CHAPTER 1
description
Transcript of CHAPTER 1
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Chapter 1: Linear DC Power Supply
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1.0 LINEAR DC POWER SUPPLY
1.1 THE OPERATION OF DC POWER SUPPLY CIRCUIT
1.1.1 THE IMPORTANCE OF DC POWER SUPPLY UNITS IN ELECTRONIC APPLIANCES
a) Electronic appliance use active components such as diode, transistor and etc. These
components need DC voltage source to operate.
b) Batteries can give constant voltage and easy to carry everywhere. But using batteries the
power will not last longer after a certain period. Electronics appliances that using high power
supply will shorter the batteries life.
c) Electronics appliances that using high power supply will use more batteries. So, its not
economical if we using batteries.
d) Electric power supply provided to public through output sockets at houses and buildings are in
AC voltage and in high value. ( 1 phase = 240 V, 3 phase = 415 V )
e) All power supply to public, factories and industries through output socket are AC power and in
high value (1 phase = 240 V, 3 phase = 415 V). Therefore, it is of great importance to convert
the AC input voltage to DC voltage because all the semiconductor devices require DC voltage
source to operate.
1.1.2 BLOCK DIAGRAM OF DC POWER SUPPLY
The power supply consists of transformer, rectifier, and filter and voltage regulator. The transformer
receives a supply of AC 240V, 50 Hz and step- down this voltage to required value AC voltage from
the transformer is fed to rectifier, a diode circuit convert AC voltage to a pulsating DC voltage. The
DC voltage is fed to the filter to reduce the ripple in it. Normally, a filter consists of passive
components such as resistors, capacitors, and inductors. The next stage is the voltage regulator
that produces stable DC. The final stage is voltage divider that divides the voltage following the
circuit necessity. Figure 1.1 shows a block diagram of a DC power supply.
Figure 1.1: Block Diagram of a DC Power Supply
Transfomer
Rectifier
Filter
Voltage
Regulator
Voltage
divider
AC voltage DC Voltage
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1.1.3 THE FUNCTION OF EACH BLOCK
a) Transformer
i. To change the 240V AC input voltage to the required value. In power supply portion, this
transformer function is to step down the input line voltage.
ii. To isolate the rectifier from the voltage source to minimize the danger of electric shock.
b) Rectifier
i. To converts the Alternate Current input signal to a pulsating Direct Current.
c) Filter
i. To eliminate the fluctuations in the rectifier voltage and produce a relatively smooth DC
voltage like voltage in battery.
ii. To reduce the ripple voltage.
iii. The output from rectifier is a pulsating DC voltage but the pulsating DC voltage from
rectifier is still not enough to get pure DC voltage.
d) Voltage Regulator
i. Stabilise the output voltage, Vo even though there is a variation of the input current or the
output current.
ii. Reduce the ripple at the output voltage of the filter circuit.
e) Voltage Divider Circuit
i. To divide the voltage following the circuit necessity.
1.2 TRANSFOMER
We use step-down transformer since the voltage is decreased from primary to secondary.
Transformer at primary windings will connect to 240V 50 Hz AC power supply and transformer at
secondary windings will step down to fit with electronics devices.
Figure 1.2: Various Type of Transformers
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Chapter 1: Linear DC Power Supply
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Since transformer consists of two winding primary and secondary that have no connection, then the
purpose of using transformers is to release the circuits at secondary windings from AC power
supply. This release can avoid the user at secondary from electric shock at high AC voltage supply.
Figure 1.3 shows the symbol of transformer.
Figure 1.3: Symbol of Transformer
Transformer connected to the voltage source is called primary winding. The coil connected to the
load called the secondary winding. The ratio of primary winding called Np turns to the secondary
winding called Ns known as transformation ratio, n.
The voltage across primary windings to secondary windings is called turn ratio,
1.2.1 CENTER TAP TRANSFORMER
Np : Ns
Vp Vs
Transformation ratio, n = Ns
Np
Np : Ns
Vp Vs
Vs
Vs
Figure 1.4: Center Tap Transformers
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1.3 THE APPLICATION OF DIODE AS HALF-WAVE RECTIFIER
Most of the electronics circuit required DC power supply to operate. The most commonly used circuit
to convert AC voltage to DC voltage is the rectifier circuit. There are two types of rectifier such as
the half-wave rectifier and full-wave rectifier.
HALF-WAVE RECTIFIER
The 240V voltage supply is connected to the primary section of the transformer.
