AB10 WHEATSTONE BRIDGE Analog Lab Experiment Board Ver ...

AB10 

WHEATSTONE BRIDGE 

Analog Lab 

Experiment Board 

Ver. 1.0 

QUALITY POLICY 

To be a Global Provider of Innovative and Affordable 

Electronic Equipments for Technology Training by 

enhancing Customer Satisfaction based on 

Research, Modern manufacturing techniques and 

continuous improvement in Quality of the products 

and Services with active participation of employees. 

An ISO 9001: 2000 company 

94-101, Electronic Complex, Pardesipura INDORE-452010, India.


Tel.: 91-731-2570301 Fax: 91-731-2555643 

Email: info@scientech.bz Web: www.scientech.bz 

Scientech Technologies Pvt. Ltd. 2




WHEATSTONE BRIDGE 


TABLE OF CONTENTS 

1. Introduction 4 

2. Theory 6 

3. Experiment 1 10 

To study the operation of Wheatstone bridge and 

measuring the value of unknown resistance. 

4. Experiment 2 12 

To measure the sensitivity of Wheatstone bridge. 

5. Warranty 14 

6. List of service Centers 15 

7. List of Accessories with AB10 16 



INTRODUCTION 

AB10 is a compact, ready to use Wheatstone Bridge experiment board. 

This board is useful for students to understand the working and operation of 

Wheatstone bridge. It can be used as stand alone unit with external DC 

power supply or can be used with SCIENTECH Analog Lab ST2612 

which has built in DC power supply, AC power supply, function generator, 

modulation generator, continuity tester, toggle switches, and potentiometer. 

Model Name 

AB01 Diode characteristics (Si, Zener, LED) 

AB02 Transistor characteristics (CB NPN) 

AB03 Transistor characteristics (CB PNP) 

AB04 Transistor characteristics (CE NPN) 

AB05 Transistor characteristics (CE PNP) 

AB06 Transistor characteristics (CC NPN) 

AB07 Transistor characteristics (CC PNP) 

AB08 FET characteristics 

AB09 Rectifier Circuits 

AB11 Maxwell’s Bridge 

AB12 De Sauty’s Bridge 

AB13 Schering Bridge 

AB14 Darlington Pair 

AB15 Common Emitter Amplifier 

AB16 Common Collector Amplifier 

AB17 Common Base Amplifier 

AB18 Cascode Amplifier 

AB19 RC-Coupled Amplifier 

AB20 Direct Coupled Amplifier 

AB21 Class A Amplifier 

AB22 Class B Amplifier (push pull emitter follower) 

AB23 Class C Tuned Amplifier 

AB28 Multivibrator ( Mono stable / Astable) 

AB29 F-V and V-F Converter 

AB30 V-I and I-V Converter 

AB31 Zener Voltage Regulator 

AB32 Transistor Series Voltage Regulator 

AB33 Transistor Shunt Voltage Regulator 

AB35 DC Ammeter 

AB39 Instrumentation Amplifier 

AB41 Differential Amplifier (Transistorized) 



AB42 Operational Amplifier (Inverting / Non-inverting / 

Differentiator) 

AB43 Operational Amplifier (Adder/Scalar) 

AB44 Operational Amplifier (Integrator/ Differentiator) 

AB45 Schmitt Trigger and Comparator 

AB51 Active filters (Low Pass and High Pass) 

AB52 Active Band Pass Filter 

AB53 Notch Filter 

AB54 Tschebyscheff Filter 

AB56 Fiber Optic Analog Link 

AB57 Owen’s Bridge 

AB58 Anderson’s Bridge 

AB59 Maxwell’s Inductance Bridge 

AB64 RC – Coupled Amplifier with Feedback 

AB65 Phase Shift Oscillator 

AB66 Wien Bridge Oscillators 

AB67 Colpitt Oscillator 

AB68 Hartley Oscillator 

AB80 RLC Series and RLC Parallel Resonance 

AB82 Thevenin’s and Maximum power Transfer Theorem 

AB83 Reciprocity and Superposition Theorem 

AB84 Tellegen’s Theorem 

AB85 Norton’s theorem 

AB88 Diode Clipper 

AB89 Diode Clampers 

AB90 Two port network parameter 

AB91 Optical Transducer (Photovoltaic cell) 

AB92 Optical Transducer (Photoconductive cell/LDR) 

AB93 Optical Transducer (Phototransistor) 

AB96 Temperature Transducer (RTD & IC335) 

AB97 Temperature Transducer (Thermocouple) 

