Electrical Measurent and Intrumentation

Schering Schering BridgeBridge

1

2

7 Introduction to AC 7 Introduction to AC Bridge.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.

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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.

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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

jXRZ

4

33

22

111 /1/1

1

13

2

3

12

3

113

2

1132

11

32

1

324

1

11)(

/1/1

1)(

RC

jR

C

CR

C

jR

CjRC

jR

jXRjXR

jXR

jXR

Z

ZZZ

x

cc

c

c

2

13

2

12

R

RCC

C

CRR

x

x

Cont’d…Cont’d…

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Example 7:Example 7: Schering Bridge.Schering Bridge.Find the equivalent series element for the unknown Find the equivalent series element for the unknown impedance of the Schering bridge network whose impedance of the Schering bridge network whose impedance measurements are to be made at null.impedance measurements are to be made at null.

RR1 1 = 470 k= 470 k CC1 1 = 0.01 mF= 0.01 mF

RR2 2 = 100 k= 100 k CC3 3 = 0.1 mF= 0.1 mF

Solution:Solution:

Find Rx and Cx ,

FFR

RCC

KC

CRR

x

x

47.010*47.010*100

)10*470(*)10*01.0(

1010*01.0

)10*01.0(*)10*100(

63

36

2

13

6

63

2

12

.