electronics devices and circuits

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MODULE 1

POWER SUPPLIES

INTRODUCTION

In general, electronic circuits using tubes or transistors require a source of d.c. power. Batteries are rarely used for this purpose as they are costly and require frequent replacement. In practice, d.c. power for electronic circuits is most conveniently obtained from commercial a.c. lines by using rectifier-filter system, called a D.C. POWER SUPPLY.

The rectifier-filter combination constitutes an ordinary d.c. power supply. The d.c. voltage from an ordinary power supply remains constant so long as a.c. mains voltage or load is unaltered. However, in many electronic applications, it is desired that d.c. voltage should remain constant irrespective of changes in a.c. mains or load. Under such situations, voltage regulating devices are used with ordinary power supply. This constitutes REGULATED D.C. POWER SUPPLY and keeps the d.c. voltage at fairly constant value.

1.1 REGULATED D. C POWER SUPPLIES

BLOCK DIAGRAM :

The regulated dc power supply mainly consists of four parts

1.TRANSFORMER

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2.RECTIFIER

3.FILTER

4.REGULATOR

→TRANSFORMER

Transformer works on the basis of ELECTROMAGNETIC INDUCTION and they are

mainly classified into two

STEPUP TRANSFORMER

STEPDOWN TRANSFORMER

Stepup transformer upconvert the input voltage where stepdown transformer downconverts.

For a DC Power Source we have to use stepdown transformers, to covert the high voltage AC

supply to low voltage DC. Transformer provides an isolation between supply lines and the

devices.

→RECTIFIER

Rectifiers are used to convert the sinusoidal AC voltage to non-sinusoidal pulsating DC. The

main component used in Rectifiers are diodes due to its switching action. They will conduct

Current only in one direction, hence the voltage. So we can use them on rectifiers to make the

alternating Current unidirectional.

Rectifiers are classified into

HALF WAVE RECTIFIERS

FULL WAVE RECTIFIERS

→FILTERS

Filters are used to eliminate or filter-out the unwanted ripples from the rectified output.

Filters play an important role in dc Power supplies, they make the pulsating dc steady. Filters

are usually made from components such as capacitor, inductor and resistors.



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→VOLTAGE REGULATOR

Voltage Regulators are used to regulate the output Voltage over load. They make the Voltage

unvaried with load connected to it. This will eliminates the remaining ripples from the filter

output. The output from Voltage Regulator may be the required DC. Voltage Regulators

includes some safety measures such as Current Limiting, short circuit etc.

1.2 HALF WAVE RECTIFIER

Circuit Diagram:

The half-wave rectifier circuit using a semiconductor diode with a load resistance RL. The

diode is connected in series with the secondary of the transformer and the load resistance R L, the

primary of the transformer is being connected to the ac supply mains.

Working of a Half wave rectifier:

The ac voltage across the secondary winding changes polarities after every half cycle.

During the positive half-cycles of the input ac voltage i.e. when upper end of the secondary

winding is positive w.r.t. its lower end, the diode is forward biased and therefore conducts

current. If the forward resistance of the diode is assumed to be zero (in practice, however, a

small resistance exists) the input voltage during the positive half-cycles is directly applied to

the load resistance RL, making its upper end positive w.r.t. its lower end. The waveforms of

the output current and output voltage are of the same shape as that of the input ac voltage.



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During the negative half cycles of the input ac voltage i.e. when the lower end of the

secondary winding is positive w.r.t. its upper end, the diode is reverse biased and so does not

conduct. Thus during the negative half cycles of the input ac voltage the current through and

voltage across the load remains zero if the reverse current, being very small in magnitude, is

neglected. Thus for the negative half cycles no power is delivered to the load.

Thus the output voltage developed across load resistance RL (VL) is a series of

positive half cycles of alternating voltage, with intervening very small constant negative

voltage levels, It is obvious from the figure that the output is not a steady dc, but only a

pulsating dc wave. Since only half-cycles of the input wave are used, it is called a half-wave

rectifier.

Power Supply Specifications.

The most important characteristics which are required to be specified for a power

supply are given below :

1.The required output dc voltage.

2.The average and peak currents in the diode.

3.The peak inverse voltage (PIV) of each diode.

