ideal | practical – voltage & current source : what is it ? | VI characteristics

Question: Explain Ideal And Practical Voltage & Current Source With V-I Characteristics Of Ideal And Practical Voltage And Current Source ?

Voltage source

Voltage source can be classified (a) Ideal voltage source (b) Practical voltage source

(a) Ideal voltage source

Ideal voltage source is defined as, a source which supply a constant value of terminal voltage for all values of current ‘l’ drawn out from it. This is because of the fact that an ideal voltage source has zero internal resistance. Fig I(a) shows the symbol used to represent an ideal voltage source and Fig.1 (b) shows its output characteristic of ideal voltage source and current source circuit with voltage source symbol

 (b)Practical voltage source

In a practical voltage source, the terminal voltage VAB reduces with the rise in output current l. The output current ‘I’ can be increased by decreasing the load resistance connected across the terminal A and B (not in figure)]. This drop in voltage  is because every practical voltage source has some internal resistance present in it. A practical voltage source can be approximated by an ideal voltage source with a resistance ‘Rs’ in series with it, where Rs is the internal resistance. Voltage source circuit or characteristic of practical voltage source is shown in fig. 2(a) .

 From fig. 2(a),

If ‘I’ increases. then from above equation VAB will decrease.

Current Source

Current source can be classified as :

(a) Ideal Current Source (b) Practical Current Source Ideal current source supply a constant value Of output current ‘ IL’ for all values of voltage appearing across it’s terminals A and B.

However, a practical current source supply a reducing current for increasing values of voltage appearing across it’s terminals A and B. This happens because of the fact that like a practical voltage source, a practical current source too has an internal in it due to which it cannot give out constant current for use. This internal resistance is absent in an ideal current Source. A practical current source can be approximated by an ideal current source with a ‘RP‘ in parallel with it. RP is the internal resistance.

Internal of ideal and practical voltage source are shown in Fig.3(a) and Fig.3(b) respectively, Fig. 3(c) shows their output characteristics of ideal current source and Ideal current source circuit with current source symbol .

From Fig. 3(b).

If VAB increases, then from above equation IL will decrease.

Conversion of Sources

While solving complicated circuits, it is normally required to convert a current source into it’s equivalent voltage source and vice-versa. This can be done by using Ohm’s law. For example consider the following current source and sec how it’s equivalent voltage source is obtained. Fig(1). conversion of current into voltage source with voltage source symbol .

V = IR = 10 x 200 = 2000V

Rs = Rp=200 ohm

Now, consider a voltage source and see how it’s equivalent voltage source can be obtained.fig (2) shows conversion of voltage source into current source with current source symbol.


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