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HOT WIRE ANEMOMETER

Project report submitted in partial fulfillment of the requirements

For the award of the degree of

BACHELOR OF TECHNOLOGY

IN

ELECTRICAL AND ELECTRONICS ENGINEERING

By

D.CHANDRASHEKAR (08241A0263)

M.DAVID LIVINGSTONE (08241A0264)

D.POORNA CHANDRA RAO (08241A0281)

CH.VISWANATH (08241A02B8)

Under the guidance of

V.HIMA BINDU

Assistant Professor

Department of Electrical and Electronics Engineering

GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING &

TECHNOLOGY, BACHUPALLY, HYDERABAD-72

2012

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GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING AND

TECHNOLOGY

Hyderabad, Andhra Pradesh.

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

C E R T I F I C A T E

This is to certify that the project report entitled HOT WIRE ANEMOMETER that is being submitted by M.DAVID LIVINGSTONE (08241A0264), D.CHANDRASHEKAR

(08241A0263), CH.VISWANATH (08241A02B8), D.POORNA CHANDRA RAO

(08241A0281) in partial fulfillment for the award of the Degree of Bachelor of

Technology in Electrical and Electronics Engineering to the Jawaharlal Nehru Technological University is a record of bonafide work carried out by him under my guidance

and supervision. The results embodied in this project report have not been submitted to any

other University or Institute for the award of any graduation degree.

Mr.P.M.Sarma Ms. V. HIMA BINDU HOD, EEE Assistant Professor.

GRIET, Hyderabad GRIET, Hyderabad

(Internal Guide)

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ACKNOWLEDGEMENT

This is to place on record my appreciation and deep gratitude to the persons without

whose support this project would never seen the light of day.

I have immense pleasure in expressing my thanks and deep sense of gratitude to my

guide Mrs. V.Himabindu, Assistant Professor Department of Electrical

Engineering, and G.R.I.E.T for her guidance throughout this project.

I also express my sincere thanks to Mr.P.M.Sarma, Head of the Department, and

Mr.M.Chakravarthy Associate Professor G.R.I.E.T for extending his help.

I express my gratitude to The Dr.S.N.Saxena, Project Supervisor G.R.I.E.T for his

valuable recommendations and for accepting this project report.

Finally I express my sincere gratitude to all the members of faculty and my friends

who contributed their valuable advice and helped to complete the project

successfully.

D.CHANDRASHEKAR (08241A0263)

M.DAVID LIVINGSTONE (08241A0264)

D.POORNA CHANDRA RAO (08241A0281)

CH.VISWANATH (08241A02B8)

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ABSTRACT

A method to obtain flow rate of a fluid is by employing hot wire anemometer.

Firstly, we designed and simulated the circuit using the software multisim for

getting error voltage and amplify it to get sufficient voltage.

Since the transformer used in hardware is not present in software,

we used a transformer which gives same output in secondary, with different

source. Once desired output was obtained that is required outputs are obtained

we proceeded for the hardware part

Hardware circuit has the power circuit where ac to dc voltage is

obtained, two Wheatstone bridges having three arm normal resistances and

fourth arm having metal oxide resistor that is thermistor and few op amps for

amplification . Thermistor was vigorously tested for resistance change before

placing it on the circuit board. Desired output was obtained with necessary

changes in the original circuit.

Anemometers can be divided into two classes: those that measure the

wind's speed, and those that measure the wind's pressure; but as there is a close

connection between the pressure and the speed, an anemometer designed for

one will give information about both.

Anemometers are mostly preferred for their wide frequency response,

miniaturization which helps in their use in inaccessible areas

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CONTENTS

S N TITLE Page

No.

a) Acknowledgement

iii

b) Abstract iv

c)

List of Figures viii

d) List of Tables ix

1. Introduction

1

2. AC-to-DC Converter

2.1 Definition

2.2 Usage

2.3 Conversion methods

2.3.1 The half wave rectifier

2.3.2 The full wave rectifier

2.3.3 The full wave bridge rectifier

2

2

2

2

2

3

4

3. Filtering and Regulation 3.1 Introduction

3.2 Filters 3.2.1 Capacitor filter

3.3 Voltage regulator

3.3.1 Linear regulators

3.3.2 Adjusting fixed linear regulator

6

6

6

6

8

9

9

4. Thermistor 4.1 Definition

4.2 Manufacture

4.3 Characteristics

4.3.1 Unloaded NTC Thermistors

4.3.1.1 Temperature dependence of resistance

4.3.1.2 B value

4.3.1.3 Temperature coefficient

4.3.1.4 Tolerance

4.3.1.4.1 Resistance tolerance

4.3.1.4.2 Temperature tolerance

4.3.1.5 Zero power measurement

4.3.2 Electrically loaded thermistors

4.3.2.1 Voltage/Current characteristic

4.3.2.2 Self heating of NTC temperature sensors

4.3.2.3 Dissipation factor

4.3.2.4 Behavior in different media

4.3.2.5 Maximum power

4.3.2.6 Thermal time constant

4.3.2.7 Measurement of thermal time constant in air

11

11

11

11

12

12

12

14

14

14

14

15

15

15

17

18

18

19

19

20

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4.3.2.8 Thermal cooling time constant

4.3.2.9 Heat capacity

21

21

5. Wheatstone bridge

5.1 Characteristic

5.2 Thermistor in Wheatstone bridge

22

22

24

6. Amplification circuit

6.1 Introduction

6.2 LM 324

6.3 Amplifiers used 6.3.1 Non-inverting amplifier

6.3.2 Differential amplifier

6.3.3 Voltage follower

25

25

26

27

27

28

29

7 Introduction to multisim software

7.1 Teaching applications

7.1.1 Analog circuits

7.1.2 Digital circuits

7.1.3 Power electronics

7.1.4 Other applications

7.2 Circuit design applications

7.2.1 Circuit designing and prototyping

7.2.2 Powerful spice simulation and analysis

7.2.3 Green energy

7.2.4 Power design

7.2.5 Design for NI hardware

7.3 Circuit simulation

7.3.1 Power circuit

7.3.2 Dc bridges

7.3.3 Amplification circuit

7.3.4 Differential amplifier

7.3.5 Total circuit

7.4 Circuit Operation

31

31

31

31

31

32

32

32

32

32

32

32

32

33

33

33

34

34

34

8. Hardware implementation

8.1 Circuit specification

8.2 Circuit description

8.2.1 Starting Power Supply 8.2.2 Wheatstone bridge

8.2.3 Normal and differential amplifier circuit

8.2.4 Total Circuit

8.3 Hardware output

37

37

37

37

37

38

38

38

9. Conclusion and Scope for work

References

40

41

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Appendix-A

Appendix-B

Appendix-C

Appendix-D

42

43

44

45

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LIST OF FIGURES

1. Fig 2.1(a) Half wave rectifier waveforms

2.Fig 2.1(b) half wave rectifier

3. Fig 2.2(a) Full wave rectifier waveforms

4. Fig 2.2(b) Full wave rectifier

5. Fig 2.3 Bridge rectifier

6. Fig 3.1 Capacitor filter operation

7. Fig 3.2 Voltage regulators

8. Fig 3.3 Voltage regulator operation

9. Fig 3.4 output of power circuit

10. Fig 4.1 R/T characteristic

11. Fig 4.2 Voltage/current characteristic

12. Fig 4.3 heat /current characteristic

13. Fig.4.4 Constant current and voltage source

14. Fig.4.5 Self heating effect in constant voltage and current source

15. Fig 4.6 temperature/thermal time constant

16. Fig 4.7 thermal time constant measurement apparatus

17. Fig 5.1 Wheatstone bridge

18. Fig 6.1 OP-AMP

19. Fig 6.2 LM 324

20. Fig 6.3 inverting amplifier

21. Fig 6.4 Differential amplifier

22. Fig 6.5 Voltage follower

23. Fig.7.1 Power circuit

24. Fig.7.2 Dc bridges

25. Fig.7.3 Amplification circuit

26. Fig.7.4 Differential amplifier

27. Fig.7.5 Total circuit

28. Fig.7.6 (a) Dc voltages

29. Fig.7.6 (b) Simulation output

30. Fig.8.1 Power supply board

31. Fig.8.2 Wheatstone bridge

32. Fig.8.3 Amplifier circuits

33. Fig.8.4 Total circuit

34. Fig.8.5 (a) Dc outputs

35. Fig.8.5 (b) with air output

36. Fig.8.5(c) Without air output

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LIST OF TABLES

1. Table-6.1 pin description

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1. INTRODUCTION

Hot wire anemometers generally use a very fine wire (on the order of several micrometers) or

