Exp. No.1: DC Bridge Measurements -...

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Name Matric no. Section:

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Instructors’ name:

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Title Time/ Date

Experiment no.: 01

Experiment title : DC Bridge Measurements

FOR INSTRUCTORS USE ONLY

Domain Item Subtotal Total

C Question /15

/20 Conclusion /05

P Pre laboratory /20

/60 Results and analysis /20

Lab activities /20

A Demonstration (understanding) /10

/20 Ethics /10

TOTAL MARKS /100

BEJ 10801 – Fundamental Electronics Laboratory

Instruction Sheet

DC Bridge Measurements

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Fundamental Electronic Laboratory (BEJ10801)

Exp. No.1: DC Bridge Measurements ii

FKEE, Sem01 Session 2019/20

Table of Content

Table of Content ii

Instruction 1

Pre Laboratory 2

1.0 Title 4

2.0 Outcomes 4

3.0 Overview 4

4.0 List of Equipment 6

5.0 Experiment Procedures 7

5.1 DETERMINING AN UNKNOWN RESISTOR USING WHEATSTONE BRIDGE 7

5.2 DETERMINING AN UNKNOWN RESISTOR USING KELVIN BRIDGE 8

6.0 Results and Analysis 12

7.0 Questions 14

8.0 Conclusions 15

9.0 Related course(s) 15

10.0 References 15

Fakulti/Pusat Pengajian (Faculty/Centre) :

Faculty of Electrical and Electronic Engineering

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Kod Kursus (Course Code):

BEJ 10801

Nama Kursus (Name of Course) :

Fundamental Electronic Laboratory

Kursus Pra Syarat (Prerequisite Courses)

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Edisi (Edition) : 2 Tarikh Keluaran (Published date) : September

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Instruction

1. Grouping: Lab group is not predetermined and consists of 3 members (maximum).

2. Pre-Lab Assignment: Must be submitted to the instructor at the beginning of lab sessions.

Verified by the instructor and returned to the students at the end of lab session. The verified

pre-lab must be attached with the final report for submission.

3. Lab Activities: All laboratory activities must be carried out in this laboratory and to be

completed within the given times.

4. Demonstration: Student must demonstrate the experiment to the respective instructor.

5. Report Submission: Report must be submitted to the respective instructor upon

COMPLETION of lab session.



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Pre Laboratory

1. Explain the purpose of using a DC Wheatstone Bridge. Name an application.

2. Name the possible measurement errors if a DC bridge is applied. Explain if the errors

could be minimized or eliminated.



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3. Sketch the connection of the circuit in Figure 3 on the following bread board diagram

with correct electronic schematic symbols



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1.0 Title

DC Bridge Measurements

2.0 Outcomes

After completing this laboratory, student should be able to:

2.1 Analyze the characteristic of a DC bridge measurement. (PLO2-PS-C4)

2.2 Organize time management in group effectively according task given. (PLO5-TS-P5)

2.3 Demonstrate good leadership skill during laboratory experiment. (PLO9-LS-A3)

3.0 Overview

DC Bridge – Wheatstone Bridge

The DC bridge shown in Figure 1 is known as the Wheatstone bridge. It is used to obtain a

precise measurement (accurate to about 0.1 %) of a resistor Rx. (usually it will be represented in

a diamond shape).

Figure 1: Wheatstone Bridge

This bridge consists of two resistor branches: on the left are two precision resistors R1 and R2.

On the right is a precisely calibrated variable resistor and an unknown (or imprecisely known)

resistor𝑅𝑥. If the current flowing between the mid-points of the branches happens to be zero, then

the branches will function as accurate voltage dividers, in which case the voltage across 𝑅2 will



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be:

𝑉2 =𝑅2

𝑅1 + 𝑅2𝑉𝑠

And this equal the voltage across the unknown resistor 𝑅𝑥 which will be:

𝑉𝑥 =𝑅𝑥

𝑅3 + 𝑅𝑥𝑉𝑠

We obtain this equality by carefully adjusting𝑅3.

In the diagram, a sensitive current meter, a galvanometer is used to detect equality of the

voltages. In order not to risk damage the meter, it is advisable to obtain approximate balance

before inserting the meter: we can estimate a safe starting value for 𝑅3 by first measuring 𝑅𝑥

approximately using a less precise instrument such as digital multimeter. A zero-centre

galvanometer is the most suitable type, as it can measure small changes in either direction as we

adjust 𝑅3.

If the meter has no significant zero error, then when it reads zero current the ratios of the resistor

in both dividers must agree, leading to the null condition formula:

𝑅1𝑅𝑥 = 𝑅2𝑅3 (1)

The current through the galvanometer is proportional to any small changes in the value of one of

resistors. This enables the indirect measurement of quantities that affect the resistivity of certain

materials.

