How it works
A Zener diode is a special semiconductor designed to reliably conduct current "backwards" once a specific reverse voltage is reached. This is called the Zener breakdown voltage.
When forward-biased (anode positive, cathode negative), it behaves exactly like a normal diode, dropping about 0.7V. But when reverse-biased, it blocks current until the voltage hits its Zener rating (e.g., 5.1V). Once hit, it abruptly begins to conduct, holding the voltage across its terminals incredibly stable at that exact 5.1V level.
This unique property makes Zener diodes the simplest way to create a stable reference voltage or regulate a power supply. They act as voltage clamps, bleeding away excess current to keep the voltage from rising above their set point.