4.The regulation.

5.The ripple factor.

Advantages and Disadvantages of Half wave rectifier:

(i)Advantages: Simple circuit and low cost.

(ii)Disadvantages:

1. The output current in the load contains, ac components in addition to dc

component. Ripple factor is high.



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2. The power output and, therefore, rectification efficiency is quite low. This is due to

the fact that power is delivered only half the time.

3. Transformer utilization factor is low.

4. The D.C output is small

1.3 FULL WAVE RECTIFIER

Full wave rectifier mainly two types

A. Centre -tap full wave rectifier

B. Bridge Rectifier

A. Centre Tap full wave rectifier:

Circuit Diagram:

The full wave rectifier circuit consists of two power diodes connected to a single load

resistance (RL) with each diode taking it in turn to supply current to the load. When point A

of the transformer is positive with respect to point C, diode D1 conducts in the forward

direction as indicated by the arrows. When point B is positive (in the negative half of the



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cycle) with respect to point C, diode D2 conducts in the forward direction and the current

flowing through resistor R is in the same direction for both half-cycles. As the output voltage

across the resistor R is the phasor sum of the two waveforms combined, this type of full wave

rectifier circuit is also known as a "bi-phase" circuit.

B.The Full Wave Bridge Rectifier

Another type of circuit that produces the same output waveform as the full

wave centre tap rectifier circuit above, is that of the Full Wave Bridge Rectifier. This type

of single phase rectifier uses four individual rectifying diodes connected in a closed loop

"bridge" configuration to produce the desired output. The main advantage of this bridge

circuit is that it does not require a special centre tapped transformer, thereby reducing its size

and cost. The single secondary winding is connected to one side of the diode bridge network

and the load to the other side as shown below.

Circuit Diagram:

The four diodes labelled D1 to D4 are arranged in "series pairs" with only two diodes

conducting current during each half cycle. During the positive half cycle of the supply, diodes

D1 and D2 conduct in series while diodes D3 and D4 are reverse biased and the current flows

through the load as shown below.



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The Positive Half-cycle

During the negative half cycle of the supply, diodes D3 and D4 conduct in series, but diodes

D1 and D2 switch "OFF" as they are now reverse biased. The current flowing through the

load is the same direction as before.

The Negative Half-cycle

As the current flowing through the load is unidirectional, so the voltage developed across the

load is also unidirectional the same as for the previous two diode full-wave rectifier.

Advantage of bridge rectifier

1. The rectification efficiency of full-wave rectifier is double of that of a half-wave rectifier.

2. The ripple voltage is low and of higher frequency in case of full-wave rectifier so simple

filtering circuit is required.



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3. Higher output voltage, higher output power and higher Transformer Utilization Factor

(TUF) in case of a full-wave rectifier.

4. No centre tap is required in the transformer secondary

5. Bridge rectifier is highly suited for high voltage applications.

COMPARISON BETWEEN RECTIFIERS

Half Wave

Rectifier

Full Wave Centre

Tap

Bridge Rectifier

1 2 4 Number of diodes

No Yes No Need of Centre tapping

Vm/π 2Vm/π 2Vm/π Average dc voltage( Vdc)

Im/π 2Im/π 2Im/π Average d.c. current( Idc)

1.21 0.48 0.48 Ripple Factor (γ)

1.57 1.11 1.11 Form Factor

Less Less More Voltage Drop in Diode



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1.4 FILTER CIRCUITS

A. SERIES INDUCTOR FILTER

Principle Of Operation:

In this arrangement a high value inductor or choke L is connected in series

with the rectifier element and the load, as illustrated in figure. The filtering action of an

inductor filter depends upon its property of opposing any change in the current flowing

through it. When the output current of the rectifier increases above a certain value, energy is

stored in it in the form of magnetic field and this energy is given up when the output current



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falls below the average value. Thus by placing a choke coil in series with the rectifier output

and load, any sudden change in current that might have occurred in the circuit without an

inductor is smoothed out by the presence of the inductor L.

The function of the inductor filter may be viewed in terms of impedances. The

choke offers high impedance to the ac components but offers almost zero resistance to the

desired dc components. Thus ripples are removed to a large extent. Nature of the output

voltage without filter and with choke filter is shown in figure.

