Ideal current and voltage sources

An ideal voltage source keeps a constant terminal voltage whatever current is drawn from it. An ideal current source delivers a constant current whatever voltage appears across it. Practical sources deviate from these ideals because of their internal resistance.

Ideal voltage source

• Internal resistance is zero, so the terminal voltage V = E for every load current.
• The V-I characteristic is a horizontal line.
• It can supply infinite current when short-circuited, so it does not exist in practice.

Ideal current source

• Internal resistance is infinite, so the current I = Is for every load voltage.
• The V-I characteristic is a vertical line.
• With an open circuit, the voltage across it would be infinite, so it does not exist in practice.

Practical sources

• A practical voltage source is an ideal voltage source E in series with an internal resistance Rs. Terminal voltage V = E − I·Rs, so it falls as load current rises.
• A practical current source is an ideal current source Is in parallel with an internal resistance Rp. Load current I = Is − V/Rp, so it falls as terminal voltage rises.

 Ideal V source      Practical V source    Practical I source
   o---+              o--[Rs]--+            o---+------+
      (E)               (E)    |              (Is)    [Rp]
   o---+              o--------+            o---+------+

A good voltage source has a very small Rs; a good current source has a very large Rp.