2. 2
7 Introduction to AC Bridge.7 Introduction to AC Bridge.
AC bridges are used to measure inductance and capacitance.
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.
Other than measurement of unknown impedance, AC bridge are
commonly used for shifting phase.
Figure : General AC Bridge Circuit.
3. 3
The Scherning Bridge is useful for measuring insulating
properties, that is for phase angles of very nearly 90o
.
Figure is the Scherning Bridge.
Arm 1 contains only a capacitor C3. This capacitor has very low
losses (no resistance) and therefore the phase angle of approximately
90o
.
Schering Bridge.Schering Bridge.
Figure: Scherning Bridge.
4. 4
The impedance of the arm of the
Schering bridge is,
Substitute the value,
Expand,
Equating the real and imaginary terms,
xx
c
c
jXRZ
jXZ
RZ
jXR
Z
−=
−=
=
−+
=
4
33
22
11
1
/1/1
1
13
2
3
12
3
1
13
2
11
32
11
32
1
32
4
1
11
)(
/1/1
1
)(
RC
jR
C
CR
C
j
R
Cj
RC
j
R
jXR
jXR
jXR
jXR
Z
ZZ
Z
x
c
c
c
c
ωω
ω
ω
−=−
−
−
=
−−=
−+
−
==
2
1
3
2
1
2
R
R
CC
C
C
RR
x
x
=
=
Cont’d…Cont’d…
5. 5
Example 7:Example 7: Schering Bridge.Schering Bridge.
Find the equivalent series element for the unknown impedance of theFind the equivalent series element for the unknown impedance of the
Schering bridge network whose impedance measurements are to beSchering bridge network whose impedance measurements are to be
made at null.made at null.
RR11 = 470 k= 470 kΩΩ CC11 = 0.01 mF= 0.01 mF
RR22 = 100 k= 100 kΩΩ CC33 = 0.1 mF= 0.1 mF
Solution:Solution:
Find Rxand Cx ,
FF
R
R
CC
K
C
C
RR
x
x
µ47.010*47.0
10*100
)10*470(*)10*01.0(
10
10*01.0
)10*01.0(*)10*100(
6
3
36
2
1
3
6
63
2
1
2
====
Ω===
−
−
−
−
.
6. 5
Example 7:Example 7: Schering Bridge.Schering Bridge.
Find the equivalent series element for the unknown impedance of theFind the equivalent series element for the unknown impedance of the
Schering bridge network whose impedance measurements are to beSchering bridge network whose impedance measurements are to be
made at null.made at null.
RR11 = 470 k= 470 kΩΩ CC11 = 0.01 mF= 0.01 mF
RR22 = 100 k= 100 kΩΩ CC33 = 0.1 mF= 0.1 mF
Solution:Solution:
Find Rxand Cx ,
FF
R
R
CC
K
C
C
RR
x
x
µ47.010*47.0
10*100
)10*470(*)10*01.0(
10
10*01.0
)10*01.0(*)10*100(
6
3
36
2
1
3
6
63
2
1
2
====
Ω===
−
−
−
−
.