heat sensitive components such as thermistors, electrically heated up to some temperature

above the ambient. Air flowing past the wire has a cooling effect on the wire. As the

electrical resistance of most metals is dependent upon the temperature of the metal, a

relationship can be obtained between the resistance of the wire and the flow speed. The

technique depends on the convective heat loss to the surrounding fluid from an electrically

heated sensing element or probe.

Several ways of implementing this exist, and hot-wire devices can be further classified as

CCA (Constant-Current Anemometer), CVA (Constant-Voltage Anemometer) and CTA

(Constant-Temperature Anemometer). The voltage output from these anemometers is thus the

result of some sort of circuit within the device trying to maintain the specific variable

(current, voltage or temperature) constant.

Additionally, PWM (pulse-width modulation) anemometers are also used, wherein the

velocity is inferred by the time length of a repeating pulse of current that brings the wire up to

a specified resistance and then stops until a threshold "floor" is reached, at which time the

pulse is sent again.

Hot-wire anemometers, while extremely delicate, have extremely high frequency-response

and fine spatial resolution compared to other measurement methods, and as such are almost

universally employed for the detailed study of turbulent flows, or any flow in which rapid

velocity fluctuations are of interest and is the most common method used to measure

instantaneous fluid velocity. basic principles of the technique are relatively straightforward

and the probes are difficult to damage if reasonable care is taken. Most sensors are operated

in the constant temperature mode

Cylindrical sensors (hot wires and hot films) are most commonly used to measure the fluid

velocity while flush sensors (hot films) are employed to measure the wall shear stress. Hot-

wire sensors are, as the name implies, made from short lengths of resistance wire and are

circular in section. Hot-film sensors consist of a thin layer of conducting material that has

been deposited on a non-conducting substrate. Hot-film sensors may also be cylindrical but

may also take other forms, such as those that are flush-mounted.

Hot-wire anemometers have been used for many years in the study of laminar, transitional

and turbulent boundary layer flows and much of our current understanding of the physics of

boundary layer transition has come solely from hot-wire measurements. Thermal

anemometers are also ideally suited to the measurement of unsteady flows such as those that

arise behind rotating blade rows when the flow is viewed in the stationary frame of reference.

.

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2. AC-TO-DC CONVERTER

2.1 DEFINITION An AC-to-DC converter is an electronic circuit which converts a source of alternating

current (AC) to source of direct current (DC), often called as rectifier. It is a class of power

converter.

2.2 USAGE Rectifiers have many uses, but are often found serving as components of DC power supplies

and high-voltage direct current power transmission systems. Rectification may serve in roles

other than to generate direct current for use as a source of power. As noted, detectors of radio

signals serve as rectifiers. In gas heating systems flame rectification is used to detect presence

of flame These are also important for portable electronic devices such as cellular phones and laptop

computers, which are supplied with power from ac supplies primarily. Such electronic

devices often contain several sub-circuits, each with its own voltage level requirement

different from that supplied by the battery or an external supply (sometimes higher or lower

than the supply voltage). AC to DC converters makes them to operate by converting AC to

DC by having some additional circuits.

2.3 CONVERSION METHODS Before the development of silicon semiconductor rectifiers, vacuum tube diodes and copper

(I) oxide or selenium rectifier stacks were used. High power rectifiers, such as are used in

high-voltage direct current power transmission, now uniformly employ silicon semiconductor

devices of various types. These are thyristors or other controlled switching solid-state

switches which effectively function as diodes to pass current in only one direction

2.3.1 The Half-Wave Rectifier

Fig 2.1 (a) Half wave rectifier waveforms

The simplest rectifier circuit is nothing more than a diode connected in series with the ac

input, as shown to the right. Since a diode passes current in only one direction, only half of

the incoming ac wave will reach the rectifier output. Thus, this is a basic half-wave rectifier.

The orientation of the diode matters; as shown, it passes only the positive half-cycle of the ac

input, so the output voltage contains a positive dc component. If the diode were to be

reversed, the negative half-cycle would be passed instead, and the dc component of the output

would have a negative polarity. In either case, the DC component of the output waveform is

vp/ = 0.3183vp, where vp is the peak voltage output from the transformer secondary winding.

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Fig 2.1 (b) half wave rectifier

It is also quite possible to use two half-wave rectifiers together, as shown in the second figure

to the right. This arrangement provides both positive and negative output voltages, with each

output utilizing half of the incoming ac cycle.

Note that in all cases, the lower transformer connection also serves as the common reference

point for the output. It is typically connected to the common ground of the overall circuit.

This can be very important in some applications. The transformer windings are of course

electrically insulated from the iron core, and that core is normally grounded by the fact that it

is bolted physically to the metal chassis (box) that supports the entire circuit. By also

grounding one end of the secondary winding, we help ensure that this winding will never

experience even momentary voltages that might overload the insulation and damage the

transformer.

2.3.2The Full-Wave Rectifier

Fig 2.2(a) Full wave rectifier waveforms

While the half-wave rectifier is very simple and does work, it isn't very efficient. It only uses

half of the incoming ac cycle, and wastes all of the energy available in the other half. For

greater efficiency, we would like to be able to utilize both halves of the incoming ac.One way

to accomplish this is to double the size of the secondary winding and provide a connection to

its center. Then we can use two separate half-wave rectifiers on alternate half-cycles, to

provide full-wave rectification. The circuit is shown to the right.

Because both half-cycles are being used, the DC component of the output waveform is now

2vp/ = 0.6366vp, where vp is the peak voltage output from half the transformer secondary winding, because only half is being used at a time.

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This rectifier configuration, like the half-wave rectifier, calls for one of the transformer's

secondary leads to be grounded. In this case, however, it is the center connection, generally

known as the center tap on the secondary winding.

2.2(b) Full wave rectifier

The full-wave rectifier can still be configured for a negative output voltage, rather than

positive. In addition, as shown to the right, it is quite possible to use two full-wave rectifiers

to get outputs of both polarities at the same time.

The full-wave rectifier passes both halves of the ac cycle to either a positive or negative

output. This makes more energy available to the output, without large intervals when no

energy is provided at all. Therefore, the full-wave rectifier is more efficient than the half-

wave rectifier. At the same time, however, a full-wave rectifier providing only a single output

polarity does require a secondary winding that is twice as big as the half-wave rectifier's

secondary, because only half of the secondary winding is providing power on any one half-

cycle of the incoming ac.

Actually, it isn't all that bad, because the use of both half-cycles means that the current drain

on the transformer winding need not be as heavy. With power being provided on both half-

cycles, one half-cycle doesn't have to provide enough power to carry the load past an unused

half-cycle. Nevertheless, there are some occasions when we would like to be able to use the

entire transformer winding at all times, and still get full-wave rectification with a single

output polarity.

