its good

1. UNIT-1
Introduction:
The measurement of any quantity plays very important role not only in science but in all
branches of engineering, medicine and in almost all the human day to day activities. The
technology of measurement is the base of advancement of science. The role of science and
engineering is to discover the new phenomena, new relationships, the laws of nature and to apply
these discoveries to human as well as other scientific needs. The science and engineering is also
responsible for the design of new equipments. The operation, control and the maintenance of
such equipments and the processes is also one of the important functions of the science and
engineering branches. All these activities are based on the proper measurement and recording of
physical, chemical, mechanical, optical and many other types of parameters.
The measurement of a given parameter or quantity is the act or result of a quantitative
comparison between a predefined standard and an unknown quantity to be measured. The major
problem with any measuring instrument is the error. Hence, it is necessary to select the
appropriate measuring instrument and measurement procedure which minimises the error. The
measuring instrument should not affect the quantity to be measured.
An electronic instrument is the one which is based on electronic or electrical principles for its
measurement function. The measurement of any electronic or electrical quantity or variable is termed
as an electronic measurement.
INSTRUMENTATION is essential in monitoring and analysis of any physical system and
its control .This course deals with different types of Transducers, Digital Voltmeters, Analog
Voltmeters, Signal Generators, Wave Analyzers and Measurement of non electrical quantities
Instrumentation is a branch of engineering, related to study of various instruments and their
control.

2. An instrument is a device that measures a physical or electrical quantity such as flow,
temperature, current, voltage, level, distance, angle, or pressure.
The measurement of a given parameter or quantity is the act of a quantitative comparison
between a predefined standard and an unknown quantity to be measured.
Measuring instrument: It is defined as the device for determining the value or magnitude of a
quantity or variable.
Electronic measurement: It is the one which is based on electronic or electrical principles for
its measurement function
Advantages of electronic instruments
Following are the advantages of electrical or electronic instrumentation.
1. Different physical quantities can be converted into electrical signal by transducers.
2. Electrical signal can amplified, multiplexed, filtered and measured easily.
3. Electrical signal can be converted from A/D or D/A signal.
4. Electrical signals can be transmitted over long distances by wire or radio link etc.
5. Many measurements can be carried simultaneously.
6. Digital signal are compatible with computers.
7. High Sensitivity, low power consumption, high reliability.
Functional Elements of an Instrument
Any instrument can be represented by a block diagram that indicates necessary elements and
its functions. The entire operation of a measuring system can be understand from the
following block diagram.
Primary Sensing Element:
An element of an instrument which makes first contact with the quantity to be
measured. In most cases a Transducer follows primary sensing element which converts the
measurand into a corresponding electrical signal.

3. Variable Conversion Element:
Output of the primary sensing element is in electrical form such as Voltage,
Frequency such an output may not be suitable for the actual measurement system. (Ex: A/D
converter)
Variable Manipulation Element:
The level of the output from the previous stage may not be enough to drive the next
stage. Thus variable manipulation element manipulates the signal, preserving the original nature
of the signal.
Data Transmission Element:
When the elements of the system are physically separated, it is necessary to transmit
the data from one stage to other. This is achieved by the data transmission element.
Data Presentation Element:
The data is monitored for analyzing purpose using data presentation element.(Ex: Visual
display)
Example: Ammeter
Moving coil senses current. Magnets & coil convert current in coil to force. Force is
transmitted to pointer through mechanical links. Pointer and scale presents the current value.

4. Performance Characteristics Static characteristics and Dynamic characteristics
Divided into two categories: Static and Dynamic characteristics.
Static characteristics: The set of criteria defined for the instruments, which are used to measure
the quantities which are slowly varying with time or mostly constant, ie., do not vary with time is
called static characteristics.
Dynamic characteristics: when the quantity under measurement changes rapidly with time, it is
necessary to study the dynamic relations existing b/w input and output which is expressed as
differential equations. The set of criteria defined based on such dynamic differential equation is
called dynamic characteristics.

