Functionally, Zener diodes are used as regulators, limiters and to control output switching. When used as regulators, the constant reverse voltage of Zener diodes allows for the regulation of output voltage against both variations in the input voltage from an unregulated power supply and from variations in the load resistance. The current traveling through Zener diodes changes to keep the voltage within the threshold of Zener action and the maximum power it can dissipate. Zener regulators tend to most efficiently function when constant voltage is applied, especially when used in conjunction with regulated power supplies, and for limiter applications.

Zener limiters are constructed with two opposing Zener diodes. Each individual diode can limit one side of a sinusoidal waveform to Zener voltage while keeping the other side near zero. When the two opposing Zener diodes are paired, the waveform is limited to Zener voltage on both polarities.

Zener diodes are also used to control output switching. In this application, Zener diodes switch the output between voltages. This is determined by the changes in input voltage through the diode. The output circuit amounts to a Zener regulator that switches from one Zener voltage to the other on transition through the Zener diode.

Kingtronics supply all kinds of Zener Diodes BZV55C & ZMM55C, http://www.kingtronics.com/pdf/BZV55C.pdf has detailed introduce Zener Diode’s features and types.