2.3.3 The Full-Wave Bridge Rectifier

The four-diode rectifier circuit shown to the right serves very nicely to provide full-wave

rectification of the ac output of a single transformer winding. The diamond configuration of

the four diodes is the same as the resistor configuration in a Wheatstone Bridge. In fact, any

set of components in this configuration is identified as some sort of bridge, and this rectifier

circuit is similarly known as a bridge rectifier.

If you compare this circuit with the dual-polarity full-wave rectifier above, you'll find that the

connections to the diodes are the same. The only change is that we have removed the center

tap on the secondary winding, and used the negative output as our ground reference instead.

This means that the transformer secondary is never directly grounded, but one end or the

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other will always be close to ground, through a forward-biased diode. This is not usually a

problem in modern circuits.

Fig 2.3 Bridge rectifier

To understand how the bridge rectifier can pass current to a load in only one direction,

consider the figure to the right. Here we have placed a simple resistor as the load, and we

have numbered the four diodes so we can identify them individually.

During the positive half-cycle, shown in red, the top end of the transformer winding is

positive with respect to the bottom half. Therefore, the transformer pushes electrons from its

bottom end, through D3 which is forward biased, and through the load resistor in the

direction shown by the red arrows. Electrons then continue through the forward-biased D2,

and from there to the top of the transformer winding. This forms a complete circuit, so

current can indeed flow. At the same time, D1 and D4 are reverse biased, so they do not

conduct any current.

During the negative half-cycle, the top end of the transformer winding is negative. Now, D1

and D4 are forward biased, and D2 and D3 are reverse biased. Therefore, electrons move

through D1, the resistor, and D4 in the direction shown by the blue arrows. As with the

positive half-cycle, electrons move through the resistor from left to right.

In this manner, the diodes keep switching the transformer connections to the resistor so that

current always flows in only one direction through the resistor. We can replace the resistor

with any other circuit, including more power supply circuitry (such as the filter), and still see

the same behavior from the bridge rectifier.

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3. FILTERING AND REGULATION

3.1 INTRODUCTION The output from rectifier is pulsating dc, which results in maloperation of the device. So its

highly recommended to maintain the dc level as constant as possible. The ripple content is

highly reduced and the voltage is maintained slightly constant by using filters. The voltage

level is maintained constant with wide variations of load by using regulators. Cascading both

of them makes the dc voltage ripple free as well as constant.

3.2 FILTERS Though filters are employed with both capacitors and inductors exist, in most of the normal

applications capacitors are preferred to inductors. It is because of high cost and bulkiness of

the inductor .In this project, Capacitor is enough to get the desired output neglecting minute

harmonics.

3.2.1 CAPACITOR FILTER The simple capacitor filter is the most basic type of power supply filter. The application of

the simple capacitor filter is very limited. It is sometimes used on extremely high-voltage,

low-current power supplies for cathode-ray and similar electron tubes, which require very

little load current from the supply. The capacitor filter is also used where the power-supply

ripple frequency is not critical; this frequency can be relatively high. The capacitor (C) shown

in figure is a simple filter connected across the output of the rectifier in parallel with the load

Fig 3.1 Capacitor filter operation

When this filter is used, the RC charge time of the filter capacitor (C) must be short and the

RC discharge time must be long to eliminate ripple action. In other words, the capacitor must

charge up fast, preferably with no discharge at all. Better filtering also results when the input

frequency is high; therefore, the full-wave rectifier output is easier to filter than that of the

half-wave rectifier because of its higher frequency.

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For you to have a better understanding of the effect that filtering has on Eavg, a comparison of

a rectifier circuit with a filter and one without a filter is illustrated in views A and B of figure

. The output waveforms in figure represent the unfiltered and filtered outputs of the half-wave

rectifier circuit. Current pulses flow through the load resistance (RL) each time a diode

conducts. The dashed line indicates the average value of output voltage. For the half-wave

rectifier, Eavg is less than half (or approximately 0.318) of the peak output voltage. This value

is still much less than that of the applied voltage. With no capacitor connected across the

output of the rectifier circuit, the waveform in view A has a large pulsating component

(ripple) compared with the average or dc component. When a capacitor is connected across

the output (view B), the average value of output voltage (Eavg) is increased due to the filtering

action of capacitor C.

The value of the capacitor is fairly large (several microfarads), thus it presents a relatively

low reactance to the pulsating current and it stores a substantial charge.

The rate of charge for the capacitor is limited only by the resistance of the conducting diode

which is relatively low. Therefore, the RC charge time of the circuit is relatively short. As a

result, when the pulsating voltage is first applied to the circuit, the capacitor charges rapidly

and almost reaches the peak value of the rectified voltage within the first few cycles. The

capacitor attempts to charge to the peak value of the rectified voltage anytime a diode is

conducting, and tends to retain its charge when the rectifier output falls to zero. (The

capacitor cannot discharge immediately.) The capacitor slowly discharges through the load

resistance (RL) during the time the rectifier is nonconducting.

The rate of discharge of the capacitor is determined by the value of capacitance and the value

of the load resistance. If the capacitance and load-resistance values are large, the RC

discharge time for the circuit is relatively long.

A comparison of the waveforms shown in figure(view A and view B) illustrates that the

addition of C to the circuit results in an increase in the average of the output voltage (Eavg)

and a reduction in the amplitude of the ripple component (Er) which is normally present

across the load resistance.

Now, let's consider a complete cycle of operation using a half-wave rectifier, a capacitive

filter (C), and a load resistor (RL). As shown in view A of figure, the capacitive filter (C) is

assumed to be large enough to ensure a small reactance to the pulsating rectified current. The

resistance of RL is assumed to be much greater than the reactance of Cat the input frequency.

When the circuit is energized, the diode conducts on the positive half cycle and current flows

through the circuit, allowing C to charge. C will charge to approximately the peak value of

the input voltage. (The charge is less than the peak value because of the voltage drop across

the diode (D1)). In view A of the figure, the charge on C is indicated by the heavy solid line

on the waveform. As illustrated in view B, the diode cannot conduct on the negative half

cycle because the anode of D1 is negative with respect to the cathode. During this interval, C

discharges through the load resistor (RL). The discharge of C produces the downward slope as

indicated by the solid line on the waveform in view B. In contrast to the abrupt fall of the

applied ac voltage from peak value to zero, the voltage across C (and thus across RL) during

the discharge period gradually decreases until the time of the next half cycle of rectifier

operation. Keep in mind that for good filtering, the filter capacitor should charge up as fast as

possible and discharge as little as possible.

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Since practical values of C and RL ensure a more or less gradual decrease of the discharge

voltage, a substantial charge remains on the capacitor at the time of the next half cycle of

operation. As a result, no current can flow through the diode until the rising ac input voltage

at the anode of the diode exceeds the voltage on the charge remaining on C. The charge on C

is the cathode potential of the diode. When the potential on the anode exceeds the potential

on the cathode (the charge on C), the diode again conducts and C begins to charge to

approximately the peak value of the applied voltage.

After the capacitor has charged to its peak value, the diode will cut off and the capacitor will

start to discharge. Since the fall of the ac input voltage on the anode is considerably more

rapid than the decrease on the capacitor voltage, the cathode quickly become more positive

than the anode, and the diode ceases to conduct.

Another thing to keep in mind is that the ripple component (E r) of the output voltage is an ac

voltage and the average output voltage (Eavg) is the dc component of the output. Since the

filter capacitor offers relatively low impedance to ac, the majority of the ac component flows

through the filter capacitor. The ac component is therefore bypassed (shunted) around the

load resistance, and the entire dc component (or Eavg) flows through the load resistance. The

largest possible capacitor will provide the best filtering.

