal (input) irrespective of the current flowing from or into the terminals. In a circuit which has a constant voltage source we get constant voltage value across any resistor which is connected. The current value will vary from resistor to resistor. Let us discuss with an example.
There is a battery voltage source of 10 Volts. If we connect a resistor of 1 ohm or 10 ohm the voltage across them will be the same. Both the voltages will be equal to 10 V. The current will be different if we change the resistance value. Following ohms law V = I/R we get 1 A current for 10 ohm resistor while we get 10 A current for resistor of 1 ohm. Constant or ideal voltage source is also referred to as independent voltage source.
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