Zener diode

A diode that conducts in reverse at a fixed voltage.

Schematic letter: D / ZD
Unit / key rating: Zener voltage (V), power (W)

Pinout

Anode: The unbanded side.
Cathode: The banded side; goes to + when used as a zener.

Real photos

What is it for?

Forward-biased, a zener behaves like a normal diode. In reverse, it starts conducting at a specific voltage (the zener voltage, e.g. 5.1 V) and holds the voltage across it there.

It's used for simple reference voltages, protecting an input from overvoltage and limiting small supplies. It always needs a series resistor.

Things to watch out for

⚠ Its code is printed on the body (e.g. 5V1, BZX55C12); a multimeter's diode mode doesn't show the zener voltage.
⚠ Exceed its power and it heats up and shorts: P = Vz × I.

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