Zener diodes are used to maintain a fixed voltage. They are designed to ‘breakdown’ in a reliable and non-destructive way so that they can be used in reverse to maintain a fixed voltage across their terminals.
Zener diodes also intended for voltage regulation purposes are sometimes used as a protection against transients. Zener diodes are found in devices such as power supplies and surge protectors.
Typical parts are: TZMC, BZT52, MMSZ