Ideal Current Source

An Ideal Current Source has several properties which allow the current source to always deliver the rated current to the circuit independent of the circuit resistance and voltage across its terminals.

These properties are:

  • Has the same current flowing through it regardless of the voltage across its terminals
  • Can supply a specified current to any circuit element connected to it
  • It has the voltage necessary to provide the rated current

The typical case is shown below:

The current source, Is, provides 1mA which flows through the 3.3kΩ resistor resulting in a 3.3V voltage drop across it.

One of the edge cases is shown below. This is a short circuit:

Current still flows in the circuit shown above, it doesn't care if there is a 0V drop across the circuit (recall it has the same current flowing through it regardless of the voltage across its terminals)

The final edge case is shown below. This is an open circuit:

This circuit has the following properties:

  • This zeroes the effect of an ideal current source
  • As an open circuit has infinite resistance and no current flowing through it and hence this circuit is logically impossible (would require the supplied current value to change which breaks the definition of an ideal current source)
  • Open Circuit => R = ∞Ω

Note: In reality, the voltage across the current source would reach a high value (V=IR) to attempt to deliver current to an infinite resistance which will damage the current source (Do not try this at home :) ).

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