DEPARTMENT OF TECHNICAL EDUCATION Unit 1: Basics...

DEPARTMENT OF TECHNICAL EDUCATION E-CONTENT

Note: This is only Basic Information for students. Please refer “Reference Books” prescribed as per syllabus

Unit 1: Basics of measurements

07 Hours

Necessity of measurements-direct and indirect methods, basic terminology, dynamic characteristics of

an instrument, generalized electronic measurement system, Errors–gross, systematic and random errors,

sources of errors. Statistical analysis–problems involving arithmetic mean, deviation, average deviation,

standard deviation. Limiting errors and probable errors. Standards-primary, secondary, working and

IEEE standards. Comparison of AC and DC bridges. Principle of Wheatstone bridge and mention its

applications.

1.1 Necessity of measurements

Measurements play a very important role in every branch of scientific research and engineering. The

whole area of automation is based on measurements. The very concept of control is based on the

comparison of the actual condition and the desired performance. The measurement confirms the validity

of a theory and also adds to its understanding. This eventually leads to new discoveries. Through

measurement a product can be designed or a process be operated with maximum efficiency, minimum

cost and with desired degree of reliability and maintainability.

1.1.1 Methods of Measurement

Measurement of any quantity involves two parameters, the magnitude of the value and unit of

measurement. For example if we have to measure voltage we can say it is 10volts. Here “10” is the

magnitude and “volts” is the unit of measurement.

There are two methods of measurement:

1. Direct comparison method

2. Indirect comparison method

Direct Comparison method:

In the direct comparison method of measurement, we compare the quantity directly with the

primary or secondary standard. For example, if we have to measure the height of a person, we do it with

the help of the measuring tape or scale that acts as the secondary standard. Here we are comparing the

quantity to be measured (height) directly with the standard.

Indirect comparison method:



In the indirect comparison method of measurement, the quantity to be measured is not measured

directly but other parameters related to the quantity are measured. For example if you want to measure

power we find voltage (V) and current (I) first and then calculate power using formula P=V*I.

1.1.2 Basic terminology

Instrument: It is defined as a device for determining the value or magnitude of a quantity or variable.

Accuracy: It is defined as the closeness with which an instrument reading approaches the true value of

the quantity being measured.

Precision: It is defined as how exactly the result is determined. i.e given a fixed value of the quantity,

precision is a measure of the degree of agreement within a group of measurements.

Sensitivity: It is defined as the ratio of the magnitude of output signal to the input signal or response of

measuring system to the quantity being measured.

Resolution: It is defined as the smallest change in measured quantity that causes a visible change in its

output.

1.1.3 Dynamic characteristics of an instrument

Dynamic characteristics of a measuring instrument refer to the case where the measured quantity

changes rapidly with time.

The dynamic characteristics of any measurement system are:

1. Speed of response

2. Measuring Lag

3. Fidelity

4. Dynamic error

Speed of Response (desirable): It is defined as the speed with which an instrument or measurement

system responds to changes in measured quantity.

Response Time (desirable): It is the time required by instrument or system to settle to its final steady

position after the application of the input.



Measuring lag: It is the delay in the response of a measurement system to changes in measured quantity.

It is of two types,

i) Retardation type: In this type of measuring lag the response begins immediately after a

change in measured quantity has occurred.

ii) Time delay: In this type of measuring lag the response of the measurement system begins after

a dead zone after the application of the input.

Fidelity: Fidelity of a system is defined as the ability of the system to reproduce the output in the same

form as the input. It is the degree to which a measurement system indicates changes in the measured

quantity without any dynamic error.

Dynamic error: It is difference between the true value of the quantity changing with time and the value

indicated by the measurement system if no static error is assumed.

**Note: Static error is the difference between the true value and the measured value of a quantity.

1.1.4 Generalized electronic measurement system

The measurement of a given quantity is the result of comparison between the quantity (whose

magnitude is unknown) & a predefined Standard. Since two quantities are compared, the result is

expressed in numerical values.

Figure 1. Shows a generalised measurement system with different elements.

Figure 1.1.4.1 Generalised measurement system

Primary sensing element:

The unknown quantity under measurement makes its first contact with the primary sensing

element of a measurement system. The sensing elements sense the condition, state or value by taking out

a small part of energy from the measured (the unknown quantity which is to be measured), and then

produce an output.



Variable conversion element:

The output from the primary sensing element may require to be converted to a more suitable form

while saving its information contents. This conversion is performed by the variable conversion element

called transducer.

Variable manipulation element:

The function of this element is to manipulate the signal presented to it preserving the original

nature of the signal. Some non-linear processes like modulation, detection, sampling, filtering, etc., are

performed on the signal to bring it to the desired form to be accepted by the next stage of measurement

system.

Data transmission element:

This element transmits the signal from one location to another without changing the physical

nature of the variable.

Data Presentation element:

This element presents a display record or indication of the output from the manipulation elements to

the person handling the instrument.

1.2 Errors

Error is defined as the difference between the actual value of a quantity and the value obtained by a

measurement.

A study of errors is the first step in finding ways to reduce them. Errors may arise from different

sources and they are mainly classified as shown below,

1.2.1 Gross errors

This type of error occurs due to human mistakes, while reading, recording and calculating

measurement results. For example the observer due to an oversight, may read the temperature as 30.50C

while the actual reading may be 30.20 C, there is 0.3

0C error in the reading.

Gross errors may be of any amount and therefore their mathematical analysis is impossible. But, the

following precautions can be taken to avoid such errors. They are:



1. Proper care should be taken while reading and recording the data.

2. More than one reading should be taken for the quantity under measurement preferably by

different observers.

1.2.2 Systematic errors

Systematic errors occur usually from the measuring instruments. They may occur because there is

something wrong with the instrument or its data handling system, or because the instrument is wrongly

used by the experimenter.

These errors can be found by conducting repeated measurements under different conditions or with

different equipment and if possible by entirely different method.

These errors are further classified as follows,

Instrumental errors:

These errors arise due to following reasons:

1. Due to inbuilt shortcomings in the instruments.

2. Due to misuse of the instruments.

3. Due to loading affects the instruments.

These errors can be minimised by using the following methods,

1. Measurement procedure must be carefully planned.

2. Correction factors should be adopted after finding the instrumental errors.

3. Instrument must be re-calibrated carefully.

Environmental errors:

These errors arise due to conditions external to the measuring device (e.g. effects of temperature,

pressure, humidity, dust etc.)


1. Temperature controlled enclosure can be used to avoid temperature variations.

2. The effect of humidity, dust etc. Can be entirely eliminated by sealing the equipment in an

airtight container.

3. By providing shields the instrument can be protected against external magnetic and electrostatic

fields.

Observational errors:

These errors arise due wrong observations. The Observational errors arise due to following

reasons:



1. Parallax error occurs on account of the pointer and the scale not being in the same plane (shown

in figure 1.2.2.1.

2. Wrong scale reading and wrong recording of data.

3. Incorrect conversion of units in between consecutive readings.

These errors can be eliminated by using digital display systems.

Figure 1.2.2.1 Parallax error

1.2.3 Random errors

Random errors are accidental, small and independent. These errors arise due to following reasons:

1. Parallax: when an observer reads a scale from an incorrect direction

2. Variation in environmental conditions

3. Friction in instrument movement

4. Mechanical vibrations


1. Taking repeated readings to obtain an average value.

2. Maintaining good experimental technique (e.g. reading from a correct position).

1.2.4 Sources of errors

1. Insufficient knowledge of process parameters and design conditions.

2. Selection of improper instrument for measurement.

3. Poor design.



4. Human error caused by person operating the instrument.

1.3 Statistical analysis

Statistical analysis of measurement is a procedure of collection, analysis, interpretation,

presentation, and organization of data.

