Question

A $9V$ stabilised voltage supply is required to run a car stereo system from its $12V$ battery. Zener diode with $V_{z}=9V$ and $P_{m a x}=0.27W$ is used as the voltage regulator in the circuit shown below. If the load resistance is $450\Omega$ , then find the value of series resistor:

• A. $60\Omega$
• B. $150\Omega$
• C. $100\Omega$
• D. $90\Omega$

Answer 1

Answer: (A)

Zener current, $I_{Z}=\frac{P_{m a x}}{V_{m a x}}=\frac{0 . 27}{9}=0.03A$
Current through $R_{L}=\frac{9}{450}=0.02A$
Current drawn from the battery, $I=I_{L}+I_{Z}=0.02+0.03=0.05A$
$\therefore $ Series resistor, $R_{S}=\frac{\left(\right. V \left(\left.\right)_{R_{S}}}{I}=\frac{3}{0 . 05}=60\Omega$