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UNIVERSITI MALAYSIA PERLIS
FUNDAMENTAL OF ELECTRICAL ENGINEERING
EMT 113/4
EXPERIMENT 7
THE WHEATSTONE BRIDGE
MARKS
Results &
Calculation
21
NAME :
MATRIC NO. :
PROGRAMME :
DATE :
Discussion
Conclusion
Total
%
10
5
36
100
EMT 113/4 Sem II 2009/2010
The Wheatstone Bridge
EXPERIMENT 7
THE WHEATSTONE BRIDGE
7.1
OBJECTIVE:
To analyze a basic Wheatstone bridge by determined the percentage error of Ig in
unbalanced condition using Thevenin’s theorem.
7.2
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
7.3
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.
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
1
EMT 113/4 Sem II 2009/2010
7.4
The Wheatstone Bridge
PROCEDURE:
1. Measure the actual value of the resistors R1, R2, R3, and R4 and record the values.
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.
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 as Ig1.
a
Rth1
Vth1
Ig1
G
b
Figure 7.3: Thevenin’s equivalent circuit
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 
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
2
EMT 113/4 Sem II 2009/2010
The Wheatstone Bridge
where,
R  2.2k
r  R 4  R
Show the step of calculation and record the value.
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
3
EMT 113/4 Sem II 2009/2010
7.5
The Wheatstone Bridge
RESULT
Your results should include:
i.
ii.
iii.
All measurements values
Percentage error
Calculation of the Thevenin’s Equivalent Voltage and Resistance
7.6
DISCUSSION:
Your discussion should include:
i)
7.7
Explanations on your results.
CONCLUSION:
Based on data, make your overall conclusion by referring to the objectives of this
experiment.
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
4