Design And Construction Of An Analogue Multimeter

ABSTRACT

This work is on an analogue multimeter. A multimeter is a tool basically used to measure current, voltage and resistance. An analog multimeter is the most commonly used instrument by technicians and engineers in the laboratory as well as other repair works. As it is clear from the name of this instrument, it can make many (multi) measurements with reasonable accuracy such as AC and DC voltages, currents, and resistances. The aim of this work is to build a hand-held device that uses moving pointer to display readings useful for basic fault finding and field service work, or a bench instrument which can measure to a very high degree of accuracy.

TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWELDGEMENT

ABSTRACT

CHAPTER ONE

1.0     INTRODUCTION

1.1     BACKGROUND OF THE PROJECT

• PROBLEM STATEMENT
• AIM AND OBJECTIVE OF THE PROJECT
• SIGNIFICANCE OF THE PROJECT
• APPLICATION OF THE PROJECT
• BENEFIT OF THE PROJECT
• PROJECT ORGANISATION

CHAPTER TWO

LITERATURE REVIEW

• OVERVIEW OF AN ANALOG MULTIMETERS
• HISTORICAL BACKGROUND OF THE STUDY
• GENERAL PROPERTIES OF MULTIMETERS
• OPERATIONAL REVIEW OF A MULTIMETER
• ANALOGUE MULTIMETER VARIOUS MEASUREMENT REVIEW

CHAPTER THREE

3.0     CONSTRUCTION METHODOLOGY

3.1     BLOCK DIAGRAM OF THE SYSTEM

3.2      CIRCUIT DIAGRAM

3.3      CIRCUIT OPERATION AND DESCRIPTION

3.5      PARTS LIST

CHAPTER FOUR

4.0     TESTING AND RESULTS

• CONSTRUCTION PROCEDURE AND TESTING
• CASING
• PACKAGING
• MOUNTING PROCEDURE
• TESTING
• RESULT ANALYSIS
• MEASUREMENT SAFETY

CHAPTER FIVE

• CONCLUSION
• RECOMMENDATION
• REFERENCES

CHAPTER ONE

• INTRODUCTION

Multimeters as the name suggest the meters that we use to measure multiple quantities with the same instrument. The most basic multimeter measures voltage, current, and resistance. Since we use it for measuring current (A), voltage (V) and resistance (Ohm), we call it as AVO meter. We can categorise the multimeters into two groups, namely analog multimeter and digital multimeter. This work discusses about analog multimeter [3].

1.1                                           BACKGROUND OF THE STUDY

A multimeter is also known as a multitester, or a VOM (volt-ohm-milliammeter), is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter can measure voltage, current, and resistance. There are two two types of multimeter: analogue and digital multimeter. Analog multimeters uses a microammeter with a moving pointer to display readings while Digital multimeters (DMM, DVOM) have a numeric display, and may also show a graphical bar representing the measured value. Digital multimeters are now far more common due to their cost and precision, but analog multimeters are still preferable in some cases, for example when monitoring a rapidly varying value [2,3].

In this work we are focusing on analogue type of multimeter. An analogue multimeter is one of the trusty workhorses of the electronics test industry. Analogue multimeters have been in use for very many years and sometimes go by the name VOA as a result of the fact that they measure volts, ohms and amps. These multimeters are extremely flexible and enable very many faults to be found in an electronic circuit [1].

Analog multimeter was first of its type, but due to latest technological development after development of digital multimeters, nowadays it is of less use. However, despite such advancements, it is still essential, and we can’t neglect it. An analog multimeter is a PMMC meter. It works based on the d’Arsonval galvanometer principle. It consists a needle to indicate the measured value on the scale. A coil moves in a magnetic field when current passes through it. The indicating needle is fastened to the coil [4]. During the flow of current through the coil, a deflecting torque gets produced due to which the coil rotates at some angle, and the pointer moves over a graduated scale. A pair of hairsprings is attached to the moving spindle to provide the controlling torque. In a multimeter, the galvanometer is a left- zero-type instrument, i.e. needle rests to the extreme left of the scale from where the scale begins with zero [3].

The meter acts as an ammeter with a low series resistance to measure direct current. For measuring high current, we connect a shunt resistor across the galvanometer so that the current through the galvanometer does not cross its maximum allowed value. Here, a significant portion of the current to be measured bypasses through the shunt. With that shunt resistance, an analog multimeter can measure even milli-ammeter or ammeter ranges of current [2].

For DC voltage measurement, the primary instrument becomes a DC voltage measuring apparatus or DC voltmeter. By adding a multiplier resistance, an analog multimeter can measure the voltage from milli-volts to kilovolts, and this meter works as a millivoltmeter, a voltmeter or even as a kilo voltmeter[2].

