Introduction to AC Bridge. AC bridge are used to measure impedances. All the AC bridges are based on the Wheatstone bridge. In the AC bridge the bridge circuit consists of four impedances and an ac voltage source. The impedances can either be pure resistance or complex impedance.

Cont’d… When the specific circuit conditions apply, the detector current becomes zero, which is known as null or balance zero. bridge circuits can be constructed to measure about any device value desired, be it capacitance, inductance, resistance the unknown component's value can be determined directly from the setting of the calibrated standard value                    

A simple bridge circuits are shown below; inductance capacitance

Similar angle Bridge. used to measure the impedance of a capacitance circuit. Sometimes called the capacitance comparison bridge or series resistance capacitance bridge

Opposite angle Bridge. From similar angle bridge, capacitor is replaced by inductance used to measure the impedance of a inductive circuit. Sometimes called a Hay bridge

Wien Bridge. uses a parallel capacitor-resistor standard impedance to balance out an unknown series capacitor-resistor combination. All capacitors have some amount of internal resistance.

Maxwell-Wien Bridge. Please prove it !!! used to measure unknown inductances in terms of calibrated resistance and capacitance. Because the phase shifts of inductors and capacitors are exactly opposite each other, a capacitive impedance can balance out an inductive impedance if they are located in opposite legs of a bridge Sometimes called a Maxwell bridge Please prove it !!!