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.
AB10
Tel.: 91-731-2570301 Fax: 91-731-2555643
Email: info@scientech.bz Web: www.scientech.bz
Scientech Technologies Pvt. Ltd.
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AB10
Scientech Technologies Pvt. Ltd.
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AB10
WHEATSTONE BRIDGE
AB10
TABLE OF CONTENTS
1.
Introduction
4
2.
Theory
6
3.
Experiment 1
To study the operation of Wheatstone bridge and
measuring the value of unknown resistance.
10
4.
Experiment 2
To measure the sensitivity of Wheatstone bridge.
12
5.
Warranty
14
6.
List of service Centers
15
7.
List of Accessories with AB10
16
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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
AB02
AB03
AB04
AB05
AB06
AB07
AB08
AB09
AB11
AB12
AB13
AB14
AB15
AB16
AB17
AB18
AB19
AB20
AB21
AB22
AB23
AB28
AB29
AB30
AB31
AB32
AB33
AB35
AB39
AB41
Diode characteristics (Si, Zener, LED)
Transistor characteristics (CB NPN)
Transistor characteristics (CB PNP)
Transistor characteristics (CE NPN)
Transistor characteristics (CE PNP)
Transistor characteristics (CC NPN)
Transistor characteristics (CC PNP)
FET characteristics
Rectifier Circuits
Maxwell’s Bridge
De Sauty’s Bridge
Schering Bridge
Darlington Pair
Common Emitter Amplifier
Common Collector Amplifier
Common Base Amplifier
Cascode Amplifier
RC-Coupled Amplifier
Direct Coupled Amplifier
Class A Amplifier
Class B Amplifier (push pull emitter follower)
Class C Tuned Amplifier
Multivibrator ( Mono stable / Astable)
F-V and V-F Converter
V-I and I-V Converter
Zener Voltage Regulator
Transistor Series Voltage Regulator
Transistor Shunt Voltage Regulator
DC Ammeter
Instrumentation Amplifier
Differential Amplifier (Transistorized)
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AB10
AB42
AB43
AB44
AB45
AB51
AB52
AB53
AB54
AB56
AB57
AB58
AB59
AB64
AB65
AB66
AB67
AB68
AB80
AB82
AB83
AB84
AB85
AB88
AB89
AB90
AB91
AB92
AB93
AB96
AB97
AB101
AB102
AB106
AB110
AB111
AB112
AB113
AB115
AB116
Operational Amplifier (Inverting / Non-inverting /
Differentiator)
Operational Amplifier (Adder/Scalar)
Operational Amplifier (Integrator/ Differentiator)
Schmitt Trigger and Comparator
Active filters (Low Pass and High Pass)
Active Band Pass Filter
Notch Filter
Tschebyscheff Filter
Fiber Optic Analog Link
Owen’s Bridge
Anderson’s Bridge
Maxwell’s Inductance Bridge
RC – Coupled Amplifier with Feedback
Phase Shift Oscillator
Wien Bridge Oscillators
Colpitt Oscillator
Hartley Oscillator
RLC Series and RLC Parallel Resonance
Thevenin’s and Maximum power Transfer Theorem
Reciprocity and Superposition Theorem
Tellegen’s Theorem
Norton’s theorem
Diode Clipper
Diode Clampers
Two port network parameter
Optical Transducer (Photovoltaic cell)
Optical Transducer (Photoconductive cell/LDR)
Optical Transducer (Phototransistor)
Temperature Transducer (RTD & IC335)
Temperature Transducer (Thermocouple)
DSB Modulator and Demodulator
SSB Modulator and Demodulator
FM Modulator and Demodulator
Log and Antilog Amplifier
Crystal Oscillator (1 MHz)
Peak Detector
Voltage Follower & Precision Rectifier
Op-Amp Oscillator (Sine / Cosine)
Sample and Hold Circuit
………… and many more
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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
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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
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If the bridge is balanced, then we have,
If IG = 0
then, I1 = I2
and I3 = I4
If IG = 0
Then eq 4. states
I1R1 = I3R3 …………………………….. Eq. 6
Similarly, from circuit bcd, we have,
I2R2 = I4R4……………………..……… Eq. 7
Dividing 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)
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Therefore the voltage difference between points b and d is:
e = Ebc - Ecd
= E [(R2 + ∆ R)/ (R1+R2 + ∆ R) - R4 / (R3+ R4)]. ……. Eq. 8
For balanced condition:
I1R1= I3 R4 and I2 R2 = I4 R4 ……………… Eq. 9
For galvanometer current to be zero, the following condition also exist:
I1= I2=E/ (R1+R2) ……………… …… Eq. 10
I3=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. 12
Let 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. 13
The 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
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Ro= { R1R3/ (R1 + R3)+R2R4 /(R2 +R4)}
G resistance of the galvanometer circuit
Si is the current sensitivity of the galvanometer.