Arithmetic mean:

It is the ratio of sum of readings taken to the total no. of readings.

Arithmetic Mean = (Sum of readings)

/ (Number of readings)

Where 1, 2, .... n are the readings taken, N is the no. of readings,

̅ is the symbol of the arithmetic mean.

Deviation:

Deviation is the departure of the given reading from the arithmetic mean of the group of readings.

Let the deviation of the first reading 1 be d1 and that of second reading 2 be d2 and so on.

Then the deviation from the mean is expressed as

Average deviation:

It is the ratio of sum of the absolute values of deviations to the no. of readings.

Where d1, d2, d3...................... dN are the deviations of readings 1, 2,.........................xN

https://en.wikipedia.org/wiki/Analysishttps://en.wikipedia.org/wiki/Data



Standard deviation:

The standard deviation also called as mean square deviation of N no. of data is defined as the

square root of the sum of individual deviations squared(d12

, d22

, ..................... dN2) divided by the no. of

readings(N).

Limiting errors:

Limiting error is used to indicate the accuracy of an instrument. The limiting error (or guarantee error) is

given by the manufacturer to define the maximum limit of the error that may occur in the instrument. For example,

if the resistance of a resistor is given as 50Ω ± 5%, it means that the resistance value falls between the limits 45Ω

and 55Ω.In other words the manufacturer of the resistor guarantees its value lie between 45 Ω to 55 Ω.

Probable errors:

It defines the half-range of an interval about a central point for the distribution, such that half of

the values from the distribution will lie within the interval and half outside. Thus it is equivalent to half

the interquartile range, or the median absolute deviation. The probable error can also be expressed as a

multiple of standard deviation σ,

i.e. Probable error ϒ= 0.675x σ

https://en.wikipedia.org/wiki/Half-rangehttps://en.wikipedia.org/wiki/Central_tendencyhttps://en.wikipedia.org/wiki/Interquartile_rangehttps://en.wikipedia.org/wiki/Median_absolute_deviation



Figure 1.3.1 Guassian distribution curve

Variance:

The square of the standard deviation is called variance.

i.e. V= (standard deviation)2

V

= σ

2

Problem 1:

A circuit was tuned for resonance by eight different students and the values of resonance

frequency in KHz were recorded as 532,548,543,535,546,531,543 and 536. Calculate the arithmetic

mean, average deviation, standard deviation and variance.

SI.NO RESONANT

FREQUENCIES(RF)

DEVIATION(d)

d=xi- ̅ 1 532(x1) d1= x1- ̅= -7.25 2 548(x2) d2= x2- ̅= 8.75 3 543(x3) d3= x3- ̅= 3.75 4 535(x4) d4= x4- ̅= -4.25 5 546(x5) d5= x5- ̅= 6.75 6 531(x6) d6= x6- ̅= -8.25 7 543(x7) d7= x7- ̅= 3.75 8 536(x8) d8= x8- ̅= -3.25 ∑RF=4314

**Note N =no. of resosonant frequencies=8

1. Arithmetic mean ( ̅) =532+548+543+535+546+531+543+536 8

=4314

8

= 539.25

2. Average deviation ( ̅) = d1+d2+d3+d4+d5+d6+d7+d8 8

=

=5.75KHZ

3. Standard deviation (σ) = N-1

= d1+d2+d3+d4+d5+d6+d7+d8)2 8-1



= 7

=6.54KHZ

4. Variance (V) = σ2

= (6.54)

2

=42.77KHZ

1.4 Standards

A standard is a physical representation of unit of measurement. Standards have been developed for all the

fundamental units as well as some of the derived mechanical and electrical units.

Standards are classified as follows:

1. Primary standards:

Primary standards are standards of such high accuracy which can be used as ultimate reference

standards. These standards are maintained by national standard laboratories in different parts of the world.

2. Secondary standards:

Secondary standards are basic reference standards used by measurement and calibration

laboratories. It is obtained by comparing with primary standard. For measurement of a quantity using

secondary standard instrument, pre-calibration is required. Calibration of a secondary standard is made by

comparing the results with a primary standard instrument or with an instrument having high accuracy or

with a known input source.

3. Working standards:

These standards are used to check and calibrate general laboratory instrument for their accuracy

and performance. Working standards are checked against the secondary standards.

4. International standards:

The Institute of Electrical and Electronics Engineers Standards Association (IEEE) is an

organization within IEEE that develops global standards in a broad range of industries,

including: power and energy, biomedical and healthcare, information

technology and robotics, telecommunication etc.

https://en.wikipedia.org/wiki/IEEEhttps://en.wikipedia.org/wiki/Technical_standardhttps://en.wikipedia.org/wiki/Electrical_powerhttps://en.wikipedia.org/wiki/Energy_developmenthttps://en.wikipedia.org/wiki/Medical_researchhttps://en.wikipedia.org/wiki/Health_carehttps://en.wikipedia.org/wiki/Information_technologyhttps://en.wikipedia.org/wiki/Information_technologyhttps://en.wikipedia.org/wiki/Roboticshttps://en.wikipedia.org/wiki/Telecommunication



These standards are not physical items that are available for comparison and checking of

secondary standards but are standard procedure, nomenclature, definitions etc. These standards have been

kept updated and some of the IEEE standards have been adopted by other organizations as standards.

One of the most important standards is the IEEE 4888 digits interface for programmable

instrumentation for test and other equipment standardising.

1.5 Comparison of AC and DC bridges

AC BRIDGES DC BRIDGES

The AC bridges are used to

measure the impedances

consisting of capacitance and

inductances.

The DC bridges are used to

measure resistances.

The AC bridges use the

alternating voltage as the

exciting voltage.

The DC bridges use the DC voltage

as exciting voltage.

The four arms of bridge

consists of resistors, inductors,

capacitors or their

combinations.

The four arms of bridge consists of

pure resistors.

The balancing equation for AC

bridges are

1. Z1Z4=Z2Z3, for magnitude

balance.

2. ɸ1+ɸ3= ɸ2+ɸ4, for phase

angle balance

The balancing equation for DC

bridges is

R1R4=R2R3

Examples of AC bridges are,

Maxwell’s bridge, Wein

bridge, etc,.

Examples of AC bridges are,

Wheatstone bridge, Kelvin bridge,

etc,.

1.6 Wheatstone bridge

The Wheatstone bridge was developed by Charles Wheatstone to measure unknown resistance. A

schematic of a Wheatstone bridge is shown below:



Figure 1.6.1 Wheatstone bridge

The bridge has four arms together with a source of EMF (V0) and a null detector i.e galvanometer

(G). The unknown resistor is Rx, the resistor Rk is known value, and the resistors R1 and R2 have a

known ratio R2/R1.A galvanometer (G) measures voltage difference VAB between points A and B.

When VAB=0 the bridge is said to be “balanced” and no current flows through the galvanometer (G).

Since VAB=0, the voltage drop from C to A must be equal to the voltage drop from C to B.VCA =

VCB. Likewise, we must have VAD = VBD. So we can write,

(1)

(2) .

Dividing (2) by (1), we have

(3) .

Thus, the unknown resistance Rx can be computed from the known resistance Rk and the known

ratio R2/R1. The resistors R1 and Rk are called Ratio arms, while the resistor R2 is called standard arm of

the bridge.

