Measurement by Wheatstone Bridge

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INSTRUMENTATION

In this experiments you will learn about basic principles of Wheatstone bridge,

operational amplifier and strain gauge.Related theory is given below, in three parts

ASSIGNMENT-I : THE BASIC WHEATSTONE BRIDGE

CONTENT:The basic behaviour of a Wheatstone Bridge is introduced. The advantages of bridge

measurement techniques over simple Ohms Law methods are investigated.

EQUIPMENT REQUIRED:

Qty Designation Description

1 TK2941M Measurements Package

1 - Power Supply, 15V dc

(eg Feedback PS 446)1 - Decade Resistance 1to 100k

1 - * DC Voltmeter 15V

1 - * Center zero dc milliammeter

1 - Resistor - 100, 1W

1 - Resistor 1k,1/8W

1 - Resistor 10k,1/8W

PRACTICALS:

2.1 The basic Wheatstone Bridge

OBJECTIVES:When you have completed this assignment you will:

1. Know the principle of operation of the basic Wheatstone Bridge.

2. Know how to measure resistance using a Wheatstone Bridge.KNOWLEDGE LEVEL:

Before starting this assignment you should:

1. Understand the theory and application of Ohms Law.2. Be familiar with the operation of series/parallel dc circuits and potential divider

circuits.

INTRODUCTION:The Wheatstone Bridge which was not direct reading:

A method of determining resistance has many sources of error. A direct reading method

with few errors would be of great advantage.A way of determining resistance value which only requires one meter is shown in the

circuit of fig 4.2.1.

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V

Vsknown

voltage

+

-

-

+ Rx

unknown

Rsstandard

resistor

Fig 4.2.1 Basic one meter measurement

Here, the unknown resistance, Rx is used in a potential divider circuit with a knownstandard resistor Rsconnected across a known source voltage Vs.

By the potential divider formula:

s

xs

x VRR

RV

+=

therefore ss

x RVV

VR

=

This circuit suffers from some disadvantages.Obviously Rs, the standard resistor, must be known precisely.

The voltmeter V must have a resistance very much greater than Rxfor accurate results.

The method does not lead to direct reading of the result.

When all the circuit values are known precisely, the final accuracy still dependsultimately on the accuracy of the meter indication.

This draw back can be over come by Wheatstone bridge circuit shown in following fig.

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Fig 4.2.5 Circuit of Wheatstone Bridge

Now, let us investigate circuit. Set up the module as shown in fig 4.2.6. this correspondsto the circuit of fig 4.2.5.

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ASSIGNMENT -II OPERATIONAL AMPLIFIER

CONTENT:

The operation of the Operational Amplifier is investigated.

EQUIPMENT REQUIRED:Qty Designation Description

1 TK2941M Measurements Package

1 - Power Supply, 15V dc(eg Feedback PS 446)

2 - DC Voltmeter 15V

PRACTICALS:5.1 The basic Op Amp5.2 Common Mode Gain

OBJECTIVES:When you have completed this assignment you will:

1. Understand the operation of the basic operational amplifier.

2. Know how to connect up an operational amplifier to act as a voltage amplifier.3. Understand the terms differential gain and common mode gain.

KNOWLEDGE LEVEL:

Before starting this assignment you should:1. Understand the theory operation of the potential divider circuit.

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INTRODUCTION:

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Connect the circuit as shown in fig 4.5.4

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PRACTICAL ASPECTS

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ASSIGNMENT-III :STRAIN GAUGEES

CONTENT:The application of variable length transducer principle to strain gauge

tranducers is investigated

EQUIPMENT REQUIRED:Qty Designation Description

1 TK2941A Instrumentation Module

1 TK294 Linear Tranducer TestRig

1 TK294E Strain Gauge Sub-unit

1 - Power Supply 15V dc

1 - DC Voltmeter 15V

PRACTICALS:

Basic Strain Gauge

OBJECTIVES:Know how the change in resistance of material,caused by a change in its

physical dimension,can be used to measure the strain in the material.

INTRODUCTION

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Connect the circuit as shown in fig 4.10.3

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Measurement by Wheatstone Bridge

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