Measurement of capacitance using Schering’s Bridge

Object Measurement of capacitance using Schering’s Bridge.

Aim

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

Apparatus Required:

Theory

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

Let the impedance of the arms be Z₁, Z₂, Z₃ & Z₄

Here the impedances are as follows:

Z₁=R₁/1+jwC₁R₁

Z₂=R₂

Z₃=-j/wC₃

Z₄=R_x- j/wC_x

Where, R_x = internal resistance of the C_x

C_x = unknown capacitance of the capacitor

At balance condition,

Z₂, x Z₃, = Z₁x Z₄

∴R_{2[-j/wc3] = [R1/1+jwc1R1 x R x -j/wcx}

Rearranging the terms and separating real & imaginary parts

We get

C_x=R_{1 x} C₃/R₂

R_x=C_{1 x} R₂/C₃

Procedure:-

1. Select one of the standard capacitor C₃ & connect on the appropriate place using two patch cords.
2. Select one of the unknown capacitor C_x using band switch.
3. Select 5 KΩ value of R₂ using 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 R₁ using Pot and select the value for null point in the headphone.
7. If the null point does not detect for selected one value of C₃ & R₂, change the value of C₃ & R₂, now again vary R₁ using Pot and select the value for null point in the headphone.
8. Now switch OFF the supply and detach all patch chords.
9. Measure the resistance R₁ & capacitance C₁ using multimeter and note down various values into the observation table.
10. Change the value or unknown capacitor C _x using band switch & repeat all above steps.

Observation Table:

Selected value of C₁= ………..

Selected value of C_x=……….

Selected value of R₂= ………..

Conclusion:-The value of unknown capacitance C_x& unknown resistance R_x has been calculated and when compared with standard values of C_x & R_x where found close to each other.

Viva-Voice Question:-

1. What are the criteria for balance the AC Bridge?
2. Define the term null as it applies to bridge measurement
3. What do u meant by dielectric loss in a dielectric ?
4. What is meant by dissipation factor ?