Source and Null Detectors in A.C Bridges

1. Source in A.C Bridges:
The source in A.C bridges provides the alternating current required for the bridge circuit. It is usually a variable frequency oscillator or a signal generator that produces an accurately known and stable sinusoidal waveform. The source should have a low internal resistance to prevent any voltage drop across it. This ensures that the full voltage is available for the bridge circuit.

2. Null Detectors in A.C Bridges:
Null detectors are used in A.C bridges to determine the point of balance or null point. The null point is the condition when the bridge is perfectly balanced, and there is no current flowing through the detector. There are different types of null detectors used in A.C bridges, depending on the specific bridge configuration and requirements.

a. Galvanometer:
The galvanometer is the most commonly used null detector in A.C bridges. It is a sensitive instrument that measures small electric currents. When the bridge is balanced, the galvanometer shows zero deflection indicating no current flow. It is connected in series with the bridge circuit and requires a high resistance shunt to prevent excessive current from flowing through it.

b. Vacuum Tube Voltmeter (VTVM):
The vacuum tube voltmeter is a highly sensitive null detector that uses vacuum tubes for amplification. It can measure very small voltages accurately and is commonly used in bridge circuits where high sensitivity is required. The VTVM is connected across the output terminals of the bridge and displays the voltage difference when the bridge is unbalanced.

c. Digital Multimeter (DMM):
Digital multimeters are versatile instruments that can be used as null detectors in A.C bridges. They provide accurate measurements of voltage, current, and resistance. DMMs are connected in parallel with the output terminals of the bridge and display the voltage difference between the terminals. They are useful when precise measurements and digital readouts are required.

d. Phase Detector:
Phase detectors are used in some specialized A.C bridges that require phase angle measurements. They compare the phase of the reference signal with the phase of the output signal. When the bridge is balanced, the phase detector shows zero phase difference, indicating the null point. This type of detector is commonly used in impedance bridges.

e. Oscilloscope:
Oscilloscopes can also be used as null detectors in A.C bridges. They display the waveform of the output signal and can be used to identify the null point visually. The oscilloscope shows a flat line when the bridge is balanced, indicating no voltage difference between the terminals.

In conclusion, A.C bridges require a suitable source to provide the alternating current, and various null detectors are used to determine the null point or point of balance. The choice of the null detector depends on the specific requirements of the bridge circuit, such as sensitivity, measurement accuracy, and the need for additional measurements like phase angles.