Applications of Wheatstone bridge:

1. To measure very low resistance values accurately.



2. Along with operational amplifiers it is used to measure physical parameters like temperature,

strain, light etc.

3. It is used by telephone companies to locate cable faults.



Unit 2: Transducers

08 Hours

Necessity of electrical transducers, selection of a transducer, active, passive, analog and digital

transducers. Strain gauge-principle, gauge factor, features of bonded, unbonded, wire and foil type strain

gauges, load cell. Principle of working & features of capacitive transducer, Hall effect type, LVDT,

thermistor, thermocouple, piezoelectric, proximity sensors, digital optical encoders & PIR sensors.

2.1 Necessity of electrical transducers

In a measurement system all the quantities being measured, could not be displayed. In such

situation, the accurate measurement of a quantity is usually done by converting the related information or

signal to another form which is more conveniently or accurately displayed. This is achieved with the help

of a device which is known as transducer.

Definition: It is a device which converts the energy of one form to another.

OR

It is a device which converts non electrical quantity (e.g. sound, light, heat) into an electrical

quantity (Voltage, current or frequency).

Non electrical quantity Electrical quantity

Figure 2.1.1 Transducer

Benefits of electrical transducer:

1. Electrical amplification and attenuation can be done easily that too using static device.

2. The effect of friction is minimised.

3. The electric or electronic system can be controlled with a very small electrical power.

4. The power can be easily used, transmitted and processed for purpose of measurement.

2.2 Selection criteria of a transducer

1. Operating principle: The transducers are so many times selected on the basis of operating principle

used by them. The operating principles used in transducer may be resistive, inductive, capacitive, opto-

electronic, and so on.

2. Sensitivity: The transducer should give a sufficient output signal per unit of measured input in order

to yield meaningful data.

Transducer



3. Operating range: The transducer should maintain the range requirements and have a better resolution

over its entire range.

4. Accuracy: High degree of accuracy is necessary for measurement.

It should not alter the quantity to be measured.

5. Stability and reliability: The transducer should exhibit a high degree of stability during its operation

and storage life. Reliability should be assured so that the instrument continues uninterrupted.

6. Insensitivity to unwanted signals: The transducer should be minimally sensitive to unwanted signal

and highly sensitive to wanted signal.

7. Cost: The transducer should be easily available at reasonable prices.

8. Loading Effects: To avoid loading effect, it is necessary that a transducer has high input impedance

and low output impedance.

9. Physical Environment: The transducer selected should be able to withstand any change in

environmental conditions and maintain its output-input relationship.

2.3 Active and passive transducers

On the basis of methods of energy conversion used the transducers are classified into following two

categories:

A transducer, which develops its output in the form of electrical current or voltage without any

auxiliary source, is called active transducer. The energy required for this is absorbed from the physical

quantity which is being measured. Therefore, active transducers are also called as self generating type

transducers.

Examples are thermocouples, piezo-electric transducers, photovoltaic cell etc.

A transducer, which derives the power required for energy conversion from an external power

source is called as a passive transducer. Therefore, passive transducers are also called as externally

powered transducers.

Examples are Resistance thermometers and thermistors, photoemission cell etc.

2.4 Analog transducers and digital transducers

On the basis of type of output, the transducers are classified into following two categories:



A transducer which converts the input physical quantity into an analog output which is a

continuous function of time is known as analog transducers.

Examples are linear variable differential transformer (LVDT), thermo-couple, strain gauge,

thermistor etc.

A transducer which converts the input physical quantity into an digital output which is in the form

of pulses is known as digital transducers.

Examples are digital tachometers, digital optical encoders etc.

2.5 Strain gauge

2.5.1 Principle

Strain Gauge is a passive transducer. It is a type of sensor whose resistance varies with applied

force. It converts force, pressure, tension etc., into a change in electrical resistance which can be

measured.

The basic principle of operation of a strain gauge is simple: when strain is applied to a thin metallic

wire, its dimension changes, thus changing the resistance of the wire. The value of resistivity of conductor

also changes.When it is strained its property is called piezo-resistance. Therefore, resistance strain gauges

are also known as piezo-resistive gauges.

2.5.2 Gauge factor

It is defined as the ratio of per unit change in resistance to per unit change in length.

Gauge factor (Gf) =

⁄

Where, R = change in resistance R,

= change in length per unit length L.

The resistance of the wire of strain gauge, R is given by

R =

Where, ρ = Resistivity of the material of wire ( of strain gauge ),

L = Length of the wire.



A = Cross-sectional area of the wire.

Problem 1:

A strain gauge has an unstrained length of 10cm and resistance of 100KΩ. When its length

reduces to 9.9cm, the resistance decreases to 98KΩ. Estimate its gauge factor.

Solution: R= Initial Resistance = 100KΩ.

L= Initial length=10Cm

ΔR= change in initial resistance

=100x103-98x10

3

=2KΩ

ΔL = The change in length

= 10-9.9 = 0.1 cm

Therefore, gauge factor = Gf =

=

= 20

2.5.3 Bonded resistance strain gauge

These strain gauges are directly bonded (that is, pasted) onto the surface of the structure under

study. Hence they are termed as bonded strain gauges.

Figure 2.5.3.1 Bonded type strain gauge

Features of bonded resistance strain gauge:

1. They are reasonably inexpensive.

2. They can pull off overall accuracy of better than +/-0.10%.

3. They are available in a short gauge length and have small physical size.

4. These strain gauges are only moderately affected by temperature changes.



5. They are extremely sensitive and have low mass.

6. Bonded resistance strain gages can be employed to measure both static and dynamic strain.

7. These types of strain gauges are appropriate for a wide variety of environmental conditions.

2.5.4 Unbonded strain gauge

These strain gauges are not directly bonded (that is, pasted) onto the surface of the structure under

study. Hence they are termed as unbonded strain gauges.

Figure 2.5.4.1 Unbonded type strain gauge

Features of unbounded strain gauge:

1. They are able to measure strains of ±1μm/m.

2. They are small in size and light in weight.

3. They are able to respond to high frequency signals.

4. They have wide range of frequency response.

5. They have stable calibration constant (gauge factor).

6. They are flexible in use and are used in wide range of applications.

7. They are low in cost.

2.5.5 Wire type strain gauge

These strain gauges consists of grid of fine resistance wire of about 0.025mm in diameter or

less directly bonded (that is, pasted) onto the surface of the structure under study.



Figure 2.5.5.1 Wire type strain gauge

Features of wire type strain gauges:

1. The wire type strain gauge should have a high value of gauge factor.

2. The resistance of wire type strain gauge should be as high as possible.

3. The wire type strain gauge should have a low resistance temperature coefficient.

4. The wire type strain gauge should not have any hysteresis effect in its response.

5. The wire type strain gauge should have linear characteristics.

6. The wire type strain gauge should have a good frequency response.

2.5.6 Foil type strain gauge

Foil type strain gauges use similar materials to wire strain gauges but have greater heat dissipation

capacity on account of greater surface area. Due to this reason they can be employed for higher operating

temperature range.

Features of foil type strain gauge:

The features of foil type strain gauge are similar to those of wire wound strain gauges except

resistance value of foil gauge are available in between 50 and 1000ohms, and maximum gauge current is

about 30mamps.

2.6 Load cells

Load cell is a passive transducer or sensor which converts applied force or load into electric

signals. These electric signals can be voltage change, current change or frequency change depending on

the type of load or circuit used.

