A Zener diode is a p-n junction diode that is designed to break down at a specified voltage. The breakdown mechanism is non-destructive provided that the current is limited to prevent overheating. The voltage at which breakdown occurs and current flows in the reverse direction is called the Zener voltage. Once current flows in the reverse direction the voltage drop across the Zener diode remains constant at the Zener voltage. Zener voltages normally fall in the range 3 volts to 20 volts. The actual figure is chosen at the design stage.

 

Connected in the forward direction, a Zener diode behaves the same as a standard diode, with little resistance to current flow and a small voltage drop of 0.3 to 0.7 volt. In the reverse direction, however, only a very small leakage current can flow between 0 volt and the Zener voltage; then, when the voltage reaches the breakdown, suddenly current can flow freely.

 

Zener diodes are used in voltage-limiting circuits.

 

Zener diode shown with typical packages. Reverse current - iZ is shown.