A  Zener diode having  breakdown voltage equal to 25V  is used in a voltage regulator circuit  shown in figure. The current through the diode  is

$\begin{array}{l}\text{ The voltage drop across }1k\mathrm{\Omega }=V_x=25V\\ \text{ The current through }1k\mathrm{\Omega }=I_L=\frac{25}{1\times {10}^3}=25\mathrm{mA}\end{array}$

$\begin{array}{l}\text{ Voltage drop across }300\mathrm{\Omega }\text{ is }30-25=5\mathrm{V}\\ \text{ The current through }300\mathrm{\Omega }\text{ is }I=\frac{5}{300}=\frac{1}{60}=16\mathrm{mA}\end{array}$

$\mathrm{The}\mathrm{current}\mathrm{through}\mathrm{the}\mathrm{zener}\mathrm{diode}\mathrm{is} I_Z{\text{=I-I}}_L=25-16=9\mathrm{mA}$