In most applications, ideal sources are approximation. The internal resistance Rin of a voltage source, which is responsible for a drop in terminal voltage of a source on load is quite small in comparison to the load resistance of a network connected across the voltage source. Hence voltage drop is considered negligible. Internal resistance needs to be taken into account when it is significant in comparison to load resistance RL.
A practical dc voltage source is represented in Fig. 2.92 (a). Vs is the internal voltage of the voltage source and Rin is the internal resistance of the voltage source. When we measure the voltage across the source terminals without any load (no resistance connected to the output terminals of the source) the terminal voltage is V = Vs as there is no current through Rin to cause a voltage drop.
Assuming Vs to be constant, the terminal voltage falls on loading by an external or load resistance RL.
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The resulting current:
As the load resistance RL reduces (load on the source increases), terminal voltage V falls linearly, if Rin and Vs are assumed constant.
When the terminals of a practical voltage source are short-circuited by a thick wire of zero resistance resulting in a short-circuit, source current Is is given as:
Similarly an ideal current source must produce infinite voltage on open-circuit. A practical current source will have a finite output voltage. A practical current source is represented as shown in Fig. 2.92 (c).
Practically, a voltage source is not different from a current source. In fact, a source can either operate as a current source or as a voltage source. It merely depends upon its operating conditions. If load impedance is very large in comparison to internal impedance of the source, it will be advantageous to treat the source as a voltage source.
On the other hand, if the load impedance is very small in comparison to the internal impedance of the source, it is better to represent the source as a current source. From the circuit point of view it does not matter at all whether the source is treated as a voltage source or a current source. In fact, it is possible to convert a voltage source into a current source and vice-versa.
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Consider a voltage source of voltage VS and internal resistance Rin shown in Fig. 2.92 (a) for conversion into an equivalent current source. The current supplied by this voltage source, when a short-circuit is put across terminals A and B, will be equal to VS/Rin. A current source supplying this current IS = VS/Rin and having the same resistance across it will represent the equivalent current source [Fig. 2.92 (c)].
Similarly a current source of output current IS in parallel with resistance Rin can be converted into an equivalent voltage source of voltage Vs = IS Rin and a resistance Rin in series with it [Fig. 2.92 (a)].
It should be noted that a voltage source series resistance combination is equivalent to a current source-parallel resistance combination if, and only if their respective open-circuit voltages are equal, and their respective short- circuit currents are equal.
For example, a voltage source branch consisting of a 10 V source in series with a resistance of 2.5 Ω may be replaced by a current source branch consisting of a 4 A source in parallel with a 2.5 Ω resistance and vice-versa, as shown in Figs 2.93 (a) and 2.93 (b) respectively.