Zener voltage regulator

A Zener voltage regulator is the simplest circuit that holds an output voltage roughly constant against changes in supply voltage and load current. It is a Zener diode connected in reverse across the load, fed from the supply through a series current-limiting resistor. It is a shunt regulator: the regulating element sits in parallel (shunt) with the load.

How it works

Connect the Zener in reverse so its Reverse breakdown polarity matches the supply, put a resistor $R$ in series between the supply and the Zener, and connect the load $R_L$ directly across the Zener. Once the supply is high enough to push the Zener into breakdown, the Zener pins its terminal voltage at $V_Z$. Because the load sees the Zener’s terminals, the load voltage is held at $V_Z$ too.

The series resistor does the regulating work. The Zener’s near-vertical breakdown curve means its voltage hardly changes even when its current changes a lot. So:

• If the supply voltage rises, the extra voltage appears mostly across $R$ (more current flows through $R$, the surplus dumped through the Zener), and the Zener voltage — hence the output — barely moves.
• If the load draws more current, that current comes out of the Zener’s share: the Zener current drops, but because its curve is steep its voltage again barely moves, so the output stays near $V_Z$.

The Zener absorbs whatever current the load does not, keeping the total through $R$ — and therefore the drop across $R$, and therefore the output — nearly fixed. The current through $R$ is approximately $(V_{supply}-V_Z)/R$, and that must always stay above the Zener’s knee current $I_{ZK}$ (otherwise the Zener falls out of breakdown and regulation collapses) and within its power limit.

How good is it

The regulation quality is set by how flat the breakdown curve really is — i.e. by the Zener dynamic resistance $r_z$. A small change $\Delta V_{supply}$ produces an output change of roughly $\Delta V_{supply}\cdot r_z/(R+r_z)$. Because $r_z$ is small (a few ohms) compared with $R$ (often hundreds of ohms to kilohms), the output change is a tiny fraction of the supply change. Choosing a Zener with a low $r_z$ gives a better regulator. The diode Voltage reference built from series forward diodes works on exactly the same principle, using the small Diode small-signal resistance in place of $r_z$. A Zener regulator forms the final, voltage-stabilising block of a simple DC power supply.