What is Voltage Source and Current Source? – Ideal & Practical

In this article, we shall discuss voltage sources and current sources, independent sources, dependent sources, and ideal & practical sources.

A source or energy source is an electric circuit element that provides the required excitation to the circuit. It has the ability to convert one form of energy like mechanical energy, chemical energy, thermal energy, etc. into electrical energy. The energy source is an active circuit element because it supplies electrical energy to the circuit.

In electrical and electronics engineering, there are two types of energy sources namely voltage source and current source. These sources may be ideal or practical and dependent or independent.

Ideal Sources

(1). Ideal Voltage Source:

A voltage source that has zero internal impedance and a constant terminal voltage is known as an ideal voltage source. Thus, the terminal voltage of an ideal voltage source is independent of the current flowing through it. The circuit symbol of the ideal voltage source is shown in figure (1).

By the definition, an ideal voltage source can supply any amount of electric current, i.e. unlimited amount of electric power. Therefore, the ideal voltage sources do not exist in the real world. In practical applications, a physical voltage source can be approximated as an ideal voltage source as long as the current drawn from it is very small compared to its terminal voltage.

Also, we cannot short-circuit an ideal voltage source because if its terminals are short-circuited, the voltage across its terminals would be zero which is contrary to its definition.

(2). Ideal Current Source:

An energy source that supplies a current of constant magnitude irrespective of the voltage across its terminals is called an ideal current source. An ideal current source has infinite internal impedance. The circuit symbol of an ideal current source is shown in figure (2).

According to the definition, an ideal current source can supply an unlimited amount of electric power which is practically not feasible. Thus, ideal current sources do not exist in actual practice. Although, we may approximate a practical current source as an ideal current source if the voltage across its terminals is very small compared to its current.

Also, an ideal current source cannot be open-circuited because when it is open-circuited, i.e. I = 0, which is contrary to its definition.

Practical Sources

(1). Practical Voltage Source:

A practical voltage source is one that has a finite internal impedance. The practical voltage source is represented by an ideal voltage source in series a resistance R_i as shown in figure (3). Where the resistance R_i is the internal resistance (or impedance) of the practical voltage source.

This internal resistance (R_i) accounts for the drop in voltage across the terminals of the voltage source when the current through it increases. Examples of practical voltage sources are cells, batteries, generators, etc.

(2). Practical Current Source:

A practical current source is one that has a finite internal resistance (or impedance). A practical current source is usually represented by an ideal current source in parallel with a resistance R_i as shown in figure (4).

The resistance (R_i) is called the internal resistance of the current source. This internal resistance accounts for the decrease in the current supplied by the source when the voltage across its terminals increases. Examples of practical current sources are semiconductor devices like transistors, Op-amps, diodes, etc.

Dependent Energy Sources

Energy sources whose magnitude of the voltage or current changes for any change in the connected network are known as dependent energy sources. They are also known as controlled sources. Therefore, the output voltage or current of dependent sources depends on the voltage or current of a certain element of the network to which they are connected.

Dependent sources are usually represented by diamond-shaped symbols as shown in figure (5). Since, the control of dependent sources is achieved by a voltage or current of some other element in the circuit, and the sources can be voltage or current. Therefore, there are four possible types of dependent energy sources as follows-

• Voltage-Dependent Voltage Source (VDVS)
• Current-Dependent Voltage Source (CDVS)
• Voltage-Dependent Current Source (VDCS)
• Current-Dependent Current Source (CDCS)

Dependent energy sources are very useful in modelling electronic elements like transistors, op-amps, integrated circuits, etc.

Key Points about Energy Sources

The following are the crucial points about energy sources-

• Energy sources are active circuit elements because they are capable of generating energy.
• An independent energy source is one whose voltage or current does not depend on any other voltage or current in the circuit.
• A dependent energy source is one whose voltage or current depends on any other voltage or current in the circuit.
• A current source is represented by a circle with an arrow inside it. The direction of the arrow indicates the direction of the current through the source.
• A voltage source is represented by a circle with polarities (+ -) inside it.
• Energy sources not only supply power to a circuit, but they can also absorb power from a circuit.
• We can convert a practical independent voltage source into a practical independent current source and vice-versa.

Conclusion

Thus, in this article, we discussed different types of energy sources, i.e. voltage sources, current sources, etc. along with their sub-classification like ideal and practical sources, and independent and dependent sources.