Electrical Measurements and Instrumentation Full course on slide share

1. Potentiometer and AC
Bridges
Electrical Measurement and Instrumentation (EE-336)
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G
Unknown Voltage Source
Battery Voltage Rheostat
KO L

2. Contents
• Basic Potentiometer
• Standardization of Potentiometer
• Measuring unknown voltage
• Crompton’s Potentiometer
• Examples
• Applications of Potentiometer
• AC Bridges
• Impedance Bridge
• Maxwell’s Bridge
• Hay’s Bridge
• Other Bridges
• Frequency Measurement using Wein’s Bridge
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3. Basic Potentiometer
• Potentiometers are used for finding unknown
voltages
• It consists of an external voltage source V, a
slide wire mechanism Rs and a rheostat Rh
• It uses the fixed battery source V for
developing voltage across the whole circuit.
• The operation develops a working current
across the whole series circuit of Rt= Rs+Rh
• Rh is used to control the total current across
the circuit
• Rs is used for measuring unknown potentials
that are connected directly to this wire.
• The connection between unknown voltage
source and the Slide wire contains a
galvanometer Both for standardizing and
measuring unknown voltage
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4. Standardizing Potentiometer
• A fixed voltage source is connected with the slide
wire via a rheostat.
• The resistance of slide wire is known so rheostat controls
current in it and can hence change the total voltage
across slide wire.
• Removing rheostat will make all the fixed battery voltage
to appear on the slide wire.
• We do not do that because the slide wire will behave as a
short circuit path.
• A known standard cell or source is connected to slide
wire with a galvanometer in between it
• When the voltage of standard source and the slide
wire becomes equal, the galvanometer shows zero
deflection. That point is marked on the slide wire.
• As resistance and voltage are linear functions of
length. So the total length gives total voltage across
the wire.
• Please note that if we do not connect the rheostat
then this process was not needed.
G
Standard Cell
Battery Voltage Rheostat
KO L
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5. Measuring Unknown voltage
• The unknown emf source is connected
and the standard source is removed
after the previous process of marking
and grading the scale.
• The Key K is moved left or right to have
zero deflection on the galvanometer G.
• After attaining zero deflecting the
length of the slide wire is used for
calculating the voltage.
𝑉𝑂𝐾 =
𝑙 𝑂𝐾. 𝑉𝑠
𝑙 𝑂𝐿
G
Unknown Voltage Source
Battery Voltage Rheostat
KO L
5

6. Crompton’s Potentiometer
• The Rheostat in a simple potentiometer is added
with a wound resistor and a graded-knobbed
voltage divider.
• Now the knob on wound wire will give us the
balance zero point, as shown
• Rheostat is used for limiting the current in the
circuit and voltage divider knob is used for reducing
the voltage on a graded scale, so both devices does
the same thing.
• The resistance of Rheostat is unknown while
standardizing the mechanism, as shown
• Crompton’s Potentiometer also posses a protection
resistance of 10KΩ. The Short-circuiting Key is
pressed while taking reading and kept in the circuit
while not taking reading so that the G doesn’t loss
its qualities by excessing usage in series connection.
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7. Example 5.1
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8. 8

9. Example 5.2
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10. 10

11. Applications of Potentiometer
• Measurement of current
• Measurement of high voltage
• Measurement of resistance
• Measurement of power
• Calibration of voltmeter
• Calibration of ammeter
• Calibration of wattmeter
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12. AC Bridges
Electrical Measurement and Instrumentation (EE-336)
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13. Contents
• Introduction to AC Bridges
• Impedance Bridge
• Maxwell’s Bridge
• Hay’s Bridge
• Other Bridges
• Frequency Measurement using Wein’s Bridge
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14. AC Bridge
• vibration galvanometers can be designed to work as detectors over the
frequency range of 5 Hz to 1000 Hz, they have highest sensitivity when
operated for frequencies below 200 Hz.
•
Head phones or audio amplifiers are popularly used as balance detectors in
ac bridges at frequencies of 250 Hz and above, up to 3 to 4 kHz.
•
Transistor amplifier with frequency tuning facilities can be very effectively
used as balance detectors with ac bridges. With proper tuning, these can be
used to operate at a selective band of frequencies with high sensitivity. Such
detectors can be designed to operate over a frequency rang
Selecting the Detector
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15. Equation for Impedance Bridge
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16. Equation for General Impedance Bridge at
Balance Condition
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17. Example
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18. Maxwell’s Inductance Bridge
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19. Maxwell’s Inductance–Capacitance Bridge
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20. Advantages and Limitations of Maxwell’s
Bridge
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21. Hay’s Bridge (Measuring Inductance)
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22. Other Bridges
• Anderson Bridge (Modified Maxwell Bridge for measuring inductance)
• Owen’s Bridge (Measuring Capacitance)
• De Sauty’s Bridge (Measuring Capacitance)
• Schering bridges (Capacitance, dissipation factor, loss angles)
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23. Frequency Measurement by Wein’s Bridge
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24. The End
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