chapter 6 _ regulated power

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Chapter 6 : Regulated Power

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Chapter 6 : Regulated Power Supplies

Contents:

Zener diode as Voltage Regulator, Transistor Series andShunt Regulators, Current limiting, Line and Load Regulation.

Points To Ponder:

Power supplies are used when dc power is required suchac in adapters, power supply circuits in TV, VCR, DVDetc.

Zener diode is used as voltage regulator because of itsproperty to operate in the breakdown region.

Regulated power supplies uses zener diodes as itprovides constant output irrespective of ac mainfluctuations.

The limitation of using zener diode as a regulator is thatits efficiency is poor at heavy loads because of lare lossesin the resistance connected at series.

The transistor series voltage regulator is an improvedversion of zener diode voltage regulator.

The output voltage appears across the load will beapproxirnataly equal to zener voltage i.e. VdC(OUt) V2.

In transistor shunt. voltage regulator the voltage acrossthe load is set by the zenerdiod and transistor baseemitter voltage i.e.VL V + VBE

Current limiting is one form oI short-circuit or over loadprotection. In this a short-circuit sensing resistance iused with a transistor which limits the maximum loadcurrent,

Q 1. What is the practical importance of voltageregulation in power supplies?

Chapter 6 : Regulated Power Supplies | Analog Electronics http://ptuece.loremate.com/ane/node/6

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OR

Explain clearly the role of regulated power supply.

Ans.

In an unregulated power supply, output voltage changeswhenever input supply voltage or load resistancechanges.

It is never constant.

The change in voltage from no-load to full load conditionis called voltage regulation.

The practical importance of voltage regulation in powersupplies is to reduce these variations to zero or, at leastto the minimum possible value.

The percentage regulation or, simply regulation of apower supply is given by

Q 2. What are the limitations of transistorized powersupphes?

Ans.

The transistorised power supplies have the limitation thatthe output voltage available is restricted by the V0 of theseries transistor used.

The power rating of the transistor used, as a series loserdepends on the voltage difference between the input andoutput voltages.

This difficulty can be minimised to a great extent byusing thyristor.

Thyristors have the ability to control large power withminimal control power, and this control power does nothave to remain continuous as in the case of the base


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current of a transistor.

Transistors for a relatively Imgh voltage, high poweroperations are rarely available.

Q 3. Discuss the V-I characteristics of Zener diode.

Ans.

V-I characteristics of zener diode between voltage andcurrent as:

1.A zener diode characteristics are always plated shown infigure above.

2. It is like an ordinary diode characteristics except that ithave a sharp breakdown voltage.

3. The sharp breakdown voltage is called zener voltage Vz.

Q 4. Draw the equivalent circuit of zener diode asvoltage regulator and explain.

Ans. Zener diode as voltage regulator:


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The major application of a zener diode is as a voltageregulator.

It provides constant voltage to the load from a sourcewhose voltage may vary by using potentiometer atinput..

The zener diode of zener voltage Vz is reverseconnected across the load RL across which constant voltage

is required (Vo = Vz).

The resistance A is connected in series with the circuitwhich absorbs output voltage fluctuations so as tomaintain constant voltage V0 across the load.

Q 5. What is the use of Bleader in Zener voltageregulator?

Ans.

In zener voltage regulator, when the input voltage (Vin)

is more than zener voltage (Vz) this will cause the zener diode to

conduct a large current Iz and more current flows through series

resistor A which increases the voltage drop across the bleeder

resistor A.

Hence the constant voltage Vo (Vz) is maintained across the

load RL

Q 6. What is PIV of a diode in a rectifier circuits?

Ans.

PIV stands for peak inverse voltage.

It is the voltage which a diode pan withstand when it is


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connected in reverse biased.

PIV for half wave rectifier is Vm across each diode and for

full wave center tap rectifier the PIV is 2Vmacross each diode.

Q 7. What is zener diode? Draw its V-Icharacteristics.

Ans.

Zener diode is the diode which is connected in revnrsebiased and provides constant voltage to the load from asource whose voltage may vary by using potentiometerat input.

It can act as a voltage regulator.

V-I characteristics of zener diode

A zener diode characteristics are always plated between-voltage and shown in fig.

It is like an ordinary diode characteristics except that ithave a sharp breakdown voltage.

The sharp breakdown voltage is called zener voltage V.

Q 8. Explain line and load regulation?

Ans.

There are two types of load line regulatibn:


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1. D.C. load line :

In order to draw the dc load line Qf a transistor, oneneed to know only its cut-off and saturation points.

It is a straight line joining these two points.

