Voltage and Current Sources of Energy | Electrical Engineering

In this article we will discuss about the independent, dependent and ideal sources of energy.

Independent and Dependent (or Controlled) Sources of Energy:

The source (voltage or current) may be independent or dependent. A source is said to be independent when it does not depend on any other quantity in the circuit. Fig. 1.15 (a) shows an independent dc voltage source whereas Fig. 1.15 (b) depicts a time-varying voltage source. The positive sign indicates that terminal A is positive with respect to terminal B i.e., the potential of terminal A is V volts higher than that of terminal B.

Similarly an ideal constant current source is shown in Fig. 1.15 (c) whereas time varying current source is shown in Fig. 1.15 (d). The arrow indicates the direction of current flow at any moment under consideration.

A dependent (or controlled) source is one which depends on some other quantity in the circuit which may be either a voltage or a current.

In electronic circuits, we very often find that the current through an element, (say collector current through a bipolar junction transistor) is dependent on a current through some other element or in a MOSFET it is a dependent on the voltage across some other element.

Such a source is called a dependent source. In a dependent source the output voltage (or current) depends on another voltage (or current). The relationship may be linear or non-linear. There are four possible dependent sources as are represented in Fig. 1.16.

Such sources can also be either constant sources or time varying sources.

Independent sources actually exist as physical entities such as an accumulator, a dc generator and an alternator. But dependent sources are parts of models that are used to represent electrical properties of electronic devices such as operational amplifiers and transistors etc.

Another major difference is that four terminals are required to define a controlled source; whereas only two are required for an independent source. Of the four dependent source terminals, one pair provides the control and the second pair exhibits the properties of the source.

Ideal Sources of Energy:

(a) Ideal Voltage Source:

A constant voltage source is an ideal source element capable of supplying any current at a given, voltage. If the internal resistance of a voltage source is zero, the terminal voltage (voltage across the load) is equal to the voltage across the source (the source emf), and is independent of the amount of load current, or in other words the voltage of an ideal voltage source is independent of load current supplied by it. For example, if the terminals are connected together the source will supply an infinite current. The symbolic representations of dc and ac ideal voltage sources are given in Figs. 1.17 (a) and 1.17 (b) respectively.

There are two worth-noting points regarding ideal voltage sources. First, ideal voltage source cannot be short-circuited (because this will be contrary to the definition of the ideal voltage source itself). Secondly (and for the same reason) two ideal voltage sources of unequal output voltages cannot be placed in parallel.

An ideal voltage source is not practically possible. There is no voltage source which can maintain its terminal voltage constant even when its terminals are short-circuited.

A lead-acid battery or dry cells are examples of an ideal voltage source when the current drawn is below a certain limit.

(b) Ideal Current Source:

Like a constant voltage source, there may be a constant current source a source that supplies a constant current to a load even if its impedance varies. Ideally, the current supplied by such a source should remain constant irrespective of the load impedance. A symbolic representation of such an ideal constant current source is shown in Fig. 1.15 (c). The arrow inside the circle indicates the direction of flow of current in the circuit, when a load is connected to the source.

There are two worth-noting points regarding ideal current sources. First, an ideal current source cannot be open-circuited (because this will be contrary to the definition of the constant current source itself). Again, for the same reason, two ideal current sources of different output currents cannot be placed in series.

An ideal current source, like an ideal voltage source, is not practically possible. There is no current source which can maintain current supplied by it constant even when its terminals are open-circuited. An ideal current source does not exist in practice.

A solar cell is an example of current source which provides a constant current to a resistance within a specified range of output voltage.