Remember, also, that the load resistance is an important consideration. If load resistance is

made small, the load current increases, and the average value of output voltage (Eavg)

decreases. The RC discharge time constant is a direct function of the value of the load

resistance; therefore, the rate of capacitor voltage discharge is a direct function of the current

through the load. The greater the load current, the more rapid the discharge of the capacitor,

and the lower the average value of output voltage. For this reason, the simple capacitive filter

is seldom used with rectifier circuits that must supply a relatively large load current. Using

the simple capacitive filter in conjunction with a full-wave or bridge rectifier provides

improved filtering because the increased ripple frequency decreases the capacitive reactance

of the filter capacitor.

3.3 VOLTAGE REGULATOR

A voltage regulator may be a simple "feed-forward" design or may include negative feedback

control loops. It may use an electromechanical mechanism, or electronic components.

Depending on the design, it may be used to regulate one or more AC or DC voltages

In an electric power distribution system, voltage regulators may be installed at a substation or

along distribution lines so that all customers receive steady voltage independent of how much

power is drawn from the line.

Many simple DC power supplies regulate the voltage using a shunt regulator such as a Zener

diode, avalanche breakdown diode, or voltage regulator tube. Each of these devices begins

conducting at a specified voltage and will conduct as much current as required to hold its

terminal voltage to that specified voltage. The power supply is designed to only supply a

maximum amount of current that is within the safe operating capability of the shunt

regulating device (commonly, by using a series resistor).If the stabilizer must provide more

power, the shunt regulator output is only used to provide the standard voltage reference for

the electronic device, known as the voltage stabilizer. The voltage stabilizer is the electronic

device, able to deliver much larger currents on demand.

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In the project we use linear regulators, as they offer high accuracy and reliability.

3.3.1 LINEAR REGULATORS

In electronics, a linear regulator is a component used to maintain a steady voltage. The

resistance of the regulator varies in accordance with the load resulting in a constant output

voltage. In contrast, the switching regulator is nothing more than just a simple switch. This

switch goes on and off at a fixed rate usually between 50 kHz to 100 kHz as set by the circuit.

The regulating device is made to act like a variable resistor, continuously adjusting a voltage

divider network to maintain a constant output voltage. The primary advantage of a switching

regulator over linear regulator is very high efficiency, a lot less heat and smaller size.

Fig 3.2 Voltage regulators

"Fixed" three-terminal linear regulators are commonly available to generate fixed voltages of

plus 3 V, and plus or minus 5 V, 6V, 9 V, 12 V, or 15 V when the load is less than 1.5

amperes.

3.3.2 ADJUSTING FIXED LINEAR REGULATOR

By adding an additional circuit element to a fixed voltage IC regulator, it is possible to adjust

the output voltage. Two example methods are listed below.

1. A Zener diode or resistor may be added between the IC's ground terminal and ground.

Resistors are acceptable where ground current is constant, but are ill-suited to regulators with

varying ground current. By switching in different Zeners, diodes, or resistors, the output

voltage can be adjusted in a step-wise fashion.

2. A potentiometer can be placed in series with the ground terminal to increase the output

voltage variably. However, this method degrades regulation, and is not suitable for regulators

with varying ground current.

The "78xx" series (7805, 7812, etc.) regulate positive voltages while the "79xx" series (7905,

7912, etc.) regulate negative voltages. Often, the last two digits of the device number are the

output voltage; e.g., a 7805 is a +5 V regulator, while a 7915 is a -15 V regulator. There are

variants on the 78xx series ICs, such as 78L and 78S, some of which can supply up to 1.5

Amps.

The LM78XX series typically has the ability to drive current up to 1A. For application

requirements up to 150mA, 78LXX can be used. As mentioned above, the component has

three legs: Input leg which can hold up to 36VDC Common leg (GND) and an output leg

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with the regulator's voltage. For maximum voltage regulation, adding a capacitor in parallel

between the common leg and the output is usually recommended. Typically a 0.1MF

capacitor is used. This eliminates any high frequency AC voltage that could otherwise

combine with the output voltage. See below circuit diagram which represents a typical use of

a voltage regulator.

Fig 3.3 Voltage regulator operation

So, after the use of voltage regulator, the slightly varying dc tends almost constant with

variation of loads.

Fig 3.4 output of power circuit

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4 .THERMISTOR

4.1 Definition

As defined by IEC 60539, NTC (Negative Temperature Coefficient) thermistors are

thermally sensitive semiconductor resistors which show a decrease in resistance as

temperature increases. With -2%/K to -6%/K, the negative temperature coefficients of

resistance are about ten times greater than those of metals and about five times greater than

those of silicon temperature sensors. Changes in the resistance of the NTC thermistor can be brought about either externally by a

change in ambient temperature or internally by self-heating resulting from a current

flowing through the device. All practical applications are based on this behavior. NTC thermistors are made of polycrystalline mixed oxide ceramics. The conduction

mechanisms in this material are quite complex, i.e. either extrinsic or intrinsic conduction

may occur. In many cases NTC thermistors have a spinal structure and then show valence

conduction effects.

4.2 Manufacture

EPCOS thermistors are produced from carefully selected and tested raw materials. The

starting materials are different oxides of metals such as manganese, iron, cobalt, nickel,

copper and zinc, to which chemically stabilizing oxides may be added to achieve better

reproducibility and stability of the thermistor characteristics. The oxides are milled to a powdery mass, mixed with a plastic binder and then compressed

into the desired shape. Standard shapes are:

Disks: The thermistor material is compressed under very high pressure on pelleting

Machines to produce round, flat pieces.

Wafers: The ceramic material is compression-molded or drawn and then cut to the

Required shape The blanks are then sintered at high temperatures (between 1000C and 1400C) to produce

the polycrystalline thermistor body. Disks are contacted by baking a silver paste onto the

flat surfaces. Depending on the application, the thermistors are fitted with leads or tab

connectors, coated or additionally incorporated in different kinds of housing. Finally the

thermistors are subjected to a special aging process to ensure high stability of the electrical

values. Otherwise the NTC resistance would possibly change even at room temperature due

to solid-state reactions in the polycrystalline material. SMD NTC thermistors are produced in ceramic multilayer technology with and without

inner electrodes. Flow charts in the quality section of this book show the individual processing steps in

detail. The charts also illustrate the extensive quality assurance measures taken during

manufacture to guarantee the constantly high quality level of our thermistor

4.3 Characteristics

A current flowing through a thermistor may cause sufficient heating to raise the

thermistor's temperature above the ambient. As the effects of self-heating are not always

negligible (or may even be intended), a distinction has to be made between the

~ 12 ~

characteristics of an electrically loaded thermistor and those of an unloaded thermistor. The

properties of an unloaded thermistor are also termed "zero-power characteristics".

4.3.1 Unloaded NTC thermistors 4.3.1.1Temperature dependence of resistance

The dependence of the resistance on temperature can be approximated by the following

equation:

The actual characteristic of an NTC thermistor can be roughly described by the exponential

relation. This approach, however, is only suitable for describing a restricted range around

the rated temperature or resistance with sufficient accuracy. For practical applications a more precise description of the real R/T curve is required.

Either more complicated approaches (e.g. the Steinhart-Hart equation) are used or the

resistance/temperature relation is given in tabulated form. Following the application notes

section you will find tables for real R/T curves. These standardized curves have been

experimentally determined with utmost accuracy over the whole specified temperature

range at a sufficient number of measuring points. They are also available for temperature

increments of 1 degree. 4.3.1.2 B value

The B value is determined by the ceramic material and represents the slope of the R/T

curve. It can be expressed from formula (1):

Formula 2 indicates that the rated B value is defined by two temperatures and can be

generalized as:

~ 13 ~

With two arbitrary temperatures T1 and T2. The specifications in this data book refer in most cases to resistance values at

temperatures of 25C (T1) and 100C (T2); i.e. B25/100 is stated. For SMD NTCs B25/50

and B25/85 values are additionally given for information. Glass-encapsulated NTCs refer

also to B0/100, B100/200 and B200/300. Inserting these temperature combinations into

(formula 3) leads to:

The B value for a particular NTC thermistor can be determined by measuring the resistance

at T1 and T2 and inserting these resistance values into the appropriate equation (formula 4). B values for common NTC materials range from 2000 through 5000 K. Figure 4.1

illustrates the dependence of the R/T characteristic on the B value.