DC Bridge – Kelvin Bridge

The Kelvin Bridge is a modified version of the Wheatstone bridge. The purpose of the

modification is to eliminate of contact and lead resistance when measuring unknown low

resistances. Resistors in the range of 1Ω to approximately 1µΩ may be measure with a high

degree of accuracy using the Kelvin bridge.



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Figure 2: Kelvin bridge

Figure 2 shows essentially the same circuit as Figure 1 but with additional resistors p and r. If the

ratio of 𝑝 𝑟⁄ is exactly the same 𝑃 𝑅⁄ , the error due to the voltage drop across Y is eliminated.

The balance equation for this bridge is a little more difficult to derive than that for the

Wheatstone bridge.

If the meter has no significant zero error, then when it reads zero current the ratios of the

resistors in both dividers must agree , leading to a null condition formula :

𝑃 × 𝑆 = 𝑄 × 𝑅

4.0 List of Equipment List of equipment used:

4.1. Breadboard

4.2. Potentiometer

4.3. Multimeter

4.4. DC power supply

4.5. Galvanometer

4.6. 1 unknown resistor (Rx)

4.7. 1 x 5.6 kΩ resistor

4.8. 4 x 10 kΩ resistor




4.12. 1 x 25 kΩ potentiometer

4.13. Wires



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5.0 Experiment Procedures

5.1 DETERMINING AN UNKNOWN RESISTOR USING WHEATSTONE BRIDGE

Procedures :

1. For observation, before connecting the circuit in Figure 3 (will be done in step 2),

measure the values of R1 and R2 (in pairs of [10 kΩ, 12 kΩ] and [20 kΩ, 47 kΩ]) and the

unknown resistor, 𝑅𝑥 with a multimeter.

2. Record your observation.

3. Connect the circuit as in Figure 3 using the components provided. In addition, R3 is a 25

kΩ potentiometer, Rs is a 10 kΩ resistor, Rx is an unknown resistor and the supply voltage

is 1 VDC. Leave S Open.

4. Adjust 𝑅3 until zero reading on the galvanometer is obtained.

5. Measure the value of R3 with a multimeter and record your observation in Table 2.

[% 𝑒𝑟𝑟𝑜𝑟 = 𝑎𝑐𝑡𝑢𝑎𝑙 𝑣𝑎𝑙𝑢𝑒−𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 𝑣𝑎𝑙𝑢𝑒

𝑎𝑐𝑡𝑢𝑎𝑙 𝑣𝑎𝑙𝑢𝑒× 100%]

6. Calculate the Experimental Value for Rx [using Equation 𝑅1𝑅𝑥 = 𝑅2𝑅3 (1)] 𝑅1𝑅𝑥 =𝑅2𝑅3 (1and record it in Table 2

7. Repeat step 3 to step 6 with different pair of 𝑅1 and 𝑅2.

8. Repeat step 3 to step 6 with S closed.

Figure 3: Experiment 1.1 – Wheatstone bridge



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5.2 DETERMINING AN UNKNOWN RESISTOR USING KELVIN BRIDGE

Operation instructions

Preparation

1. Make sure that the INT, BA as well as 𝑃2𝑠 terminal are shorted securely. Place the BA

switch to OFF position, open the 𝑅𝑥 terminal.

2. Set the GA sensitivity dial to CH position. By doing this, check that the galvanometer

driving battery is good. When the pointer of the galvanometer deflects to the blue zone on

the scale, this shows that the battery is good.

Notes : Its take 1 to 2 second for stabilized indication.

3. Set the GA sensitivity dial to G2 and check that the galvanometer indication is in the zero

position. If deviated, turn carefully the zero adjusting screw of the galvanometer to obtain

a true zero point.

4. Connect the (very low-valued) unknown resistance provided [i.e. (i) audio cable and (ii)

coaxial cable] to the 𝑅𝑥 terminal (refer to Figure 4)

Operation for measurement:

1. Select the multiplying factor by using the plug to fit the approximating value of the

unknown resistance according Table 1.

2. Turn the BA switch ON

3. Set the measuring dial to a position near the center, push the GA button switch

momentarily and observe the galvanometer. If the deflection is on the (+) side, increase

the dial value to obtain zero indication by repeating the above operation. When the

indication comes near the zero position, push and turn the GA button switch either

clockwise or counterclockwise to lock the switch. Then move the measuring dial to obtain

zero indication on the galvanometer. If the indication is or the (-) side in the beginning of

this adjustment, reduce the dial value and obtain zero indication in the same manner as

above. If more sensitivity is required, select G1 position first and G0

4. When the galvanometer indication comes to zero through adjustment of the measuring

dial, the resistance value of 𝑅𝑥 is calculated from the following equation.