AB101 DSB Modulator and Demodulator 

AB102 SSB Modulator and Demodulator 

AB106 FM Modulator and Demodulator 

AB110 Log and Antilog Amplifier 

AB111 Crystal Oscillator (1 MHz) 

AB112 Peak Detector 

AB113 Voltage Follower & Precision Rectifier 

AB115 Op-Amp Oscillator (Sine / Cosine) 

AB116 Sample and Hold Circuit 

………… and many more 



THEORY 

Bridges are among the most accurate types of measuring devices used in the 

measurement of impedance. In addition, bridges are also used to measure 

DC resistance, capacitance, and inductance. Certain types of bridges are 

more suitable for measuring a specific characteristic, such as capacitance or 

inductance. 

Wheatstone bridge : A very important device used in the measurement of 

medium resistances is the Wheatstone bridge. Wheatstone bridge has-been 

in use longer than almost any electrical measuring instrument. It is still an 

accurate and reliable instrument and is extensively used in industry. The 

Wheatstone bridge is an instrument for making comparison instruments and 

operates upon a null indication principle. This means the indication is 

independent of the calibration of the null indicating instrument or any of its 

characteristics. For this reason, very high degrees of accuracy can be 

achieved using Wheatstone bridge. Accuracy of 0.1% is quite common with 

a Wheatstone bridge as opposed to accuracies of 3% to 5% with ordinary 

ohmmeter for measurement of medium resistances. The Wheatstone bridge 

is well suited also for the measurement of small changes of a resistance and, 

therefore, is also suitable to measure the resistance change in a strain gauge. 

It is commonly known that the strain gauge transforms strain applied to it 

into a proportional change of resistance. It is widely used across industry 

even today. 

The basic circuit of a Wheatstone bridge is shown below. It has four 

resistive arms, consisting of resistances R1, R2, R3 and R4 together with a 

source of emf (a battery) and a null detector, usually a galvanometer G or 

other sensitive current meter. 

Since there is a current in each of the arms abc and adc of the circuit, there 

is a potential drop (IR) in the direction of the current. That is, the point a is 

at a higher potential than the point b and likewise b is at a higher potential 

than the point c. Note therefore, that both the points b and d are at potentials 

lower than that of a and higher than that of c. But the potentials of these two 

points b and d are not necessarily equal. 

Consider the arm bd containing the galvanometer G. The current in this arm 

would be from b to d if b is at a higher potential than d. It would be from d 

to b if d is at a higher potential than b. 

In the event that the potential of the point b is exactly equal to that of the 

point d, there will be no current in the arm bd and the galvanometer would 



indicate this lack of current. In this particular case, the Wheatstone bridge is 

said to be balanced. 

This condition is obtained only if the ratio of the resistances R1 and R2, 

happens to be equal to the ratio of the resistances R3 and R4. 

Thus whenever a circuit is connected as in the figure above, one of the four 

resistances R1, R2, R3 or R4 being unknown, and at least one of the other 

three resistances being a variable resistance, the required condition of 

R1 / R2 = R3 / R4 …….………………Eq. 1 

can be attained by altering the value of the variable resistor. Once the 

galvanometer indicates a zero deflection, if the values of the other 

resistances are known then the value of one unknown resistance can be 

calculated from the above condition. 

From Kirchhoff's first law applied to the point b we have, 

Similarly, for the point d, we have, 

Applying Kirchhoff's second law to the network abd, 

Likewise, for the network bcd, we have, 

Fig. 1 

I1 = IG + I2.............................................. Eq. 2 

I4 = IG + I3.............................................. Eq. 3 

I1R1 + IGRG - I3R3 = 0........................................... Eq. 4 

I4R4 + IGRG - I2R2 = 0........................................... Eq. 5 



If the bridge is balanced, then we have, 

If IG = 0 

then, I1 = I2 

and I3 = I4 

If IG = 0 

Then eq 4. states 

Similarly, from circuit bcd, we have, 

Dividing eq 6 by eq 7, we get 

I1R1 = I3R3 …………………………….. Eq. 6 

I2R2 = I4R4……………………..……… Eq. 7 

I1R1 / I2R2 = I3R3 / I4R4 

This is called the balance condition of the bridge. If this condition is 

satisfied then the galvanometer gives no deflection. 

