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Question:
Published on: 28 November, 2022

What are the special features of a high voltage Schering bridge? What is meant by “Wagner’s Earth”?

Answer:

  1. High voltage supply is obtained from a transformer. Usually at the power frequency and detector used is vibration galvanometer.
  2. It is must for preventing dangerous high voltages appearing across arms BC and AC in the case of breakdown of either of the high voltage capacitors. It is done by connecting a spark gap across each of the arms BC and DC.
  3. The impedances of arms AB and AD are high so current from the source is less.
  4. In operating the Schering bridge at the high voltages, and in the precise measurements at low voltages it is extremely important that the effect of stray capacitances between the bridge elements be eliminated.
  5. Schering bridge has an advantage for high voltage tests due to the fact that the arms AB and AD, each contains only capacitors, which can be specially constructed for high voltage work. These 2 arms will have very high impedance, at power frequencies in comparison with that of the arms BC and CD. Thus the major portion of the source potential difference exists across the arms AB and AD and the potential differences across the arms BC and CD, which contains the control, is small. For operators’ safety from high voltage hazards, while carrying out control, junction point C is earthed.

Wagner’s Earthling: For elimination of some of the effects of stray capacitances in A.C. bridge circuits, the method is widely used. For eliminating the troublesome capacitances between the detector and ground this device is used. Fig. shows the circuit of a capacitance bridge with Wagner earthling device. \(C_1\) and \(C_2\) represents stray capacitance between detector and ground. The oscillator is changed from its usual ground connection and bridge by a series combination of resistor \(R_W\) and capacitor \(C_W\). The junction of \(R_W\) and \(C_W\) is grounded.

For balancing the bridge, the detector is first connected to point 1 and \(R_1\) is adjusted for null. The defector is then connected to position 2 connecting it to Wagner earth, and then the resistance \(R_W\) is now adjusted for null. The procedure is repeated till final balance is adjusted at positions 1 and 2 changing \(R_1\), \(R_2\) and \(R_W\).

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