Transcript Lecture 10 - AC bridges

Introduction to AC Bridge.

AC bridge are used to measure impedances.

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.
Cont’d…

When the specific circuit conditions apply, the detector current becomes
zero, which is known as null or balance zero.

bridge circuits can be constructed to measure about any device value
desired, be it capacitance, inductance, resistance

the unknown component's value can be determined directly from the
setting of the calibrated standard value
A simple bridge circuits are shown
below;
inductance
capacitance
Similar angle Bridge.

used to measure the impedance of a capacitance circuit.

Sometimes called the capacitance comparison bridge or series
resistance capacitance bridge
R2
Rx 
R3
R1
R1
Cx 
C3
R2
Opposite angle Bridge.
 From similar angle bridge, capacitor is replaced by
inductance
 used to measure the impedance of a inductive circuit.
 Sometimes called a Hay bridge
 2 R1 R2 R3C12
Rx 
2
2
1   2 R1 C1
R2 R3C1
Lx 
2
2
1   2 R1 C1
Wien Bridge.
 uses a parallel capacitor-resistor standard impedance to
balance out an unknown series capacitor-resistor
combination.
 All capacitors have some amount of internal resistance.

R1 
1


Rs 
Rx  2
2 

R2 
 Rx C x 

R2 
1

C
Cs 
2
2  x
2

R1  1   Rx C x 
2
Rx 

Rs
R2 


2
2 
2

R1  1   Rs Cs 
Cx 

R1 
1
 Cs 

2
2 
2

R2 
 Rs Cs 
1
Maxwell-Wien Bridge.

used to measure unknown inductances in terms of calibrated
resistance and capacitance.

Because the phase shifts of inductors and capacitors are exactly
opposite each other, a capacitive impedance can balance out an
inductive impedance if they are located in opposite legs of a bridge

Sometimes called a Maxwell bridge
3
R2 R3
Rx 
Rs
Lx  R2 R3Cs
2
Please
prove it !!!