Fig 4.1 R/T characteristic

~ 14 ~

4.3.1.3 Temperature coefficient The temperature coefficient of the resistance is defined as the relative change in

resistance referred to the change in temperature.

4.3.1.4 Tolerance

The rated resistance RR and the B value are subject to manufacturing tolerances. Due to

this tolerance of the B value, an increase in resistance spread must be expected for

temperatures that lie above or below the rated temperature TR. For practical examples

concerning this topic see chapter "Standardized R/T characteristics".

4.3.1.4.1 Resistance tolerance

The resistance tolerance for an NTC thermistor is specified for one temperature point,

which is usually 25C. Upon customer request other temperatures than those specified in

the data sheets are possible. Generally, the resistance tolerance can be expressed by the following relation:

If the third temperature-dependent term in (formula 6) is neglected, the equation can be

simplified as follows:

In this formula RB denotes the resistance tolerance resulting from the spread of the B

value.

For practical usage of (formula 6) the partial derivatives can be calculated from the

exponential model given in (formula 1), which leads to

As can be seen from this equation, the resistance tolerance at a certain temperature is influenced by two variables: the manufacturing tolerance of the rated resistance and the variation of the B value. 4.3.1.4.2 Temperature tolerance

By means of (formula 5) the temperature tolerance can be calculated for small temperature

intervals.

~ 15 ~

For practical application we recommend that the standardized R/T curves (see chapter

"Standardized R/T characteristics") be used; the temperature steps tabulated there are small

enough to permit calculation by the approximation formula given above. 4.3.1.5 Zero-power measurement

Zero-power resistance is the resistance value measured at a given temperature T with the

electrical load kept so small that there is no noticeable change in the resistance value if the

load is further decreased. At too high a measuring load the test results will be distorted by

the self-heating effect.

4.3.2 Electrically loaded NTC thermistors

When a current flows through the thermistor, the device will heat up more or less by power

dissipation. This self-heating effect depends not only on the load applied, but also on the

thermal dissipation factor th and the geometry of the thermistor itself.

The general rule is:

The smaller the device, the smaller is the permissible maximum load and the measuring

load (zero power).

The following general rule applies to self-heating of an NTC thermistor by an electrical

Load:

4.3.2 .1 Voltage/current characteristic

If a constant electrical power is applied to the thermistor, its temperature will first increase

considerably, but this change declines with time. After some time a steady state will be

reached where the power is dissipated by thermal conduction or convection.

~ 16 ~

This is the so called parametric description of the voltage/current curve with R (T) being

the temperature dependent NTC resistance. With the aid of the above equations these

curves can be calculated for different ambient temperatures.

By plotting the voltage values obtained at constant temperature as a function of current one

obtains the voltage/current characteristic of the NTC thermistor.

Fig 4.2 voltage/current characteristic

On a log scale the curves for constant power and constant resistance take the shape of a

straight line. The voltage/current characteristic of an NTC thermistor has four different sections:

1. The straight rise section where the dissipation power only produces negligible self-

heating. Voltage and current are proportional to each other. The resistance value is

~ 17 ~

exclusively determined by the ambient temperature. Use of this curve section is made when

NTC thermistors are employed as temperature sensors. (dV/dI = R = constant)

2. The section of non-linear rise up to maximum voltage where resistance already begins

to drop. (R > dV/dI > 0)

3. At maximum voltage the incremental resistance is zero. (dV/dI = 0)

4. The falling-edge section where the decrease in resistance is greater than the relative

increase in current. This curve section in the operating area of NTC thermistors when a

self-heating effect is desired (e.g. inrush current limiters, liquid level sensors). (dV/dI <

0) 4.3.2 .2 Self-heating of NTC temperature sensors

All considerations in this section refer to the NTC B57861S0103F045 as an example, whose

rated resistance is RR = 10 k 1.0% and whose B value is B = 3988 K 0.3%. The self-

heating effect as a function of applied current is plotted logarithmically in figure 3 for

different ambient temperatures.

Fig 4.3 heat /current characteristic

There is a major difference in self-heating depending on whether the feed-in of the NTC is

a constant current supply or a constant voltage supply combined with a series resistor

(figure 4)

The self-heating effect of both cases is compared in figure 5. The constant current is 200 A and the constant voltage is 5 V. In the case of constant voltage the self-heating effect is shown for three different series resistors (RS = 5 k , RS = 10 k and RS = 20 k ).

~ 18 ~

4.3.2 .3 Dissipation factor

Dissipation factor th is defined as the ratio of the change in power dissipation and the

resultant change in the thermistor's body temperature. It is expressed in mW/K and serves as

a measure of the load that causes a thermistor in steady state to raise its body temperature by

1 K. The higher the dissipation factor, the more heat is dissipated by the thermistor to its

surroundings.

For measuring th the thermistor is loaded such that the V/I ratio corresponds to the

resistance value measured at T2 = 85C.

Designing an NTC thermistor into a circuit will always produce some kind of increase in its

body temperature that leads to falsification of the measured result in a temperature sensor

application. To keep this small, make sure the applied power is as low as possible. No

general details can be given for optimal wiring in a specific application because our

products have a wide bandwidth of both resistance and thermal conductivity. Simulation

with PSpice may be found helpful. Please note that all figures for the thermal

characteristics of our NTC thermistors refer to still air. As soon as other ambient conditions

apply (e.g. agitated air) or once a component obtained from EPCOS is subsequently

prepared, the thermal characteristics illustrated in our library are no longer valid. 4.3.2.4 Behavior in different media

As shown by the equations (formula 12a) and (formula 12b) the voltage/current curve is

influenced not only by the NTC resistance R (T) but also by the dissipation factor th. The

dissipation factor, in turn, depends on size, shape and leads of the device as well as on the

medium surrounding the thermistor.

~ 19 ~

The voltage/current curves specified in the data sheets apply to still air. In agitated air or in

a liquid the dissipation factor increases and the V/I curve shifts towards higher values of

voltage and current. The opposite applies when the thermistor is suspended in a vacuum. The voltage/current curve thus indicates by which medium the thermistor is surrounded.

This means that NTC thermistors can be used for sensing the flow rate of gases or liquids,

for vacuum measurement or for gas analysis.

4.3.2.5 Maximum power P25

P25 is the maximum power an NTC thermistor is capable of handling at 25C ambient

temperature. When the maximum power P25 is applied to the NTC thermistor, it

operates in the self heating regime. 4.3.2.6 Thermal time constant a

Thermal time constant a can be a crucial parameter when selecting a temperature

sensor to match an application. The thermal time constant (thermal response time) of a

temperature sensor is mainly influenced by:

Its design (e.g. sensor element, material used to assemble the sensor element in the sensor

case, connection technology, housing), its mounting configuration (e.g. immersed, surface-

mounted), the environment it will be exposed to (e.g. air flow, inactive air, fluid).

When a temperature sensor with temperature T1 is immersed in a medium (air, water) with

temperature T2, the change in temperature of the sensor as a function of time follows to a

first approximation an exponential law:

After the thermal time constant a the temperature change of the sensor is 1- 1/e = 63.2%

of the temperature difference T1 - T2, this means T (a) = T1 + (T2- T1). (1- 1/e).

Fig 4.6 temperature/thermal time constant

Temperature increase from T1 to T2 of a sensor modeled with an exponential

law.EPCOS possesses extensive and sophisticated in-house facilities to test

~ 20 ~

the performance and reliability of temperature sensors. Test stations exist to

carry out thermal response time measurement in air/water) or air/air.