𝑅𝑥 = (𝑖𝑛𝑑𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑜𝑛 𝑡ℎ𝑒 𝑚𝑒𝑎𝑠𝑢𝑟𝑖𝑛𝑔 𝑑𝑖𝑎𝑙) × (𝑚𝑢𝑙𝑡𝑖𝑝𝑙𝑦𝑖𝑛𝑔 𝑓𝑎𝑐𝑡𝑜𝑟)



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5. When measurement is finished, first release the GA button switch. Then, turn off the BA

switch and set GA sensitivity dial to 𝑂𝐹𝐹𝐺𝐴 position.

Table 1: Multiply factor

𝑅𝑥 Multiply

0 ~ 1.1mΩ 0.0001*

~ 11mΩ 0.001

10 ~ 110mΩ 0.01

0 ~ 1.1Ω 0.1

~ 11Ω 1

10 ~ 110Ω 10

* with 2771



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COMPONENT NAMES AND FUNTIONS

Figure 4: Kelvin Bridge: Yokogawa 276910

1. Measuring dial - this dial is used to adjust the measuring arm.

2. Multiplying factor switching plug – the multiplying factor is selected by this plug.

3. Battery switch – this switch turn on/off the power to the bridge.

4. Galvanometer button switch – the galvanometer is connected to the bridge by

depressing this switch. This switch is locked by turning it clockwise or counterclockwise

while keeping depressed.

5. Galvanometer sensitivity dial – the galvanometer sensitivity is selected by this dial. G0

gives the highest sensitivity. G1 is medium and G2 is lower than G1. With this switch, the

galvanometer driving battery can be checked through observing the meter. When the

pointer is deflected to the blue zone, this shows that the battery is good. Also this switch

turns on/off the galvanometer driving battery.

6. INT.BA –A- Terminal - when the self-contained battery is used to supply power to the

1

10 9

2

𝑅𝑥 8

6

7

Zero adjustor

3

4 5



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bridge, these terminals are short circuited by the short-bar.

7. EXT .BA –A- Terminal – when an external power supply is used ,these terminal are

short –circuited by the short bar.

8. 𝑹𝒙 Terminal – an unknown resistance is connected to these terminals. The Rx terminals

consist of current terminal C1,C2 and potential terminal P1 and P2.

9. P2s Terminal – this is one of the potential terminals to switch the standard F2771 is

connected.

10. EXT. BA Terminal – an external battery is connected across these terminals. The (-) side

of the terminal is one of the current terminals to which the external standard resistor is

connected.



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6.0 Results and Analysis

1. List down and label properly the results from experiment 5.1 along with relevant

calculations.

Table 2: Observations from Experiment 5.1 [Hint: Appropriate units must be stated]

Actual (measured) Value Experimental Value

R1 R2 S Open S Closed S Open S Closed

R3 Rx R3 Rx Rx % error Rx % error

2. Analyze your results.



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3. List down and label properly (with appropriate unit) the results from experiment 5.2.

Table 3: Observations from Experiment 5.2 [Hint: Appropriate units must be stated]

Type of Resistor Dial Reading Multiplying Factor Experimental Value (Rx)

4. Analyze your results.



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7.0 Questions

1. Based on the result and analysis of experiment 5.1, what cause the unknown value, 𝑅𝑥,

varies?

2. Based on the results and analysis of experiment 5.1, compare your results obtained for the

value of Rx with S open and with S closed. Which one provides better accuracy? Why?

3. Why is Kelvin Bridge able to measure much lower values of resistances than a

Wheatstone bridge?

4. What are possible errors and sources of errors when conducting the experiment? Justify

your answers.



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8.0 Conclusions

9.0 Related course(s)

BEJ10702 - Instrumentation and Measurement.

10.0 References

1. J. P. Bentley, Principles of Measurement System, 3rd Edition. Pearson Prentice Hall, 1995. Call

number: QC53 .B46 1995

2. Principles of Measurement System, (Fourth Edition); J.P. Bentley, Pearson Prentice Hall,

2005.

3. R. G. Gupta, Electronic Instruments and Systems: Principles, Maintenance and Troubleshooting,

McGraw Hill, 2001. Call number: TK7870.2 .G86 2001

4. D. Jones, and C. A. Foster, Electronic Instruments and Measurements. Prentice Hall International

Edition, 1991. Call number: TK7878.4 .J66 1991 5. D. A. Bell, Electronic Instrumentation and Measurements, 2nd Edition. Prentice Hall Career and

Technology, 1994. Call number: TK7878 .B45 1994

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