A constant current source is an electronic circuit that supplies constant current regardless of the load. No matter whether a constant current source is loaded with a high or a low load resistance, the constant current does not change, even if the load resistance changes.
The condition for generating constant current is when the internal resistance( Ri) of the current source goes to infinity, and is thus much larger than the load resistance (Rl), Ri "Rl. In this case, the extremely high internal resistance and the much lower load resistance determine the current flow.
Constant current sources are realized by feedback control loops with a control element, a transistor, field effect trans istor( FET) or operational amplifier. The control elements are regulated so that the current delivered to the load is always constant and independent of the load. If the load resistance becomes lower, then the control element becomes more highly resistive.