A constant current diode also called a current limiting diode (CLD), is an electronic device that regulates or limits current to a maximum value. This is important as it protects your circuit from harmful effects in the event of a short-circuit.
Constant current diodes are made from a junction field-effect transistor, the simplest type of three-terminal semiconductor transistor which functions like a current limiter. They allow the current running through them to rise to a certain point and level off at a specific value. Unlike zener diodes constant current diodes maintain a constant current instead of the constant voltage.
Constant current diodes can be used in several circuit applications. Examples of devices using constant current diodes include waveform generator circuits, timing circuits and battery chargers. Constant current diodes can also be used as a constant current source for LED drivers and can be used to replace holding coils in telephone connection devices.