1) Note that only NTCs with special protection (e.g. K504, K276) can be exposed to liquid.

4.3.2.7Measurement of thermal time constant in air

The thermal response time is determined by a double air channel method whose

temperatures can be set separately. Furthermore, the air speed in each channel can be

adjusted and measured with a calibrated anemometer.

Fig 4.7 thermal time constant measurement apparatus Figure shows the two air channels from the top side. The temperature sensor can be moved

horizontally from one air channel to the other. A slider between the two air channels

ca n be moved vertically and opens a gap between the two air channels during movement of

the sensor.

The resistance values of the NTC thermistor are determined at three different temperatures

in a temperature controlled bath. When the test run starts, the temperature sensor is placed

in one air channel with defined air speed and stabilized at temperature T1 until it reaches

the temperature of the ambient air. The sensor is then quickly moved to the other air

channel with the same air speed at upper temperature T2. When the experiment is finished

the software calculates the thermal time constant a. By default T1 is set to 40C, T2 is set to 80C, and air speed is adjusted to 5

m/s.

~ 21 ~

4.3.2.8 Thermal cooling time constant c

The thermal cooling time constant refers to the time necessary for an unloaded thermistor to

vary its temperature by 1-1/e = 63.2% of the difference between its mean temperature and

the ambient temperature.

c depends to a large extent on component design. The values of c specified in this data

book were determined in still air at an ambient temperature of 25C. The NTC thermistor is internally heated to 85C to measure subsequently the time it

requires to cool down to 47.1C at an ambient temperature of 25C. This adjustment to the

ambient is asymptotic and occurs all the faster, the smaller the device is.

4.3.2.9 Heat capacity Cth

The heat capacity Cth is a measure of the amount of heat required to raise the NTC's

mean temperature by 1K. Cth is stated in mJ/K.

The relationship between heat capacity, dissipation factor and thermal cooling time constant

is expressed by:

~ 22 ~

5. WHEATSTONE BRIDGE

5.1 CHARACTERISTIC

A Wheatstone bridge is generally an electrical circuit used to measure an unknown electrical

resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown

component. Its operation is similar to the original potentiometer.

Fig 5.1 Wheatstone bridge

In the figure, Rx is the unknown resistance to be measured; R1, R2and R3are resistors of

known resistance and the resistance of R2 is adjustable. If the ratio of the two resistances in

the known leg (R2/R1)is equal to the ratio of the two in the unknown leg (Rx/R3), then the

voltage between the two midpoints (B and D) will be zero and no current will flow through

the galvanometer Vg. If the bridge is unbalanced, the direction of the current indicates

whether R2is too high or too low. R2 is varied until there is no current through the

galvanometer, which then reads zero.

Detecting zero current with a galvanometer can be done to extremely high accuracy.

Therefore, if R1, R2and R3 are known to high precision, then Rx can be measured to high

precision. Very small changes in Rx disrupt the balance and are readily detected.

At the point of balance, the ratio of R2/R1=Rx/R3, Therefore, Rx=(R2/R1)xR3

A Wheatstone bridge circuit makes a very good general purpose measurement circuit. Some

of the physical quantities that it can be used to measure are stress and strain, pressure, fluid

flow and, temperature. This article discusses using a Thermistor in conjunction with a

Wheatstone bridge to measure voltage difference

Generally, one or more of the resistances are variable and change in accordance with some

physical phenomenon, such as strain in this case. The Wheatstone bridge then converts this

change in resistance to a change in voltage.

In the circuit, Vin is the excitation voltage and Vg is the output voltage. The voltage Vg

measures the potential difference between the outputs of the voltage dividers ADC and ABC,

connected across the excitation voltage source. The voltage at terminal D is equal to the drop

across resistor R2 and the voltage at terminal B is equal to the drop across R1.

~ 23 ~

Initially, the bridge is balanced, which implies that the output voltage Vg is zero. This is

because the resistors are chosen in such a way that the ratio of R1:R3 is equal to the ratio

R2:Rx. As a result, the voltage at terminals B and D is equal. If there is a change in any of the

resistances, the output voltage Vg is no longer zero, due to an imbalance in the bridge.

First, a brief mathematical calculation of the functioning of the Wheatstone bridge is useful.

The voltage drop in the arms AB and AD is given by:

The equations are obtained by using the current-divider rule to determine the current flowing

through arms ABC and ADC of the bridge, IABC and IADC, respectively. The output voltage

Vgcan then be obtained from the:

Simplifying gives

This equation shows that if the ratio R1:R3 is equal to the ratio R2: Rx, then Vg will be zero.

Any change in the values of any of the resistances will disturb this balance and the output

voltage will no longer be zero.

~ 24 ~

In the project, above circuit is implemented twice with same component values. two bridges

are enclosed in a special box with one thermistor fixed and other which can be enclosed in

fluid flowing tube

One bridge is given +12 volts where as other bridge is given with -12 volts. As a result

voltage difference in one case is positive and other is negative.

5.2 THERMISTOR IN WHEATSONE BRIDGE

The principle of the Wheatstone bridge is used to design a error voltage generator. One of the

four resistors in the Wheatstone bridge are replaced by identical thermistor, which is laid out

on Rx.

In the presence of normal rated temperature, the Wheatstone bridge output voltage is zero,

since the bridge is balanced in the rated temperature condition. To balance, the bridge the

circuit needs to be calibrated for the rated temperature condition.

When temperature changed by fluid flow is applied to the circuit board, the nominal

resistance of the thermistor changes in accordance with the fluid flow rate. As a result, the

Wheatstone bridge is unbalanced and an output voltage proportional to the change in

resistance of the resistance, and hence proportional to the applied fluid flow rate, is generated

at the output of the bridge. The change in resistance of thermistor is typically very small, of

the order of micro-ohms. Therefore, the output voltage of the bridge is also very small. Thus

this output voltage needs to be amplified using a low noise precision amplifier. The project

uses LM324 op-amp to do the job

~ 25 ~

6. AMPLIFICATION CIRCUIT

6.1 INTRODUCTION

Generally, an amplifier, or simply amp, is a device for increasing the power of a signal by use

of an external energy source. In popular use, the term usually describes an electronic

amplifier, in which the input "signal" is usually a voltage or a current. In audio applications,

amplifiers drive the loudspeakers used in PA systems to make the human voice louder or play

recorded music. Amplifiers may be classified according to the input (source) they are

designed to amplify (such as a guitar amplifier, to perform with an electric guitar), the device

they are intended to drive (such as a headphone amplifier), the frequency range of the signals

(Audio, IF, RF, and VHF amplifiers, for example), whether they invert the signal (inverting

amplifiers and non-inverting amplifiers), or the type of device used in the amplification

(valve or tube amplifiers, FET amplifiers, etc.).

A related device that emphasizes conversion of signals of one type to another (for example, a

light signal in photons to a DC signal in amperes) is a transducer, a transformer, or a sensor.

However, none of these amplify power.

Though an amplifier can be realized through transistors and other stuff, operational amplifiers

are mostly preferred due to their very good characteristics and their compactness as well.

OPERATIONAL AMPLIFIER

Fig 6.1 OP-AMP

An operational amplifier ("op-amp") is a DC-coupled high-gain electronic voltage amplifier

with a differential input and, usually, a single-ended output. An op-amp produces an output

voltage that is typically hundreds of thousands times larger than the voltage difference

between its input terminals.

Operational amplifiers are important building blocks for a wide range of electronic circuits.

They had their origins in analog computers where they were used in many linear, non-linear

and frequency-dependent circuits. Their popularity in circuit design largely stems from the

fact that characteristics of the final op-amp circuits with negative feedback (such as their

~ 26 ~

gain) are set by external components with little dependence on temperature changes and

manufacturing variations in the op-amp itself.

