In-Class Exercise 6
Diode Circuits
1
Zener Regulator (1)
1.  A zener regulator uses a 9-V zener diode to maintain constant 9 V
across the load with the input varying from 18 to 25 V and the
output current varying from 400 to 800 mA. Assume RZ = 0 and
the minimum current IZ,min required for the zener diode to operate
in the breakdown region, where IZ,min = 0.1IZ,max.
(a) Select the value for Ri needed and determine its minimum
power requirement.
(b) Determine the power rating of the zener diode.
(c) Calculate the peak-to-peak output variation if RZ = 1 Ω.
2
Two extreme conditions:
1.  The current through the
zener diode must be less
than its maximum
allowable current. This
occurs when iRi is largest
and iL is smallest.
2.  The current must be
greater than Iz,min in order
to have a constant output
voltage Vz. This occurs
when iRi is smallest and iL
is largest.
3
Condition 1:
IRi,min = Iz,min + IL,max
18 − 9
= Iz,min + 0.8 = 0.1Iz,max + 0.8
Ri
Condition 2 :
IRi,max = Iz,max + IL,min
⇒
25 − 9
= Iz,max + 0.4
Ri
⇒ Ri = 9.74 Ω and Iz,max = 1.24 A
⇒
PRi =
(25 − 9) 2
= 26.28 W
9.74
PZ = 9 × 1.24 = 11.2 W
The voltage across Rz varies from Iz,min × 1
to Iz,max × 1 or 0.124 V to 1.24 V
The output voltage is then :
VL,min = 9 + 0.124 = 9.124 V
VL,max = 9 + 1.24 = 10.24 V
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2. Sketch the output waveform when vi = 9 sin 1000t V.
Show the maximum and minimum values on the sketch.
Assume the diode is ideal.
I
i. v i positive v o = 0 as D1 conducts
ii. v i negative v i = v o when v i ≥ -4 V
iii. v i negative and v i ≤ -4 V
v − (−4) v i + 4
I= i
=
2k + 4k
6k
⎛ v + 4⎞
v o = −4 + 4k i
= −1.33 + 0.667v i
⎝ 6k ⎠
v o (−9) = −7.33 V
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3. Sketch the output waveform when vi = 9 sin 1000t V.
Show the maximum and minimum values on the sketch.
Assume the diode is ideal.
i. v i positive
v i = v o when v i ≤ 5 V
when v i > 5 V
v − (5) v i − 5
I= i
=
2k + 2k
4k
⎛ v − 5⎞
v o = 5 + 2k i
= 2.5 + 0.5v i
⎝ 4k ⎠
v o (+9) = 6.5 V
ii. v i negative
v i = v o when v i ≥ −6 V
when v i < − 6 V
v − (−6) v i + 6
I= i
=
2k + 4k
6k
⎛ v + 6⎞
v o = −6 + 4k i
= −2 + 0.667v i
⎝ 6k ⎠
v o (−9) = −8 V
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Self-study:
1. Find the output of the clipping circuit for vi = 8 sin 1000t
V, assuming that the diode has a forward voltage of (a) 0 V
and (b) 0.7 V.
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2. For the circuits below, sketch the output waveform
when vi varies linearly from 0 to 150 V. Assume ideal
diodes.
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