Strain gauge load cells:

Figure shows a strain gauge load cell. It consists of a steel cylinder, on which four identical strain

gauges. The gauges R1 and Rg are along the direction of applied load and the gauges R2 and R3 are

attached circumferentially to gauges R1 and Rg. All the four gauges are connected electrically to the four

limbs of a wheat stone bridge circuit.



Figure 2.6.1 Load cell

When there is no load on the cell, all the four gauges have the same resistance

(R1=R2=R3=Rg).under these conditions the A and B terminals are at the same potential, the bridge is

balanced and the output voltage is zero.

Applications:

1. Road vehicle weighing devices.

2. Draw bar and tool-force dynamometers.

3. Crane load monitoring.

2.7 Capacitive transducers

Capacitive transducers are passive transducers with a variable capacitance. These are mainly

used for the measurement of displacement, pressure etc,.

The capacitive transducer comprises of two parallel metal plates that are separated by a dielectric

material.

2.7.1 Principle of working

The principle of operation of capacitive transducers is based upon the equation for capacitance of

a parallel plate capacitor.

Where, A = Overlapping area of plates; m2,

d = Distance between two plates; m,



= 0rPermittivity (dielectric constant); F/m.

0 = Permittivity of free space=8.854*10-12

F/m.

r = Relative Permittivity.

Capacitive transducers using change in Area of the plates:

Figure (a) shows a capacitive transducer, where capacitance changes due to change in area (A) of

the plates. Since capacitance is directly proportional to the effective area of the plates, response of such

system is linear.

Capacitive transducers using change in distance of the plates:

Figure (b) shows a capacitive transducer, where capacitance changes due to change in distance(D)

between the plates. Here, one is a fixed plate and another is a movable plate. The displacement to be

measured is applied to the movable plate. Since the capacitance varies inversely as the distance between

the plates the response of the transducer is not linear.

Capacitive transducers using change in dielectric medium:

Figure (c) shows, if the area (A) and the distance (D) between the plates of a capacitor remain

constant, capacitance will vary only as a function of the dielectric constant (e) of the substance filling the

gap between the plates. Physical variables such as displacement, force or pressure can cause the

movement of dielectric material in the capacitor plates, resulting in changes in the effective dielectric

constant which in turn will change the capacitance.

Figure 2.7.1.1 Capacitive transducers

2.7.2 Features of capacitive transducer

1. Accuracy - provide accuracies as high as ±0.02% Full Scale (FS).

2. Minimal mechanical motion.

3. Range capabilities.

4. Long term stability.

5. High-level output.

6. Electromagnetic compatibility.



7. Resistant to harsh environments.

2.7.3 Advantages of capacitive transducer

1. Requires extremely small force for operation.

2. Requires small power for operation.

3. Frequency response is good.

4. A resolution of the order of 2.5*10-3

.

5. Higher input impedance. Therefore loading effects are minimum.

2.7.4 Disadvantages of capacitive transducer

1. Sensitivity to temperature variations.

2. The possibility of erratic or distortion signals due to long lead length.

3. They show nonlinear behavior on account of Edge effects. This can be eliminated using guard

rings.

2.7.5 Applications

1. As frequency modulator in RF oscillator.

2. In capacitance microphone.

3. Use the capacitance transducer in an ac bridge circuit.

4. To measure force and pressure.

5. To measure humidity in gases.

2.8 Hall effect transducers

2.8.1Hall effect

When a magnetic field is applied at right angles to the direction of electric current, an electric field

is setup which is perpendicular to both the direction of electric current and the applied magnetic field.

This phenomenon is called Hall effect.

Fig 2.8.1.1 Current carrying semiconductor bar subject to transverse magnetic field



In the above figure, A thin sheet of semiconductor bar (called Hall element) is carrying a current (I)

and is placed into a magnetic field (B) which is perpendicular to the direction of current flow. Due to the

presence of force, the distribution of current is no more uniform across the Hall element and therefore a

potential difference is created across its edges perpendicular to the directions of both the current and the

field. This voltage is known Hall voltage and its typical value is in the order of few microvolts. The Hall

voltage is directly proportional to the magnitudes of I and B. So if one of them (I and B) is known, then

the observed Hall voltage can be used to estimate the other.

VH

VH =

Where, RH = Hall co-efficient.

B = Magnetic field strength.

I = Current carried by the semiconductor bar.

B = width of the specimen along the magnetic field.

The Hall effect may be used:

To find whether semiconductor is N-type or P-type.

To determine charge carrier concentration.

Hall effect transducer:

These are transducers in which “Hall Effect” is used to measure various electrical and non-electrical

quantities.

The following are the applications of Hall effect transducers:

2.8.2 Hall effect displacement transducer

Hall effect element may be used for measuring a linear displacement.



Figure 2.8.2.1 Hall effect displacement transducer

The Hall Effect element is located in the gap, adjacent to the permanent magnet and the field

strength produced in the gap, due to the permanent magnet, is changed by changing the position of the

ferromagnetic plate. The voltage output of the Hall effect element is proportional to the field strength of

the gap which is a function of the position of ferromagnetic plate with respect to the structure.Thus

displacement can be measured by the Hall effect transducer. Very small displacements (as small as 0.025

mm) can be measured by this method.

2.8.3 Current measurement

Hall effect transducer is used to measure current in a conductor, without disturbing the circuit and

without making electrical connection between the conductor circuit and the meter.

Figure 2.8.3.1 Measurement of current using Hall effect In the above figure when a DC or AC current flows through the conductor wound around a core it

sets up a magnetic field. This magnetic field is proportional to the current. A Hall effect sensor is placed

in the slot which acts as a magnetic concentrator. The voltage produced at the output terminals is

proportional to the magnetic field strength and hence is proportional to the current, flowing through the

conductor.This method is used to measure current from less than a mA to thousands of amperes.

2.8.4 Fluid level measurement

The below figure shows how a Hall effect transducer can be used to measure fluid(fuel) level in an

automobile fuel tank.



Figure 2.8.4.1 Measurement of fluid level using Hall effect

A fuel level indication can be obtained with a Hall-effect sensor by attaching a magnet to the float

assembly. As the float moves up and down with the fuel level, the gap between the magnet and the Hall

element will change. The gap changes the Hall-effect and thus the output voltage.

2.9 LVDT (Linear Variable Differential Transducer)

LVDT works under the principle of mutual induction. It is used to translate the linear motion into

electrical signals.

2.9.1 Construction

Figure 2.9.1.1 shows the construction of LVDT. LVDT consists of a cylindrical former where it is

surrounded by one primary winding in the centre of the former and the two secondary windings at the

sides. The number of turns in both the secondary windings are equal, but they are opposite to each other,

i.e., if the left secondary windings is in the clockwise direction, the right secondary windings will be in

the anti-clockwise direction, hence the net output voltages will be the difference in voltages between the

two secondary coil. The two secondary coils are represented as S1 and S2. An iron core is placed in the

centre of the cylindrical former which can move in to and fro motion as shown in the figure. The AC

excitation voltage is 5 to 12V and the operating frequency is given by 50 to 400 HZ.



Figure 2.9.1.1 LVDT construction and circuit

2.9.2 Working of LVDT

As shown in the above figure, an ac voltage with a frequency between (50-400Hz) is supplied to the

primary winding. Thus, two voltages VS1 and VS2 are obtained at the two secondary windings S1 and S2

respectively. The output voltage will be the difference between the two voltages (VS1-VS2) as they are

combined in series. Let us consider three different positions of the soft iron core inside the former.