Op-amps are among the most widely used electronic devices today, being used in a vast array

of consumer, industrial, and scientific devices. Many standard IC op-amps cost only a few

cents in moderate production volume; however some integrated or hybrid operational

amplifiers with special performance specifications may cost over $100 US in small

quantities. Op-amps may be packaged as components, or used as elements of more complex

integrated circuits.

The op-amp is one type of differential amplifier. Other types of differential amplifier include

the fully differential amplifier (similar to the op-amp, but with two outputs), the

instrumentation amplifier (usually built from three op-amps), the isolation amplifier (similar

to the instrumentation amplifier, but with tolerance to common-mode voltages that would

destroy an ordinary op-amp), and negative feedback amplifier (usually built from one or more

op-amps and a resistive feedback network).

6.2 LM 324

Fig 6.2 LM 324

LM324 is a 14 pin IC consisting of four independent operational amplifiers (op-amps)

compensated in a single package. Op-amps are high gain electronic voltage amplifier with

differential input and, usually, a single-ended output. The output voltage is many times higher

than the voltage difference between input terminals of an op-amp.

These op-amps are operated by a single power supply LM324 and need for a dual supply is

eliminated. They can be used as amplifiers, comparators, oscillators, rectifiers etc. The

conventional op-amp applications can be more easily implemented with LM324.

~ 27 ~

Pin Description: Pin No

Function Name

1 Output of 1st comparator Output 1

2 Inverting input of 1st comparator Input 1-

3 Non-inverting input of 1st comparator Input 1+

4 Supply voltage; 5V (up to 32V) Vcc

5 Non-inverting input of 2nd

comparator Input 2+

6 Inverting input of 2nd

comparator Input 2-

7 Output of 2nd

comparator Output 2

8 Output of 3rd

comparator Output 3

9 Inverting input of 3rd

comparator Input 3-

10 Non-inverting input of 3rd

comparator Input 3+

11 Ground (0V) Ground

12 Non-inverting input of 4th comparator Input 4+

13 Inverting input of 4th comparator Input 4-

14 Output of 4th comparator Output 4

Tab 6.1 Pin description

6.3 AMPLIFIERS USED

The amplifiers used in the project are discussed below

6.3. 1 Inverting amplifier

Fig 6.3

An inverting amplifier inverts and scales the input signal. As long as the op-amp gain is very

large, the amplifier gain is determined by two stable external resistors (the feedback resistor

Rf and the input resistor Rin) and not by op-amp parameters which are highly temperature

dependent. In particular, the RinRf resistor network acts as an electronic seesaw (i.e., a class-1 lever) where the inverting (i.e., ) input of the operational amplifier is like a fulcrum about

which the seesaw pivots. That is, because the operational amplifier is in a negative-feedback

configuration, its internal high gain effectively fixes the inverting (i.e., ) input at the same 0 V (ground) voltage of the non-inverting (i.e., +) input, which is similar to the stiff

mechanical support provided by the fulcrum of the seesaw. Continuing the analogy,

~ 28 ~

Just as the movement of one end of the seesaw is opposite the movement of the other

end of the seesaw, positive movement away from 0 V at the input of the RinRf network is matched by negative movement away from 0 V at the output of the

network; thus, the amplifier is said to be inverting.

In the seesaw analogy, the mechanical moment or torque from the force on one side of

the fulcrum is balanced exactly by the force on the other side of the fulcrum;

consequently, asymmetric lengths in the seesaw allow for small forces on one side of

the seesaw to generate large forces on the other side of the seesaw. In the inverting

amplifier, electrical current, like torque, is conserved across the RinRf network and relative differences between the Rin and Rf resistors allow small voltages on one side

of the network to generate large voltages (with opposite sign) on the other side of the

network. Thus, the device amplifies (and inverts) the input voltage. However, in this

analogy, it is the reciprocals of the resistances (i.e., the conductances or admittances)

that play the role of lengths in the seesaw.

Hence, the amplifier output is related to the input as in

.

So the voltage gain of the amplifier is where the negative sign is a

convention indicating that the output is negated. For example, if Rf is 10 k and Rin is 1 k, then the gain is 10 k/1 k, or 10 (or 10 V/V).Moreover, the input impedance of the

device is because the operational amplifier's inverting (i.e., ) input is a virtual ground.

In a real operational amplifier, the current into its two inputs is small but non-zero (e.g., due

to input bias currents). The current into the inverting (i.e., ) input of the operational amplifier is drawn across the Rin and Rf resistors in parallel, which appears like a small

parasitic voltage difference between the inverting (i.e., ) and non-inverting (i.e., +) inputs of the operational amplifier. To mitigate this practical problem, a third resistor of value

can be added between the non-inverting (i.e., +) input and

the true ground. This resistor does not affect the idealized operation of the device because no

current enters the ideal non-inverting input. However, in the practical case, if input currents

are roughly equivalent, the voltage added at the inverting input will match the voltage at the

non-inverting input, and so this common-mode signal will be ignored by the operational

amplifier (which operates on differences between its inputs).

6.3.2 Differential amplifier

~ 29 ~

Fig 6.4 Differential amplifier

The name "differential amplifier" should not be confused with the "differentiator," The

"instrumentation amplifier," which is also shown on this page, is a modification of the

differential amplifier that also provides high input impedance.

The circuit shown computes the difference of two voltages multiplied by some constant. In

particular, the output voltage is:

The differential input impedance Zin (i.e., the impedance between the two input pins) is

approximately R1 + R2. The input currents vary with the operating point of the circuit.

Consequently, if the two sources feeding this circuit have appreciable output impedance, then

non-linearities can appear in the output. An instrumentation amplifier mitigates these

problems.

Under the condition that the Rf/R1 = Rg/R2, the output expression becomes:

where is the differential gain of the circuit.

Moreover, the amplifier synthesized with this choice of parameters has good

common-mode rejection in theory because components of the signals that have

V1 = V2 are not expressed on the output. Although this property is described here with

resistances, it is a more general property of the impedances in the circuit. So, for

example, if a compensation capacitor is added across any resistor (e.g., to improve

phase margin and ensure closed-loop stability of the operational amplifier), similar

changes need to be made in the rest of the circuit to maintain the ratio balance.

Otherwise, high-frequency components common to both V1 and V2 can express

themselves on the output. Additionally, because of leakage or bias currents in a real

operational amplifier, it is usually desirable for the impedance looking out each input

to the operational amplifier to be equal to the impedance looking out of the other

input of the operational amplifier. Otherwise, the same current into each operational

amplifier input will generate a parasitic differential signal and thus a parasitic output

component. Consequently, choosing R1 = R2 and Rf = Rg is common in practice.

In the special case when Rf/R1 = Rg/R2, as before, and Rf = R1, the differential gain

A = 1, and the circuit is a differential follower with:

6.3.3 Voltage follower (Unity Buffer Amplifier)

~ 30 ~

Fig 6.5 Voltage follower

Used as a buffer amplifier to eliminate loading effects (e.g., connecting a device with a high

source impedance to a device with a low input impedance).

(

(Realistically, the differential input impedance of the op-amp itself, 1 M to 1 T)

Due to the strong (i.e., unity gain) feedback and certain non-ideal characteristics of real

operational amplifiers, this feedback system is prone to have poor stability margins.

Consequently, the system may be unstable when connected to sufficiently capacitive loads. In

these cases, a lag compensation network (e.g., connecting the load to the voltage follower

through a resistor) can be used to restore stability. The manufacturer data sheet for the

operational amplifier may provide guidance for the selection of components in external

compensation networks. Alternatively, another operational amplifier can be chosen that has

more appropriate internal compensation.

