A Zener diode allows current in the forward direction like a regular diode

but also conducts in reverse when the voltage exceeds a specific Zener breakdown voltage (Zener voltage).

Common Zener diodes: 500mW BZX55 and 1.3W BZX85, like BZX55C7V5 (7.5V).

Zener diodes can be series-connected with silicon diodes for various reference voltages.