Question

A zener diode voltage regulator operated in the range $120-180V$ produces a constant supply of $110V$ and $250mA$ to the load. If the maximum current is to be equally shared between the load and zener, then the values of series resistance ($R_S$) and load resistance ($R_L$) are:

• A. $R_L=70\Omega ;R_S=280\Omega$
• B. $R_L=440\Omega ;R_S=140\Omega$
• C. $R_L=140\Omega ;R_S=440\Omega$
• D. $R_L=280\Omega ;R_S=70\Omega$

Answer 1

The correct option is B $R_L=440\Omega ;R_S=140\Omega$

The resistance offered by Zener diode will be: $R_Z=\frac{110}{0.25}\Omega =440\Omega$

Hence, for equal current in load and Zener diode, $R_L=R_Z=440\Omega$

Now, total resistance will be: $\frac{440}{2}+R_S=R_S+220$

Since current through Zener diode is $0.25A$, current through combination will be: $0.25A\times 2=0.5A$.

Hence, for current to be maximum, the maximum voltage can be $180V$.

Therefore, $180=0.5(R_S+220)$

$R_S=140\Omega$