Sensitivity of Wheatstone bridge : 

It is frequently desirable to know the galvanometer response to be expected 

in a bridge which is slightly unbalanced so that a current flow in the 

galvanometer branch of the bridge network. This may be used for: 

1. Selecting a galvanometer with which a given unbalance may be 

observed in a specified bridge arrangement, 

2. Determining the minimum unbalance which can be observed with a 

given galvanometer in the specified bridge arrangement, and 

3. Determining the deflection to be expected for a given unbalance. 

The sensitivity to unbalance can be computed by solving the bridge circuit 

for a small unbalance. Assume that the bridge is balanced when the branch 

resistances are R1, R2, R3 and R4 so that R1 / R2 = R3 / R4. Suppose the 

resistance R2 is changed to R2 + ∆ R creating an unbalance. This will to 

cause an emf (e) to appear across the galvanometer branch. With 

galvanometer branch open, the voltage drop between points b and c is 

Ebc = I2 (R2 + ∆ R) = E (R2 + ∆ R)/ (R1+R2 + ∆ R), 

Where 

E = emf of battery 

Similarly, 

Ecd=I4 (R4) =E R4 / (R3+ R4) 



Therefore the voltage difference between points b and d is: 

For balanced condition: 

e = Ebc - Ecd 

= E [(R2 + ∆ R)/ (R1+R2 + ∆ R) - R4 / (R3+ R4)]. ……. Eq. 8 

I1R1= I3 R4 and I2 R2 = I4 R4 ……………… Eq. 9 

For galvanometer current to be zero, the following condition also exist: 

Combining the equation we obtain: 

Substituting the values in eq(8) 

I1= I2=E/ (R1+R2) ……………… …… Eq. 10 

I3=I4=E/ (R3+R4) ………………..……. Eq. 11 

R2 / ( R1+R2)= R4/( R3+R4 ) 

e = E [(R2 + ∆ R)/ (R1+R2 + ∆ R) - R2 / ( R1+R2)] 

as ∆ R( R1+R2)


Ro= { R1R3/ (R1 + R3)+R2R4 /(R2 +R4)} 

G resistance of the galvanometer circuit 

Si is the current sensitivity of the galvanometer. 

Objective : 

EXPERIMENT 1 

To study the operation of Wheatstone bridge and measuring the value 

of unknown resistance. 

Apparatus required : 

1. Analog board, AB10. 

2. DC power supply +5V from external source or ST2612 Analog 

Lab. 

3. Galvanometer. 

4. Multimeter. 

5. 2mm patch chords. 

Circuit diagram : 

Circuit used to study the operation of Wheatstone Bridge is shown below : 



Procedure : 

Fig. 2 

1. Connect +5V variable dc power supplies at their indicated position 

from external source or ST2612 Analog Lab. 

2. Connect either socket ‘a’ or ‘b’ to c socket with 2mm patch cord. 

3. Connect either socket ‘g’ or ‘h’ to f socket. with 2mm patch cord. 

4. Connect patch cord between socket ‘d’ and ‘e’. 

5. Connect positive terminal of galvanometer to ‘i’ socket and negative 

terminal of galvanometer to ‘j’ socket. 

6. Set R2 at some fixed value of resistance. 

7. Vary R unknown till the galvanometer gives the null deflection for 

accurate null deflection fine tuning pot is given on the board. 

8. Measure the value of resistance R1, R2 and R3 with the help of 

multimeter. 

9. Calculate the value of R UNKNOWN as shown in Eq1. 

10. Measure the value of R UNKNOWN with help of multimeter and verify 

that the value of R UNKNOWN as calculated by the Eq.1 and measured 

by multimeter are same. 

11. Repeat the process for different values of resistance R1, R2 and R3. 

Result : 

The value of R UNKNOWN resistance is = ………………….. 



Objective : 

EXPERIMENT 2 

To measure the sensitivity of Wheatstone bridge. 

Apparatus required : 

1. Analog board AB10. 

2. DC power supply +5V from external source or ST2612 Analog Lab. 

3. Galvanometer. 

4. Multimeter. 

5. 2mm patch chords. 

Circuit diagram : 

Circuit used to study the sensitivity of Wheatstone Bridge is shown below: 

Fig. 3 



Procedure : 

1. Connect +5V variable dc power supplies at their indicated position 

from external source or ST2612 Analog Lab. 

2. Connect either socket ‘a’ or ‘b’ to ‘c’ socket with 2mm patch cord. 

3. Connect either socket ‘g’ or ‘h’ to ‘f’ socket with 2mm patch cord. 

4. Connect patch cord between socket ‘d’ and ‘e’. 

5. Connect positive terminal of galvanometer to ‘i’ socket and negative 

terminal of galvanometer to ‘j’ socket. 