5. Calibration: Calibration is the process of making an adjustment or making a scale so that the
reading of an instrument agree with the accepted and certified standard.
Note: if the device is repaired, aged or modified then recalibration is carried out.
STATIC CHARACTERSTICS
1. Accuracy
2. Precision
3. Resolution
4. Error
5. Sensitivity
6. Expected Value
Accuracy: Degree of closeness which the instrument reading approaches the true value of the
quantity to be measured. It indicates the ability of an instrument to indicate true value of the
quantity. Accuracy refers to how closely the measured value of a quantity corresponds to its
“true” value.
Precision: Precision is a measure of the reproducibility of the measurement i.e., its measure of
the degree to which successive measurements differ from one other. It is the degree of agreement
within a group of measurements or instruments. It is the measure of consistency or repeatability
of measurement. It denotes the closeness with which individual measurements are departed or
distributed about the average of numbers of measured values. High precision may not have high
accurate.
Error is the difference between the true value of the size being measured and the value found by
measurement. (or) The deviation of the true value from the desired value.
Sensitivity: This is the relationship between a change in the output reading for a given change of
the input. (This relationship may be linear or non-linear.) The sensitivity of measurement is a
measure of the change in instrument output that occurs when the quantity being measured
changes by a given amount The ratio of the change in output of an instrument to a change in the
value of the quantity to be measured.

6. Note: if the calibration curve is linear, then sensitivity of the instrument is the slope of the
calibration curve.
Resolution is the smallest amount of input signal change that the instrument can detect reliably.
If the input is slowly increased from some arbitrary input value, it will again be found that output
does not change at all until a certain increment is exceeded. This increment is called resolution or
discrimination of the instrument. Thus the smallest increment in input which can be detected
with certainty by an instrument is its resolution or discrimination.
-The smallest change in a measured variable to which an instrument will respond.
-This min change which cause change in output is called resolution.
-Resolution means smallest measurable input change.
Errors in Measurement
No measurement can be made with perfection and accuracy, but it is important to find out
what the accuracy actually is and how different errors have entered into the measurement.
Error occurs due to several sources like human carelessness in taking reading, calculating and
in using instrument etc. Some of the time error is due to instrument and environment effects.
Errors come from different sources and are classified in three types:
1. Gross Error
2. Systematic Errors
3. Random Errors

7. Gross Error: These errors occur due to the human mistakes in reading or using the instruments.
These errors cover human mistakes like in reading, calculating and recordings etc. It sometimes
occurs due to incorrect adjustments of instruments.
The complete elimination of gross errors is impossible, but we can minimize them by the
following ways:
1. It can be avoided by taking care while reading and recording the measurement data.
2. Taking more than one reading of same quantity. At least three or more reading must be taken
by different persons.
Systematic Errors:
A systematic error is divided in three different categories:
1. Instrumental Errors.
2. Environmental Errors.
3. Observational Errors.
Instrumental Errors: The instrument error generate due to instrument itself. It is due to the
misuse of the instruments, loading effects of instruments.
Instrumental errors may be avoided by
(a) Selecting a suitable instrument for the particular measurement application.
(b) Applying correction factors after determining the amount of instrumental error.
(c) Calibrating the instruments against a standard.
Environmental Errors: Environmental errors arise as a result of environmental effects on
instrument. It includes conditions in the area surrounding the instrument, such as the effects of
changes in temperature, humidity, barometric pressure or of magnetic or electrostatic fields.
Environmental errors may be avoided by
(a) Using the proper correction factor and information supplied by the manufacturer of the
instrument.
(b) Using the arrangement which will keep the surrounding condition constant like use of air
condition, température controlled enclosures etc.
(c) Making the new calibration under the local conditions.

