A zener diode functions as a voltage regulator by operating in its breakdown region, where the voltage across it remains constant despite significant variations in current. In this configuration, when the input voltage exceeds the breakdown voltage of the zener diode, it maintains a stable output load voltage. The relationship between currents and voltages is defined using Kirchhoff's Current Law (KCL) at node A: total current equals load current plus zener current. Small changes in output or load voltages occur due to variations in zener resistance and corresponding shifts in zener or load currents. However, this regulation method is effective only for minor fluctuations in line and load conditions.