A zener diode of power rating 2 W is to be used as a voltage regulator. If the zener diode has a breakdown of 10 V and it has to regulate voltage fluctuated between 6 V and 14 V, the value of $$R_s$$ for safe operation should be _________ $$\Omega$$.

Maximum allowed current through the Zener diode: $$I_{Z,\text{max}} = \frac{P}{V_Z} = \frac{2}{10} = 0.2\text{ A}$$

Under no-load condition, the maximum current through the series resistor $$R_s$$ equals $$I_{Z,\text{max}}$$:

$$I_{s,\text{max}} = \frac{V_{\text{in,max}} - V_Z}{R_s}$$

$$0.2 = \frac{14 - 10}{R_s} \implies 0.2 = \frac{4}{R_s} \implies R_s = \frac{4}{0.2} = 20\ \Omega$$