EXPERIMENT 1
Objective :
To study the operation of Wheatstone bridge and measuring the value
of unknown resistance.
Apparatus required :
1.
2.
Analog board, AB10.
DC power supply +5V from external source or ST2612 Analog
Lab.
3.
4.
Galvanometer.
5.
2mm patch chords.
Multimeter.
Circuit diagram :
Circuit used to study the operation of Wheatstone Bridge is shown below :
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AB10
Fig. 2
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 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 = …………………..
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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
Scientech Technologies Pvt. Ltd.
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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 ) = ………………………………
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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.
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AB10
LIST OF SERVICE CENTERS
1. Scientech Technologies Pvt. Ltd.
90, Electronic Complex
Pardesipura,
INDORE – 452010
2. Scientech Technologies Pvt. Ltd.
First Floor, 14, Uday Park,
NEW DELHI – 110049
3. Scientech Technologies Pvt. Ltd.
New no.2, Old no.10, 4th street
Venkateswara nagar, Adyar
CHENNAI – 600025
4. Scientech Technologies Pvt. Ltd.
202/19, 4th main street
Ganganagar,
BANGALORE- 560032
5. Scientech Technologies Pvt. Ltd.
8,1st floor, 123-Hariram Mansion,
Dada Saheb Phalke road,
Dadar (East), MUMBAI –400014
6. Scientech Technologies Pvt. Ltd.
988, Sadashiv Peth,
Gyan Prabodhini Lane,
PUNE – 411030
7. Scientech Technologies Pvt. Ltd
SPS Apartment, 1st Floor
2, Ahmed Mamoji Street,
Behind Jaiswal Hospital,
Liluah, HOWRAH-711204 W.B.
8. Scientech Technologies Pvt. Ltd
Flat No. 205, 2nd Floor,
Lakshminarayana Apartments
‘C’ wing, Street No. 17,
Himaytnagar,
HYDERABAD- 500029
Scientech Technologies Pvt. Ltd.
Ph: (0731) 5202959
Email: info@scientech.bz
Ph.: (011) 26513912, 26864943
Fax: (011) 26864943.
Email: ndel@scientech.bz
Ph.: (044) 42187548, 42187549
Fax: (044) 42187549
Email: chennai@scientech.bz
Ph.: (080) 51285011
Fax: (080) 51285022
Email: bangalore@scientech.bz
Ph.: (022) 56299457
Fax: (022) 24168767
Email: stplmum@scientech.bz
Ph.: (020) 24461673
Fax: (020) 24482403
Email: pune@scientech.bz
Ph.: +913355266800
Email: kolkata@scientech.bz
Ph.: (040) 55465643
Email: hyd@scientech.bz
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LIST OF ACCESSORIES
1.
2mm patch cord (red)................................................................. 1 No.
2.
2mm patch cord (blue)............................................................. 3 Nos.
3.
2mm patch cord (black).......................................................... 1 No.
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