Finding Value of Capcitance and Dissipation Factor of a Given Capacitor by Schering Bridge

Our aim here is to determing the value of the unknown electrical capacitance and dissipation factor of a given capacitor by the Schering’s bridge.

The circuit diagram of the schering’s bridge is as shown in the figure below:

Schering's Bridge Circuit Diagram

Its connection diagram in the kit is as below:

Schering's-bridge-connections-as-in-the-kit

Determination of “C” :-

The unknown capacitor C and the unknown resistor R can be found in the equipment. When the bridge is balanced, the following equation is satisfied.

Balanced Schering's Bridge Equations

Equating the real and imaginary parts on either sides of the equation we get,

equation for capacitance after balancing real and imaginary parts

Determination of Dissipation Factor :-

A practical capacitor can be considered as a pure capacitor in series with a small resistance as shown below:

equivalent-circuit-of-a-practical-capacitor

The phasor diagram is as below:

phasor diagram for practical capacitor

Dissipation Factor (D.F) = Power factor of capacitor.

Dissipation Factor

Procedure for Finding Capacitance :-

  1. Make connections as shown in the figure. Connect the unknown capacitor using 0.01μFm
  2. Set C2 = 0 and R = 0
  3. Set R1 = 1000Ω and adjust R2. Adjust both R1 and R2 till you get zero or minimum sound in the headphones.
  4. Note down R1, R2, C1 values.
  5. Calculate C using the formula C = (C1 R2) / R1
  6. Choose different unknown capacitor, repeat the above procedure and find the value of C using equation for C given above.

Procedure for Finding Dissipation Factor (D.F) :-

  1. Without disturbing the setting of the bridge (null points with R1 and R2) introduce R with a initial value say of 500Ω
  2. Adjust C2 till you get balance as indicated by minimum or zero sound in the headphones
  3. Find D.F using the formula D.F = ω.R.C
  4. Repeat above steps with different values of R.

Other Main Applications of Schering’s Bridge:-

Various other applications of Schering’s bridge include measurements of the properties of insulators, capacitor brushings, insulating oil and other insulating materials.

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