For the CE circuit of following fig.1 the load line is drawnin fig. 2.current as

Point A is the cut-off point and B is the saturation point.

Consider the following two particular cases

lf Vcc and RL are known only then load line can be drawn.Slope of load line is

0 point i.e. the Quiescent point is point which lies betweencut off and saturation i.e. in the midway

2. AC load line: This line is steeper than the dc line but thetwo intersect at the Q-point determined by biasing dc voltageand currents.

In AC load line the cut off is given by


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Q 9. Explain zener diode breakdown mechanism.

Ans.

Zener breakdown takes place in a very thin junction i.e.when both sides of the junction are very heavily dopedand the depletion layer is narrow.

In the zener breakdown mechanism, the electric fieldbecomes as high as jQ7 V/rn in the depletion layer withonly a small applied reverse bias voltage.

In this process it becomes possible for some, electrons tojump across the barrier from the valence band inp-material to some of the unfilled conduction band inn-material.

This process is known as zener-breakdown. In thisprocess, the junction is not damaged.

The junction regains its original position when thereverse voltage is removed.

This process is used in the zener diodes.

The zener breakdown does not results in the destructionof the diode, as long as current through the diode islimited by the external circuit to a level within its powerhandling capabilities.


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Q 10. Explain in detail the effect of temperature onzener diode.

Ans.

A junction having a narrow depletion layer width andhence high field intensity (E 1 Q6 V/cm even at low‘voltages) will break down by the zener mechanism.

An increase in temperature increases the energies of thevolume electrons and hence makes it easier for theseelectrons to escape from the covalent bonds.

Less applied voltage is therefore required to pull theseelectrons from their positions in the crystal lattice andconvert them into conduction electrons.

Thus the zener breakdown voltage decreases withtemperature:A junction with broad depletion layer and therefore a lowfield intensity will breakdown bythe avalanche mechanism.

In this case, intrinsic carriers to concide with valenceelectrons and create avalanche multiplication.

As the temperature increases, the vibrationaldisplacement of atoms in the crystal grows.

This vibration increases the probability of collision withthe lattice atoms of the intrinsic particles as they crossthe deflectron width.

The intrinsic holes and electrons thus have lessopportunity to gain sufficient energy between collisionsto start the avalanche process.

Therefore the value of the avalanche voltage mustincrease with increase in temperature.

Q 11. Draw and design a zener voltage regulator circuitW provide output of 12V for a maximum load current of0.5A, when the input voltage variation is 15 to 18 V.

Ans.


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To regulate 12V across the load,

We must select a zener with

Vz= 1 2V

To meet the load current, the current supplied by the source is

Is = 0.5 A

Q 13. What is the need of voltage regulator circuit in apower supply? Discuss the application of zener diode asvoltage regulator.

Ans.


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In an unregulated power supply, output voltage changeswhenever input supply voltage or load resistancechanges.

It is never constant.

The change in voltage from no- load to tuli-ioad conditionis called voltage regulation.

The practical importance of voltage regulation in powersupplies is to reduce these variations to zero or, at least,to the minimum possible value

The percentage regulation or, simply, regulation ot apower supply is given by

Zener diode as voltage regulator:


It provides constant voltage to the load from a sourcewhose voltage may vary by using potentiometer at input.

The zener diode of zener voltage Vz is reverse connected

across the load RL across which constant voltage is required

(Vo = Vz).

The resistance R is-connected in series with the circuitwhich absorbs output voltage fluctuations so as tomaintain constant voltage V0 across the load.


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Q 14. Write a short note on transistor series and shuntregulators.

OR

With the help of a circuit diagram explain with workingof a transistor series regulator.

Ans.

Transistor series voltage regulator :

The circuit is as shown in fig.

It is also called as emitter-follower regulator because thevoltage at the emitter follows the base voltage.

In this, the transistor behaves like a variable resistancewhose resistance is determined by the base current.

It is called pass transistor because total current to beregulated passes through it.

Keeping in mind the polarities of different voltages theyare related by the equation derived by KVL

Current increases by decreasing RL, VL decreases.Thus, VBE will

increase because Vz is fixed. (‘.‘ VL = ILRL if RL increases

VLdecreases).

R is used for limiting current passing through the zenerdiode,

Transistor shunt voltage regulator:It is as shown in fig. Ituses transistor in shunt, Since path AB is in parallel acrossVL, we from KVL


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Since V is fixed, any decrease or increase in VL wiH havea corresponding effect on1 VBE, suppose VL decreases,then as seen from the above relation, VBE alsodecreases.

