dual power supply final

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UNIT: 1

D.C.POWER SUPPLY

Introduction:

Most of the electronic devices and circuits require D.C. sources for their operation. Dry

cell and batteries are on form of the source but their voltages are low and they are expensive as

compared to conventional D.C. power supplies.

The regulated power supply is to provide the necessary dc voltage and current, with low

levels of ac ripple and with stability and regulation.

There are two types of power supplies

There are two types of power supplies:

(i) Unregulated power supply and

(ii) Regulated power supply.

i) Unregulated power supply:

In unregulated power supply as the A.C. input varies, the output D.C. voltage also varies.

As load varies the D.C. voltages also vary. The output D.C. voltage does not remain constant.

Such supply is called as unregulated supply.

The unregulated supply consists of three blocks

• Transformer

• Rectifier

• Filter

(Unregulated)

ii) Regulated power supply:

A regulated power supply will provide constant D.C. output voltage irrespective of line

voltage or load resistance variation. Such supply is called regulated power supply. An



unregulated power supply can be converted into a regulated power supply. The regulated power

supply can be made by adding a voltage regulator circuit.

A typical D.C. regulated power supply at least consists of the following Regulated

1. Transformer:

The job of the transformer is to step up or step down the A.C. voltage to suit the

requirements of the solid state device and circuit fed by the D.C. power supply. It also provides

isolation from the supply line which is an important safety consideration.

2. Rectifier:

It is a circuit which employs one or more diodes to convert A.C. voltage in to pulsating

D.C. voltage.

3. Filter:

The function of filter is to remove the fluctuation or pulsation (called Ripple) present in

the D.C. output voltage. Of course no filter can give an output voltage of D.C. battery in practice

but it approaches it so closely that the power supply performs well.

Voltage Regulator:

The function of voltage regulator is to keep yhe output voltage of D.C. supply constant

even when

• A.C. input to the transformer varies

• The transformer output vary upto certain value

• The load varies.

If at the output of D.C. regulated power supply different voltages are required a voltage

divider electronic circuits are used. It consists of a number of resistors connected in series across

the output terminal of the voltage regulator. Sometimes a potentiometer is included across the

output terminal so as to vary the output D.C. voltage.



UNIT: 2

TRANSFORMERS

These are components which only work with alternating currents and are used to

transform or change an alternating voltage up or down.

Working Principle of a Transformer

A transformer is a device that

(i) Transfer electric power from one circuit to another

(ii) It does so without a change of frequency

(iii) It accomplishes this by electromagnetic induction and(iv) Is when the two electric circuits are in mutual, inductive of each other.

Half-wave Rectifier with Transformer Coupled Input Voltage

RL

Vin VPri VSec

Fig: (2.1) Half-wave Rectifier with Transformer Coupled Input Voltage

A transformer is often used to couple the ac input voltages from the source to the

rectifier circuits. Transformer coupling provides two advantages. First, it allows the source

voltage to be stepped up or stepped down as needed. Second, the ac power source is electrically

isolated from the rectifier circuit, thus reducing the shock hazard.

Basic ac circuit recall that the secondary voltage of a transformer equals the turns ratio

i

Sec

N N

Pr times the primary voltage.

If NSec > NPri, the secondary voltage is greater than the primary voltage.

ii

Sec V N

N V Pr

Pr sec )(=



If NSec < NPri, the secondary voltage is less than the primary voltage. If NSec = NPri, then

VSec = VPri.

UNIT: 3

RECTIFIER CIRCUITS

Rectifier circuits are found in all dc power supplies that operate from an ac voltage

source. They convert the ac input voltage to a pulsating dc voltage. The most basic type of

rectifier circuit is the half-wave rectifier. Although half-wave rectifiers have some applications,

the full-wave rectifiers are the most commonly used type in dc power supplies. These are two

types of full-wave rectifiers:

(1) full-wave center-tapped rectifier

(2) full-wave bridge rectifier

Full-wave Center-tapped Rectifier

The full-wave center-tapped rectifier uses two diodes connected to the secondary of a

center-tapped transformer, as shown in Fig: (3.1). The input voltage is coupled through the

transformer to the center-tapped secondary. Half of the total secondary voltage appears between

the center tap and each end of the secondary winding.

CT

D1

D2

RL

2

secV

2

secV

Vin

-

-

+

+

F

Fig: (3.1) A Full-wave Center-tapped Rectifier

For a positive half-cycle of the input voltage, the polarities on the secondary are as shown

in Fig: (3.2.a). This condition forward-biases the diode D1 and reverse-biases the diode D2 .The

current path is through D1 and the load resistor R L, as indicated.



