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Course Title: 19EEE343- Electrical Measurements
Topic for the class: Unit 4: Potentiometers
Dr. T Padmavathi
Asst Professor
Department of Electrical, Electronics and Communication Engineering
GITAM Institute of Technology (GIT)
Visakhapatnam – 530045
Email: ptadi@gitam.edu
12 October 2022
Department of EECE, GIT Course Code and Course
Title:19EEE343 EM
1
Unit IV
Potentiometers:
 General principle
 Vernier dial
 principle of standardization
 AC potentiometers coordinate type and polar type
 Application of DC and AC potentiometers
 Display devices: CRT display, DSO, Digital multimeter.
POTENTIOMETER
• A potentiometer is an instrument designed to
measure an unknown voltage by comparing it
with a known voltage.
• The known voltage may be supplied by a
standard cell or any other known voltage -
reference source.
• Measurements using comparison methods are
capable of a high degree of accuracy because the
result obtained does not depend upon the actual
deflection of a pointer, as is the case in deflection
methods, but only upon the accuracy with which
the voltage of the reference source is known.
• Another advantage of the potentiometers is
that since a potentiometer makes use of a
balance or null condition, no current flows
and hence no power is consumed in the circuit
containing the unknown emf when the
instrument is balanced.
• Thus the determination of voltage by a
potentiometer is quite independent of the
source resistance.
• Since a potentiometer measures voltage, it can
also be used to detect current simply by
measuring the voltage drop produced by the
unknown current passing through a known
standard resistance.
• The potentiometer is extensively used for a
calibration of voltmeters and ammeters and has
in fact become the standard for the calibration of
these instruments.
• For the above mentioned advantages the
potentiometer has become very important in the
field of electrical measurements and calibration.
Principle of Potentiometer
• It works on the principle of
opposing the unknown emf by a
known emf with the negative
terminals of both the emfs
connected together, while the
positive terminals connected
together through a galvanometer
as shown in figure.
• When the emfs are of same values,
there is no deflection on
galvanometer.
• To measure the
unknown emf by using
above method, the
unknown emf must be
variable.
• The main advantage of this method is that the
current in the resistor can be varied easily to
obtain any desired voltage with very fine
adjustment.
• The voltage drop across resistor can be
determined by calibrating the resistor with
standard cell.
TYPES
• DC Potentiometers
• AC Potentiometers
Slide Wire DC Potentiometer
• A basic potentiometer
circuit consists of a slide
wire AB of uniform cross
section and unit length.
• Generally slide wire is
made up of a manganin.
• With switch 'S' in the
"operate" position and the
galvanometer key K open, the
battery supplies the "working
current" through the rheostat
R and the slide wire.
• The working current through
the slide wire may be varied
by changing the rheostat
setting.
• The method of measuring the
unknown voltage, E, depends
upon finding a position for the
sliding contact such that
galvanometer shows zero
deflection, i.e., indicates null
condition, when the
galvanometer key, K, is closed.
Zero galvanometer
deflection or a null means
that the unknown voltage,
E, is equal to the voltage
drop E1 across portion ac
of the slide wire.
Thus determination of the
value of unknown voltage
now becomes a matter of
evaluating the voltage
drop E1 along the portion
ac of the slide wire.
• The slide wire has a
uniform cross-section
and hence uniform
resistance along its entire
length.
• A calibrated scale in cm
and fractions of cm, is
placed along the slide
wire so that the sliding
contact can be placed
accurately at any desired
position along the slide
wire.
• Since the resistance of slide
wire is known accurately,
the voltage drop along the
slide wire can be controlled
by adjusting the value of
working current.
• The process of adjusting
the working current so as
to match the voltage drop
across a portion of sliding
wire against a standard
reference source is known
as "Standardisation".
DC potentiometers
https://www.youtube.com/watch?v=ZkQIIvhu
V-s
Standardization of potentiometer
Standardization of a potentiometer is a process of
adjusting the working current supplied by the supply
battery such that the voltage drop across a portion of
sliding wire matches with the standard reference
source.
