Zener Diode as a Voltage Regulator

The regulator circuit. A basic Zener voltage regulator connects an unregulated DC supply voltage $V_i$ (one that may fluctuate somewhat) through a series resistor $R_S$ to a Zener diode held in REVERSE bias, with the load resistor $R_L$ connected in PARALLEL with the Zener diode -- so the regulated output voltage $V_O$, taken directly across the Zener diode (and hence across the load too, since they share the same two nodes), is simply the Zener voltage $V_Z$ so long as the diode stays in breakdown.

How regulation works. The resistor $R_S$ is chosen so that, under normal operating conditions, MORE current flows into the parallel Zener-plus-load combination than the load alone actually needs -- the surplus is absorbed by the Zener diode, which draws whatever current, $I_Z$, is left over after the load has taken its share: $I = I_Z + I_L$, where $I$ is the total current supplied through $R_S$. If the unregulated input $V_i$ rises somewhat, the extra voltage appears almost entirely across $R_S$ (since $V_O$ stays clamped at $V_Z$), which simply increases the total current $I$ -- and, because the load current $I_L = V_O/R_L$ is unchanged, the entire extra current is absorbed by a larger $I_Z$ through the Zener diode, leaving the OUTPUT voltage unchanged. If instead the load itself draws more current (a smaller $R_L$), the Zener diode simply gives up some of its own current $I_Z$ to supply the extra $I_L$, again leaving the total current $I$ through $R_S$, and hence the output voltage, essentially unaffected. …