4.Electrical Instrumentation lab manual for diploma electrical engineering 3rd sem as per GTU

PRACTICAL 4

AIM: - TEST THE CAPACITANCE BY USING UNIVERSAL IMPEDANCE BRIDGE

v OBJECTIVE: - To find the capacitance of unknown capacitor using Schering bridge. As a universal impedance bridge.

v APPARATUS :

Sl. NO.	NAME	TYPE	RANGE	QTY.
1	Schering bridge circuit			1 no
2.	Head phones			1 no.
3.	Decade capacitance box			1 no.
4.	DMM	digital		1 no.
5.	Patch cards			1set
6	RPS		230v	1 no
7	Galvanometer			1 no

v CIRCUIT DIAGRAM:

For circuit diagram in PDF formate please click on the download link

v THEORY:

The Schering Bridge is very widely used for the measurement of capacitance, dielectric loss and power factor of capacitors. The advantage of using this bridge is that it can be employed in both low voltage and high voltage measurements.

v FROM THE CIRCUIT :

            C₁ is the unknown capacitance and

            R₁ is a resistance representing its loss component

            C₂­ is a standard air capacitor and it is loss free

            R₃ & R₄ are non-inductive standard resistors.

            R₃ is fixed and R₄ is variable

            C₄ is variable capacitors.

            The detector may be a head phone or vibration galvanometer

            E is a low voltage a.c. source   r 230v,50hz,supply

            The bridge is balanced by adjusting

C₄ and R₄

For detail theory in PDF formate please click on the download link

v PROCEDURE :

1.         Connections are made as per the circuit diagram.

2.         Connect the unknown capacitance ‘C_x’ between the terminals A & D.

3.         Switch on the supply and vary the Resistance knob (R_1­) until minimum sound is obtained from the loud speaker.

4.         Switch off the supply and measure the Resistance (R₁) between C & B by using DMM.

5.         Repeat the above steps for different values of capacitance & tabulate the readings.

6.         Switch off the supply.

v PRECAUTIONS :  

1.         Avoid loose connections.

2.         Resistance should be varied very smoothly.

3.         Switch off the supply when the resistance R₁ is measured.

v FORMULA USED:

            r₁R₄=   &   C1= Cx =(R4/R3) x C2

NOTE: the value of unknown capacitors is

C = 0.01μF

       0.02 μF

       0.03 μF

       0.04 Μf

v OBSERVATION TABLE:-

sr.no	Cs (known capacitance)	R4	R3	Cx=CsR4/R3

v CONCLUSION:-

FOR CIRCUIT DIAGRAM ...

PLEASE CLICK ON THE BELOW LINK...

DOWNLOAD link for practical 4

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