Null Position - This is also called the central position as the soft iron core will remain in the exact

center of the former. Thus the linking magnetic flux produced in the two secondary windings will be

equal. The voltage induced because of them will also be equal. Thus the resulting voltage VS1-VS2 = 0.

Right of Null Position - In this position, the linking flux at the winding S2 has a value more than

the linking flux at the winding S1. Thus, the resulting voltage VS1-VS2 will be in phase with VS2.

Left of Null Position - In this position, the linking flux at the winding S2 has a value less than the

linking flux at the winding S1. Thus, the resulting voltage VS1-VS2 will be in phase with VS1.

From the working it is clear that the difference in voltage, VS1-VS2 will depend on the right or left

shift of the core from the null position. Also, the resulting voltage is in phase with the primary winding

voltage for the change of the arm in one direction, and is 180 degrees out of phase for the change of the

arm position in the other direction.

The magnitude and displacement can be easily calculated or plotted by calculating the magnitude

and phase of the resulting voltage.

2.9.3 Advantages of LVDT

1.High resolution.



2. High output.

3. High sensitivity.

4. Very good linearity.

5. Ruggedness.

6. Less friction.

7. Low hysteresis.

8. Low power consumption.

2.9.4 Disadvantages of LVDT

1. Very high displacement is required for generating high voltages.

2. Shielding is required since it is sensitive to magnetic field.

3. The performance of the transducer gets affected by vibrations

4. It is greatly affected by temperature changes.

2.9.5 Applications of LVDT

LVDT is used to measure displacement ranging from fraction millimetre to centimetre. Acting as a

secondary transducer, LVDT can be used as a device to measure force, weight and pressure, etc,.

Multiple LVDT‟s are used for measurement of pressure or weight applied by liquid in a tank.

2.10 Thermistors

Thermistors are transducers which are thermally sensitive variable resistance made of

semiconducting materials. A thermistor is a type of resistor whose resistance is dependent on temperature.

It exhibits high negative temperature coefficient of resistance.

Figure 2.10.1 Types of thermistors

2.10.1 Applications of thermistors

1. Measurement of temperature.

2. Measurements of level, flow and pressure of liquids.

https://en.wikipedia.org/wiki/Resistorhttps://en.wikipedia.org/wiki/Electrical_resistancehttps://en.wikipedia.org/wiki/Temperature



3. Voltage Regulation.

4. Circuit Protection.

5. Volume Control.

6. Time Delay.

2.11 Thermocouple

A Thermocouple is a sensor used to measure temperature. Thermocouples consist of two wire legs

made from different metals. The wires legs are welded together at one end, creating a junction. This

junction is where the temperature is measured. When the junction experiences a change in temperature, a

voltage is created.

Figure 2.11.1 Thermocouple

The thermocouple works on the principle of Seebeck effect. Seebeck effect is the phenomenon in

which a voltage difference is produced between two dissimilar electrical conductors or semiconductors

due to temperature difference between the two substances. When heat is applied to one of the two

conductors or semiconductors, heated electrons flow towards the cooler one. Direct current will flow

through an electric circuit, if the pair is connected through it. Seebeck effect usually produces small

voltages that are a few microvolts per kelvin of temperature difference at junction.

2.11.1 Advantages

1. Low cost.

2. Small size.

3. Robust.

4. Wide range of operation.

5. Provide fast response.

6. Accurate for large temperature changes.



2.11.2 Disdvantages

1. Very weak output.

2. Limited accuracy for small temperature.

3. Sensitive to electrical noise.

2.11.3 Applications

1. Temperature measurement for kilns, gas turbine exhaust, diesel engines.

2. Steel industry.

3. Heating appliance safety.

4. Power production in thermoelectric generation.

5. Thermoelectric cooling.

2.12 Piezoelectric Transducers

Piezoelectric transducer is based on principle of piezoelectric effect

Piezoelectric effect:

Piezoelectric effect states that when mechanical stress or forces are applied on quartz, crystal,

produce electrical charges on quartz crystal surface. The rate of charge produced will be proportional to

rate of change of mechanical stress applied on it. Higher will be stress higher will be voltage. Certain

crystals namely Quartz, Rochelle salt and tourmaline, which exhibits piezoelectric effect are called

piezoelectric crystals.

There are two main groups of piezoelectric crystals,

Natural crystals: Quartz and tourmaline.

Synthetic crystals: Rochelle salt, lithium sulphate etc.

Working of piezoelectric transducer:

https://en.wikipedia.org/wiki/Kilnhttps://en.wikipedia.org/wiki/Gas_turbinehttps://en.wikipedia.org/wiki/Diesel_engine



In the above figure, Piezo electric crystal is used for measuring varying force applied to a simple

plate. The magnitude and polarity of the induced charge on the crystal surface is proportional to

magnitude and direction of applied force.

The charge at the electrode gives rise to voltage (E) is given by,

E=

= gtP

Where, g = Voltage sensitivity

T = Thickness of the crystal

F = Force in Newton

A = Area of the crystal

P = pressure =

2.12.1 Advantages

1. No need of external force.

2. Easy to handle due to small size.

3. High frequency response.

4. High output.

5. They can be cut into variety of shapes and sizes.

2.12.2 Disadvantages

1. It is not suitable for measurement in static condition.

2. It is affected by temperatures.

2.12.3 Applications

1. Microphones.

2. Medical diagnostics.

3. It is used in electric lighter used in kitchens.

4. They are used for studying high speed shock waves and blast waves.

5. Used in Inkjet printers.

6. It is also used in restaurants or airports where when a person steps near the door the door opens

automatically. In this the concept used is when person is near the door a pressure is exerted

persons weight on the sensors due to which the electric effect is produced and the door opens

automatically.

7. Under water detection system.



2.13 Proximity sensors

A proximity sensor is a sensor able to detect the presence of nearby objects without any physical

contact. The object being sensed is often referred to as the proximity sensor's target. The maximum

distance that this sensor can detect is defined "nominal range".

Types of proximity sensors:

The Eddy current proximity sensor:

This uses the effect of eddy (circular) currents to sense the proximity of non-magnetic but

conductive materials. A typical eddy current transducer contains two coils: an active coil (main coil) and

a reference coil. When a coil is supplied with an alternating current, an alternating magnetic field is

produced. If there is metal object in close proximity to this alternating magnetic field, then eddy currents

are induced in it.Therefore impedance of the coil changes thereby changing the amplitude of the

alternating current.

Figure 2.13.1 Eddy current proximity sensor

Advantages of Eddy current proximity sensor:

1. Non-contacting measurement.

2. High resolution.


Disadvantages of Eddy current proximity sensor:

1. Effective distance is limited to close range.

2. The relationship between the distance and the impedance of the coil is nonlinear and

temperature dependent.

https://en.wikipedia.org/wiki/Sensor



3. Only works on conductive materials with sufficient thickness.

Capacitance proximity sensors:

Figure 2.13.2 below shows capacitance proximity sensor. Capacitive Proximity Sensors detect

changes in the capacitance between the sensing object and the sensor. The amount of capacitance varies

depending on the size and distance of the sensing object. An ordinary capacitive proximity sensor is

similar to a capacitor with two parallel plates, where the capacity of the two plates is detected. One of the

plates is the object being measured (with an imaginary ground), and the other is the Sensor's sensing

surface. The changes in the capacity generated between these two poles are detected. The objects that can

be detected depend on their dielectric constant.