B.SHUNT CAPACITOR FILTER



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Principle of operation:

This is the most simple form of the filter circuit and in this arrangement a high value

capacitor C is placed directly across the output terminals, as shown in figure. During the

conduction period it gets charged and stores up energy .During non conducting period it give

up energy to load.

The function of the capacitor filter may be viewed in terms of impedances. The large

value capacitor C offers a low impedance shunt path to the ac components or ripples but

offers high impedance to the dc component. Thus ripples get bypassed through capacitor C

and only dc component flows through the load resistance RL.



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Half-Wave Rectifier With Shunt Capacitor Filter.

The waveforms of ac input voltage, rectified and filtered output voltages and

load current are shown in figure. During the positive half cycle of the ac input, the diode of

the rectifier is forward biased and so it conducts. This quickly charges the capacitor C to peak

value of the supply voltage VSmax because of almost zero charging time constant. This is

shown by point b in figure. After being fully charged, the capacitor holds the charge till input

ac supply to the rectifier goes negative. During the negative half cycle, the diode gets reverse

biased and so stops conduction. So the capacitor C discharges through load resistance RL and

loses charge. Voltage across RL (VL) or across C (vc), both being equal, decreases

exponentially with time constant CRL along the curve be, as illustrated.

Because of the large discharge time constant CRL, the capacitor does not

have sufficient time to discharge appreciably. Due to this fact the capacitor maintains a

sufficiently large voltage across RL, even during the negative half-cycle of the input supply.

During rectified voltage exceeds the capacitor voltage vc represented by point C in fig. The

capacitor again gets quickly charged to to Vg max (or VLmax) as represented by point d in

the figure.This process of charging and discharging is repeated for each cycle of input supply

voltage. seen, from the figure, that nearly constant dc voltage appears across load resistance

RL at all times and also the dc component of output voltage is increased considerably.



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C. CHOKE –INPUT OR L-SECTION FILTER

Principle of Operations:

Choke-input filter consists of a choke L connected in series with the rectifier

and a capacitor C connected across the load . This is also sometimes called the L-section

filter because in this arrangement inductor and capacitor are connected, as an inverted L. ln

figure only one filter section is shown. But several identical sections are often employed to

improve the smoothing action. (The choke L on the input side of the filter readily allows dc to

pass but opposes the flow of ac components because its dc resistance is negligibly small but

ac impedance is large. Any fluctuation that remains in the current even after passing through

the choke are largely by-passed around the load by the shunt capacitor because Xc is much

smaller than RL. Ripples can be reduced effectively by making XL greater than Xc at ripple

frequency. However, a small ripple still remains in the filtered output and this is considered

negligible if it than l%. The rectified and filtered output voltage waveforms from a full-wave

re with choke-input filter are shown in figure.



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D. CAPACITOR- INPUT OR π -FILTER

Principle Of Operations:

Such a filter consists of a shunt capacitor C1 at the input followed by an L-

section filter formed by series inductor L and shunt capacitor C2. This is also called the n-

filter because the shape of the circuit diagram for this filter appears like Greek letter n (pi).

Since the rectifier feeds directly into the capacitor so it is also called capacitor input filter.

In this filter, the input capacitor C1 is selected to offer very low reactance to

the ripple frequency. Hence major part of filtering is accomplished by the input capacitor C1.

Most of the remaining ripple is removed by the L-section filter consisting of a choke L and

capacitor C2.)



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1. The capacitor C1 offers low reactance to the AC component of the rectifier output while it

offers infinite resistance to the DC component. As a result the capacitor shunts an

appreciable amount of the AC component while the DC component continues its

journey to the inductor L

2. The inductor L offers high reactance to the AC component but it offers almost zero

resistance to the DC component. As a result the DC component flows through the

inductor while the AC component is blocked.

3. The capacitor C2 bypasses the AC component which the inductor had failed to block.

Advantages: More output voltage & Ripple less output

Disadvantages: Large in size and weight & High cost

1.5 VOLTAG REGULATOR

A voltage regulator is a device that maintains a relatively constant output voltage even

though its input voltage may be highly variable. There are a variety of specific types of

voltage regulators based on the particular method they use to control the voltage in a circuit.