D1

D2

RL

Vin

-

+

+

F

-

+

- +

+ -

0

0

Vout

Fig: (3.2.a) During positive half-cycles, D1 is forward-biased and D2 is reverse-biased.

For a negative half-cycle of the input voltage, the voltage polarities on the secondary are

as shown in Fig: (3.2.b). This condition reverse-biases D1 and forward-biases D2 .The current

path is through D2 and R L, as indicated. Because the output current during both the positive and

negative portions of the input cycle is in the same direction through the load, the output voltage

developed across the load resistor is a full-wave rectified dc voltage.

D1

D2

RL

Vin

F

-

+

- +

+ -

0

0

Vout

Fig: (3.2.b) During negative half-cycles, D2 is forward-biased and D1 is reverse-biased.

• Peak value of output voltage for the full-wave center-tapped rectifier:

V sceVp

out Vp 7.02

)()( −=

• Average value of output for the full-wave center-tapped rectifier:

π

)(2 out Vp AGV V =

• Diode peak inverse voltage for the full-wave center-tapped rectifier:

V V PIV out P 7.02 )( +=



Full-wave Bridge Rectifier

The full –wave bridge rectifier uses four diodes, as shown in Fig: (3.3.a). When the input

cycle is positive, diodes D1 and D2 are forward-biased and conduct current through R L. During

this time, diodes D3 and D4 are reverse-biased.

RL

Vin

F

-

+ 0

Vout

D1

D4D2

D3

Fig: (3.3.a) During positive half-cycles of the input, D1 and D2 are forward-biased and conduct current, D3 and D4

are reverse-biased.

When the input cycle is negative as shown in Fig: (3.3.b), diodes D3 and D4 are forward-

biased and conduct current in the same direction through R L as during the positive half-cycle.

During the negative half-cycle, D1 and D2 are reverse-biased. A full-wave rectifier output voltage

appears across R L as a result of this action.

RL

Vin

F

-

+ 0

Vout

-

+

-

+

D1

D4

D2

D3

Fig: (3.3.b) During negative half-cycles of the input, D3 and D4 are forward-biased and conduct current, D1 andD2 are reverse-biased.

• Peak value of output voltage for the full-wave bridge rectifier:

V Vpout Vp 4.1(sec))( −=

• Average value of output voltage for the full-wave bridge rectifier:

π

)(2 out VpV AVG =

• Diode peak inverse voltage for the full-wave bridge rectifier:

PIV = VP (out) + 0.7



UNIT: 4

POWER SUPPLY FILTER

A power supply filter ideally eliminates the fluctuations in the output voltage of a half –

wave rectifier and produces a constant-level dc voltage. The 60Hz pulsating dc output of a half-

wave rectifier or the 120Hz pulsating output of a full-wave rectifier must be filtered to reduce the

large voltage variations. Fig: (4.1) illustrates the filtering concepts showing a nearly smooth dc

output voltage from the filter. The small amount of fluctuation in the filter output voltage is

called ripple.

Vin

0 VFull-wave

Rectifier Filter

0 VVout

0

Fig: (4.1) Power supply filteringCapacitor Filter

A half wave rectifier with a capacitor filter is shown in Fig: 4.2. During the positive first

quarter-circle of the input, the diode is forward bias and presents a low resistance path, allowing

the capacitor to charge to within 0.7V of the input peak. When the input begins to decrease

below its peak, the capacitor retains its charge and the diode becomes reversed biased since the

cathode is more positive than the anode. During the remaining part of the cycle, the capacitor can

discharge only through the load resistor at a rate determines by the R LC time constant.

RLVin

+

-

+

-

+

-

+ -

0

VP(in)

t0

0t0

VP(in) –0.7VVc

Fig: (4.2) (a) Initial charging of capacitor (diode is forward-biased) happens only once when power is turn on.

RLVin

+

-

+

-

+ -

0

VP(in)

t0 0t0

Vc

t1

t1

Fig: (4.2) (b) Discharging through R L after peak of positive alternation (diode is reverse biased)



RLVin

+

-

+

-

+ -

0

VP(in)

t0 0t0

Vct1

t1

t2t2

Fig: (4.2) (c) Charging back to peak of input (diode is forward-biased)

Fig: (4.2) Operation of a half-wave rectifier with a capacitor filter

Ripple Voltage

The capacitor quickly at the beginning of a cycle and slowing discharges after the

positive peak. The variation in the output voltage due to charging and discharging is called the

ripple voltage.