8. Observational Errors: These errors occur due to carelessness of operators while taking the
reading. There are many sources of observational errors such as parallax error while reading a
meter, wrong scale selection, the habits of individual observers etc.
To eliminate such observational errors, one should use the instruments with mirrors, knife edged
pointers, etc. Now a day’s digital display instruments are available, which are much more
versatile.
Random Errors: These errors are due to unknown causes and occur even when all systematic
errors have been accounted for. In some experiments some random errors usually occur, but they
become important in high-accuracy work. These errors are due to friction in instrument
movement, parallax errors between pointer and scale, mechanical vibrations, hysteresis in elastic
members etc.
Absolute Error: Measurement is the process of comparing an unknown quantity with an
accepted standard quantity. Absolute error may be defined as the difference between the
measured value of the variable and the true value of the variable.
e = At – Am , where
e – error (or) absolute error
Am – measured value of quantity
At – true value of quantity
Note: instead of specifying absolute error, the relative or percentage of error is specified.
The Relative error is the ratio of absolute error to the true value of the quantity to be measured.
Mathematically, the relative error can be expresses as,

9. Relative error:

10. P1) The expected value of the voltage across a resistor is 80V. However the measurement gives a
value of 79V. Calculate
i. Absolute Error
ii. % Error
iii. Relative Accuracy
iv. % of Accuracy
P2) The expected value of the Current through a resistor is 20mA. However the measurement
yields a current value of 18mA. Calculate
i. Absolute Error
ii. % Error
iii. Relative Accuracy
iv. % of Accuracy
Dynamic characteristics:
1. Speed of response
2. Fidelity
3. Lag
4. Dynamic error
Speed of response:
It gives information about how fast the system reacts to the changes in the input.
or
It is defined as a rapidity with which a measurement system responds to a change in measured
quantity. It gives information about how fast the system reacts to the changes in the input.
Fidelity:
It is defined as the degree to which an instrument indicates the changes in the measured variable
without dynamic error. (or) It is the ability of an instrument to produce a wave shape identical to
wave shape of input with respect to time. It also shows the change in quantity without dynamic
error.
Lag: Delay in the response of a system. Every instrument takes some time to respond to the
change in the measured variable. This retardation or delay in the response of the instrument is
called measuring lag. The measuring lag is of the following two types:
Retardation Lag: The response of measurement system begins immediately after a change in
measured quantity has occurred.
Time Delay: The measurement lags of this type are very small and are of the order of a fraction
of a second and hence can be ignored. In this case, response begins after the application of input

11. and is called after “Dead Time”. Such a delay shifts the response along time axis and hence
causes the dynamic error. The largest change of input quantity for which there is no change in
the measured quantity is known as Dead Zone.
Dynamic Error: Difference between the true value of the variable to be measured changing with
time and the value indicated by the measurement system assuming zero static error.
Basic meter: A basic d.c. meter uses a motoring principle for its operation. It stntes that any
current carrying coil placed in a magnetic field experiences a force, which is proportional to the
magnitude of current passing through the coil. This movement of coil is called D'Arsonval
movement and basic meter is called D'Arsonval galvanometer.
D.C instruments:
a) Using shunt resistance, d.c. current can be measured. The instrument is d.c. microammeter,
milliammeter or ammeter.
b) Using series resistance called multiplier, d.c. voltage can be measured. The instrument is d.c.
millivoltmeter, voltmeter or kilovoltmeter.
c) Using a battery and resistive network, resistance can be measured. The instrument is
ohmmeter.
A.C instruments:
a) Using a rectifier, a.c. voltages can be measured, at power and audio frequencies. The
instrument is a.c. voltmeter.
b) Using a thermocouple type meter radio frequency (RF) voltage or current can be measured.
c) Using a thermistor in a resistive bridge network, expanded scale for power line voltage can be
obtained.
DC Voltmeter: Voltmeter is used for measuring voltage or the potential difference. Fig. shows
the symbol of voltmeter. And the circuit diagram of dc voltmeter.

12. The basic d.c. voltmeter is nothing but a permanent magnet moving coil (PMMC) D' Arsonval
galvanometer. The resistance is required to be connected in series with the basic meter to use it
as a voltmeter. This series resistance is called a multiplier. The main function of the multiplier is
to limit the current through the basic meter so that the meter current does not exceed the full
scale deflection value. The voltmeter measures the voltage across the two points of a circuit or a
voltage across a circuit component.