Figure 2.13.2 Capacitive proximity sensor

Inductive proximity sensors:

In the below figure a coil is wound around a core. When the end of a inductive coil is close to a

metal object its inductance changes. This change can be monitored by its effect on a oscillator circuit, and

this change is used to trigger a switch.

Figure 2.13.3 Inductive proximity sensor

Pneumatic sensors:

Pneumatic sensors are used to measure displacement, as well as sense the objects close to it. The

displacement and proximity are transformed into change in air pressure. Figure below shows a schematic

of such a sensor. It comprises of three ports. Low pressure air is allowed to escape through port A. In the

absence of any obstacle / object, this low pressure air escapes and in doing so, reduces the pressure in the

port B. However, when an object obstructs the low pressure air (Port A), there is rise in pressure in output

port B. This rise in pressure is calibrated to measure the displacement.

https://www.ia.omron.com/support/glossary/atoz/178/index.html



Figure 2.13.4 Pneumatic sensor

2.14 Digital optical encoder

A digital optical encoder is a device that converts motion into a sequence of digital pulses. By

counting a single bit or by decoding a set of bits, the pulses can be converted to relative or absolute

position measurements.

Encoders have both linear and rotary configurations, but the most common type is rotary.

Rotary encoders are manufactured in two basic forms:

Absolute encoder where a unique digital word corresponds to each rotational position of the shaft,

The incremental encoder, which produces digital pulses as the shaft rotates, allowing

measurement of relative position of shaft. Most rotary encoders are composed of a glass or plastic code

disk with a photographically deposited radial pattern organized in tracks. As radial lines in each track

interrupt the beam between a photo emitter-detector pair, digital pulses are produced.



Figure 2.14.1 Digital optical encoder

Figure above shows the construction of an optical encoder. It comprises of a disc with three

concentric tracks of equally spaced holes. Three light sensors are employed to detect the light passing

through the holes. These sensors produce electric pulses which give the angular displacement of the

mechanical element e.g. shaft on which the Optical encoder is mounted. The inner track has just one hole

which is used locate the „home' position of the disc. The holes on the middle track offset from the holes of

the outer track by one-half of the width of the hole. This arrangement provides the direction of rotation to

be determined. When the disc rotates in clockwise direction, the pulses in the outer track lead those in the

inner; in counter clockwise direction they lag behind.

2.15 PIR sensors

All objects with a temperature above absolute zero emit heat energy in the form of radiation.

Usually this radiation isn't visible to the human eye because it radiates at infrared wavelengths, but it can

be detected by electronic devices designed for such a purpose. A passive infrared sensor (PIR sensor) is

an electronic sensor that measures infrared (IR) light radiating from objects in its field of view. They are

often referred to as PIR, "Passive Infrared", "Pyroelectric", or "IR motion" sensors.

PIRs are basically made of a pyroelectric sensor (which is shown in figure below as the round metal

can, with a rectangular crystal in the centre), which can detect levels of infrared radiation. The PIR sensor

itself has two slots in it, each slot is made of a special material that is sensitive to IR. The lens used here

consists of two slots that can 'see' out past some distance. When the sensor is idle, both slots detect the

same amount of IR. When a warm body like a human or animal passes by, it first intercepts one half of

the PIR sensor, which causes a positive differential change between the two halves. When the warm body

https://en.wikipedia.org/wiki/Absolute_zerohttps://en.wikipedia.org/wiki/Heathttps://en.wikipedia.org/wiki/Human_eyehttps://en.wikipedia.org/wiki/Sensorhttps://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Pyroelectric



leaves the sensing area, the reverse happens, whereby the sensor generates a negative differential change.

These change pulses are what is detected.

Figure 2.15.1 PIR sensors

2.15.1 Applications

1. All outdoor Lights.

2. Lift Lobby.

3. Multi Apartment Complexes.

4. Common staircases.

5. For Basement or Covered Parking Area.

6. Shopping Malls.

2.15.2 Features

1. Low Noise and High Sensitivity.

2. Supply Voltage – 5V.

3. Delay Time Adjustable.

4. Standard TTL Output.



Unit 3: Analog meters 11 Hours

Principle of PMMC meters, DC ammeters and voltmeters using PMMC. Shunt and series resistors, multi

range voltmeters/ammeters, loading effect and voltmeter sensitivity, problems on extending range.

Working of electrodynamometer type voltmeter, ammeter and wattmeter.

Electronic voltmeters: Pros and cons, working of FET input, chopper type DC amplifier voltmeter,

solid-state voltmeter using op-amp, AC voltmeter using full-wave rectifier, Peak responding and true

RMS voltmeters. Ohmmeters series and shunt type. Concept of Calibration of meters.

3. ANALOG METERS:

An instrument which measures and indicates values by means of a continuous scale and a movable

pointer are called analog meters.

“Ammeters”, are connected in series in the circuit whose current is to be measured. “Voltmeters”

are connected in parallel with the circuit whose voltage is to be measured. “Ohmmeters” are used for

measurement of resistance.

3.1 PMMC meters

Figure 3.1.1 PMMC meter and D’Arsonval movement

Principle of PMMC meters:

When current carrying conductor is placed in a magnetic field, a mechanical force acts on the

conductor, if it is attached to a moving system, with the coil movement, the pointer moves over the scale.

Thereby, the basic PMMC movement is called as D‟Arsonal movement. It can be used for D.C

measurements.

Construction:

It consists of a permanent horse shoe magnet with soft iron pole pieces attached to it.

A cylinder shaped soft iron core, is placed in between two pole pieces around which a coil of fine

wire moves wound on a light metal frame. A light pointer attached to the moving coil moves up-scale as

http://www.electrical4u.com/what-is-magnetic-field/



the coil rotates when the current is passed through it. The rotating coil is prevented from continuous

rotation by a spring which provides restoring torque.

Working:

This meter movement works on “MOTOR PRINCIPLE” (when a current carrying conductor is

placed in a magnetic field, it is acted upon by a force which tends to move it to one side and out of the

field).When the instrument is connected in the circuit to measure current or voltage, the operating current

flows through the coil. Since the current carrying coil is placed in the magnetic field of the permanent

magnet, a mechanical torque acts on it. As a result of this torque, the pointer attached to the moving

system moves in clockwise direction over the graduated scale to indicate the value of current or voltage

being measured.

The deflecting torque is given by,

Td = NBldI

where N is number of turns,

B is magnetic flux density in air gap,

l is the length of moving coil, d is the width of the moving coil,

I is the electric current.

Thus, Td α I

The instrument is spring controlled so that,

Tc α θ

The pointer will comes to rest at a position, where

Td =Tc

Therefore,

Thus, the deflection is directly proportional to the operating current. Hence, such instruments have

uniform scale.

Advantages:

1. Uniform scale.ie, evenly divided scale.

2. Very effective eddy current damping.

3. High efficiency.

4. Require little power for their operation.

5. No hysteresis loss (as the magnetic field is constant).

6. Very accurate and reliable.

Disadvantages:

1. Cannot be used for a.c measurements.

θ α I

http://www.electrical4u.com/what-is-magnetic-field/#Magnetic-Flux-or-Magnetic-Lines-of-Force



2.More expensive (about 50%) than the moving iron instruments because of their accurate design.

3. Some errors are caused due to variations (with time or temperature) either in the strength of

permanent magnet or in the control spring.

