EE Pro for TI-89, 92 Plus
Equations - Meters and Bridge Circuits
34
20.3 Wien Bridge
A Wien bridge circuit is designed to measure an unknown capacitance Cx using a
bridge arrangement shown here. The circuit has two methods of varying parameters
of the bridge to achieve null. In the first approach, Cx can be computed in terms of
RR3, RR1, Rs and Rx while in the second equation, the source frequency can be
used as a key controlling parameter. The bridge can also be used to measure the
frequency f with the third equation after setting Cx=Cs, Rx=Rs, and RR3=2*RR1.
The final equation relates f to radian frequency
ω
.
Eq. 20.3.1
Eq. 20.3.2
Eq. 20.3.3
Eq. 20.3.4
Example -20.3
A set of measurements obtained using a Wien bridge is based on the following input. All measurements are carried
out at 1000_Hz. The known resistors RR1 and RR3 are 100
Ω
each, the series resistance is 200
Ω
, and Cs is
1.2_
µ
F. Find the values of the unknown RC circuit components and the radian frequency.
Entered Values
Calculated Results
Solution -
Use the first, second and fourth equations to solve the problem. Press
„
to display the input
screen, enter all the known variables and press
„
to solve for the unknowns. The computed results are
shown in the screen display above.
-PQYP8CTKCDNGU%U
A
µ
(
H
A*\
44
A
Ω
44
A
Ω
4U
A
Ω
%QORWVGF4GUWNVU%Z
A
µ
(
4Z
A
Ω
ω
ω
ATU
Cx
Cs
RR
RR
Rs
Rx
=
−
F
HG
I
KJ
3
1
Cs Cx
Rs Rx
⋅
=
⋅ ⋅
1
2
ω
f
Cs Rs
=
⋅ ⋅
1
2
π
ω
π
= ⋅ ⋅
2
f