AC bridges ( Weins bridge, Desauty bridge, Maxwell's inductance bridge)

An ac bridge consist of four arms of impedances, a source of excitation and a balance detector.

1. AC BRIDGE An ac bridge consist of four arms of impedances, a source of excitation and a balance detector. Used for measurement of unknown impedances like inductance, capacitance, for storage factor, dissipation factor, phase shifting etc. All the ac bridges are based on the Wheatstone bridge.

2. MAXWELL’S INDUCTANCE CAPACITANCE BRIDGE In this bridge, an inductance is measured by comparison with a standard variable capacitance. Balance equation – ( R 3 + j 3 ) = R 2 R 4 Separating real and imaginary- = and L 3 =R 2 R 4 C Q factor - Q= R 1 = C 4 R 4  

3. ADVANTAGES The two balance equation are independent if we choose R 4 and C 4 as variable element. The frequency does no appear in any of the two equations. It very useful in measurement of wide range of inductance at power and audio frequency.

4. DISADVANTAGES This bridge requires a variable standard capacitor which may be expensive if calibrated to high degree of accuracy. The bridge is limited to measurement of low Q coils. (Q>10 )

5. DE SAUTY’S BRIDGE The De Sauty’s bridge is an A.C Bridge works on the principle of Wheat stone’s bridge . This bridge is used to determine the capacity of an unknown capacitor C1 in terms of a standard known capacitor C2. R1 and R2 are non - inductive resistors . R1, R2 , C1 and C2 are connected in a Wheat stone’s bridge. Headphone is used as a detector.

6. When the bridge is balanced, the ratios of impedances are equal as given below- = = = = C  

7. ADVANTAGES The advantage of this bridge is its simplicity to obtain a perfect balance. DISADVANTAGES A perfect balance can be obtained, only if both the capacitors are free from dielectric losses i.e. when air capacitors are used .

8. WEIN’S BRIDGE Wien bridge is primarily known as a frequency determining bridge and find its application as harmonic distortion analyzer, audio and HF oscillators etc. At balance- R 3 = R 4 Equating real and imaginary parts- = + , = When R 1 =R 2 =R and C 1 =C 2 =C =2 and =1/2 RC  

9. ADVANTAGES This may be used for measurement of capacitance also DISADVANTAGES Because of frequency sensitivity, the Wien's bridg e may be difficult to balance unless the waveform of the applied voltage is sinosuidal.

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