This document summarizes several AC bridge circuits used for measuring unknown impedances. It describes Maxwell's inductance-capacitance bridge which measures inductance by comparing it to a standard variable capacitor. De Sauty's bridge is used to determine the capacity of an unknown capacitor in terms of a standard known capacitor. Wein's bridge is primarily a frequency determining bridge but can also measure capacitance. It provides equations for calculating resistance ratios and oscillation frequency at balance.

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 –
(R3 + j 𝜔𝐿3)
𝑅1
1+𝑗𝜔𝐶𝑅1
=R2R4
Separating real and imaginary-
R3 =
R2R4
R1
and L3=R2R4C
Q factor - Q=𝜔𝐿3/R1=𝜔C4R4

3. ADVANTAGES
The two balance equation are independent if we
choose R4 and C4 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-
Z1
Z2
=
Z3
Z4
1
𝑗𝜔𝐶1
𝑅1
=
1
𝑗𝜔𝐶2
𝑅2
C2
C1
=
R1
R2
C2 =
R1
R2
C1

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-
𝑅2
1+𝑗𝜔𝐶2
𝑅2
R3 = 𝑅1 −
𝑗
𝜔𝐶1
R4
Equating real and imaginary parts-
𝑅4
𝑅3
=
𝑅1
𝑅2
+
𝐶3
𝐶4
, 𝑓=
1
2 𝜋√𝑅1
𝑅2
𝐶1
𝐶2
When R1=R2=R and C1=C2=C
𝑅4
𝑅3
=2 and 𝑓=1/2 𝜋RC

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

10. THANK YOU