Applications:

1. In the measurement of direct currents and voltages.

2. In DC galvanometers to detect small currents.

3.2 DC ammeters and voltmeters using PMMC

3.2.1 DC Ammeters

The basic movement of DC ammeter is the PMMC D‟Arsonal movement. Since the coil winding

in PMMC meter is small and light, they can carry only small currents (μA-1mA). Measurement of large

current requires a shunt external resistor to connect with the meter movement, so only a fraction of the

total current will passes through the meter.

Figure 3.2.1.1 DC ammeters

Let, Rm=Internel resistance of the meter

Rsh=shunt resistance

Im= current through the meter

Ish= shunt current

I=Current of the circuit to be measured.

Thus, I= Im+Ish

Since voltage drop across the shunt and the meter is same,

Vsh=Vm

Ish*Rh=Im*Rm

Rsh=



Therefore,

3.2.2 D.C Voltmeter

A voltmeter is always connect in parallel with the element being measured, and measures the

voltage between the points across which it is connected. Most d.c voltmeter use PMMC meter with series

resistor as shown in figure below. The series resistance should be much larger than the impedance of the

circuit being measured, and they are usually much larger than Rm.

Figure 3.2.2.1 DC voltmeter

Let, Rm=Internel resistance of the meter

Rs=series resistance

Im= current through the meter

V=Voltage of the circuit to be measured

Now, V=Im*(Rs+Rm)

Rs =

Therefore,

3.3 Multirange ammeters

The current range of the DC ammeter can be extended by a number of shunts selected by a

switch(S).Such meter is called multirange ammeters.

Rs=



Figure 3.3.1 Multirange ammeters

Let, Rm = Internel resistance of the meter

R1, R2, R3, R4 = shunt resistors

Im = current through the meter

I1, I2, I3, I4 = shunt currents

I = Current of the circuit to be measured.

We know that, Rsh =

Ish=I-Im

We can write, Rsh =

- 1=

But,

= m = multiplying power,

Therefore, Rsh =

Let m1, m2, m3, m4 be the shunt multiplying powers for current I1, I2, I3, I4.

R4=

R2=

R1=

R3=



3.3.1 Multirange DC ammeter using universal shunt (Ayrton shunt)

The universal shunt or the Ayrton shunt eliminates the possibility of having the meter in the

circuit without shunt.

Figure 3.3.1.1 Aryton shunt

The selector switch S, selects the appropriate shunt required to change the range of the meter.

When the position of the switch is '1' then the resistance R1 is in parallel with the series combination

of R2 , R3 and Rm. Hence current through the shunt is more than the current through the

meter, thus protecting the basic meter.

When the switch is in the position '2', then the series resistance of R1 and R2, is in parallel with the

series combination of R3 and Rm. The current through the meter is more than through the shunt in this

position.

When the switch is in the position '3', the resistances R1 , R2 and R3 are in series and acts as the

shunt. In this position, the maximum current flows through the meter. This increases the sensitivity of the

meter.

The voltage drop across the two parallel branches is always equal.

Thus, Ish Rsh = Im Rm

But in position 1, R1 is in parallel with R2 + R3 + Rm

Where, I1 is the first range required.

In position 2, R1 + R2 is in parallel with R3 + Rm .

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where I2 is the second range required.

In position 3, R1 + R2 + R3 is in parallel with Rm .

where I3 is the third range required.

The current range I3 is the minimum while I1 is maximum range possible. Solving the equations

(1), (2) and (3) the required Ayrton shunt can be designed.

3.3.2 Multirange DC voltmeter

A DC voltmeter is converted into a multirange voltmeter by connecting a number of resistors

(multipliers) in series with the meter movement.

Figure 3.3.2.1 Multirange DC voltmeter

In the above figure, the multipliers are connected in series with the meter. The selector switch is

used to select the required voltage range.

When the switch S is at position V1, R1 + R2 + R3 + R4 acts as a multiplier resistance. While when the

switch S is at position V4 then the resistance R4 only acts as multiplier resistance. The V4 is

the lowest voltage range while V1 is the maximum voltage range.

The multiplier resistances can be calculated as :

In position V4, the multiplier is R4 only. The total resistance of the circuit is say RT.

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In position V3, the multiplier is R3 + R4.

In position V2, the multiplier is R2 + R3 + R4.

In position V1, the multiplier is R1 + R2 + R3 + R4.

Using equations (1), (2), (3) and (4) multipliers can be designed.

3.3.3 Loading effect and voltmeter sensitivity

Loading effect:

When selecting a meter for certain voltage measurement it is important to consider the sensitivity

of a DC voltmeter. A low sensitivity meter may give a correct reading if the circuit resistance is low but it

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will produce defective readings in a high resistance circuit because the meter acts as a shunt which in turn

reduces the total equivalent circuit. Than the meter indicates low readings. This is called loading effect.

Voltmeter sensitivity:

In a, the ratio of the total resistance RT to the voltage range remains same. This ratio is nothing but

the reciprocal of the full scale deflection current of the meter i.e. 1/Im. This value is called sensitivity of

voltmeter.

Thus the sensitivity of voltmeter is defined as,

S =

Ω/V

The sensitivity range is specified on the meter dial and it indicates the resistance of the meter for a

one volt range.

Where, S=sensitivity of the voltmeter (Ω/V)

V=Voltage rang, as set by the range switch

Rm=internal resistance of the meter

Rs=Resistance of the multiplier

RT=Total circuit resistance

RT =

------------------------------------------------------------(1)

S =

---------------------------------------------------------------(2)

Sub (2) in (1)

RT = S*V-------------------------------------------------------------(3)

Since Rs and Rm are in series, RT=Rs+Rm---------------------(4)

Sub (4) in (3), Rs+Rm = S*V ------------------------------------(5)

Solving eq (5) for Rs,

3.4 Electrodynamometer

Rs=(S*V)-Rm



Electrodynamometer type instruments are used as AC voltmeters and ammeters both in the range

of power frequencies and lower part of the audio frequency range. In order that the instrument should be

able to read a.c. quantities, the magnetic field in the air gap must change along with the change in current.

This principle is used in the electrodynamometer type instrument.

Figure 3.4.1 Electrodynamometer

Construction:

The necessary field required for the operation of the instrument is produced by the fixed coils. A

uniform field is obtained near the centre of coil due to division of coil in two sections. These coils are air

cored. Fixed coils are wound with fine wire for using as voltmeter, while for ammeters and wattmeter‟s it

is wound with heavy wire. Ceramic is usually used for mounting supports. If metal parts would have been

used then it would weaken the field of the fixed coil. The moving coil is wound either as a self-sustaining

coil or else on a non-metallic former. If metallic former is used, then it would induce eddy currents in it.

The construction of moving coil is made light as well as rigid. It is air cored. The moving coil is mounted

on a aluminium spindle. The moving system carries a pointer.

The electrodynamometer is used as Ac voltmeter, ammeter and with a slight modification it can

also be used as a wattmeter, power factor meter, frequency meter.

3.4.1 Electrodynamometer type ammeter

Figure 3.4.1.1 Electrodynamometer type ammeter



In the above circuit arrangement, when the instrument is used as an ammeter, fixed coils and

moving coil are connected in series and therefore carry the same current

i.e.I1=I2=I

Hence, angular deflection,

To measure heavy currents shunt is used to limit current through the moving coil. For small currents

shunt is not needed.

