In a Zener regulated power supply, a Zener diode with $V_{z} = 6.0 \; V$ is used for regulation. The load current is to be $4.0 \; mA$ and the unregulated input is $10.0 \; V$. What should be the value of the series resistor $R_{S}$?

(C) The value of $R_{S}$ should be such that the current through the Zener diode is significantly larger than the load current to ensure good load regulation.
Typically, we choose the Zener current $(I_{z})$ to be five times the load current $(I_{L})$, i.e., $I_{z} = 5 \times 4.0 \; mA = 20 \; mA$.
The total current through $R_{S}$ is $I = I_{z} + I_{L} = 20 \; mA + 4.0 \; mA = 24 \; mA$.
The voltage drop across $R_{S}$ is $V_{in} - V_{z} = 10.0 \; V - 6.0 \; V = 4.0 \; V$.
Using Ohm's law, $R_{S} = \frac{V_{in} - V_{z}}{I} = \frac{4.0 \; V}{24 \times 10^{-3} \; A} \approx 167 \; \Omega$.
The nearest standard value for a carbon resistor is $150 \; \Omega$. Therefore, a series resistor of $150 \; \Omega$ is appropriate.