Modern Physics 6 Question 1
1. Figure shows a DC voltage regulator circuit, with a Zener diode of breakdown voltage $=6 V$. If the unregulated input voltage varies between $10 V$ to $16 V$, then what is the maximum Zener current?
(Main 2019, 12 April II)
(a) $2.5 mA$
(b) $1.5 mA$
(c) $7.5 mA$
(d) $3.5 mA$
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Solution:
- In given voltage regulator circuit breakdown of Zener occurs at $6 V$.
After breakdown, voltage across load resistance $\left(R _L=4 k \Omega\right)$ is,
$$ V=V _Z=6 V $$
$\therefore$ Load current after breakdown,
$$ I _L=\frac{V _Z}{R _L}=\frac{6}{4000}=1.5 \times 10^{-3} A $$
When unregulated supply is of $16 V$, potential drop occurring across series resistance $\left(R _S=2 k \Omega\right)$ is;
$$ V _S=V-V _Z=16-6=10 V $$
So, current across series resistance is
$$ I _S=\frac{V _S}{R _S}=\frac{10}{2 \times 10^{3}}=5 \times 10^{-3} A $$
So, current across Zener diode is
$$ I _Z=I _S-I _L=5 \times 10^{-3}-1.5 \times 10^{-3}=3.5 \times 10^{-3} A $$