1.5.1 TRANSISTOR VOLTAGE REGULATOR

Basically there are two types of transistor voltage regulators. They are

(1) Series Voltage Regulators and

(2) Shunt Voltage Regulators.

Each type of circuit can provide an output dc voltage that is regulated or maintained at

a predetermined value even if the input voltage varies or the load connected to the output

terminal changes.

(1)Transistor Series Voltage Regulator or Emitter Follower Voltage Regulator

The basic connection of a series voltage regulator circuit is shown in the block

diagram given in the figure below.



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BLOCK DIAGRAM

Block Diagram Explanation:

The series element controls the magnitude of the input voltage that gets to the output. The

output voltage is sampled by a circuit that provides feedback voltage to be compared to a

reference voltage.

If the output voltage increases the comparator circuit provides a control signal to

cause the series control element to reduce the magnitude of the output voltage — thereby

maintaining the output voltage. On the other hand, if the output voltage falls, the comparator

circuit provides a control signal to cause the series control element to increase the magnitude

of output voltage.



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CIRCUIT DIAGRAM

CIRCUIT EXPLANATION

A simple series voltage regulator using an NPN transistor and a Zener diode is shown

in the figure. This circuit is called a series regulator because collector and emitter terminals of

the transistor are in series with the load, as illustrated in the figure. This circuit is also called

an emitter follower voltage regulator because transistor Q is connected in emitter follower

configuration. The unregulated dc supply is fed to the input terminals and regulated output

voltage Vout is obtained across the load resistor RL. Zener diode provides the reference voltage

and the transistor acts as a variable resistor, whose resistance varies with the operating

conditions .

PRINCIPLE OF OPERATION

Keeping in mind the polarities of different voltages we have

VOUT = IL*RL ( 2)



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CASE 1: LINE REGULATION (ASSUME RL CONSTANT).

Let the supply (or input) voltage increase which will cause the output voltage V out

to increase. An increase in output voltage Vout will result in decrease of VBE because Vz is

fixed and decrease in VBE will reduce the level of conduction. When the level of conduction

decreases, the output load current IL get reduced. Then IL*RL (VOUT) get reduced. So the

output voltage maintains a constant value.

CASE 2: LOAD REGULATION (ASSUME VIN CONSTANT).

Now let us consider the effect of change in load on the output voltage. Let load

resistance RL decreases. Under such a situation the output voltage Vout tends to fall and,

therefore, VBE tends to increase. As a result the conduction level of the transistor will increase

leading to decrease in the collector-emitter resistance. When the conduction level increases ,

IL also increased. Then IL*RL (VOUT) get increased. So the output voltage maintains a constant

value.

Limitations

The output voltage cannot be maintained absolutely constant because both VBE and Vz

decrease with the increase in room temperature.

The output voltage cannot be changed as there is no provision for it in the circuit.

It cannot provide good regulation at high currents because of small amplification

provided by one transistor.

It has poor regulation and ripple suppression with respect to input variations as

compared to other regulators.



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(2) Transistor Shunt Voltage Regula tor

The basic connection of a shunt voltage regulator circuit is shown in the block diagram given in the figure below.

BLOCK DIAGRAM

Block Diagram Explanation:

Shunt voltage regulator provides regulation by shunting current away from the

load. The block diagram of such a voltage regulator is depicted in the figure. The input

unregulated voltage provides current to the load. Some of this current is shunted away by the

control element to maintain the regulated output voltage across the load. If the output voltage

tends to change due to change in load, the sampling circuit provides a feedback signal to a

comparator circuit which then provides a control signal to vary the magnitude of current

shunted away from the load. For example, the output voltage tends to fall, the sampling

circuit provides a feedback signal to the comparator circuit which then provides a control

signal to draw lesser shunt current, providing more load current, thereby keeping the

regulated voltage constant.



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CIRCUIT DIAGRAM

CIRCUIT EXPLANATION

A shunt voltage regulator using an NPN transistor and a Zener diode is shown in the

figure. A series resistance RSE is connected in series with the unregulated (or input), supply.

Zener diode is connected across the base and collector terminals of the NPN transistor and

the transistor is connected across the output, as shown in the figure.