As a result, I decreases, hence, 1c = Bl8 decreases,thereby decreasing I and hence VR

Consequently, VL increases because at all times

Same line of logic applies in case VL increases.

Q 15. A zeher diode regulator has vz = 12V. The inputvoltage may vary from 20 to 35 V and load current from15 to 75 mA. To hold voltage constant under allconditions, what should be the value of seriesresistance?

Ans.

Zener regulator must operate in the breakdown region inorder to hold output voltage constant under all operatingconditions. Thus, there must be zener current for allinput voltages and load currents.

The worst case occurs when the input voltage isminimum and load current is maximum because thenzener current drops to a minimum value.


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Q 16. Write note on: Voltage regulator.

Ans.

Voltage regulator:

In case of power supplies, the dc output voltage varieswithi the change in ac input voltage.

To get the constant oUtput voltage i.e. withoutfluctuation voltage regulator circuit is used.

The simplest regulator circuit consists of a resistor Rconnected in series with the input voltage and a zenerdiode connected in parallel with load as shown in fig.

Let be the voltage to the regulator circuit. As long asthe voltage across AL is than the zener breakdown voltage V,the zener diode does not conduct. If the zener diode don’tconduct, the resistor R5 and RL make a potential divideracross V. At an increased the voltage across RL becomesgreater than the zener breakdown voltage. The resistor R5bnits the zéner current from exceeding its rated maximum i.eIzman From fig.

If load resistance RL is reduced, IL should increase,the current lz through zener falls by the same percentage in order to

maintain constant current Is. Thus voltage drop across Rsbecomes constant. Hence, the output voltage Vo remainsconstant.

If on the other hand, the load current should decrease,the zener diode passes an extra current Iz such that thecurrent Is is kept constant. The output voltage of the circuitis thus stabilized.


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Q 17. Show that a full-wave rectifier is twice as efficientas a half-wave rectififer?

Ans.

Efficiency is given by:

The efficiency is maximum if rf is negligible as comparedto RL. Maximum efficiency = 81.2%

This is double the efficiency of halt wave rectifier (i.e.40.6%).

Therefore, a full-wave rectifier is twice as effective as ahalf-wave rectifier.


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Q 18. Determine the value of ripple factor in thefull-wave rectifier operating at 50 Hz with a 100 uFcapacitor filter and 100 ohm load.

Ans.

The ripple factor for full wave rectifier circuit is given by:

Q 19. What is a regulated power supply? Draw theblock diagram of a regulated power supply.

Ans.

The power supply that maintains constant output-voltage irrespective of mains fluctuations or changes isknown as regulated power supply.

Block diagram is as shown

Transformer may be step-up or step-down dependingupon the requirement of output.

Rectifier circuit may be half wave or full wave.

Filter circuit has capacitor and inductor to filter thefrequency components.

Regulator stage may be a zener diode as voltageregulator or regulator IC’s of particular voltage value.


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Q 20. Define the term voltage regulation.

Ans.

Voltage regulation is defined, as the change in dc outputvoltage from no load (VNL) to full load (VFL) with respect tofull load vàltage (VFL) of a power supply circuit.

The percentage voltage regulation is given by

Where ,VNL = No load dc voltage at the output

VFL = Full load dc voltage at the output.

Q 21. Calculate the regulation of a power supply if itsdc output voltage decreases 50 V to 48 V when loaded.

Ans.

Given


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Q 22. Draw the block diagram for series voltageregulator.

Ans.

Q 23. What are the limitations of using zener diode asvoltage regulator?

Ans.

Limitations of using zener diode as voltage regulator are

1. As the loss in series resistor is very large thus, itsefficiency is poor for heavy loads.

2. Due to change in load the output voltage also changesslightly.

Q 24. What are the advantages and disadvantages ofzener diode regulator?

Ans.

Advantages:


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1. It is simple ir cirOuit.

2. It is light in weight.

3. It-is more reliable and provides- regulation over a widerange of current.

Disadvantages:

1. As there is power dissipation in series resistor and thediode, it results in poor efficiency.

2. The stabilized output is determined by the zenerbreakdown voltage and cannot be varied.

Q 25., How does zener diode maintain constant voltageacross load in the breakdown region?

Ans.

Zener chode as voltage regulator:


It provides, constant voltage to the load from a sourceWhose voltage may vary by using potentiometer atinput.

The zener diode of zener voltage V is reverse connectedacross the load RL across which ‘constant voltage isrequired (V0 Vi).

The resistance R is connected in series with the circuitwhich absorbs output voltage fluctuations so as tomaintain constant voltage V0 across the ‘load.