Figure 1.5: A Half-Wave Rectifier Circuit with Load RL
OPERATION
During the positive cycle of the input, the polarity of voltage sources is as shown in figure 1.6.
D(diode) is in forward-biased. D operates as closed switch so that current can flow in the direction
shown through the load resistor in the circuit. Voltage drop at RL is same with input signal in
positive cycle magnitude if we neglect the voltage drop at the diode.
Figure 1.6: Positive half cycle of the input in half-wave rectifier
+
-
Vin
t
Vo
t
Vin
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During the negative cycle of the input, the polarity of voltage sources is as shown in figure 1.7.
D(diode) is in reverse bias. D operates as opened switch so that current cant flow through the
circuit. There is no current flow in the diode and there is no voltage drop across the RL.
Figure 1.7: Negative half cycle of the input in half-wave rectifier
Figure 1.8: Input and output of Half-wave rectifier
OUTPUT VOLTAGE
Output voltage for half wave rectifier obtained only in positive cycle. Since current across the diode
and voltage drop at diode is 0.7V (assumed silicon diode), Output voltage is:-
FREQUENCY
Output signal frequency is equivalent to the frequency of the input source.
Vout = Vin - 0.7V
The diode is
forward biased
The diode is
reverse biased
-
+
Vo
t t
Vin
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1.4 THE APPLICATION OF DIODE AS FULL-WAVE RECTIFIER
The difference between full-wave and half-wave rectification is that a full-wave rectifier allows
unidirectional current to the load during the entire input cycle and the half-wave rectifier allows this
only during one- half of cycle. The result of full- wave rectification is a DC output voltage that
pulsates every half-cycle of the input, as shown in figure1.9. A full-wave rectifier produces ripple
voltage during both positive and negative input cycle.
There are two types of full-wave rectifiers:
a. Center Tap Full-Wave Rectifier
b. Full-Wave Bridge Rectifier
Figure 1.9: Full-Wave Rectification
CENTRE TAP FULL WAVE RECTIFIER
The transformer supplies an input voltage to the full-wave rectifier. In a center tapped transformer,
the voltage at half number of turn (M to G) is half the voltage from point M to N. If the voltage at
point between M and N is Vmax, the secondary voltage between the point M and G is Vmax /2. The
polarities of the waveform at MG and GN are out of phase as shown in figure 1.10.
Figure 1.10: Full-wave rectifier circuit by using center tap transformer
Vin
Vout
M
N
C A
B
G
D1
D2 RL
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OPERATION
When the AC voltage is given to the circuit, the end of M and N on the secondary transformer will be
a positive and negative in turn. During the positive half cycle of the input voltage, terminal M be a
positive, G be a potential zero (Ground) and N be a negative. Diode D1 is in forward bias and
conducting and D2 is in reverse bias and cut off. Current flows from point M, D1, C, A, B, G. A
positive wave cycle will result at RL load.
Figure1.11: Positive and Negative half cycle of the input
During the negative half cycle, the polarity of the voltage source changes (terminal M be a
negative, G be a potential zero and N be a positive). Diode D1 is in reverse bias and D2 is in
forward biased. Current flow from point N, D2, C, A, B, G. Since the direction current flow through
RL is similar to the current flow through the positive cycles, so similar waveform will produced.
OUTPUT VOLTAGE
Output voltage for full wave rectifier will result in positive and negative cycles. Since at one cycle,
current across the diode and voltage drop at diode is 0.7V (assumed silicon diode), output voltage
is:-
FREQUENCY
Output signal frequency is twice with input signal frequency.
Vo = VM-G - 0.7V
Vout
Vin
M
N
C A
B
G
D1
D2 RL
Vin
VMG
t
VNG
t
t
Vout
t
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1.5 THE APPLICATION OF DIODE AS BRIDGE RECTIFIER
OPERATION
During the positive half cycle of the input voltage cycle, M is positive and N is negative. D1 and
D3 are forward biased. D2 and D4 are reversed biased. Current flow from point M, E, D1, A, B,
C, D3, F, N. A positive wave cycle will result at RL load.