By adding a battery and a resistance network, this instrument can work as an ohmmeter. We can change the range of the ohmmeter by connecting a switch to a suitable shunt resistance. By selecting different values of shunt resistance, we can obtain different scales of resistance measurement.

Analog multimeters, like digital ones have a variety of ranges. They are described in terms of Full Scale Deflection or FSD. This is the maximum that the range can read. In order to get the best reading, it is necessary to have the scale reading somewhere between about a quarter and all of the FSD. In this way the optimum accuracy and significant number of figures can be read. As a result of this meters have a variety of ranges, that may appear to be reasonably close to each other [2,3].

1.2                                                  PROBLEM STATEMENT

All functioning electronic gadget or electric device operates with electronic parts or electric wiring which can fail at any time due to some abnormalities such as aging, under or over-voltages and intermittent power loss. When an electric or electronic circuit fails, the maintenance personnel needs not to troubleshoot the circuit or circuit’s components just by assumption. Assumption in an electric or electronic circuit can lead to failure or loss of lives by electrocution. In order to avoid this, an analogue multimeter was invented.

1.3                                                   AIM OF THE PROJECT

The main aim of the project is to build a hand-held device that uses moving pointer to display readings useful for basic fault finding and field service work, or a bench instrument which can measure to a very high degree of accuracy.

1.4                                            OBJECTIVES OF THE PROJECT

At the end of this work, the student involves shall be able to build a simple analogue multimeter that is capable of measuring current, voltages and resistance. The device shall be able to measure resistance, current and voltages in the following range:

2. DC Voltage: 2.5V, 10V, 25V, 100V, 250V, 1000V
3. AC voltage: 10V, 25V, 100V, 250V, 1000V

• DC Current: 50µA, 1mA 10mW, 100mA

1. Resistance: R, 100R, 10 000R

1.5                                         SIGNIFICANCE OF THE PROJECT

Analogue meters are older designs, but are still preferred by many engineers and troubleshooters. One reason for this is that analog meters are more sensitive to changes in the circuit that is being measured

Analogue multimeters continuously read the test value. If there are slight changes in readings, the needle of an analog multimeter will track them.

Resistance measurements on an analog meter, in particular, can be of low precision due to the typical resistance measurement circuit which compresses the scale heavily at the higher resistance values. Inexpensive analog meters may have only a single resistance scale, seriously restricting the range of precise measurements. Typically, an analog meter will have a panel adjustment to set the zero-ohms calibration of the meter, to compensate for the varying voltage of the meter battery, and the resistance of the meter’s test leads [4,5,6].

1.6                                           LIMITATION OF THE PROJECT

1. One of the challenges of an analogue multimeter is its sensitivity. This comes about because the meter must draw a certain amount of current from the circuit it is measuring in order for the meter to deflect. Accordingly the meter appears as another resistor placed between the points being measured. The way this is specified is in terms of a certain number of Ohms (or more usually kOhms) per volt. The figure enables the effective resistance to be calculated for any given range [2].

Thus if a multimeter had a sensitivity of 20 kOhms per volt, then on the range having a full scale deflection of 10 volts, it would appear as a resistance of 10 x 20 kohms, i.e. 200 kohms.

When making measurements the resistance of the meter should be at the very least ten times the resistance of the circuit being measured. As a rough guide, this can be taken to be the highest resistor value near where the meter is connected [3][4].

1. It requires manual reading of parameters from the scale which is cumbersome unlike digital multimeter.

• The results are not accurate unlike DMM. The inaccurate results are caused due to three types of errors viz. improper range setting, improper counting on the scale, wrong setting on AC/DC.

1. The meter uses needle which is rotated by magnetic field. It gets damage if meter gets dropped by mistake.
2. Analogue multimeter does not have any digital circuitry. Hence it cannot perform advanced measurements such as frequency, impedance, waveform analysis etc. which are commonly measured by digital multimeter.
3. The pointer movement is slow, can’t be used to measure voltages with frequencies higher than 50 HZ

• Analog meters are bulky in size.
• Inaccurate due to the effect of earth magnetic field.

1. They are vulnerable to shock and vibration.

1.7                                    BENEFITS OF ANALOG MULTIMETER

Following are the benefits or advantages of Analogue Multimeter:

1. They are cheaper compare to digital meters.
2. They respond very quickly to measurement.

• They do not require batteries unless resistance needs to be measured.

1.7                                          APPLICATION OF THE PROJECT

Analogue multimeter is one of the trusty workhorses of the electronics test industry for basic fault finding and field service work which is also used by:

1. Electronics hobbyist
2. Electronics technicians

• Electronics repairers

1. Electronics labs
2. Electricians, etc.

1.8                                                         PROJECT ORGANISATION

The work is organized as follows: chapter one discuses the introductory part of the work, chapter two presents the literature review of the study, chapter three describes the methods applied, chapter four discusses the results of the work, chapter five summarizes the research outcomes and the recommendations.