Zener Diode

A Zener diode is heavily doped so it breaks down at a precise, repeatable reverse voltage $V_Z$ (available from about 2.4 V to over 200 V). In breakdown the voltage stays close to $V_Z$ across a wide current range, and the process is not destructive as long as the power limit is respected.

Simple shunt regulator:

Vin ──[ R ]──┬── Vout ≈ Vz
             │
            Zener (cathode up)
             │
GND ─────────┴──

$R = \frac{V_{in,min} - V_Z}{I_{Z,min} + I_{L,max}} \qquad P_Z = V_Z \cdot I_Z$

Example: 12 V in, 5.1 V Zener, 20 mA load, 5 mA minimum Zener current → $R = (12 - 5.1)/0.025 = 276$ Ω (use 270 Ω).

Zeners are also used for over-voltage clamping and protecting inputs. For better regulation, use a linear regulator IC.