PRINCIPLE OF OPERATION

Keeping in mind the polarities of different voltages we have

VOUT = IL*RL ( 2)



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VR= I *RSE (3)

CASE 1: LINE REGULATION (ASSUME RL CONSTANT).

If the input (or supply) voltage increases, it causes increase in Vout and VBE

resulting in increase in base current IB and therefore, increase in collector current Ic (Ic = β IB).

Thus with the increase in supply voltage, supply current I increases causing more voltage

drop in series resistance RSE and thereby reducing the output voltage. This decrease in output

voltage is enough to compensate the initial increase in output voltage. Thus output voltage

remains almost constant.

CASE 2: LOAD REGULATION (ASSUME VIN CONSTANT).

If the load resistance RL decreases, it causes decrease in Vout and VBE resulting

in decrease in base current IB and therefore, decrease in collector current Ic (Ic = β IB). We

know in load regulation , input is constant, means “I” is constant.

I=IB+IC+IL (4)

So when IB and IC decreases IL get increased to make I constant. If IL increases

VOUT increased (IL *RL). Thus output voltage remains constant.

1.6 ZENER DIODE SHUNT VOLTAGE REGULATOR

CIRCUIT DIAGRAM:



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Here zener diode is connected parallel to the load resistor . So this is an example of

shunt voltage regulator. A resistor Rs is necessary to limit the current through the zener. Here

zener diode is reverse biased, hence it give a constant output voltage Vout.

PRINCIPLE OF OPERATION:

Consider the current through Rs is,

Is= Iz + IL (2)

CASE 1: LINE REGULATION

When Vin increases , source current Is increased. (from equation 1)

IL can not be changed , to make output constant.

So only available option is increase the zener current Iz (from equation 2)

So when the input voltage increase , zener conducts more amount of current and make

the output constant.

CASE 1: LOAD REGULATION

When we reduce the load resistance RL , zener conducts less.

So the zener current get reduced and more current is passed to the load.

So in output a constant voltage maintained.

1.7 IC VOLTAGE REGULATOR



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Fixed output IC voltage regulator:

They are mainly classified in to two types

1. Fixed output positive voltage regulator (78XX series)

2. Fixed output negative voltage regulator (79XX series)

Fundamental block diagram of three terminal( 78XX or 79XX) ic

voltage regulator

The error amplifier is used to maintain a constant voltage through a negative

feedback. The internal voltage reference is tightly controlled during the fabrication of IC. The

series-pass element is driven by the output of the error amplifier. IF acts as an automatically

controlled variable resistor. This resistance varies as required for maintaining the output

voltage constant. The series-pass element is typically a BJT that is rated to pass the maximum

load current.



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1. Fixed output positive voltage regulator (78XX series)

The 78xx family is commonly used in electronic circuits requiring a regulated power

supply due to their ease-of-use and low cost. For ICs within the family, the xx is replaced

with two digits, indicating the output voltage (for example, the 7805 has a 5 volt output,

while the 7812 produces 12 volts). The 78xx line are positive voltage regulators: they

produce a voltage that is positive relative to a common ground.

Different types and voltage range

Part Number Output Voltage (V)

7805 +57806 +67808 +87810 +107812 +127815 +157818 +187824 +24



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BASIC BLOCK CONNECTION

An unregulated, input voltage Vin is filtered by capacitor C1, and connected to the pin .1 (IN terminal) of IC. The pin 2 (OUT terminal) of the IC provides a regulated dc voltage which is filtered by capacitor C2 (mostly for any high frequency noise). The third pin (GND terminal) of the IC is connected to ground. While the input voltage may vary over some permissible voltage range, and the output load may vary over some acceptable range, the output voltage remains constant within specified voltage variation limits.

2. Fixed output negative voltage regulator (79XX series)

The 79xx family is commonly used in electronic circuits requiring a regulated power supply

due to their ease-of-use and low cost. For ICs within the family, the xx is replaced with two

digits, indicating the output voltage (for example, the 7905 has a -5 volt output, while the

7912 produces -12 volts). The 79xx line are positive voltage regulators: they produce a

voltage that is positive relative to a common ground.

Different types and voltage range

Part Number Output Voltage (V)



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7905 -57906 -67908 -87910 -107912 -127915 -157918 -187924 -24

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