Draw the circuit of a Zener diode voltage regulator. Explain how it keeps the output voltage constant against changes in load current and input voltage.

1. 1 · State the principle. In reverse breakdown, the voltage across a Zener stays at Vz over a wide range of current.
2. 2 · Draw the circuit. [Diagram: input Vi; series resistor Rs; Zener reverse biased (cathode towards +) across the output; load RL in parallel with the Zener; currents Is, Iz and IL and output Vo = Vz marked]
3. 3 · Write the currents. Is = (Vi − Vz)/Rs, and Is = Iz + IL.
4. 4 · Load change. If IL rises, Iz falls by the same amount; Is and Vo stay the same.
5. 5 · Input change. If Vi rises, Is rises; the extra voltage drops across Rs and the extra current flows in the Zener, so Vo stays at Vz.
6. 6 · Real life. Chargers and set-top boxes use this to keep chips at a steady voltage.
7. 7 · Close in one line. So, the output stays at Vz as long as Vi > Vz and Iz stays above its minimum.

Scoring tip: Draw the Zener with its cathode towards the positive side. A forward-biased Zener in your diagram loses the diagram mark.