Figure 1.13 : Positive Half Cycle in Bridge Rectifier
Figure 1.12 : Bridge Rectifier with input and output waveform
4
4
4
Vin
Vout
M
N
A
B
C
E
F
D1
D2 D3
D4
RL -
+
1 2
3
4 5
6
7
B
Vin
M
N
A C
E
F
D1
D2 D3
D4
RL
+
-
+
-
Vin
t
Vout
t
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During the negative half cycle of the input voltage cycle, M is negative and N is positive. D2 and
D4 are forward biased. D1 and D3 are reversed biased. Current flow from point N, F, D2, A, B, C,
D4, E, M. A positive wave cycle will result at RL load. Since the direction current flow through RL is
similar to the current flow through during the positive cycles, so similar wave will produced.
Figure 1.14: Negative Half Cycle in Bridge Rectifier
OUTPUT VOLTAGE
Output voltage for bridge rectifier will result in both positive and negative cycles. Since current
across the two diode at one cycle and voltage drop at diode is 1.4V (assumed silicon diode), Output
voltage is:-
FREQUENCY
Output signal frequency is twice with input signal frequency.
Vo = VM-N - 1.4V
+
-
Vin
Vout
A
E
4
4
M
N
B
C
F
D1
D2 D3
D4
RL
4
1 2
3
4 5
6
7
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1.5.1 BRIDGE RECTIFIER INTEGRATED CIRCUIT
The bridge rectifier integrated circuit is available in special packages containing the four diodes
required. Bridge rectifiers are rated by their maximum current and maximum reverse voltage. They
have four leads or terminals: the two DC outputs are labelled + and -, the two AC inputs are labelled
. Figure 1.15 show a various types of Bridge Rectifier Integrated Circuit.
Note that some have a hole through their centre for attaching to a heat sink
Figure 1.15: Types of Bridge Rectifiers
PIN CONFIGURATION
Figure 1.16: Bridge Rectifier Pin Configuration
APPLICATION OF BRIDGE RECTIFIER IC
a) DC power for instruments
b) Industrial heating controlling
c) Various rectifying power supply
d) Light change
e) Non-touch switch
f) Soft-starters for electric machines
g) Static compensation of zero power
h) Arc welders
i) Frequency converters
j) UPS power supply
k) Charging and discharging of battery
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Chapter 1: Linear DC Power Supply
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DATA SHEET OF BRIDGE RECTIFIER IC
This is an example of data sheet of Bridge rectifier IC
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1.6 FILTER
The pulsating DC from the rectifier is generally still not suitable to power the actual load circuit. The
pulsations typically vary from 0 volts to the peak output voltage of the transformer. Therefore, we
insert a circuit to store energy during each voltage peak, and then release it to the load when the
rectifier output voltage drops. This circuit is called a filter and its job is to reduce the pulses from the
rectifier to a much smaller ripple voltage.
Ripple Voltage is the small variation in the DC voltage on the output of the filtered rectifier caused
by the slight charging and discharging action of the filter capacitor.
The filtered output has DC value and some AC variation (ripple) after passing through filter circuit.
Figure 1.17 shows output waveform before and after passing filter circuit.
Filter circuit is used to eliminate the fluctuations in the rectifier voltage and produce a relatively
smooth DC voltage like voltage in battery. The output from rectifier is a pulsating dc voltage but the
pulsating DC voltage from rectifier is still not enough to get pure DC voltage.
No filter configuration can be absolutely perfect, but a properly designed filter will provide a DC
output voltage with only a small ac ripple. The DC voltage derived from AC source signal by
rectifying and filtering will have some AC variation (ripple) as shows in Figure 1.19.
Figure 1.17: Output waveforms before and after passing filter circuit
VD.C
V
t
Figure 1.18: pure DC Voltage Figure 1.19: Ripple DC Voltage
V
t
V
t t Dc voltage Ripple dc voltage
V Ripple
Rectifier
Circuit
Filter
circuit
t
( Vr )p-p
VD.C
V
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Good filter circuit can reduce Vr p-p values that obtain from ripple dc voltage. The most commonly
types of filters used are RC filter, LC filter and filter.
1.6.1 RC FILTER
The Resistor-Capacitor (RC) filter is limited to applications in which the load current is small. This
type of filter is used in power supplies where the load current is constant and voltage regulation is
not necessary. For example, RC filters are used in high-voltage power supplies for cathode-ray
tubes and in decoupling networks for multistage amplifiers.