• A battery of sufficient capacity B2 is
connected in series with a rheostat
Rh which regulates the working or
standard current flowing through
the slide wire.
• A standard cell B1 usually a Weston
standard cell of emf 1.0186 volts is
connected to galvanometer and a
switch K through a series resistance
Rs.
• By properly adjusting Rs full
sensitivity of the galvanometer can
be obtained.
• A slide wire with total length of
200cm and resistance of 200ohms is
connected which is indicated by
points A & B.
• During standardization process,
switch K is closed and the sliding
contact is placed at the mark of
101.86 cm long the slide wire as
indicated by point C in Fig.
• Thus we can observe some deflection
in the galvanometer.
• Now by adjusting the Rh we can get
null deflection in the galvanometer.
• Under the condition of null deflection,
the voltage drop along 101.86 cm
portion of the slide wire equals the emf
of standard cell.
• Once the potentiometer is standardized
, the rheostat is not disturbed.
• After standardizing a potentiometer , it
is used as direct reading potentiometer
as the voltage along the slide wire at
any point is proportional to the length
of the slide wire where the point is
obtained by moving sliding contact
along the wire to get null deflection in
the galvanometer for any battery whose
emf is to be measured.
Vernier Potentiometer
• Slide wire is eliminated.
• Two ranges-
• Normal range of 1.6V
down to 10μV
• Lower range of 0.16V
down to 1 μV
DC potentiometers
Three measure dials
 Measures up to 1.5V in
steps of 0.1V-XI range
 Measures up to 0.1V in
steps of 0.001V has 102
studs
 Measures up to
0.0001V to 0.001V in
steps of 0.00001V has
102 studs.
• The resistance of the
middle dial shunts two of
the coils of first dial.
• The moving arm of
middle dial carries two
arms spaced two studs.
• The vernier
potentiometer reads to
increment of 10 μV on
range of X1 and has
readability of 1 μV on
X0.1 range
• If a third range of X0.01 is
provided, the readability
becomes 0.1 μV.
• Measurements are
subjected to stray thermal
and contact emfs in the
potentiometer,
galvanometer and the
measuring circuits.
• These emfs can be
minimised by properly
selecting metals for
resistors, terminals and
connecting leads.

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DC potentiometers

  • 1. Course Title: 19EEE343- Electrical Measurements Topic for the class: Unit 4: Potentiometers Dr. T Padmavathi Asst Professor Department of Electrical, Electronics and Communication Engineering GITAM Institute of Technology (GIT) Visakhapatnam – 530045 Email: ptadi@gitam.edu 12 October 2022 Department of EECE, GIT Course Code and Course Title:19EEE343 EM 1
  • 2. Unit IV Potentiometers:  General principle  Vernier dial  principle of standardization  AC potentiometers coordinate type and polar type  Application of DC and AC potentiometers  Display devices: CRT display, DSO, Digital multimeter.
  • 4. • A potentiometer is an instrument designed to measure an unknown voltage by comparing it with a known voltage. • The known voltage may be supplied by a standard cell or any other known voltage - reference source. • Measurements using comparison methods are capable of a high degree of accuracy because the result obtained does not depend upon the actual deflection of a pointer, as is the case in deflection methods, but only upon the accuracy with which the voltage of the reference source is known.
  • 5. • Another advantage of the potentiometers is that since a potentiometer makes use of a balance or null condition, no current flows and hence no power is consumed in the circuit containing the unknown emf when the instrument is balanced. • Thus the determination of voltage by a potentiometer is quite independent of the source resistance.
  • 6. • Since a potentiometer measures voltage, it can also be used to detect current simply by measuring the voltage drop produced by the unknown current passing through a known standard resistance. • The potentiometer is extensively used for a calibration of voltmeters and ammeters and has in fact become the standard for the calibration of these instruments. • For the above mentioned advantages the potentiometer has become very important in the field of electrical measurements and calibration.
  • 7. Principle of Potentiometer • It works on the principle of opposing the unknown emf by a known emf with the negative terminals of both the emfs connected together, while the positive terminals connected together through a galvanometer as shown in figure. • When the emfs are of same values, there is no deflection on galvanometer.
