First slide

Diodes

Question

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

Moderate

A

20mA
B

5mA
C

10mA
D

15mA

Solution

$V o l t a g e a c r o s s Z e n e r d i o d e r e m a i n s c o n s \tan t . Z e n e r d i o d e a n d l o a d r e s i s \tan c e a r e i n p a r a l l e l .$
$∴ I_{L} = \frac{V_{0}}{R_{L}} = \frac{15}{10^{3}} = 15 m A$
$A l s o, v o l t a g e a c r o s s s e r i e s r e s i s \tan c e = V_{s} = E - V_{o} = 20 - 15 = 5 V$
$∴ C u r r e n t t h r o u g h s e r i e s r e s i s \tan c e = I_{S} = \frac{V_{s}}{R_{s}} = \frac{5}{250} = 20 m A$
$H e n c e, c u r r e n t t h r o u g h d i o d e = I_{Z} = I_{S} - I_{L} = 20 - 15 = 5 m A$

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