Figure 1.20 shows an RC filter circuit and Figure 1.21 shows associated waveforms. Half-wave
rectifiers are used to provide the inputs. The waveform shown in view (a) of the figure 1.21
represents the unfiltered output from a typical rectifier circuit. Note that the dashed lines in view (a)
indicate the average value of output voltage (V avg) for the half-wave rectifier. The average output
voltage (V avg) is less than half (approximately 0.318) the amplitude of the voltage peaks. With no
filter circuit connected across the output of the rectifier circuit (unfiltered), the waveform has a large
value of pulsating component (ripple) as compared to the average (or dc) component.
The RC filter consists of a capacitor (C1), a series resistor (R1), and a capacitor (C2). Views (a), (b),
and (c) of figure 1.21 show the output waveforms of a half-wave rectifier. Each waveform is shown
with an RC filter connected across the output. The following explanation of how a filter works will
show you that an RC filter of this type does a much better job than the single capacitor filter.
Figure 1.21: Output Waveforms of RC filter
Figure 1.20: RC Filter Circuit
Half-Wave
Rectifier
Circuit
C2 RL Vo
R1
C1
V avg
(a) Unfiltered output from
rectifier ( AC + DC)
( AC + DC)
(c) Voltage across Capacitor C2
( DC output)
Vr
V avg
Peak
0.318 Peak
1 Cycle
(b) Voltage across Capacitor C1
Charging process Discharging process
V avg
Vr
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Chapter 1: Linear DC Power Supply
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C1 performs exactly the same function as it did in the single capacitor filter. It is used to reduce the
percentage of ripple to a relatively low value. Thus, the voltage across C1 might consist of an
average DC value of +100 volts with a ripple voltage of 10 volts peak-to-peak. This voltage is
passed on to the R1-C2 network, which reduces the ripple even further.
C2 offers infinite impedance (resistance) to the dc component of the output voltage. Thus, the DC
voltage is passed to the load, but reduced in value by the amount of the voltage drop across R1.
However, R1 is generally small compared to the load resistance. Therefore, the drop in the DC
voltage by R1 is not a drawback.
Component values are designed so that the resistance of R1 is much greater than the reactance
(XC) of C2 at the ripple frequency. C2 offers very low impedance to the AC ripple frequency. Thus,
the AC ripple senses a voltage divider consisting of R1 and C2 between the output of the rectifier
and ground. Therefore, most of the ripple voltage is dropped across R1. Only a trace of the ripple
voltage can be seen across C2 and the load. In extreme cases where the ripple must be held to an
absolute minimum, a second stage of RC filtering can be added. The RC filter is extremely popular
because smaller capacitors can be used with good results.
The RC filter has some disadvantages. First, the voltage drop across R1 takes voltage away from
the load. Second, power is wasted in R1 and is dissipated in the form of unwanted heat. Finally, if
the load resistance changes, the voltage across the load will change.
1.6.2 LC FILTER
A LC filter or L- Section or Choke Input filter is the combination of inductor filter and capacitor filter.
In inductor filter, the ripple factor increase with the increase in the load,( RL decrease). But the ripple
factor decrease in case of capacitor filter. The combination of these filters into L- C filter would make
ripple independent of load.
Figure1.22 show a typical L-C filter circuit. It consists of L connected in series with the rectifier
output and a filter capacitor, C across the load. Here, a single filter section is shown, but several
identical sections are often used to reduce the ripple effectively.
Figure 1.22: LC Filter Circuit
T2
T1
Rectifier
Circuit
C RL Vo
L T3
3
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Chapter 1: Linear DC Power Supply
16
The output of the full wave rectifier is applied across the terminals T1, T2, of the filter circuit. The
rectifier output contains AC as well as DC component. The inductor L offers high resistance to the
flow of AC component but allows the DC component to pass.
Figure 1.23: Output Waveforms of LC filter
As a result of which the AC component appears across the choke ( inductor) while the entire DC
component passes through it towards the load. At terminal T3, the rectifier output contains the dc
component and the remaining part of AC component. The capacitor,C offer low reactance and
hence bypass the AC component but prevent the DC component to flow the through it. Therefore,
only DC component reaches the load. In this way AC component has been filtered out by the choke
input filter (LC filter) and only DC component is obtained across the load.
1.6.3 FILTER
(Pi) filter or capacitor input filter consists of a capacitor filter followed by a LC filter. The addition of
the input filter with full wave diode rectifier results in a high load voltage which is higher than that
obtained from a L-C filter. The circuit Pi filter is shown in figure 1.24.
Figure 1.24: Filter Circuit
The pulsating output from the rectifier is applied across the input terminal T1, and T2 of the filter.