13. Sensitivity: The sensitivity of a voltmeter is given in ohms per voltage. It is the reciprocal of the
full-scale deflection current.
P) Calculate the sensitivity of a 200 μA meter movement which is to be used as a dc voltmeter.

14. P) Calculate the value of multiplier resistance on the 50 V range of a dc voltmeter that uses a
200 µA meter movement with an internal resistance of 100 Ω.
P) A basic D’Arsonval movement with a full deflection of 50 μA and internal resistance of 500
Ω is used as a voltmeter. Determine the value of the multiplier resistance needed to measure a
voltage range of 0–10 V.
Multi-range Voltmeter: Fig. Shows the multi-range voltmeter. The range of dc voltmeter is
extended by using number of multipliers and a selector switch. Such a meter is called multirange

15. voltmeter. The R1, R2, R3 and R4 are the four series multipliers. When connected in series with
the meter, they can give four different voltage ranges as V1, V2, V3, and V4. The selector switch
S is multiposition switch by which the required multiplier can be selected in the circuit.
Precautions when using voltmeter in measurement work.
1. Place the voltmeter across the circuit or component whose voltage is to be measured.
2. When using a multi-range voltmeter, always use the highest voltage range and then decrease
the range until a good up-scale reading is obtained.
3. Always be aware of the loading effect.
P) Calculate the value of multiplier resistance for the multiple range dc voltmeter
circuit shown.

16. P) A basic D’arsonval movement with internal resistance, Rm = 100 Ω, and fullscale current,
Ifsd = 1 mA, is to be converted into a multirange dc voltmeter with voltage ranges of 0-10 V, 0-
50 V, 0-250 V, and 0-500 V. The circuit arrangement is shown.
Given: Rm = 100 Ω; Ifsd = 1 mA and voltage ranges = 0-10 V, 0-50 V, 0-250 V, and 0-500 V.

18. Loading Effect:
When selecting a meter for a certain voltage measurement, it is important to consider the
sensitivity of a dc voltmeter. A low sensitivity meter may give a correctly reading when
measuring voltages in a low resistance circuit, but it is certain to produce unreliable readings in a
high resistance circuit. A voltmeter when connected across two points in a highly resistive
circuits, acts as shunt for that portion of the circuit, reducing the total equivalent resistance of
that portion. The meter then indicates a lower reading than what existed before the meter was
connected. This is called loading effect of an instrument and caused mainly by low sensitivity
instruments.

20. Extending Voltmeter Ranges:
The range of a voltmeter can be extended to measure high voltages, by using a high voltage
probe or by using an external multiplier resistor, as shown in figure below.
In most meters the basic movement is used on the lowest current range. The basic meter
movement can be used to measure very low voltages. However, great care must be taken not to
exceed the drop for full scale deflection of the basic movement.
Solid State Voltmeter:
Figure shows the circuit of an electronic voltmeter using IC 741C.This is a directly coupled very
high gain amplifier. The gain of the OpAmp can be adjusted to any suitable lower value by

21. providing appropriate resistance between its output terminal PinNo.6 and inverting input
PinNo.2 to provide a negative feedback.
The ratio R2/RI determines the gain the 0.1µf capacitor across the 100k resistance R2 Is for
stability under stray pickups. Terminals 1 and 5 are called offset null terminals .A 10kΩ
potentiometer is connected between these two offset null terminals with its centre tap connected
to a 5V supply. This potentiometer is called zero set and is used for adjusting zero output for
zero input conditions.
The two diodes used are for IC protection. Under normal conditions they are
non conducting as the maximum voltage across them is 10mV. If an excessive voltage say more
than 100mV appears across them then depending upon the polarity of the voltage one of the
diodes conducts and protects the IC. A µA scale of 50 -1000 μA full scale deflection can be used
as an indicator. R4 Is adjusted to get maximum full scale deflection

