Zener Diode Voltage Regulator

A zener diode regulates voltage by operating in reverse breakdown. When reverse-biased beyond its zener voltage (Vz), it clamps the voltage across it to Vz regardless of current variations. The series resistor Rs limits the current flowing through the circuit and absorbs the voltage difference between Vin and Vz.

The design process starts with choosing Rs so that the zener always carries at least 5–10 mA of current, even at maximum load. This minimum current keeps the zener in its regulation zone. If the load current increases, less current flows through the zener; if the load disconnects, all the current flows through the zener — so its power rating must handle the worst case.

Zener regulators are simple but inefficient for high-current loads because Rs dissipates power as heat. They work best for low-current voltage references, protection clamps, and circuits drawing under 100 mA. For higher currents, consider a three-terminal regulator like the 7805 or LM317.

Key Points

• Rs must supply both Iz (min 5 mA) and Iload
• Zener power rating must handle Pz = Vz × (Iz_max) when load disconnects
• Regulation degrades if Iz drops below the minimum knee current
• Choose a zener with Pz rating ≥ 2× the calculated dissipation

Applications

• Low-current voltage references
• Overvoltage protection clamps
• Bias voltage generation for transistor circuits
• Simple voltage regulation for sensors