First slide

Diodes

Question

In figure, what is the voltage needed at the source, to maintain 15 V across the load resistance $R_{L}$ , assuming that the series resistance R is $200 Ω$ and Zener requires a maximum current of 10mA to work satisfactory?

Moderate

A

16.5 V
B

18.5 V
C

30 V
D

15 V

Solution

$L o a d r e s i s \tan c e a n d z e n e r d i o d e a r e i n p a r a l l e l ∴ P o t e n t i a l d r o p a c r o s s L o a d = 15 V$
$O h m' s l a w a c r o s s L o a d : V_{Z} = I_{L} R_{L} \Rightarrow 15 = I_{L} (2000) \Rightarrow I_{L} = 7.5 m A$
$A s p e r K i r c h h o f f' s J u n c t i o n R u l e : I = I_{Z} + I_{L} = 10 + 7.5 = 17.5 m A$
$O h m' s L a w a c r o s s S e r i e s R e s i s \tan c e : V_{S} = I R = (17.5 \times 10^{- 3} \times 200) = 3.5 V$
$H e n c e, V o l t a g e a t s o u r c e = E = V_{S} + V_{Z} = 3.5 + 15 = 18.5 V$

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