CHAPTER 1

Chapter 1: Linear DC Power Supply

1

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


2

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


3

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


4

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


5

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


6

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


7

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


8

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


9

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


10

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


11

DATA SHEET OF BRIDGE RECTIFIER IC

This is an example of data sheet of Bridge rectifier IC


12


13

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


14

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


15

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


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



17

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



18

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


19

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


20

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


21

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


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


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


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


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


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.

Date post:	09-Nov-2015
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CHAPTER 1

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