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Ideal and Practical Voltage Source Explained

The voltage source supplies current to the electrical circuit. When load is connected across the voltage source, the voltage source supplies current to the load.
Ideal Voltage Source
An ideal voltage source is capable to maintain the constant voltage across its terminals. The voltage across the voltage source terminals remains constant and the voltage is independent of the current. The symbol of voltage source is as given below.
The voltage across the terminals of an ideal voltage source remains constant and the voltage does not drop with increase in the circuit current. The voltage across an ideal voltage source with change in the circuit current is as depicted below.

The voltage of an ideal voltage source remains constant if there does not happen voltage drop across the internal resistance of the voltage source. The voltage source has certain resistance which cause voltage drop across the internal resistance. An ideal voltage source must have zero internal resistance. In this condition, the voltage across the load will be equal to the voltage across the terminals of the voltage source.

Internal resistance of an Ideal Voltage  Source
An ideal voltage source is capable to maintain the constant voltage across its terminal from the no load to full load condition. An ideal source circcuit is as given below.
An ideal voltage source voltage across load(Vo) is equal to the voltage across the terminal voltage(Vt).

Thus the internal resistance of an ideal voltage source is zero. The source voltage is equal to the voltage across the load and no voltage drop takes place in the internal resistance because an ideal voltage source has zero internal resistance.

Practical Voltage Source
The internal resistance of an ideal voltage source is zero. However, practically the voltage source has some internal resistance. The symbol of the practical voltage source is as depicted below.
The internal resistance (r) of an ideal voltage source is zero. However, practically the voltage source has some internal resistance. The symbol of the practical voltage source is as depicted below.

The terminal voltage at the source when current is delivered to the load no more remains constant, and the voltage at the source terminal is somewhat less than the terminal voltage at no load condition. The practical voltage source at load condition is depicted as below.

The voltage across the load can be found by applying Kirchoff’s volatge law (KVL). According to KVL, the algebric sum of the voltages in a loop  is equal to zero.

The voltage across the load can be found by applying Kirchoff’s volatge law(KVL). According to KVL, the algebric sum of the voltages in a loop is equal to zero.

The voltage across the load can not be equal to the source voltage when current is delivered by the voltage source. The difference in the voltage across the load and the source voltage depends on the magnitude of the current flowing in the circuit. The voltage across the load is equal to the source voltage minus the voltage drop in internal resistance. The higher the circuit current, the more is the voltage drop in the internal resistanece and the voltage across load will get dropped accordingly. The same has been shown in the below graph.

The voltage across the load can not be equal to the source voltage when current is delivered by the voltage source. The difference in the voltage across the load and the source voltage depends on the magnitude of the current flowing in the circuit.