Modelling Guide for Timber Structures - FPInnovations
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.
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.