Wheatstone Bridge

Dividing equation 5 by equation 1 gives
Now and solving for R m we get
R3 m R
= (6)
R R
1 2
R2
Rm = R3
(6)
R
Typically, 1 R and 2 R are fixed resistors and 3 R is a variable resistor. R m is the resistance that
is being measured. R 3 is adjusted until the detector indicates that the bridge is balanced. Then
the value of R m is determined using equation 6.
Example
Consider using a Wheatstone bridge having R 1 = 200 Ω and R 2 = 2000 Ω to measure a
resistance R m . The bridge is balanced by adjusting 3 R until R 3 = 250 Ω . What is the value of
R m ?
Solution
From equation 6
R2
2000
Rm= R3=
250 = 2500 Ω
R 200
1
Example
Consider using a Wheatstone bridge having R 1 = 200 Ω and R 2 = 2000 Ω to measure a
resistance, R m , of a temperature sensor. Suppose the resistance of the temperature sensor, R m ,
in Ω, is related to the temperature T, in °C, by the equation
R = 1500 + 25T
m
The bridge is balanced by adjusting 3 R until R 3 = 250 Ω . What is the value of the temperature?
Solution
From equation 6
Next, the temperature in °C is given by
R2
2000
Rm= R3=
250 = 2500 Ω
R 200
1
m 1500 R − 2500 −1500
1000
T = = = = 40 ° C
25 25 25
1
2