i&m lab 4 handout

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Instrumentation & Measurement Lab (EE-304L) Page 1

International Islamic University,

IslamabadInstrumentation & Measurement LAB

EXPERIMENT # 04: Introduction to Galvanometers (DC Voltmeter)

Name of Student: ..

Roll No.:

Date of Experiment: ..

Report submitted on: ..

Marks obtained:

Remarks:

Instructors Signature: ...


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Introduction to Galvanometers (DC Voltmeter)

1. Objective

To design and construct a DC Voltmeter from a normal galvanometer and then analyze the

loading effect of connecting a voltmeter on a simple circuit.

2. Equipment RequiredDMM

DC Power Supply

Bread Board

Galvanometer

Wires

Resistors: as needed

3. Introduction

Galvanometer is an instrument used to indicate the presence, direction, or strength of a small

electric current. The typical galvanometer is a sensitive laboratory instrument used mainly to

detect and compare currents. It is used for DC measurements and is one of the earliest measuring

instrument mechanisms.

3.1 Specifications of a Galvanometer

Galvanometers are specified with certain full scale deflection Ifsd (e.g. 1mA) and internal

resistance Rg(e.g. 100).
http://www.bing.com/images/search?q=Galvanometer&view=detail&id=9CBE0B5A29282E8ECD69759E4DF309CC2CCAFECA&FORM=IDFRIR


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3.2 Voltmeter

Galvanometers can be turned into voltmeters if they are connected with a suitably high series

resistance. The purpose of the series resistance is to limit the current through the meter and

extend the range to measure voltage.

3.3 Multi-Range Voltmeter

The addition of a number of multipliers, together with a range switch, provides the instrument

with a workable number of voltage ranges. For a four-range voltmeter, we find the resistances as:


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3.4 Loading Effect

Each voltmeter has a certain input impedance. When a voltmeter is used to measure the voltage

across a circuit component, the voltmeter circuit itself is in parallel with the circuit component.

Total resistance will decrease, so the voltage across component will also decrease. This is called

voltmeter loading. The resulting error is called a loading error. The voltmeter loading can be

reduced by using a high sensitivity voltmeter.

4. Procedure

1. First of all, find the parameters of galvanometer. Use DMM to find Rg and then connectthe galvanometer in series with a potentiometer and power supply. Change the value of

potentiometer (or the input supply) slowly until you get full deflection on the

galvanometer. Measure the input voltage and potentiometer resistance and put in the

following equation to find Ifsd.

2. For the ranges of 10V, 20V, 50V and 100V for given galvanometer, calculate and notedown multiplier resistors R1, R2, R3 and R4. Remember as the value of multiplierresistor increases, less current flows through the galvanometer and therefore the range is

increased.

3. Using the closest values of the resistor, construct the voltmeter.4. Generate test voltages at about 80% of the ranges using suitable supply.5. Take the readings using the constructed voltmeter and then with DMM for comparison.


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6. Construct the above circuit. Take those values of R1 and R2 that are comparable to themultiplier resistances. First measure using the DMM (which has higher input impedance)

and then measure using the constructed voltmeter. Record the values in the table.

5. Observations

Test Data

Sr No. Range - 4 Range - 3 Range - 2 Range - 1

1. V (Range)

2. V (Applied)

3. V (Read)

Input Data

Sr No. Parameter Nominal Value Measured Value

1. Vin

2. R1

3. R2


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Output Data

Sr No. Parameter Equation

1. Vout

2. R2 || RV

3. VoutLoaded

4. %age Error

Sr No. Parameter Unloaded Load - DMM LoadVoltmeter

(constructed)

1. Vout (Calc)

2. Vout (Read)

3. %age Error

i&m lab 4 handout

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