22. Differential Voltmeter:
The differential voltmeter technique, is one of the most common and accurate methods of
measuring unknown voltages. In this technique, the voltmeter is used to indicate the difference
between known and unknown voltages, i.e. an unknown voltage is compared to a known voltage.
Figure below shows a basic circuit of a differential voltmeter based on the potentiometric
method; hence it is sometimes also called a potentiometric voltmeter.
In this method, the potentiometer is varied until the voltage across it equals the unknown voltage,
which is indicated by the null indicator reading zero. Under null conditions, the meter draws
current from neither the refer source nor the unknown voltage source To detect small differences
the meter movement must be sensitive, but it need not be calibrated, since only zero has to be
indicated. The reference source used is usually a 1Vdc standard source or a zener controlled
precision supply. A high voltage reference supply is used for measuring high voltages.
The usual practice, however, is to employ voltage dividers or attenuators across an
unknown source to reduce the voltage. The input voltage divider has a relatively low input
impedance, especially for unknown voltages much higher than the reference standard. The
attenuation will have a loading effect and the input resistance of voltmeter is not infinity when an
attenuator is used.
In order to measure ac voltages, the ac voltage must be converted into dc by incorporating a
precision rectifier circuit. A block diagram of an ac differential voltmeter is shown in Fig. below

23. AC Voltmeter:
An AC electronic voltmeter is used to measure AC voltage. Note that the PMMC meter
movement is used for measurement of AC voltage by inserting a rectifier in the measuring
circuit. Such meters are widely used and more accurate. AC analog voltmeters are one of the
most popular electronic measuring instruments in use today. They are used to measure the r.m.s
value of voltage of many waveforms commonly found in electronics.
Shows the block diagram of an alternating current electronic voltmeter.
Here the voltage divider allows selection of voltage range. The amplifier provides the necessary
gain to establish voltmeter sensitivity as well as high input impedance. The negative circuit is

24. for stability and accurate overall gain. A rectifier and filter is used to convert ac to dc. AC
voltmeters are designed to respond to one of these three values: average value, rms value or peak
value of ac input voltage.
Accordingly, this classifies the AC meters into:
1. Rectifier Type AC Voltmeter
2. Average Responding AC Voltmeter
3. Peak Responding AC Voltmeter
4. True RMS AC Voltmeter
The average and peak responding voltmeters are designed to measure only sine waves.
Rectifier Type AC Voltmeter: Shows a simple rectifier type AC voltmeter.
Here the diodes D1 and D2 are used for rectification. For the positive half cycle the diode D1 is
ON and meter deflection is proportional to the average value of the half cycle. In the negative
half cycle, the diode D2 is ON and D1 is OFF. The current through the meter is in opposite
direction and hence meter movement is bypassed. It produces pulsating dc and the meter
indicates the average value of the input.
The rectifier type AC voltmeters are of different types, these are:
(a) AC voltmeter using Half-wave rectifier
(b) AC voltmeter using Full-wave rectifier
(c) Multirange AC voltmeter using rectifier

25. A.C. Voltmeter using half-wave rectifier: The circuit of an AC Voltmeter using half-wave diode
rectifier is shown below.
The half-wave rectifier circuit has been combined in series with a dc meter movement. When
used as a DC voltmeter (i.e. without rectifier) it would have a range of 10 V. However, if an ac
voltage of rms value of 10 V is applied across input terminals AB, it would read 4.5 V. This can
be explained as follows, we know that r.m.s value of input voltage, Erms = 10 V
Then the peak value is given by
Epeak = Erms √2 = 10 × √2 = 14.14
Therefore an average value of half-wave rectifier
Eavg = 0.636 × Epeak
= 0.636 × 14.14 = 8.99 V
Since in the half-wave rectified output, one half-cycle is absent, the average for the full cycle is
Eavg =8.99/2= 4.5 V
The meter movement will, therefore read 4.5 V i.e. 45% of the dc value. It may also be noted that
ac sensitivity of a half-wave ac meter is only 45 per cent of the dc sensitivity.

