AB10 WHEATSTONE BRIDGE - Specjalizowane … · AB10 WHEATSTONE BRIDGE Analog Lab Experiment Board...

AB10WHEATSTONE BRIDGE

Analog LabExperiment 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.

AB10

Tel.: 91-731-2570301 Fax: 91-731-2555643Email: [email protected] Web: www.scientech.bz

Scientech Technologies Pvt. Ltd. 2

http://www.scientech.bz/

mailto:[email protected]

AB10


AB10

WHEATSTONE BRIDGE

AB10

TABLE OF CONTENTS

1. Introduction 4

2. Theory 6

3. Experiment 1 10To study the operation of Wheatstone bridge and measuring the value of unknown resistance.

4. Experiment 2 12To measure the sensitivity of Wheatstone bridge.

5. Warranty 14

6. List of service Centers 15

7. List of Accessories with AB10 16


AB10

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 characteristicsAB09 Rectifier CircuitsAB11 Maxwell’s BridgeAB12 De Sauty’s BridgeAB13 Schering BridgeAB14 Darlington PairAB15 Common Emitter AmplifierAB16 Common Collector AmplifierAB17 Common Base AmplifierAB18 Cascode AmplifierAB19 RC-Coupled AmplifierAB20 Direct Coupled AmplifierAB21 Class A AmplifierAB22 Class B Amplifier (push pull emitter follower)AB23 Class C Tuned AmplifierAB28 Multivibrator ( Mono stable / Astable)AB29 F-V and V-F ConverterAB30 V-I and I-V ConverterAB31 Zener Voltage Regulator AB32 Transistor Series Voltage RegulatorAB33 Transistor Shunt Voltage RegulatorAB35 DC AmmeterAB39 Instrumentation AmplifierAB41 Differential Amplifier (Transistorized)


AB10

AB42 Operational Amplifier (Inverting / Non-inverting / Differentiator)

AB43 Operational Amplifier (Adder/Scalar)AB44 Operational Amplifier (Integrator/ Differentiator)AB45 Schmitt Trigger and ComparatorAB51 Active filters (Low Pass and High Pass)AB52 Active Band Pass Filter AB53 Notch Filter AB54 Tschebyscheff Filter AB56 Fiber Optic Analog LinkAB57 Owen’s BridgeAB58 Anderson’s BridgeAB59 Maxwell’s Inductance BridgeAB64 RC – Coupled Amplifier with FeedbackAB65 Phase Shift OscillatorAB66 Wien Bridge OscillatorsAB67 Colpitt OscillatorAB68 Hartley OscillatorAB80 RLC Series and RLC Parallel Resonance AB82 Thevenin’s and Maximum power Transfer TheoremAB83 Reciprocity and Superposition TheoremAB84 Tellegen’s TheoremAB85 Norton’s theoremAB88 Diode ClipperAB89 Diode ClampersAB90 Two port network parameterAB91 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 DemodulatorAB102 SSB Modulator and DemodulatorAB106 FM Modulator and DemodulatorAB110 Log and Antilog AmplifierAB111 Crystal Oscillator (1 MHz)AB112 Peak DetectorAB113 Voltage Follower & Precision RectifierAB115 Op-Amp Oscillator (Sine / Cosine)AB116 Sample and Hold Circuit

………… and many more


AB10

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


AB10

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.

Fig. 1

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

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

Similarly, for the point d, we have,

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

Applying Kirchhoff's second law to the network abd,

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

Likewise, for the network bcd, we have,

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


AB10

If the bridge is balanced, then we have,

If IG = 0then, I1 = I2

and I3 = I4 If IG = 0Then eq 4. states

I1R1 = I3R3 …………………………….. Eq. 6Similarly, from circuit bcd, we have,

I2R2 = I4R4……………………..……… Eq. 7Dividing eq 6 by eq 7, we get

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)


