How a zener regulator works
A zener diode conducts in reverse once the voltage reaches its rated value, holding the voltage steady. A series resistor drops the rest of the input voltage and sets the total current, which is shared between the zener and the load. The resistor must be small enough that, at the lowest input and highest load, there's still a few milliamps left for the zener: R = (Vin min − Vz) ÷ (I load + I zener min).
Power ratings
The worst case for the zener is the highest input voltage with no load, when all the current flows through it. The worst case for the resistor is also the highest input. Choose parts rated for at least twice the calculated power. If the zener needs to be bigger than about 1 W, a regulator such as the LM317 is usually a better choice: see the LM317 calculator.
Where they're used
Zener regulators suit voltage references, bias supplies, meter circuits and protection against overvoltage, where the current is small. In a car or shack supply, where the voltage varies from about 11 to 14.4 V, they give a steady few volts for logic or a reference.
Questions
How do I calculate a zener resistor?
R = (lowest input − zener voltage) ÷ (load current + minimum zener current).
What wattage zener do I need?
At least twice the power it takes at the highest input with no load: (Vin max − Vz) ÷ R × Vz.
Why does my zener output drop under load?
The resistor is too big for the load current, so the zener runs out of current. Recalculate with the real load.
Can a zener regulate high currents?
Not efficiently. Above a few tens of milliamps, use a linear regulator, or a zener driving a transistor.