Question

A 9v stabilized voltage supply is required to run a car stereo system from car 12 v battery.A zener diode with Vz = 9v and Pmax. = 0.25 W is ued as a voltage regulator as shown in figure .find the value of the series resistor.

Answer 1

Solution;

The figure is as below;

R can be calculated using the equation;

$V_s=I_{z_{max}}R+V_z$

But;

$P_{max}=I_{z_{max}}V_z$

Hence;

$I_{z_max}=\frac{P_max}{V_z}=\frac{0.25}{9}=0.0278A$

By direct substitution;

$12=0.0278R+9$

$0.0278R=12-9=3$

$R=\frac{3}{0.0278}=108\Omega$