AB10

Therefore the voltage difference between points b and d is:e = Ebc - Ecd

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

I1R1= I3 R4 and I2 R2 = I4 R4 ……………… Eq. 9For galvanometer current to be zero, the following condition also exist:

I1= I2=E/ (R1+R2) ……………… …… Eq. 10I3=I4=E/ (R3+R4) ………………..……. Eq. 11

Combining the equation we obtain:R2 / ( R1+R2)= R4/( R3+R4 )

Substituting the values in eq(8)e = E [(R2 + ∆ R)/ (R1+R2 + ∆ R) - R2 / ( R1+R2)]

= E [R1∆ R/{( R1+R2)2+∆ R( R1+R2)}]as ∆ R( R1+R2)<< ( R1+R2)2

=E [R1∆ R/( R1+R2)2] ………….……… Eq. 12Let Sv be the voltage sensitivity of galvanometer.Therefore, deflection of galvanometer is :

θ= Sv e= Sv E [R1∆ R/( R1+R2)2]The bridge sensitivity SB is defined as the deflection of the galvanometer per unit fractional change in unknown resistance.Bridge sensitivity SB = θ/ (∆ R /R2)

= Sv E [R1/ (R1+R2)2] ………….………. Eq. 13The sensitivity of the bridge is dependent upon bridge voltage, bridge parameters and the voltage sensitivity of the galvanometer. Sensitivity is maximum for bridges with equal arms.Note :

If current sensitivity is given instead of voltage sensitivity, voltage sensitivity can be calculated by the following formula:

Sv = Si /(Ro+G)

Where Ro is the thevenin equivalent of the bridge


AB10

Ro= { R1R3/ (R1 + R3)+R2R4 /(R2 +R4)}G resistance of the galvanometer circuitSi is the current sensitivity of the galvanometer.

EXPERIMENT 1Objective : 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 :


AB10

Fig. 2Procedure :

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 = …………………..


AB10

EXPERIMENT 2

Objective :

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


AB10

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 ) = ………………………………


AB10

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.


AB10

LIST OF SERVICE CENTERS

1. Scientech Technologies Pvt. Ltd.90, Electronic Complex Ph: (0731) 5202959Pardesipura, Email: [email protected] INDORE – 452010

2. Scientech Technologies Pvt. Ltd. Ph.: (011) 26513912, 26864943First Floor, 14, Uday Park, Fax: (011) 26864943.NEW DELHI – 110049 Email: [email protected]

3. Scientech Technologies Pvt. Ltd.New no.2, Old no.10, 4th street Ph.: (044) 42187548, 42187549

Venkateswara nagar, Adyar Fax: (044) 42187549CHENNAI – 600025 Email: [email protected]

4. Scientech Technologies Pvt. Ltd. 202/19, 4th main street Ph.: (080) 51285011 Ganganagar, Fax: (080) 51285022 BANGALORE- 560032 Email: [email protected]. Scientech Technologies Pvt. Ltd. Ph.: (022) 56299457 8,1st floor, 123-Hariram Mansion, Fax: (022) 24168767 Dada Saheb Phalke road, Email: [email protected] Dadar (East), MUMBAI –400014 6. Scientech Technologies Pvt. Ltd. 988, Sadashiv Peth, Ph.: (020) 24461673 Gyan Prabodhini Lane, Fax: (020) 24482403 PUNE – 411030 Email: [email protected]. Scientech Technologies Pvt. Ltd SPS Apartment, 1st Floor Ph.: +913355266800 2, Ahmed Mamoji Street, Email: [email protected]

Behind Jaiswal Hospital, Liluah, HOWRAH-711204 W.B.8. Scientech Technologies Pvt. Ltd Ph.: (040) 55465643

Flat No. 205, 2nd Floor, Email: [email protected] Apartments‘C’ wing, Street No. 17,Himaytnagar, HYDERABAD- 500029


AB10

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 - Specjalizowane … · AB10 WHEATSTONE BRIDGE Analog Lab Experiment Board...

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