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Figure shown 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 ?.......$mA$
$2.5$
$3.5$
$7.5$
$1.5$
Solution
Maximum current will flow from zener if input voltage is maximum.
When zener diode works in breakdown state, voltage across the zener will remain same.
$\therefore $ ${V_{{\text{across}}}}4{\mkern 1mu} K\Omega = 6{\text{V}}$
$\therefore $ Current through $4\, K\Omega = \frac{6}{4000}\,A$ $=\frac{6}{4}\,mA$
since input voltage $=16\,V$
$\therefore $ Potential difference across $2 \mathrm{K} \Omega=10 \mathrm{V}$
$\therefore $ Current through $2 \mathrm{k} \Omega=\frac{10}{2000}=5 \mathrm{mA}$
$\therefore $ Current through zener diode $=\left(I_{s}-\mathrm{I}_{\mathrm{L}}\right)=3.5 \mathrm{mA}$
