Voltage Source and Current Source

Ideal Voltage Source

• An ideal voltage source is one in which terminal voltage remains constant regardless current flow through its terminals. 
• The ideal voltage source has zero internal resistance. 
• The symbol for the ideal voltage source is shown in the Figure A. 
• Its terminal voltage remains constant with respect to current as shown in the Figure B.

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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