The three circuit component C1, L and C2 exhibit filtering action, ie, separation of AC from DC.
V r 0.318 Peak
V avg
Peak
1 Cycle
(b)Unfiltered output from rectifier
( AC + DC )
(c) Voltage across Capacitor C ( DC output)
V avg
Charging process Discharging process
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Chapter 1: Linear DC Power Supply
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The capacitor C1 offers low reactance to AC component of rectifier while it offers infinite reactance
to the DC component. As a result of which capacitor C1 bypass AC and block DC. Thus DC
component flows the L. The inductor L offers high reactance to the AC component but almost zero
reactance to the DC component. Therefore it allows the DC component to flow through it while
blocks AC. The filter capacitor C2 by pass the AC component, if any present from that obtained from
L. Therefore only pure DC will appear across the load.
Figure 1.25 : Output Waveforms of LC Filter
ADVANTAGES OF PI FILTER
1. Reduction in the ripples
2. Increase in the average load voltage
3. Simple circuit
DISADVANTAGE OF PI FILTER
More expensive than the RC filter because L(an iron-core choke) costs more than a resistor.The
second disadvantage is size. The iron-core choke (L) is bulky and heavy, facts which may render
the Pi filter unsuitable for many applications.Regulation is relatively poor
1.7 VOLTAGE REGULATOR
The voltage regulator is one block of a power supply. Its input voltage come from filtered output of
rectifier derived of an ac voltage. The function of a voltage regulator is to stabilise the output
voltage, Vo even though there is a variation of the input current or the output current and reduce the
ripple at the output voltage of the filter circuit. There are 3 types of voltage regulator used:
i. Zener Diode
ii. Serial Transistor
iii. Integrated Circuit ( 78XX series)
V r Peak
0.318 Peak
V avg
1 Cycle
V avg
Vr
(a)Unfiltered output from rectifier
(AC +DC)
(b) Voltage across Capacitor C1 (c) Voltage across Capacitor C2
(only DC)
V avg
Charging process Discharging process
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Chapter 1: Linear DC Power Supply
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1.7.1 ZENER DIODE VOLTAGE REGULATOR
A simple voltage regulation circuit that uses a zener diode is shown in figure 1.26. This circuit
consists of a series resistor (R) and zener diode (Dz) connected to the output of a rectifier circuit. In
order to operate in zener area, input voltage must be larger than zener voltage and load resistance;
RL cannot make zener current decreased to zero. However, we should review zener diode operation
before discussing the circuit operation.
Zener diode is similar to conventional diodes when they are forward biased. When they are reverse
biased, no conduction takes place until a specific value reverse breakdown voltage (or zener
voltage) is reached. The zener is designed so that it will operate in reverse breakdown region of its
characteristic curve.( see figure 1.27).
Figure 1.27: IV Curve for Zener Diode
The reverse breakdown voltage is predetermining by manufacturer. When used as a voltage
regulator, the zener diode is reverse biased so that it will operate in the breakdown region. In this
region, changes in current through the diode have little effect on the voltage across it. Since the
diode is connected parallel to the load, the voltage across the load is always equal to the breakdown
voltage, Vz. The function of R is to control the current flow in the circuit not exceeding the maximum
allowable current. There is a large ripple voltage in the filter output. When this rippling waveform is
fed to zener diode, the output voltage is more constant. Figure 1.28 shows the output voltage of a
regulator circuit. The constant-voltage characteristic of a zener diode makes it desirable for use as a
regulating device.
Figure1.26: Zener Diode Voltage Regulator
Rectifier
circuit
Filter
Dz
R
RL
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1.7.2 SERIAL TRANSISTOR VOLTAGE REGULATOR
Recall that a Zener diode is a diode that block current until a specified voltage is applied. Remember
also that the applied voltage is called the breakdown, or zener voltage. Zener diodes are available
with different Zener voltages. When the Zener voltage is reached, the Zener diode conducts from its
anode to its cathode (with the direction of the arrow).
Figure 1.29: Serial Transistor Voltage Regulator
This regulator has transistor Q1 placed in series with load device (RL). Transistor Q1, then acts to
produce variable resistance to compensate for changes in the input voltage. The collector-emitter
resistance of Q1 varies automatically with changes in the circuit conditions. The zener diode
establish the DC biased placed on the base of transistor Q1.When this circuit is operating properly,
if the voltage across the load increase, the rise in emitter voltage makes the base less positive.