0

Ripple

(a) Half-wave

0

Ripple

(b) Full-wave

Same Slope

Fig: (4.3) Comparison of ripple voltage for half-wave and full-wave signals with the same filter

capacitor and load and derived from same sine wave input



UNIT: 5

THE VOLTAGE REGULATORS

There are many type of circuit to regulate a certain dc voltage. Discrete circuits can be

constructed using feedback transistors to get a voltage regulator. There also exits many IC types

of voltage regulators. The well-known types of voltage regulator ICs are;

(1) The 78XX series - for positive regulators

(2) The 79XX series - for negative regulators

(3) The LM 317 - for adjustable positive regulators

(4) The LM 337 - for adjustable negative regulators

Fixed Positive Linear Voltage Regulators

The 78XX series of IC regulators is representative of three terminal devices that providea fixed positive output voltage. The three terminals are input, output and ground as indicated in

the standard fixed voltage configuration in Fig: (5.1.a).The last two digits in the part number

designate the output voltage. For example, the ‘7805’ is a +5V regulator. Other available output

voltages are given in Table: 5.1.

Capacitors although not always necessary are sometime used on the input and output as

indicated in Fig: (5.1.b). The output capacitor acts basically as a line filter to improve transient

response. The input capacitor is use to prevent unwanted oscillations when the regulator is some

distance from the power supply filter such that the line has a significant inductance.

The 78XX can produce output current in excess of 1A when used with an adequate heat

sink. The 78LXX series can provide up to 100mA, the 78MXX series can provide up to 500mA,

and the 78TXX series can provide in excess of 3A.

The input voltage must be at least 2V above the output voltage in order to maintain

regulation. The circuits have internal thermal overload protection and short-circuit current-

limiting features. Thermal overload occurs when the internal power dissipation becomes

excessive and the temperature of the device exceeds a certain value.

78 XX+ INPUT + OUTPUT

IN OUT

GNDC1 C2

78 XX

1 2 3

PIN 1 - INPUT

2 - GROUND

3 - OUTPUT

Fig: (5.1.a) Pin Layout Fig: (5.1.b) Standard configuration



TYPE NUMBER OUTPUT VOLTAGE

7805 +5V

7806 +6V

7808 +8V

7809 +9V

7812 +12V

7815 +15V

7818 +18V

7824 +24V

Table (5.1) 78XX series

Fixed Negative Linear Voltage Regulators

The 79XX series is typical of three-terminals IC regulators that provide a fixed negative

output voltage. This series is the negative counterpart of the 78XX series and shares most of the

same features and characteristics. Fig: (5.2.a & b) and Table (5.2) indicate the pin layout; the

standard configuration and part numbers with corresponding output voltage that are available.

79 XX

1 2 3

79 XX- INPUT - OUTPUT

IN OUT

GND

PIN 1 - GROUND

2 - INPUT

3 - OUTPUT

C1 C2


Table (5.2) 79XXseries

5.3 Adjustable

Positive Linear

Voltage Regulators

Fig: (5.3.a)

Pin Layout

Fig: (5.3.b) Standard configuration

The LM317 is an excellent example of the three- terminal positive regulator with an

adjustable output voltage. Notice that there is an input, an output and an adjustable terminal.

The external fixed resistor R 1 and the external variable resistor R 2 provide the output voltage

adjustment. Vout can be varied from 1.2V to 37V depending on the resistor values. The LM317

can provide over 1.5A of output current to a load.

Type Number Output Voltage

7905 -5V

7905.2 -5.2V

7906 -6V

7908 -8V

7912 -12V

7915 -15V

7918 -18V

7924 -24V



The LM317 is operated as a “floating” regulator because the adjustment terminal is not

connected to ground, but floats to whatever voltage is across R 2. This allows the output voltage

to be much higher than that of a fixed-voltage regulator.

IREF =11

REFR

V25.1R

V =

A constant 1.25V reference voltage (VREF), is maintained by the regulator between the

output terminal and the adjustment terminal. This constant reference voltage produces a constant

current (IREF) through R 1 regardless of the value of R 2. IREF also flows through R 2

2REF1REF2R 1R OUT R IR IVVV +=+=

)R R (I 21REF +=

)R R (R

V21

1

REF+=

)1( 2

R

RV V REF OUT +=

Adjustable Negative Linear Voltage Regulators

LM337

1 2 3

PIN 1 -ADJ2 - INPUT3 - OUTPUT

- OUTPUT- INPUT IN OUT

LM337

ADJ

R2

R1


The LM 337 is the negative output counterpart of the LM 317 and is a good example of

the type of IC regulator. Like the LM 317, the LM 337 requires two external resistors for output

voltage adjustments as shown in Fig: (5.4.b). The output voltage can be adjusted from -1.2V to

-37V, depending on the external resistors values.



