Question

A Zener diode, having breakdown voltage equal to $15V$, is used in a voltage regulator circuit shown in figure. The current through the diode is:

• A. $5mA$
• B. $10mA$
• C. $15mA$
• D. $20mA$

Answer 1

The correct option is A $5mA$

Current passes in reverse direction in a Zener diode when the Zener voltage is reached. Here the Zener voltage is given as $15V$.

Hence we can redraw the circuit diagram to note the potentials and the current through different segments.


Here we have grounded the point F and all the points F, E and D have voltage is equal to 0, since all of them are connected to each other through a wire.
Point A has potential of $20V$, B has potential of $15V$ and C again has potential of $15V$ (again because points B and C are connected through a wire).

The current through the resistance $250\Omega$ is
$i_3=\frac{V_A-V_B}{R}=\frac{20-15}{250}$
$=\frac{1}{50}=0.02A$

The current through the $1k\Omega$ resistance will be
$i_2=\frac{V_C-V_D}{R}=\frac{15-0}{1000}$
$=0.015A$

According to the junction rule at B:
$i_3=i_1+i_2$
Hence, $i_1=i_3-i_2=0.005A=5mA$