  • 8. • To measure the unknown emf by using above method, the unknown emf must be variable.
  • 9. • The main advantage of this method is that the current in the resistor can be varied easily to obtain any desired voltage with very fine adjustment. • The voltage drop across resistor can be determined by calibrating the resistor with standard cell.
  • 10. TYPES • DC Potentiometers • AC Potentiometers
  • 11. Slide Wire DC Potentiometer
  • 12. • A basic potentiometer circuit consists of a slide wire AB of uniform cross section and unit length. • Generally slide wire is made up of a manganin.
  • 13. • With switch 'S' in the "operate" position and the galvanometer key K open, the battery supplies the "working current" through the rheostat R and the slide wire. • The working current through the slide wire may be varied by changing the rheostat setting. • The method of measuring the unknown voltage, E, depends upon finding a position for the sliding contact such that galvanometer shows zero deflection, i.e., indicates null condition, when the galvanometer key, K, is closed.
  • 14. Zero galvanometer deflection or a null means that the unknown voltage, E, is equal to the voltage drop E1 across portion ac of the slide wire. Thus determination of the value of unknown voltage now becomes a matter of evaluating the voltage drop E1 along the portion ac of the slide wire.
  • 15. • The slide wire has a uniform cross-section and hence uniform resistance along its entire length. • A calibrated scale in cm and fractions of cm, is placed along the slide wire so that the sliding contact can be placed accurately at any desired position along the slide wire.
  • 16. • Since the resistance of slide wire is known accurately, the voltage drop along the slide wire can be controlled by adjusting the value of working current. • The process of adjusting the working current so as to match the voltage drop across a portion of sliding wire against a standard reference source is known as "Standardisation".
  • 19. Standardization of potentiometer Standardization of a potentiometer is a process of adjusting the working current supplied by the supply battery such that the voltage drop across a portion of sliding wire matches with the standard reference source.
  • 20. • A battery of sufficient capacity B2 is connected in series with a rheostat Rh which regulates the working or standard current flowing through the slide wire. • A standard cell B1 usually a Weston standard cell of emf 1.0186 volts is connected to galvanometer and a switch K through a series resistance Rs. • By properly adjusting Rs full sensitivity of the galvanometer can be obtained.
  • 21. • A slide wire with total length of 200cm and resistance of 200ohms is connected which is indicated by points A & B. • During standardization process, switch K is closed and the sliding contact is placed at the mark of 101.86 cm long the slide wire as indicated by point C in Fig. • Thus we can observe some deflection in the galvanometer. • Now by adjusting the Rh we can get null deflection in the galvanometer.
  • 22. • Under the condition of null deflection, the voltage drop along 101.86 cm portion of the slide wire equals the emf of standard cell. • Once the potentiometer is standardized , the rheostat is not disturbed. • After standardizing a potentiometer , it is used as direct reading potentiometer as the voltage along the slide wire at any point is proportional to the length of the slide wire where the point is obtained by moving sliding contact along the wire to get null deflection in the galvanometer for any battery whose emf is to be measured.
  • 23. Vernier Potentiometer • Slide wire is eliminated. • Two ranges- • Normal range of 1.6V down to 10μV • Lower range of 0.16V down to 1 μV
  • 25. Three measure dials  Measures up to 1.5V in steps of 0.1V-XI range  Measures up to 0.1V in steps of 0.001V has 102 studs  Measures up to 0.0001V to 0.001V in steps of 0.00001V has 102 studs.
  • 26. • The resistance of the middle dial shunts two of the coils of first dial. • The moving arm of middle dial carries two arms spaced two studs. • The vernier potentiometer reads to increment of 10 μV on range of X1 and has readability of 1 μV on X0.1 range
  • 27. • If a third range of X0.01 is provided, the readability becomes 0.1 μV. • Measurements are subjected to stray thermal and contact emfs in the potentiometer, galvanometer and the measuring circuits. • These emfs can be minimised by properly selecting metals for resistors, terminals and connecting leads.