UNIT: 6

THE REGULATED DUAL VOLTAGE DC POWER SUPPLY

By combining the step down transformer, rectifier, filters and voltage regulators 7812 and

7912 together, we get a regulated dual voltage ( ±12V) dc power supply circuit as shown in

Figure: (6.1).

This is a simple circuit, which gives regulated -12 -0- +12V supply. ICs 7812 and 7912

are used here as positive and negative regulators respectively. The unregulated input voltage

must always be higher than the regulators output voltage by at least 3V in order for it to work. If

the input/output voltage difference is greater than 3V then the excess potential must be dissipated

as heat. The maximum power dissipated in this type of series regulator is the power dissipated in

the internal pass transistor, which is approx.(VS max - Vout) IL max. Hence, as the load current

increases, the power dissipated in the internal pass transistor increases. If ILoad exceeds 0.75 A,



the IC package should be secured to a heat sink. When this is done, I Load can increase to about 1.5

A.

The filter capacitors, C1 & C2, are chosen to maintain the voltage input range to the

regulator. Capacitors C5 & C6 improve the regulators ability to react to sudden changes in load

current and to prevent uncontrolled oscillations. C3 and C4 are small film capacitors to improve

transient response and filter high frequency line noise.

Fig: (6.1) Circuit diagram of the regulated dual voltage DC power supply

PCB fabrication:

The PCB is fabricated using positive laser print of the circuit layout. The steps followed are

discussed below. The PCB layout is shown in Fig. 6.2.

The printed circuit layout is printed on the photo paper using a laser printer.

On the clean copper clad of the size of circuit layout, the mirror image of the layout is

transferred using a household hot electric iron.

The photo paper is removed by dipping the copper clad in warm water.

For etching the copper clad is immersed in a tray containing ferric chloride aqueous

solution. The copper clad plate is moved up and down continuously. When etching is

complete the copper clad plate is removed and rinsed with water and then dried in open

air.

The laser toner ink is removed by using a smooth sand paper.

The drilling is made by using a hand drill of 0.8 mm diameter.



Again the circuit layout is scrubbed with a fine sand paper.

Thus the P.C.B. is ready for mounting the components.

Fig: (6.2) PCB layout of the regulated dual voltage DC power supply

The component mounting view of the P.C.B. is shown in Fig. 6.3.

Fig: (6.3) Component Mounting on PCB

The components and their values used to build the circuit are given in the following table.

Components Value

• Capacitors

C1, C2

C3,C4

C5, C6

• Diodes

D1, D2, D3, D4

• ICs

IC1

2200µ F/35V

100 nF = 0.1µ F

10µ F/25V

1N 4001 diodes



IC2

• Miscellaneous

Transformer

Switch

LM 7812T, positive voltage regulator

LM 7912T, negative voltage regulator

220V AC Pri: to 17V-0-17V, 1A Sec:

ON/OFF switch

UNIT: 7

TEST AND RESULTS

This Unit will discuss some of the tests carried out on the final circuit which was

discussed in chapter (6).

The equipments used is analyzing circuit is vital in yielding the correct information about

the advantages and any design. During the course of final test, the equipments used were a digital

multi-meter (DMM). This was used to measure the line regulation, the load regulation, and

output DC voltages. The results of line regulation and load regulation are shown in Fig. 7.1 and

Fig. 7.2 respectively. The study of Fig. 7.1 shows that the output of the power supply remains

constant for the input of line voltage from 160V A.C. Similarly the study of Fig. 7.2 shows that

the output voltages of the power supply remain constant and the current decreases with increase

in load resistance.



Conclusion:

The circuit used to fabricate the dual power supply using ICs 7812 and 7912 work well.

It can be used for the study of electronic circuits.

REFERENCES

(1) Floyd, Thomas L.

"Electronic Devices"

Fourth Editions, Prentice Hall, Inc, 1996

(2) Wason Kamal

"Electronics Projects"

Second Edition, EFY Enterprises Pvt Ltd, 1999

(3) http://www.electronicsforu.com

dual power supply final

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