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Friday, 23 August 2019

Universal Impedance Bridge

The universal impedance bridge can be found in any communication laboratory. It is a very useful laboratory instrument. Universal Impedance Bridge is used for the measurement of resistance, capacitance, and inductance. It uses different bridge configurations to measure the values of D.C resistance, A.C resistance, inductance, capacitance, Q factor of coils, dissipation factor of capacitors. Dials and multiplier switches are provided to directly read the value of the component under measurement.

It consists of suitable impedance standards, A.C. and D.C. sources, along with A.C. and D.C. null detectors. The appropriate bridge circuit will be utilised for measurement of the particular component by the switching arrangement provided in the bridge.

The universal impedance bridge uses four bridge configurations for its operation. The Wheatstone bridge circuit is used for measurement of both the A.C. and D.C. resistance. A standard capacitor and a resistor of the precision type is used in a four arm network to measure capacitance. This determines the losses in the unknown capacitor. Maxwell bridge circuit is used for low Q inductors. Hay bridge circuit is used for measurements of inductors with Q factor above 10.

A suspension galvanometer with a current sensitivity of 0.5 per division is used for D.C. resistance measurements. A selective amplifier operating an electron ray indicator tube is used as null detector for all the A.C. measurements. Provision is made to connect external A.C. and D.C. null detectors. High impedance headphones can also be connected to this bridge as null detector. The D.C. source is a small power supply. The alternating voltage source consists of an oscillator with plug in RC networks that can be used for required frequency. The standard frequency used in the bridge is 10 kHz.

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