6. Set R2 at some fixed value of resistance. 

7. Vary RUNKNOWN till the galvanometer gives the null deflection for 

accurate null deflection fine tuning pot is given on the board. 

8. Measure the value of resistance R1, R2 and R3 with the help of 

multimeter. 

9. Calculate the value of R unknown as shown in eq1. 

10. Now slightly vary R2 and measure R2 with help of multimeter, 

calculate the change in resistance R2 . 

11. Measure the voltage between socket ‘i’ and ‘j’ with multimeter. 

12. Calculate the sensitivity of bridge by substituting the value in Eq.13 

Result : 

Sensitivity of Wheatstone bridge (SB ) = ……………………………… 



WARRANTY 

1) We guarantee the instrument against all manufacturing defects during 

24 months from the date of sale by us or through our dealers. 

2) The guarantee covers manufacturing defects in respect of indigenous 

components and material limited to the warranty extended to us by the 

original manufacturer, and defect will be rectified as far as lies within 

our control. 

3) The guarantee will become INVALID. 

a) If the instrument is not operated as per instruction given in the 

instruction manual. 

b) If the agreed payment terms and other conditions of sale are not 

followed. 

c) If the customer resells the instrument to another party. 

d) Provided no attempt have been made to service and modify the 

instrument. 

4) The non-working of the instrument is to be communicated to us 

immediately giving full details of the complaints and defects noticed 

specifically mentioning the type and sr. no. of the instrument, date of 

purchase etc. 

5) The repair work will be carried out, provided the instrument is 

dispatched securely packed and insured with the railways. To and fro 

charges will be to the account of the customer. 

DISPATCH PROCEDURE FOR SERVICE 

Should it become necessary to send back the instrument to factory please 

observe the following procedure: 

1) Before dispatching the instrument please write to us giving full details 

of the fault noticed. 

2) After receipt of your letter our repairs dept. will advise you whether it 

is necessary to send the instrument back to us for repairs or the 

adjustment is possible in your premises. 

Dispatch the instrument (only on the receipt of our advice) securely packed 

in original packing duly insured and freight paid along with accessories and 

a copy of the details noticed to us at our factory address. 



LIST OF SERVICE CENTERS 

1. Scientech Technologies Pvt. Ltd. 

90, Electronic Complex Ph: (0731) 5202959 

Pardesipura, Email: info@scientech.bz 

INDORE – 452010 

2. Scientech Technologies Pvt. Ltd. Ph.: (011) 26513912, 26864943 

First Floor, 14, Uday Park, Fax: (011) 26864943. 

NEW DELHI – 110049 Email: ndel@scientech.bz 


New no.2, Old no.10, 4 th street Ph.: (044) 42187548, 42187549 

Venkateswara nagar, Adyar Fax: (044) 42187549 

CHENNAI – 600025 Email: chennai@scientech.bz 


202/19, 4 th main street Ph.: (080) 51285011 

Ganganagar, Fax: (080) 51285022 

BANGALORE- 560032 Email: bangalore@scientech.bz 

5. Scientech Technologies Pvt. Ltd. Ph.: (022) 56299457 

8,1st floor, 123-Hariram Mansion, Fax: (022) 24168767 

Dada Saheb Phalke road, Email: stplmum@scientech.bz 

Dadar (East), MUMBAI –400014 


988, Sadashiv Peth, Ph.: (020) 24461673 

Gyan Prabodhini Lane, Fax: (020) 24482403 

PUNE – 411030 Email: pune@scientech.bz 

7. Scientech Technologies Pvt. Ltd 

SPS Apartment, 1 st Floor Ph.: +913355266800 

2, Ahmed Mamoji Street, Email: kolkata@scientech.bz 

Behind Jaiswal Hospital, 

Liluah, HOWRAH-711204 W.B. 

8. Scientech Technologies Pvt. Ltd Ph.: (040) 55465643 

Flat No. 205, 2 nd Floor, Email: hyd@scientech.bz 

Lakshminarayana Apartments 

‘C’ wing, Street No. 17, 

Himaytnagar, 

HYDERABAD- 500029 



LIST OF ACCESSORIES 

1. 2mm patch cord (red)................................................................. 1 No. 

2. 2mm patch cord (blue)............................................................. 3 Nos. 

3. 2mm patch cord (black).......................................................... 1 No.

AB10 WHEATSTONE BRIDGE Analog Lab Experiment Board Ver ...

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