A.C. bridges are circuits used to measure unknown resistances, capacitances, inductances, frequencies, and mutual inductances. Some common bridges include the Wheatstone bridge for measuring resistance using direct current, Maxwell's bridges for measuring inductance using alternating current, and the De Sauty, Schering, Wien series, and Wien parallel bridges for measuring capacitance using various balanced circuit configurations. Each bridge uses a balanced circuit equation to relate known components to unknown values.

1. A.C.Bridges

2. A.C.Bridges are those circuits which are
used to measured the unknown resistances,
capacitance ,inductance ,frequency and
mutual inductance.

3. Wheat Stone Bridge
Resistances can be measured by direct-
current ,as shown in fig. a

4. Fig. a
R 1 R4 R 1 R 3 = R 2 R4
R 2 R 3

5. Inductance and capacitance can also
be measured by a similar four-arm
bridge, as shown in fig.b.
In this case the alternating current
source is employed by a vibration
galvanometer.

6. Fig.b

7. Maxwell’s inductance
Bridge
In the Maxwell’s inductance bridge ,there
are two pure resistances used for balance
relations but on other side or arms the two
known impedances are used.
The known impedances and the resistances
make the unknown impedances as Z1 and
Z2.Such a network is known as Maxwell’s
A.C.. Bridge. As shown in fig.

8. Fig.
(R1 + jwL1 )R3 = (R4 + jwL4 )R2

9. Maxwell’s Wien
Bridge
In the Maxwell’s Wien Bridge the positive
phase angle of the inductance may be
compensated by the negative phase angle
of the capacitance impedance put in the
opposite arm.
The unknown inductance then becomes
known in terms of the capacitance. As
shown in fig.

10. Fig.
R2 R4 + jwL1 R2 = R1 R3 + jwCR1 R2 R3

11. Anderson Bridge
In the Anderson Bridge the unknown
inductance is measured in terms of a known
capacitance and resistance.
this method is capable of precise measurements
of inductance over a wide range of values from
a few micro-henrys to several henrys and is the
best bridge method.

13. Capacitance Bridge
We will consider only De Sauty bridge
method of comparing two capacitances the
bridge has maximum sensitivity when C2 = C3.
The simplicity of this method is offset by the
impossibility of obtaining a perfect balance if
both the capacitors are not free from the
dielectric loss.
A perfect balance can only be obtained if air
capacitors are used. as shown in fig.

14. Fig.
C2= C3 R1
R2

15. Schering Bridge
Schering bridge used for the measurement
of capacitance and dielectric loss of a
capacitor.
It is a device for comparing an imperfect
capacitor C2 in terms of a loss-free
standard capacitor C1. As shown in fig.

16. Fig.
C3 = C2 ( R1 / R2 ) R3 = R2 ( C1 / C2 )

17. Wien Series Bridge
It is a simple ratio bridge and is used for
audio-frequency measurement of
capacitance over a wide range. As shown
in fig.

18. Fig.
R1=R2R4/R3 C1=C4(R3/R2)

19. Wien Parallel Bridge
It is also a ratio bridge used mainly as the
feedback network in the wide range audio-
frequency R-C oscillators.
It is may be used for the measurement of
the audio-frequency but it is not as
accurate as the modern digital frequency
meters. As shown in fig.

20. Fig.
C2 = R2 = R3
C1 R1 R4