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4-1
Electronics
Principles & Applications
Eighth Edition
Charles A. Schuler
Chapter 4
Power Supplies
(student version)
©2013
McGraw-Hill
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4-2
INTRODUCTION
• The System
• Half-Wave Rectification
• Full-Wave Rectification
• RMS to Average
• Filters
• Multipliers
• Ripple and Regulation
• Zener Regulator
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4-3
Dear Student:
This presentation is arranged in segments. Each segment
is preceded by a Concept Preview slide and is followed by a
Concept Review slide. When you reach a Concept Review
slide, you can return to the beginning of that segment by
clicking on the Repeat Segment button. This will allow you
to view that segment again, if you want to.
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4-4
Concept Preview
• The power supply energizes the system.
• Rectifiers change ac to dc.
• A half-wave rectifier conducts for half of the ac
input cycle.
• The cathode end of the load circuit is positive.
• A full-wave rectifier conducts for the entire ac
input cycle.
• Full-wave rectifiers use two diodes and a center
tapped transformer.
• A bridge rectifier uses four diodes and provides
full-wave performance without a transformer.
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4-5
Circuit A
Power Supply
Circuit B
Circuit C
The power supply energizes the other circuits in a system.
Thus, a power supply defect will affect the other circuits.
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4-6
dc
ac
Power Supply
Circuit A
dc
Circuit B
dc
Circuit C
Most line operated supplies change ac to dc.
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4-7
The cathode makes this
the positive end of the load.
+
0
-
+
ac
0
Half-wave pulsating dc
A series rectifier diode changes ac to dc.
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4-8
The cathodes make this
the positive end of the load.
+
0
Full-wave pulsating dc
+
0
ac
-
Two diodes and a transformer provide full-wave rectification.
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4-9
Only half of the transformer secondary conducts at a time.
C.T.
VLOAD is equal
to one-half the
total secondary
voltage.
VTOTAL
½ VTOTAL
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4-10
+
0
-
ac
+
0
Full-wave
pulsating dc
-
The bridge circuit eliminates the need for a transformer.
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4-11
+
0
-
ac
+
Full-wave
pulsating dc
0
-
Reversing the diodes produces a negative power supply.
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4-12
Power supply basics quiz
Most line-operated power supplies change
ac to ________.
dc
A single diode achieves ________ -wave
rectification.
half
Two diodes and a center-tapped transformer
provide ________ -wave rectification. full
A bridge rectifier uses ________ diodes.
four
The positive end of the load is the end in
contact with the diode ________. cathodes
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4-13
Concept Review
• The power supply energizes the system.
• Rectifiers change ac to dc.
• A half-wave rectifier conducts for half of the ac
input cycle.
• The cathode end of the load circuit is positive.
• A full-wave rectifier conducts for the entire ac
input cycle.
• Full-wave rectifiers use two diodes and a center
tapped transformer.
• A bridge rectifier uses four diodes and provides
full-wave performance without a transformer.
Repeat Segment
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4-14
Concept Preview
• When a meter is connected to the output of a
rectifier, it will respond to the average value of the
pulsating dc waveform.
• The average dc load voltage is 45% of the rms
input voltage for half-wave rectifiers.
• The average dc load voltage is 90% of the rms
input voltage for full-wave rectifiers.
• The average dc load voltage is 135% of the rms
input voltage for 3f full-wave rectifiers.
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4-15
Vac
Vac
Vdc
Vdc
Ignoring diode loss,
the average dc is 45%
of the ac input for
half-wave.
ac
Converting rms
to average
Vac
Vdc
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4-16
Vac
Vdc
ac
Ignoring diode loss,
the average dc is 90%
of the ac input for
full-wave.
Vac
Vdc
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4-17
Three-phase rectification is used in commercial,
industrial and vehicular applications.
Full-wave, 3 f bridge
3f
Vdc = 1.35 x Vrms = 162 V
120 V
60 Hz
+200 V
0V
-200 V
0 ms
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20 ms
40 ms
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4-18
Three-phase rectifier output
200
160
Volts
120
Vdc = 1.35 x Vrms = 162 V
80
40
0
0
10
20
30
40
Time in milliseconds
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Average dc quiz
The average dc voltage with half-wave is
45%
equal to ______ of the ac voltage.
4-19
The effective ac voltage in a two-diode,
full-wave rectifier is _______ of
the secondary voltage.
half
The average dc voltage with a full-wave
rectifier is _________ of the
effective ac voltage.
90%
The average dc voltage with a bridge rectifier
is equal to ________ of the ac voltage. 90%
The average dc voltage with a 3 f bridge rectifier
is equal to ________ of the ac voltage. 135%
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4-20
Concept Review
• When a meter is connected to the output of a
rectifier, it will respond to the average value of the
pulsating dc waveform.
• The average dc load voltage is 45% of the rms
input voltage for half-wave rectifiers.
