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UNIVERSITI MALAYSIA PERLIS
ELECTRICAL ENGINEERING TECHNOLOGY
EMT 113/4
EXPERIMENT 7
THE WHEATSTONE BRIDGE
MARKS
Table 7.1
Table 7.2
Calculation
Discussion
Conclusion
Total
%
12
8
20
5
5
50
100
NAME
PROGRAMME
MATRIK #
GROUP
DATE
EMT 113/4
The Wheatstone Bridge
EXPERIMENT 7
TITLE : THE WHEATSTONE BRIDGE
OBJECTIVE :
To analyze a basic Wheatstone bridge by determined the percentage error of Ig in
unbalanced condition using Thevenin’s theorem.
EQUIPMENT & COMPONENT :
1 dc power supply,1 galvanometer (uA),1 digital multimeter,3 resistors, 2.2kΩ ± 5%,
1 resistor, 2.7kΩ ± 5%,1 potentiometer 10kΩ
INTRODUCTION :
When the Wheatstone bridge is in unbalanced condition, current flows through the
galvanometer, causing a deflection of its pointer. The amount of deflection is a function
of sensitivity of the galvanometer. A more sensitive galvanometer will deflects by a
greater amount for the same current.
To determine the amount of deflection that would result for particular degree of
unbalance, the Thevenin’s theorem is applied. Since we are interested in determine the
current through the galvanometer or the meter movement, the Thevenin’s equivalent is
finding as seen by the galvanometer.
The Thevenin’s equivalent voltage is found by disconnecting the galvanometer from
the bridge circuit, as shown in Fig.7.1, and determining the open-circuit voltage
between terminals a and b.
R2
R1
V
a
R3
b
R4
Figure 7.1 Open-circuit Wheatstone bridge
Applying the voltage divider equation, the voltage at point a and b can be determined.
Therefore, the voltage between a and b is the difference between Va and Vb which
represents Thevenin’s equivalent voltage.
Vth  Vb  Va  Va  Vb
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EMT 113/4
The Wheatstone Bridge
Thevenin’s equivalent resistance can be determined by replacing the voltage source, V
with short-circuited the terminal a b, then finding the resistance looking into terminals a
and b.
PROCEDURE :
1. Measure the actual value of the resistors R1, R2, R3, and R4 and record in the Table 7.1
2. Construct the circuit as shown in Fig. 7.2 below.
Figure 7.2: Basic Wheatstone bridge
3. Apply 4V to the circuit and record the galvanometer current as Ig3.
4. Disconnect the galvanometer from the circuit of Fig. 7.2 and calculate Thevenin’s
equivalent circuit as looking back into the terminal a and b. Use the measured value for
the resistor R1, R2, R3, and R4. Show the step of calculation. Record Vth1, and Rth1 in
the table.
5. Construct the Thevenin’s equivalent circuit as calculated in previous step. Connect the
galvanometer to the output terminals of a and b as in Fig.7.3 below. Measure the
galvanometer current and record in the table as Ig1.
a
Rth1
Vth1
Ig1
G
b
Figure 7.3: Thevenin’s equivalent circuit
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
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EMT 113/4
The Wheatstone Bridge
6. Apply and calculate the following approximations of Thevenin’s equivalent voltage and
resistance to your original circuit of Fig.7.2.
r
R RR  r 


and R th 2  
Vth 2  V  

2
2R  r
 4R  2r 
R  2.2k
where,
r  R 4  R
Show the step of calculation and record the value in table 7.1.
7. Next determine deflection current in the galvanometer, Ig2 using following formula.
Vth2
Ig 2 
; which, Rg = internal resistance of galvanometer = 2.3Ω
R th2  R g
8. Calculate and record the percentage of error between the Ig2 and Ig3, and between Ig1
and Ig2.
9. Calculate and record the percentage of error between the Vth1 and Vth2.
10. Calculate and record the percentage of error between the Rth1 and Rth2.
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
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EMT 113/4
The Wheatstone Bridge
Name:______________________________ Matric No.:_____________
Date:________________
Course : ________________
RESULT
Resistor
Values
R (kΩ)
Galvanometer
Current
Ig (A)
Thevenin’s
Equivalent Circuit
Approximate
Thevenin’s
Equivalent Circuit
R1 =
Ig1 =
Vth1 =
Vth2 =
R2 =
Ig2 =
Rth1 =
Rth2 =
R3 =
Ig3 =
∆R =
R4 =
12 mark
Table 7.1
Percent of Error, e% 
calculated  measured
 100%
calculated
Ig2 and Ig3
Ig1 and Ig2
Vth1 and Vth2
Rth1 and Rth2
8 mark
Table 7.2
Instructor Approval: …………………………….
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
Date: ………………
4
EMT 113/4
Name:______________________________
Date:_________________________
The Wheatstone Bridge
Matric No.:_____________
Course : ___________________
CALCULATION
Instructor Approval: …………………………….
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
Date: ………………
5
EMT 113/4
Name:______________________________
Date:_______________________
The Wheatstone Bridge
Matric No.:_____________
Course : ____________________
CALCULATION
20 mark
Instructor Approval: …………………………….
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
Date: ………………
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EMT 113/4
Name:______________________________
Date:______________________
The Wheatstone Bridge
Matric No.:_____________
Course : _________________
DISCUSSION:
1. What does bridge null or balanced mean? What happens in a bridge circuit to make it
balance?
2.5 mark
2. Name the bridge which is modified version of the Wheatstone bridge and the purpose
of its modification?
2.5 mark
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
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EMT 113/4
Name:______________________________
Date:______________________
The Wheatstone Bridge
Matric No.:_____________
Course : _________________
CONCLUSION:
Based on measurement data, make your overall conclusion by referring to the objective of
this experiment ( your answer should be in simple notes ).
5 mark
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