JEE Main · 2026 · 23 Jan 2026 · Shift 1 · Current Electricity

The series resistor must drop the excess of the maximum supply over the Zener voltage while keeping the Zener current within its rating. The Zener can carry at most four times the load current, $I_{Z,\max }=4\times 5\mathrm{mA}=20\mathrm{mA}$. The severest case is the load disconnected, when the entire current through $R_S$ passes through the Zener; limiting this to $20\mathrm{mA}$ at the maximum input of $25\mathrm{V}$ gives $R_S=\frac{V_{\mathrm{in},\max }-V_Z}{I_{Z,\max }}=\frac{25-5}{20\times 10^{-3}}=1000\Omega$.