26. A.C. Voltmeter using full-wave rectifier:
The circuit of an AC voltmeter using full-wave rectifier is shown above In this case, the meter
reading would be 90% of r.m.s input voltage i.e. 90% of the dc value.
The voltmeter will indicate 90 % of the r.m.s input voltage. This is explained as below:
We know that the peak value is of an input voltage with an r.m.s value of 10,
Epeak = Erms √2
= 10 × √2 = 14.14
Average value of half wave rectifier
Eavg = 0.636 × Epeak
= 0.636 × 14.14 = 8.99 V ≈ 9 V
It may also be noted that ac sensitivity of a full-wave ac meter is only 90 per cent of the dc
sensitivity.

27. Multirange A.C. Voltmeter:
The main purpose of the multirange ac voltmeter is for measuring ac voltage for different ranges.
Fig. shows the circuit diagram of such an electronic instrument.
The rectifier type ac voltmeter is used with series of multiplier resistance R1, R2, R3, R4 and R5.
Due to different multiplier resistances various voltage ranges is achieved. The resistance R5 acts
as a basic multiplier resistance and corresponds to the multiplier Rs. AC Analog voltmeters are
one of the most popular electronic measuring instruments in use today. They are used to
measure the rms voltage of the many waveforms commonly found in electronics.
Average Responding A.C. Voltmeter: Fig. shows the average responding AC voltmeter. As
seen from this diagram a sine wave being measured is fed through a DC blocking capacitor,
amplified or attenuated, rectified by the diode bridge and fed to the meter. The meter then
responds to this rectified average or DC value.

28. The applied waveform is amplified with a high gain stabilized amplifier to a required high level.
This voltage is rectified using diodes D1 and D2. The rectified voltage is fed to a dc mA used as
a measuring meter. In this meter instrument, the rectifier current is averaged by a filter to
produce a steady deflection of the meter pointer. This dc component deflects a D’Arsonval
(moving coil) meter to indicate the rms value of a sine wave. The blocking capacitor used at the
input side blocks the dc component of the input voltage. The negative feedback is used for the
amplifier to ensure stability for measurement. Capacitors C1 and C2 in the rectifier circuit act as
storing capacitors or filter capacitors. The dc milliammeter is calibrated in terms of rms value of
the input voltage.
Advantages
1. The diode nonlinearity is minimized using meter in feedback path.
2. Variations in the meter impedance are compensated by the negative feedback
3. High frequency range of operation is provided.

29. Disadvantages
Errors in the reading of an average responding voltmeter may be due to the application of
complex waveforms like distorted or non sinusoidal input or presence of noise etc.
Peak Responding Voltmeter:
Peak responding voltmeter is also designed to indicate the RMS value of a sine wave. The
difference between average responding meter and this meter is the use of storage capacitors with
the rectifying diode. A capacitor is charged through a rectifying diode to the positive peak of the
applied sine wave. The voltmeter then responds to the DC output.
The two types of peak responding voltmeter are
1. DC coupled peak responding voltmeters.
2. AC coupled peak responding voltmeters.
D.C Coupled Peak Responding Voltmeters:
Figure Shows the dc coupled peak voltmeter, in which the capacitor charges to the total peak
voltage above ground reference. The meter reading will be affected by the presence of dc with ac
voltage.