The current through Q1 will then reduced, which result in an increase in the collector-emitter
resistance of Q1. The increase in resistance will cause a larger voltage drop across the load.
Opposite conditions would occur if the load voltage were to decrease. Many variation of this circuit
are used in regulated power supply today.
As changes in the circuit output voltage occur, they are sensed at the emitter of Q1 producing a
corresponding change in the forward bias of the transistor. In other words, Q1 compensates by
increasing or decreasing its resistance in order to change the circuit voltage division.
C
B
E
Q1 NPN
R
Rectifier
circuit
Filter
Unregulated
DC volatage
Dz
RL
Figure 1.28: A zener diode voltage regulator with its output voltage
Vz
t
-
+
From filter output
Vz
Dz
Rs
RL
Vo
t
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Chapter 1: Linear DC Power Supply
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1.7.3 INTEGRATED CIRCUITS VOLTAGE REGULATOR
The most popular IC voltage regulator is the three terminal IC voltage regulators which includes the
function of modified zener diode voltage regulator. The voltage regulator commonly used is the
78XX series. It provides a constant positive output voltage. The two final XX digits designate the
value of the output voltage. For example a LM 7805 IC voltage regulator will produce an output DC
voltage of +5V. Other output values of the 78XX series are shown in Table 1.
Figure 1.30: Physical Form of IC Voltage Regulator
IC regulator number Output voltage
7805 +5V
7806 +6V
7808 +8V
7809 +9V
7812 +12V
7815 +15V
7818 +18V
7824 +24V
Table 1: The output voltage of 78XX series voltage regulator
Figure 1.30 shows a 78XX series positive voltage regulator. The pulsating DC input voltage from
rectifier is filtered by the capacitor C1 which filters the undesired ripples before it is connected to
terminal 1. The regulated output voltage of +5V (for example using LM7805) is produced at terminal
2 filter by the capacitor C2. C2 will also filter all the high frequency distortion in the system. Terminal
3 is grounded. The IC 79XX series have the same characteristics but it produces constant negative
DC output.
Figure 1.31: IC Voltage Regulator (78XX series)
1 2
3
C1
Vk
C2
Rectifier
circuit
Filter
LM7805
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Chapter 1: Linear DC Power Supply
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1.8 VOLTAGE DIVIDER CIRCUIT
In electronics system equipment, especially in large and complicated devices, they consist of circuit
stages with different dc voltage values. As example, in TV system it use more than ten circuit stages
with different function and need dc voltage around 100V, 48V, 12V and etc.
By using dc power supply, all the requirements can achieve by using voltage divider after the
highest value was obtained. Figure 1.32 shows a fix voltage divider circuit and figure 1.34 shows a
variable voltage divider circuit.
1.8.1 FIX VOLTAGE DIVIDER CIRCUIT
Based on figure1.33, three different voltages can produced from this circuit such as 20V, 13V and
3V, according to equipment requirements. This circuit are consisting of a few numbers of resistors.
The value of resistor (R1, R2, and R3) will determine the voltage value of each stage. The voltage at
point A to ground is equal to 20V. This is because R1, R2 and R3 are connected in series. In
Kirchhoff Voltage Law, the voltage drop across any resistor or combination of resistor in series
circuit is equal to the voltage source. This law also used in voltage divider concept. The voltage
value at point B to ground is 13V and if 3V voltage is required, Point C to ground can be used.
Figure 1.33: Fix Voltage Divider Circuit
Figure1.32: Fix Voltage Divider Circuit
Voltage
regulator
circuit
R1
R2
R3
80V
40V
20V
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Chapter 1: Linear DC Power Supply
22
1.8.2 VARIABLE VOLTAGE DIVIDER
Variable voltage divider circuits are consisting of R1 and variable resistor (VR). Based on figure
below, voltage at point A is 80V (equal to voltage from voltage regulator circuit). While voltage at
point B is depending on the value set at variable resistor. The value at point B can adjusted
according to equipment or component requirements
Figure1.34: Variable voltage divider circuit
B
Voltage
regulator
circuit
R1
VR
1
80V
0 - 40V
A
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Chapter 1: Linear DC Power Supply
23
1.9 COMPLETE CIRCUIT FOR DC POWER SUPPLY
a) DC Power supply using step down center tap transformer, full wave rectifier, filter,
zener diode voltage regulator and fix voltage divider.
b) DC Power supply using step down center tap transformer, bridge rectifier, LC filter,
zener diode voltage regulator and variable voltage divider.
