## 25/11/2017

### Voltage Source and Current Source

Ideal Voltage Source

Practical Voltage Source
• As the practical voltage source has some finite internal resistance, its terminal voltage does not remains constant but falls as the current through the circuit increases.
• The practical voltage source has internal resistance r is shown in the Figure C.
• The terminal voltage V is equal to emf E minus voltage drop across resistance r. Its terminal voltage with respect to current is shown in the Figure D.

V = E – Ir

### Ideal current source An ideal current source is one in which current through its terminals remains constant regardless of terminal voltage.  The symbol of ideal current source is shown in the Figure E.  The ideal current source has infinite internal resistance.  The relation between voltage and current is shown in the Figure F for ideal current source.

Practical Current Source
• The practical current source is one in which current through its terminal decreases as the voltage across it increases.
• The practical current source is considered by connecting parallel resistance across an ideal current source as shown in the Figure G.
• The current through terminal is I1 is equal to

I1 = I – V / R
• Where R is internal resistance of voltage source and V is terminal voltage of an ideal current source.
• The voltage – current relation is shown in the Figure H in which current falls as voltage across terminal increases.
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