~ 31 ~

7 INTRODUCTION TO MULTISIM SOFTWARE

NI Multisim (formerly Multisim) is an electronic schematic capture and simulation program

which is part of a suite of circuit design programs, along with NI Ultiboard. Multisim is one

of the few circuit design programs to employ the original Berkeley SPICE based software

simulation. Multisim was originally created by a company named Electronics Workbench,

which is now a division of National Instruments. Multisim includes microcontroller

simulation (formerly known as MultiMCU), as well as integrated import and export features

to the Printed Circuit Board layout software in the suite, NI Ultiboard.

Multisim is widely used in academia and industry for circuits education, electronic schematic

design and SPICE simulation.

7.1 Teaching Applications Multisim is used for a variety of application areas for educators. Below you will find

resources to learn how Multisim is used in each of the following application areas.

7.1.1 Analog Circuits

The intuitive graphical environment of Multisim gives students the ability to quickly place

basic components to understand fundamental circuit concepts and theory. These components,

including resistors, capacitors, inductors, power sources, switches, oscilloscopes, and probes,

help to enhance learning of the following concepts:

Resistive, capacitive, and inductive circuits

Steady-state and transient circuit analysis techniques

AC analysis and frequency response

Operational amplifier and filter circuits

7.1.2 Digital Circuits

The programmable logic device (PLD) schematic environment is dedicated to teaching digital

logic. Logic gates, encoders, decoders, adders, counters, and shift registers provide students

with low-level components. The ability to generate VHDL, programming files, and deploy

logic to a Xilinx FPGA completes the sequence, which allows education in the following:

Boolean algebra and minimization of logic

Sequential and combinational logic design

State machines and arithmetic logic

Hardware description languages and field-programmable gate arrays (FPGA)

7.1.3 Power Electronics

With the release of Multisim 12.0, many components now give students the ability to explore

the concepts of power electronics directly. Included are example circuits with

parameterizable machines, transistors, thyristors, voltage controlled switches, PWM

generators, and phase-angle controllers, which give educators the ability to focus on teaching

the following:

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AC to DC conversion

DC to DC switch mode power supply topologies

DC to AC for motor drive and renewable energy

Rectifiers, inverters, and power switches

PWM and motor drives

7.1.4 Other Applications

Bioinstrumentation

Student Design

Mechatronics and Control Design

7.2 Circuit Design Applications

7.2.1 Circuit Design and Prototyping

Using Multisim and NI Ultiboard simulation and layout environments, you can rapidly design

and prototype sophisticated analog circuitry that performs to specification

7.2.2 Powerful SPICE Simulation and Analyses

Multisim features a comprehensive suite of simulation tools and an expandable selection of

custom analyses for examining and optimizing circuit behavior. Visualize a database of

accurate models from leading semiconductor manufacturers.

7.2.3 Green Energy

Multisim is used for the simulation and prototyping of electronic circuits directly integrated

into energy-efficient systems and platforms such as wind energy, solar energy, and energy

storage systems.

7.2.4 Power Design

Simulate and prototype power electronics systems ranging from simple power supplies to

motor controllers used in automotive applications to power grid systems.

7.2.5 Design for NI Hardware

Design breakout boards, daughter cards, and connector accessories for NI hardware with pin-

accurate connectors for NI data acquisition, NI Single-Board RIO, and NI CompactRIO.

7.3 Circuit simulation

Practical circuit is simulated in this software; first the AC to DC conversion is constructed

and the circuit is as follows,

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7.3.1 Power circuit

Fig 7.1

As there is no presence of required transformer in multisim, a transformer which gives same

required dc with different voltage source is used.

7.3.2 Dc bridges

Fig 7.2

7.3.3 Amplification circuit

Fig 7.3

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7.3.4 Differential amplifier

Fig 7.4

7.3.5 Total circuit

Fig 7.5

7.4 Circuit Operation First terminal of the transformer is directly connected to anode of In4007 diode. The second

one is connected to ground and the last one to cathode of other In4007.Due to these diodes

rectification occurs, as a result fluctuating positive dc and negative dc occurs at the 3 and 6

terminals respectively

This fluctuating dc is is made to constant dc using capacitor filter. And then output is handled

to LM7815 and LM7915 voltage regulators for positive and negative dc outputs in order to

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regulate the voltage level at constant .so the output obtained is a pure dc 0f +15 volts and -15

volts without any ripple content.

Fig 7.6 a Dc voltages

Fig 7.6 b simulation output

The obtained Dc outputs are used for biasing as well as for power requirement with slight

modifications. The error voltage from two bridges is given to two inverting amplifiers which

differ from supply polarity i.e. +12 and -12volts.

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The outputs obtained are then given to differential amplifier. The resistances

connected to amplifier define the amplification.

The circuit simulation is done with varying a resistor precision value to 10 %,

Which introduces sufficient error in the circuit.

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8. HARDWARE IMPLEMENTATION

8.1 CIRCUIT SPECIFICATION This section covers a simple hot wire anemometer

Input: 230 volts Ac.

Output: approximately 0.1 to 1 volts

Protection: None

8.2 CIRCUIT DESCRIPTION 8.2.1 Starting Power Supply The starting power supply is obtained from a 15V transformer connected to a rectifier circuit.

The rectifier circuit consists of 100 microfarad capacitors, 7815, 7915 voltage regulators for

obtaining positive and negative voltages. Consists of 10nf capacitors and also jumpers to

connect to the circuit.

Fig.8.1.Power Supply Board

8.2.2 Wheatstone bridge The bridges are constructed by 1k and 10k ohm resistances respectively. The thermistor is

about 10k ohm under normal conditions. The bridges are initially approximately balanced

owing to values of the resistances.

Thermistor of one of the bridge is taken out and is exposed to air flowing, where as

rest of the circuit is enclosed in a box.

fig 8.2 Wheatstone bridge

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8.2.3 Normal and differential amplifier circuit The error voltage from bridges is given to two inverting amplifiers by using suitable

resistances. The voltage outputs from these are given to differential amplifier. The amplifier

output is dependent on the fluid speed passing through thermistor

Fig 8.3 amplifier circuits

8.2.4 Total Circuit

Fig 8.4 total circuit

8.3 HARDWARE OUTPUT The outputs obtained from the hardware circuit are shown below,

Fig 8.5 a Dc outputs

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The above figure shows dc voltage obtained from power circuit ,which is used for biasing as

well as input supply.

Fig 8.5 b without air output

Fig 8.5 c with air output

The figures 8.5 b and 8.5 c shows output terminal voltage of differential amplifier with and

without exposure of fluid flow.

By properly calibrating the device with a standard instrument, it can be used to

measure velocity of a fluid.

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9. CONCLUSION AND SCOPE FOR FUTURE WORK

We obtained a voltage variation ranging from 0.1V to 1V with exposure of thermistor to air

thus working as a hot wire anemometer.

The scope of future work would be to include a microcontroller. In a microcontroller based

solution, you can get good range of outputs as well as output can be displayed on LCD

screen. The device should be calibrated using standard instrument, so as to measure fluid

flow rate

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9.REFERENCES

1. U.A.Bakshi, Electronic Devices and circuits: Converters.

2. A.k.sawhney, Measurements and Instrumentation

3. M.D.Singh /Khanchandani, Power Electronics, Tata-McGraw-Hill publications.

4. Wikipedia.org

5.www.datasheetcatalog.org,TL494,

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APPENDIX-A

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APPENDIX-B

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APPENDIX-C

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APPENDIX-D Device Features

Resistance values from 100 Ohms to 9.8 M Ohms Epoxy Coating Typical Dissipation Constant = 2mw/C in still air Typical time constant in still air = 10 Seconds #28 AWG tin/lead plated copper leads Interchangeable tolerances available