• The average dc load voltage is 90% of the rms
input voltage for full-wave rectifiers.
• The average dc load voltage is 135% of the rms
input voltage for 3f full-wave rectifiers.
Repeat Segment
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4-21
Concept Preview
• A filter capacitor will charge to the peak value of
the ac input.
• The peak value of the ac input is 141% of the rms
value.
• The peak value is 141% for both half-wave and
full-wave circuits.
• Voltage doublers produce a peak value that is
282% of the rms input value.
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4-22
Filter
capacitor
Discharge
VP
+
ac
0
Charge
-
A relatively large filter capacitor will maintain the
load voltage near the peak value of the waveform.
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4-23
Filter capacitors
must have a low
ESR to be effective
in eliminating
ac ripple.
0.04 
is typical
for this
capacitor
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4-24
Discharge time is less.
VP
+
ac
0
-
Full-wave is easier to filter since the discharge
time is shorter than it is for half-wave rectifiers.
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4-25
Vac
Vdc
ac
Adding a filter capacitor
increases the dc output
voltage.
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Ignoring diode loss
and assuming a large
filter, the dc output is
equal to the peak value
for both half-wave and
full-wave.
Vac
Vdc
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4-26
Vac
Vdc
Ignoring diode loss
and assuming a large
filter, the dc output is
equal to the peak value
for both half-wave and
full-wave.
ac
Vac
Vdc
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4-27
Vac
Vdc
ac
Full-wave doubler
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Ignoring diode loss and
assuming large filters,
the dc output is twice the
peak ac input.
Vac
Vdc
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4-28
The charge on C1 adds to the ac line voltage
is charged.
and C2 C
is1 charged
to twice the peak line value.
ac
C1
C2
Half-wave voltage doubler
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4-29
Capacitive filter dc output quiz
(Ignore diode loss and assume a light load for this quiz.)
The dc output in a well-filtered half-wave
supply is _____ of the ac input.
141%
The dc output in a well-filtered full-wave
supply is _____ of the ac input.
141%
The dc output in a well-filtered half-wave
doubler is _____ of the ac input. 282%
The dc output in a well-filtered full-wave
doubler is _____ of the ac input. 282%
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4-30
Concept Review
• A filter capacitor will charge to the peak value of
the ac input.
• The peak value of the ac input is 141% of the rms
value.
• The peak value is 141% for both half-wave and
full-wave circuits.
• Voltage doublers produce a peak value that is
282% of the rms input value.
Repeat Segment
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4-31
Concept Preview
• The ideal dc power supply has no ac ripple.
• The percentage of ac ripple is a measure of how
closely a real power supply approaches the ideal.
• The dc output voltage of an ideal supply never
changes..
• The percentage of voltage regulation is a measure
of how closely a real power supply approaches the
ideal.
• Zener diodes are used as voltage regulators.
• The voltage drop across a zener diode remains
almost constant, if it is conducting.
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4-32
Vac
Vdc
An ideal dc power
supply has no ac ripple.
ac ripple = 0%
Vac
Vdc
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4-33
Vac
Vdc
ac
Real power supplies
have some ac ripple.
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1.32 Vac
x 100% = 11 %
ac ripple =
12 Vdc
Vac
Vdc
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4-34
An ideal power supply has perfect voltage regulation.
The voltage
does not change.
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Vac
Vdc
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4-35
Voltage Regulation =
=
ac
DV
VFL
x 100 %
1V
x 100 %
12 V
= 8.33 %
The output of a
real supply drops
under load.
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Vac
Vdc
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4-36
12
Reverse bias in Volts
6
2
4
8
10
0
5
10
Vrev
I
15 Reverse
current
20 in mA
25
30
35
V
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The voltage across a conducting
zener is relatively constant.
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4-37
Vac
Vdc
A shunt zener diode can
be used to regulate voltage.
ac
If the zener stops conducting, the regulation is lost.
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4-38
Power supply quality quiz
The voltage regulation of an ideal power
supply is ___________.
0%
The ac ripple output of an ideal power
supply is ___________.
0%
DV in a real power supply should be
as ___________ as is feasible.
small
The ac component of an ideal dc power supply
should be as ________ as is feasible. small
A device that is commonly used to regulate
voltage is the ________ diode.
zener
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4-39
Concept Review
• The ideal dc power supply has no ac ripple.
• The percentage of ac ripple is a measure of how
closely a real power supply approaches the ideal.
• The dc output voltage of an ideal supply never
changes..
• The percentage of voltage regulation is a measure
of how closely a real power supply approaches the
ideal.
• Zener diodes are used as voltage regulators.
• The voltage drop across a zener diode remains
almost constant, if it is conducting.
Repeat Segment
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4-40
REVIEW
• The System
• Half-Wave Rectification
• Full-Wave Rectification
• RMS to Average
• Filters
• Multipliers
• Ripple and Regulation
• Zener Regulator
McGraw-Hill
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.