3.4.2 Electrodynamometer type voltmeter

Figure 3.4.2.1 Electrodynamometer type voltmeter

In the above circuit arrangement, when the instrument is used as an voltmeter, fixed coils and

moving coil are connected in series along with a high resistance.

i.e I1=I2=I

I =

in DC circuits

And I =

in AC circuits

Hence, angular deflection,

3.4.3 Electrodynamometer type wattmeter

Figure 3.4.3.1 electrodynamometer type wattmeter

ɵ

ɵ V2

ɵ



The fixed coils are connected in series with the circuit, while the moving coil is connected

in parallel. Also, on analog wattmeters, the moving coil carries a needle that moves over a scale to

indicate the measurement. A current flowing through the fixed coil generates an electromagnetic

field around the coil. The strength of this field is proportional to the line current and in phase with it.

For a dc circuit the deflection of the needle is proportional to both current(I) and the voltage(V).

Thus power, P=VI

For an ac circuit the deflection is proportional to the average instantaneous product of voltage and

current,

Thus power, P=VI cos φ. Here, cosφ represents the power factor.

Advantages of electrodynamic instruments:

1. As the coils are air cored, these instruments are free from hysteresis and eddy current losses. These

instruments can be used on both a.c. and d.c.

2. Electrodynamometer voltmeters are very useful where accurate r.m.s values of voltage, irrespective

of waveforms, are required.

3. Low power consumption

4. Light in weight.

Disadvantages of electrodynamic instruments:

1. These instruments have a low sensitivity also it introduces increased frictional losses. To get

accurate results, these errors must be minimized.

2. They are more expensive than other type of instruments.

3. These instruments are sensitive to overload and mechanical impacts.

4. The operation current of these instruments is large due to the fact that they have weak magnetic

field.

3.5 Electronic voltmeters

Pros:

1. Low level signal detection.

2. Low power consumption.

3. Less loading effect.

4. High sensitivity and high input impedence.


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6. Parallax error is eliminated.

Cons:

1. Special circuits are needed to convert analog to digital

2. These are expensive.

3. Effected by temperature changes

4. Large bandwidth is required

3.5.1 FET voltmeter using differential amplifier

Figure 3.5.1.1 FET voltmeter using differential amplifier

FET Voltmeter using differential amplifier is as shown in figure above. It consists of two identical

FET‟s Q1 and Q2. Increase in the current of one FET is offset by corresponding decrease in source current

of the other. The two FET‟s form the lower arms of the balanced bridge circuit where as the two drain

resistors RD form the upper arms.

The circuit is balanced under zero input voltage condition provided the tw FET‟s are identical.

Under such conditions there would be no current through the PMMC meter. Current flows through the

meter when positive voltage is applied to the gate of FET Q1. The magnitude of this current is found to be

proportional to the voltage being measured.

3.5.2 Chopper type DC amplifier voltmeter



Figure 3.5.2.1 Chopper type DC amplifiervoltmeter

Figure above shows the circuit diagram of a Chopper type DC amplifier voltmeter.

Two photo diodes are used in input stage which acts as half-wave modulators because of its

alternate switching action by the neon lamps at the frequency of oscillator.

In dc voltmeter circuit two neon lamps are used, these are supplied by an oscillator for alternate half

cycles. Output of chopper modulator is a square wave voltage (proportional to the input signal) which is

supplied to the ac amplifier through a capacitor and the amplified output is again passed through a

capacitor and then fed to chopper demodulator. The capacitor is used to smooth the output from the

amplifier.

The Chopper demodulator gives a dc output voltage (proportional to the input voltage) which is

passed through the low pass filter to remove any residual ac component and this dc output voltage is

supplied to the PMMC meter for measurement of input voltage.

Advantages:

1. It has very high input impedance of the order of 10MΩ.

2. It allows input signal in the range of 0.01mV.

3.5.3 Solid-state voltmeter using op-amp



Figure 3.5.3.1 Solid state voltmeter using op-amp

Figure above shows an electronic voltmeter using op-amp 741. This is directly coupled very high

gain amplifier. The gain can be adjusted to any suitable value by providing appropriate resistance

between its output terminal, Pin No.6, and inverting input pin No.2, to provide negative feedback. The

ratio R2/R1 determines the gain. The 0.1μF capacitor across the 100KΩ resistance R2 is used for stability

under stray pickups.

A 10KΩ potentiometer is connected between the offset null terminal 1 and 5 with its centre tap

connected to -5V supply for adjusting zero output for zero input conditions. The two diodes used are for

IC protection. If an excessive voltage say more than 100mV appears across them then depending on the

polarity of the voltage one of the diode conducts and protects the IC. A μA scale 50-1000μA full scale

deflection can be used as an indicator. R4 is adjusted to get maximum full scale deflection.

3.5.4 AC voltmeter using full-wave rectifier



Figure 3.5.4.1 AC voltmeter using full-wave rectifier

A full wave rectifier type AC voltmeter consists of four diodes and a PMMC meter as shown in

above fig. The current through the moving coil instrument flows in the same direction for both cycles of

the input voltage. The indication of instrument depends upon the mean value of the current flowing

through it.

Here, the meter reading would be 90% RMS i.e., 90% of the DC value. When the input is positive

D1 and D3 conducts, and the current flows through the meter from top to bottom. When the input is

negative D2 and D4 conducts through the meter from top to bottom. In both the cycles of the input

voltage current flows in same direction.

Advantages:

1. The frequency range extends from about 20Hz to high audio frequencies.

2. They have much lower operating current.

3. They have practically uniform scale for most ranges.

3.5.5 AC voltmeter using peak responding voltmeter

Figure 3.5.5.1 DC and AC coupled peak voltmeter respectively

Figure3.5.5.1 shows the two most common types of peak responding voltmeters. The capacitor

charges through the diode to the peak value of the applied voltage.

In both the circuits, the capacitor discharges very slowly through the high impedance input of DC

amplifiers, so that a negligible small amount of current supplied by the circuit under test keeps the

capacitor charged to the peak AC voltage. In DC coupled peak voltmeter the reading of the meter is

affected by the presence of DC with AC voltage.

Advantages:

The rectifying diode and the storage capacitor may be taken out of the instrument and placed in

probe when no pre-amplification is needed. The measured AC signal then travels no further than the

diode. The meter is then able to measure frequencies upto hundreds of MHz with a minimum of circuit

loading.

Disadvantages:



1. Error caused due to harmonic distortion.

2. Limited sensitivity.

3.5.6 True RMS reading voltmeter

Figure 3.5.6.1 Block diagram of true RMS reading voltmeter

Figure above shows the block diagram of a RMS Reading Voltmeter. It consists of two

thermocouples called main thermocouple (MT) and balancing thermocouple (BT). BT is used in the

feedback loop to cancel out the non-linear effects of the MT.

The unknown A.C voltage is amplified and fed to the heating element of the main thermocouple.

The heat produced by the wire is sensed by the measuring thermocouple which produces a proportional

DC voltage. This DC voltage upsets the bridge balance. The unbalance voltage is amplified by the DC

amplifier and fed back to the heating element of the balancing thermocouple.

Bridge balance is reestablished when the two thermocouples produce the same output voltages. At

this point the DC current in the heating element of the feedback thermocouple is proportional to the AC

current in the input thermocouple i.e., the DC is proportional to the rms value of the input AC signal. This

DC value is indicated by the meter movement in the output circuit.

Advantages:

1. Sensitivities in the mV range is possible.

2. The non-linear behaviour is avoided.



3. Complex waveforms are accurately mea

DEPARTMENT OF TECHNICAL EDUCATION Unit 1: Basics...

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