Figure 1.36: DC Power Supply Circuit (b)
Center Tap
Transformer
Rectifier
Circuit
( Bridge Rectifier)
Filter Circuit
( LC Filter)
Voltage Regulator
Circuit
( Zener Diode)
Variable Voltage
Divider
D2 D1
D4 D3
R3
R2
15V
12V
Dz
R1
C
L
M
N
Figure 1.35: DC Power Supply Circuit (a)
Filter
Circuit
( Filter)
Voltage
Regulator
Circuit
( Zener Diode)
AC
Voltage
Fix
Voltage
Divider
Center Tap
Transfomer
Rectifier
Circuit
(Full wave)
C1 C2
L
Dz
R
DC
Voltage
R1
R2
R3
D1
D2
M
N
G
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Chapter 1: Linear DC Power Supply
24
c) Output waveform at each stage in DC power supply
Figure 1.37: DC power supply circuit with output waveform
L
C1 C2 RL
R
Filter
Circuit
(LC Filter)
Voltage
Regulator
circuit
Voltage
Divider Circuit
Center Tap
Transfomer Full-Wave
Rectifier
(Center Tap)
Circuit
DC
Output
Voltage
N
G AC
Input
Voltage
M D1
V V V
V V
240
LM7805
C D2
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Chapter 1: Linear DC Power Supply
25
TUTORIAL
Answer the question below.
1. State 3 reasons why DC power supply needed in human life.
2. Draw block diagram for DC power supply.
3. State the function of power supply unit.
4. State 2 functions of transformers in dc power supply.
5. In addition to stepping up or stepping down the input line voltage, what additional purpose does
the transformer serve.
6. What factors determine wheather transformers in step up or in step down?
7. Explain the function of rectifier circuit.
8. List down 3 types of rectifier circuits and draw the circuits.
9. What main advantage does a bridge rectifier have over a conventional full-wave rectifier?
10. What is the name of the simplest type of rectifier which uses one diode?
11. Is a full-wave rectifier output easier to filter than that of a half-wave rectifier? Explain.
12. Explain the function of filter circuit.
13. List down 3 types of filter circuits and draw the circuits.
14. Why does larger capacitor filter can decrease ripple voltage in the circuit?
15. What determines the rate of discharge of the capacitor in a filter circuit
16. Does low ripple voltage indicate good or bad filtering?
17. What is the function of regulator in DC power supply unit?
18. Name 3 regulator circuits and draw them.
19. Explain why voltage divider circuit is needed in DC power supply unit.
20. Determine the regulator output for following IC voltage regulator :
a. 7809
b. 7909
c. 7815
d. 7918
21. If the line frequency is 60Hz, the output frequency of half-wave rectifier is
a. 30Hz
b. 60Hz
c. 120Hz
d. 240Hz
22. If the line frequency is 60Hz, the outout frequency of full-wave rectifier is
a. 30Hz
b. 60Hz
c. 120Hz
d. 240Hz
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Chapter 1: Linear DC Power Supply
26
23. What is the purpose of using pairs of diodes in the bridge rectifier circuit.
a. As a safety factor for the circuit
b. To amplify the output voltage
c. To stabilize the voltage
d. To make sure the current flows in the same direction through the loads during each half of
the AC input cycles.
24. The circuit is used to reduce the voltage difference to zero or at least to a minimum value. The
statement refers to _______ circuit.
a. Transformer
b. Rectifier
c. Filter
a. Regulator
25. Draw the half-wave rectifier
a. Draw the current direction in the circuit
b. Draw the output waveform of the circuit
c. Explain the operation of the circuit
26. Draw a bridge rectifier circuit
a. Explain the operation of the circuit
b. Draw the output waveform if the input voltage is 240V AC and the transformer output is 24V
AC.
27. Draw a full-wave rectifier with filter and voltage regulator
a. Explain the operation of the circuit
b. Draw the output waveform if the input voltage is 240V AC and the transformer output is 40V
AC.
28. Draw the typical circuit connection of LM7805 voltage regulator and state the formula for
determining output voltage of resistive divider circuit.
29. Draw and label the schematic diagrams of a power supply that consists of step down
transformer, half wave rectifier, LC filter, IC voltage regulator and variable voltage divider circuit.
30. Explain briefly the function of each block for DC power supply.