30. A.C Coupled Peak Responding Voltmeters:
Figure shows AC coupled peak responding voltmeter.
In this the circuit if the positions of diode and capacitor are interchanged. The capacitor still
charges to the peak value of the ac input. In both the circuits, capacitor discharges very slowly
through the high impedance input of the dc amplifier. So a negligible small amount of current
supplied by the circuit under test keeps the capacitor charged to the peak ac voltage.
Advantages
1. The primary advantage of peak responding voltmeter is that the rectifying diode and
the storage capacitor may be taken out of the instrument and placed in the probe when
no ac pre-amplification is required.
2. The peak responding voltmeter is able to measure frequencies up to several hundreds
of MHz with a minimum of circuit loading
Disadvantages
1. The major disadvantage is caused due to harmonic distortion in the input waveforms and
limited sensitivity of the instrument. This is because of imperfect diode characteristics.
D.C. Ammeter: The basic movement of a dc ammeter is a PMMC D’Arsonval galvanometer.
The coil winding of a basic movement is very small and light it can carry very small value of

31. currents. When the large currents are to be measured it is necessary to bypass the major part of
the current through a low resistance called shunt resistor. Symbol is shown below
The shunt resistor is connected parallel with D’Arsonval movement. The ammeter is always
connected in series with the load in the circuit. The dc ammeter is shown in Fig. The resistance
of the shunt can be calculated by circuit analysis.
Where Rm = internal resistance of the movement coil
Rsh = resistance of the shunt
Im = Ifsd = full scale deflection current of the movement
Ish = shunt current
I = current to be measured
Since the shunt resistance is in parallel with the meter movement, the voltage drop across the
shunt and movement is the same.

32. Main properties of shunt resistor are given below:
1. Resistance of the shunt should not vary with time.
2. Temperature co-efficient of shunt and instrument should be low and should be same.
Multi-range Ammeter:
The current range of the dc ammeter is further extended by a number of shunts, selected by a
range switch. This type of meter is called a Multi-range Ammeter .Figure Shows the multi-range
ammeter and circuit diagram.

33. It has four shunts (Rsh1, Rsh2, Rsh3, Rsh4) parallel with the meter movement and gives four
different current ranges (I1, I2, I3, I4). If m1, m2, m3 and m4 be the shunt multiplying powers
for currents I1, I2, I3 and I4.
Ammeter uses a multi position make-before-break switch. This type of switch is essential in
order that the meter movement is not damaged when it change from one resistor to other resistor.
If we use ordinary switch the meter remain without shunt when we change from one resistor to
other resistor, this may damage the ammeter.This ammeter used for ranges 1 – 50 A. While using
the multi-range ammeter we use the highest current range first then decrease the current range.
Precautions when using ammeter in measurement work
1. Never connect an ammeter across a source of EMF. Because of its low resistance it draws
damaging high currents and destroys the delicate movement. It is always connected in series with
a load.
2. Always connect in right polarity. Reverse polarity may damage the pointer.
3. When using the multi-range meter, first use the highest current range; then decrease the
current range until substantial deflection is obtained.

34. P) A 1 mA meter movement with an internal resistance of 100 Ω is to be converted into 0–100
mA. Calculate the value of shunt resistance required.
P) Design a multi-range DC milli-ammeter with a basic meter having a resistance 75 Ω and full
scale deflection for the current of 2 mA. The required ranges are 0-10 mA, 0-50 mA and 0-100
mA.

35. Series Ohmmeter:
A D’Arsonval movement is connected in series with a resistance and a battery to a pair of
terminals to which the resistance under test is connected. This forms the basic type of series
Ohmmeter. So that indication of the instrument depends on the magnitude of current flowing
through the meter which ultimately depends on the value of resistance under test.
When the terminal A and B is shorted
When the terminal A and B is shorted (unknown resistor Rx = 0), the maximum current is flows
in the circuit. In this condition the shunt resistor is adjust until the movement indicates full-scale
current (Ifsd). The full scale current position of the pointer is marked “0 Ω” on the scale.

