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Instrumentation & Measurement Lab Experiments

Measurement of capacitance De Sauty’s Bridge



Aim

To determine unknown capacitance of given capacitor by De sauty’s Bridge Method.

Apparatus Required:

S.No Name Type Quantity
1 De sauty’s Bridge training kit EE-123 1
2 Patch cords - 5-8
3 Multimeter Digital 1
4 Audio oscillator 1

Theory

The De sauty’s Bridge is one of the most important A.C bridge which is extensively Used for the measurement of unknown capacitance. De sauty’s Bridge contains four arms each having a resistance or capacitor or a combination of both.

Circuit Diagram :

de-sauty-bridge

C2= Capacitor whose capacitance is to be measured

C3= Standard capacitor

R1= Non inductive resistor of one arm

R4= Non inductive resistor of other arm

Balance is obtained by varying either R1 or R4

So For balance, points B & D are at the same potential

I1R 1= I 2R 4

R1[1/jwc3] = R4[1/jwc2]

R1C3= R4C2

∴C2= C3R4/R1

Where,

I1= Current in the resistor R1of one arm

I2= Current in the resistor R4of one arm

Procedure:-

  1. Select one of the standard capacitor C 3& connect on the appropriate place using two Patch cords.
  2. Select one of the unknown capacitor C 3using band switch.
  3. Select 5 KΩ value of R 4using band switch.
  4. Connect headphone provided with this model at the place indicated.
  5. Connect audio oscillator at the appropriate place indicated & switch it ON.
  6. Now vary R1using Pot and select the value for null point in the head phone. If the null point does not detect for selected one value of C3& R4change the value of C 3& R 4now again vary R 1 using Pot and select the value for null point in the head phone.
  7. Now switch OFF the supply and detach all patch chords.
  8. Measure the resistances R1& R 4using multimeter and note down various value into the observation table.
  9. Change the value or unknown capacitor C2 using band switch & repeat all above steps.

Observation Table:

Selected value of C3= ………..

Selected value of C2=……….

Selected value of R4= ………..

S.No R1 (Ohms) R 4 (Ohms) C 3 (Farads) Calculated Value of Unknown capacitance C2= C3R4/R1
1
2
3
4

Conclusion:-The value of unknown capacitor C2has been calculated and when compared with standard values were found close to each other.

Viva-Voice Question:-

  1. Name the commonly used detector for AC Bridge?
  2. Name the detector used for bridge measurement at audio frequency?
  3. What is meant by time constant of a coil ?
  4. List the factor that may be lead to inaccuracies in measurement by AC bridge