Schering bridge Experiment


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Schering bridge Experiment

  1. 1. 2010 SPARK-KING Knowledge is NO-WHERE ….. (Engineering Lectures and Engineering Solutions) 09833066325 / 08796149007 VIJAY BALU RASKAR BE-Electrical / MBA - Operation [Engineering Assignments]
  2. 2. SPARK-KING ….. Unique solution for Learning. VBR Experiment No: Title: Measurement of Tangent of Dielectric Loss Angle (tan  ) by Schering Bridge. Roll No: Batch: Date: ___________________________________________________ DESIGNED BY, PROF. VIJAY BALU RASKAR
  3. 3. SPARK-KING ….. Unique solution for Learning. VBR EXPERIMENT NO: Measurement of Tangent of Dielectric Loss Angle (tan  ) by Schering Bridge TITLE: Measurement of Tangent of Dielectric Loss Angle (tan  ) by Schering Bridge. AIM: Measuring the value of unknown capacitance with the help of Schering Bridge. APPARATUS:  Analog Board, AB13  DC Power Supply  12V from external source ST2612 Analog Tab  Function Generator  2mm patch cords  Digital multimeter THEORY: Schering Bridge is the simplest method of comparing two capacitances and to determine unknown capacitance. In figure, Zx consisting of an unknown capacitance Cx in series with the resistance Rx and second arms are consisting of capacitor C3 and third arm consisting of variable resistance R2 and fourth arm consists parallel combination of resistance R1 and capacitor C1. The balance can be obtained by varying the resistance R2 of third arm. Cx = Capacitance with unknown capacitance Rx = Effective resistance C3 = Standard capacitor R1, R2 = Non-inductive resistance At balance, Z1Zx = Z2Z3 The value of D (Dissipation Factor for the capacitor) indicates the quality of the capacitor. This bridge is used for measurement of small valued capacitors at low voltages with high Precision. ___________________________________________________ DESIGNED BY, PROF. VIJAY BALU RASKAR
  4. 4. SPARK-KING ….. Unique solution for Learning. VBR An AC Bridge, in its basic form, consists of four arms, a source of excitation and a balance detector. In an AC bridge each of four arms are impedance, and the AC source and a detector sensitive to small alternating potential differences. The usefulness of AC bridge circuits is not restricted to the measurement of unknown impedances and associated parameters like inductance, capacitance, storage factor, etc. These circuits find other application in communication system and complex electronic circuits. A.C. bridge are commonly used for phases shifting, providing feedback paths for oscillators and amplifiers, filtering out undesirable signals and measuring the frequency of audio signals. For measurement at low frequencies, the power line may act as the source of the supply to bridge circuits. For higher frequencies electronic oscillators are universally used as bridge source supplies. These oscillators have the advantage that the frequency is constant easily adjustable and determinable with accuracy the waveform is very close to a sine wave and their power output is sufficient for most bridge measurements. Detectors most commonly used for AC Bridge are, 1. Head Phones 2. Vibration Galvanometers 3. Tunable Amplifiers Detectors Head phones are widely used as detectors at frequencies of 250Hz and up to 3 to 4KHz. They are most sensitive detectors for this range when working at a single frequency a tuned detector normally gives the greatest sensitivity and discrimination against harmonics in the supply. Vibration galvanometers are extremely using for power and low audio frequency ranges. Vibration galvanometers are manufactures to work at various frequency ranges from 5 Hz to 1000Hz but are most commonly used below 200Hz as below this frequency they are more sensitive than the head phones. Tunable amplifiers detectors are the most versatile of the detectors. The transistors amplifiers can be tuned electrically and thus can be made to respond to a narrow bandwidth at the bridge frequency. The output of the amplifier is fed to a pointer type instrument this detector can be used, over a frequency range of 10Hz to 100 KHz. General Equation for Bridge balance: Basic AC Bridge circuit is shown below the four arm of the bridge are impedance Z1, Z2, Z3 and Z4. The condition for balance of bridge requires that there should be no current through the detector. This requires that the potential difference between point b ___________________________________________________ DESIGNED BY, PROF. VIJAY BALU RASKAR
  5. 5. SPARK-KING ….. Unique solution for Learning. VBR and point d should be zero. This will be the case when the voltage drop from a to b equals the voltage drop from a to d both in magnitude and phase. E1 = E2 I1Z1 = I2Z2 Also, at Balance, I1 = I3 = E / (Z1 + Z2) And I2 = I4 = E / ( Z2 + Z4) Substitute the equation, We get, Z1Z4 = Z2Z3 CONDITION FOR AC BRIDGE BALANCING: Z1Z4 = Z2Z3 and 1   4   2   3 The phase angles are positive for inductive impedance and negative for capacitance impedance. PROCEDURE:- 1. Connect  12V DC power supply at their indicated position from external source OR ST2612 analog Lab. 2. Connect function generator probes between Vin terminals. 3. Connect 2mm path chord between sockets d and a to calculate the value of Cx1 and Rx1. 4. Switch on power supply and function generator. 5. Adjust 1KHz frequency at function generator. 6. Voice will coming out from Speaker then rotate the potentiometer R2 up to getting very low voice from Speaker. 7. Note down the readings and calculate the resistance R2 between potentiometer (point e and point f) 8. Calculate the value from observation table. 9. Repeat all the steps from 1st to 9th 10. Switch off the power supply for the both. RESULT: The capacitance of capacitor Cx = _________ The effective resistance Rx = _________ ___________________________________________________ DESIGNED BY, PROF. VIJAY BALU RASKAR
  6. 6. SPARK-KING ….. Unique solution for Learning. VBR The dissipation factor D = _________ CONCLUSION: We studied the measurement of unknown capacitance, Effective Resistance and dissipation factor using Schering Bridge. CIRCUIT DIAGRAM: ___________________________________________________ DESIGNED BY, PROF. VIJAY BALU RASKAR
  7. 7. SPARK-KING ….. Unique solution for Learning. VBR OBSERVATION TABLE:- SR R1() C1( F ) C 3( F ) R2() R 2C1 R1C 3 D  CxRx Rx  Cx  NO C3 R2 1 2 3 CALCULATION:- R 2C1 R1C 3  Rx  and Cx  C3 R2  D  CxRx ___________________________________________________ DESIGNED BY, PROF. VIJAY BALU RASKAR