36. When the terminal A and B is opened
When the terminal A and B is opened (unknown resistor Rx = ∞), the current in the circuit drops
to zero and the movement indicates zero current, which is then marked “∞Ω” on the scale.By
connecting different known resistance across the terminals A B, intermediate marking can be
done on scale. The accuracy of the instrument can be checked by measuring different values of
the standard resistance.
The current is inversely proportional to the resistance, the scale is marked from ∞ to 0 as shown
in Figure.
A convenient quantity to use to design of a series type ohmmeter is the value of Rx which causes
half-full scale deflection of the instrument. At this position the resistance across terminals A and
B is defined as the half-scale position resistance Rh. IF full-scale deflection current of the meter,
fm, internal resistance of the meter, Rm, the battery emf E and the half-scale resistance Rh are
given then the circuit can be analyzed and the values of Rse and Rsh can be determined.
A convenient quantity to use in design of a series type ohmmeter is the value if Rx which causes
half-scale deflection of the meter. At this position, the resistance across terminals A and B is
defined as the half-scale position resistance Rh. Its value is equal to the total resistance Rse in
series with parallel combination of Rm and Rsh.

37. ---(1)
The total resistance presented to the battery then equals 2Rh, and the battery current needed to
supply the half-scale deflection is
For full scale deflection, the battery current must be doubled

38. Shunt Type Ohmmeter:
The shunt type ohmmeter is shown below. It consists of a battery in series with an adjustable
resistance Rse and a D’Arsonval movement. The Unknown resistance is connected in parallel
with the meter. The switch disconnects the battery when the instrument is not in use.
When the terminal A and B is shorted:
When the terminal A and B is short then the entire current flows through the short circuit and
the meter current is zero. This pointer position is marked as zero and the corresponding RX = 0Ω
as terminals AB are shorted.
When the terminals A and B are open:
When the terminals A and B are open, then the entire current flows through the meter and pointer
deflects to maximum. The resistance Rse is then adjusted such that current through the meter is
full scale deflection current. This position of pointer is marked as ∞Ω.
The scale is marked as 0 to∞¥ as shown in Figure

41. Electronic Multimeter:
For the measurement of d.c. as well as a.c. voltage and current, resistance, an electronic
multimeter is commonly used. It is also known as Voltage-Ohm Meter (VOM) or multimeter
The important salient features of VOM are as listed below.
1. The basic circuit of VOM includes balanced bridge d.c. amplifier.
2. To limit the magnitude of the input signal, RANGE switch is provided. By properly
adjusting input attenuator input signal can be limited.
3. 3) It also includes rectifier section which converts a.c. input signal to the d.c. voltage.
4. 4) It facilitates resistance measurement with the help of internal battery and additional
circuitry.
5. 5) The various parameters measurement is possible by selecting required function
using FUNCTION switch.
6. 6) The measurement of various parameters is indicated with the help of indicating
Meter.
A Multimeter is basically a PMMC meter. To measure dc current the meter acts as an ammeter
with a low series resistance. Range changing is accomplished by shunts in such a way that the
current passing through the meter does not exceed the maximum rated value. A multimeter
consists of an ammeter, voltmeter, and ohmmeter combined with a function switch to connect the
appropriate circuit to the D’Arsonval movement.
Multimeter as Ammeter:

42. Multimeter as Voltmeter:

43. Multimeter as Ohmmeter:

44. Digital Multimeter:
Basic block diagram of a digital multimeter (DMM) is shown in Figure below.
The DMM is made up of following three basic elements:
(a) Signal conditioning
(b) Analog-to-digital (A/D) conversation
(c) Numeric digital display
Features of Basic Digital Multimeter:
The main features of any digital multimeter is the types of measurement and the ranges over
which it will operate. Most DMMs will offer a variety of measurements. The basic
measurements will include:
(a) Current (DC)
(b) Current (AC)
(c) Voltage (DC)
(d) Voltage (AC)
(e) Resistance
Block Diagram of Digital Multimeter:
The digital multimeter can measure ac voltage, dc voltage, ac current, dc current and resistance
over several ranges. The basic circuit is shown in Figure.

45. Advantage of Digital Multimeter (DMM):
Following are the main advantages of Digital Multimeter:
1. DMM offer high measurement accuracy.
2. These instruments have a high input impedance.
3. They are smaller in size.
4. These meters eliminate observational, parallax and approximation errors.
5. The output of these instruments can be directly feed to a computer for further analysis and use.