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1. A zener diode characteristics are always plated betweenvoltage and current

shown in figS

2. It is like an ordinary diode characteristics except that ithave a sharp breakdown voltage.

3. The sharp breakdowr voltage is called zener voltage V.

Q 26. What is transformation utilisation factor?

Ans.

It is defined as


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If Rf << RL, then TUE = 0 286 or 28 6%

Q 27. Write the classification for voltage regulators?

Ans.

Voltage can be controlled either at the source or near the,load. There are types of regulators

1. Generator automatic voltage regulator It is used forregulating the output voltage of generator.

2. Supply automatic voltage regulator.

It is used for industrial application.

The supply automatic voltage regulator is divided into threetypes:-

Type 1: It maintains the output voltage constant in spite ofchanges in the input voltage

Type 2: It maintains the output voltage constant in spite ofchanges in the load

Type 3: It maintains the output voltage constant in spite ofchanges in input voltage and the load.

Q 28. What are the disadvantages of voltage andcurrent regulators?

Ans.

1. The regulator operates continuously leading to wastage ofpower.

2. While bulky step down transformer have to be used itbecomes uneconomical to have more than one regulator forvoltage requirements of different values for different circuits.

3. The input voltage has to be higher than output voltageand this leads to poor efficiency.


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4. In remote locations such as in satellite, the supply is frombattery and dc to ac converter has to be used.

Q 29. A transistor supplies 2 watts to a load of 4 K2 Thezero signal dc collector current is 35 mA and with signalit is 39 mA Determine % second harmonic distortion

Ans.

This s the fundament B1 = 31.6 mA. , Increase in DCcollector current with signal = 39 — 35 =. 4 mA, B0 = B =4mA

Q 30. Differentiate between line and load regulation.Draw the circuits of a series ulator and explain how itregulates the supply Incorporate current limiting factorthe circuit to have overload protection.

Ans.

There are two types of load line regulation

1. D C load line: In order to draw the dc load line of atransistor, one need to know i its cut-off and saturationpoints It is a straight line joining these two points For the CEcuit of following fig.1 the load line is drawn in fig. 2.


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Q point i.e. the Quiescent point is point which lies betweencut off and saturation i.e. in the midway

2. AC load line : This line is steeper than the dc line but thetwo intersect at the Q-point determined by biasing dc voltageand currents.

In AC load line the cut off is given by


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Transistor series voltage regulator :

The circuit is as shown in fig. It is also called as emitter-follower regulator because the voltage at the emitterfollows the base voltage.

In this, the transistor behaves like a variable resistancewhose resistance is determined by the base current.

It is called pass transistor because total current to beregulated passes through it.

Keeping in mind the polarities of different voltages, theyare related by the equation derived by KVL:

Current increases by decreasing RL, VL decreases.

Thus, VBE will increase because V fixed. VL = 1L L if RLincreases VL decreases).

R is used for limiting current passing through the zenerdiode.

Transistor shunt voltage regulator:

It is as shown in fig. It uses transistor in shunt. Sincepath AB is in parallel across VL, we have from KVL. –


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Since V is fixed, any decrease or increase in VL will havea corresponding effect on VBE, suppose VL decreases,then as seen from the above relation, VBF alsodecreases.

As a fesult, l decreases, hence, IC = BI decreases,thereby decreasing I and hence VA = IR.

Consequently, VL increases because at all times

Same line of logic applies in case VL increases.

Q 31. Write short note on Zener diode as voltageregulator.

Ans.

Voltage regulator:

In case of power supplies, the dc output voltage varieswith he change in ac input voltage.

To get the constant output voltage i.e. withoutfluctuation age regulator circuit is used. -

The simplest regulator circuit consists of a resistor Rconnected in series with the input tae and a zener diodeconnected in parallel with load as shown in fig.


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Let V be the voltage to the regulator circuit As long asthe voltage across RL is than the zener breakdownvoltage V, the zener diode does not conduct If the zenerdi dont conduct the resistor R5 and RL make a potentialdivider across V At an increased the voltage across RLbecomes greater than the zener breakdown voltage.

The resistor limits the zener current from exceeding itsrated maximum.

From fig.

If load resistance RL Is reduced L should increase thecurrent l through zener falls the same percentage inorder to maintain constant current Is

Thus voltage drop across becomes constant Hence, theoutput voltage V0 remains constant

If on the other hand, the load current should decrease,the zener diode passes extra current l such that thecurrent I is kept constant.

The output voltage of the circuit thus stabilized.


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chapter 6 _ regulated power

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