Grade 11 Learner Book
All rights reserved. No part of this book may be reproduced in any form, electronic, mechanical, photocopying, or otherwise, without prior permission of the copyright owner.
ISBN 978-1-920540-46-3 First published 2012
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Please note that this is a sample draft copy and may still undergo minor changes.
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Chapter 1 – Safety ... 1
Chapter 2 – Tools and measuring instruments ...15
Chapter 3 – Single-phase AC generation ...39
Chapter 4 – Single-phase transformers ...51
Chapter 5 – Protective devices ...71
Chapter 6 – Single-Phase Motors ...89
Chapter 7 – RCL ...101
Chapter 8 – Semiconductors ...139
Chapter 9 – Power supplies ...179
Chapter 10 – Amplifi ers ...201
Chapter 11 – Logic ...225
Chapter 12 – Communication ...261
Icon Description
Key word
Did you know?
Take note
Activity
Safety
Personal safety
Signage
A
B
Ergonomics
Safety and the
OHS Act
Introduction
Workshops and laboratories are places where workers are constantly designing, making, manufacturing or repairing industrial tools or equipment. It is in these places that safety and fi rst aid are of the utmost importance to both the employer and the employee. We must remember that safety is the responsibility of every single person, not just of the employer or factory owner. Many accidents occur in the workplace because of negligence on the part of the worker and, sometimes, the employer. In South Africa about 230 000 serious accidents occur every year. It is the aim of the Occupational Health and Safety Act (OHS Act) to eliminate, prevent or reduce such accidents so as to make our workshops accident-free and safe places to work in. According to the Act, an accident is an unplanned, uncontrolled event caused by unsafe acts and conditions. It is the aim of this chapter to look at general safety and fi rst aid in the workplace and how to make the workshops and laboratories safer for everybody.
Safety and the OHS Act
Safety can generally be described as: the condition of being safe; free from danger, risk, or injury. Working in electrical workshops can be very dangerous as
carelessness and neglect can lead to serious injury or even death. When working with electrical equipment, care must be taken to adhere to all possible safety measures for personal safety and protection. It is commonly agreed that, if unsafe acts and unsafe conditions can be eliminated, 98% of all accidents could be prevented. A workshop is only as safe as the persons operating in it make it. One must ensure proper workshop safety, since ultimately the safety in any workshop is the responsibility of everybody in the workshop, including visitors.
According the OHS Act both the employer and the employee are responsible for safety in the workshop. Th e OHS Act states that: “every employer shall provide and maintain, as far as is reasonably practicable, a working environment that is safe and without risk to the health of his employees.”
To maintain safe working conditions in the workplace, the employer must ensure he does the following:
• Provide and maintain systems of work, plant and machinery that are safe and without risks to health.
• Take steps to eliminate or reduce any danger or potential hazard to the safety or health of employees.
• Make arrangements to ensure the safety and absence of risks to the health of employees in connection with the production, processing, use, handling, storage or transport of articles or substances.
• Provide training and supervision as may be necessary to ensure the health and safety at work of the employees.
• Ensure that work is performed and that plant or machinery is used under the general supervision of a person trained to understand the hazards associated with it and who has the authority to ensure that precautionary measures taken by the employer are implemented.
Th e employee, on the other hand, must also adhere to certain conditions to maintain safety in the workplace. Each employee has the following responsibilities: • Ensure the health and safety of themselves and of other persons who may be
aff ected by their acts.
• As regards any duty or responsibility imposed on the employer or any other person by this Act, co-operate with such employer or person to enable that duty or responsibility to be performed or complied with.
• Carry out any offi cial order given to them, and obey the health and safety rules and procedures laid down by the employer or by anyone authorised by their employer, in the interest of health or safety.
• If any situation which is unsafe or unhealthy comes to their attention, report such situation to their employer or to the health and safety representative as soon as possible.
• If they are involved in any incident which may aff ect their health or which has caused an injury to themselves, report such incident to their employer or to anyone authorised by the employer, or to their health and safety representative, as soon as possible.
Th e general safety in the electrical workshop can also be improved by having workshop rules to ensure proper conduct in the workshop. It is important not only to post your workshop safety rules, but to enforce them to reduce accidents and injuries. Th e following rules will ensure that your workshop is a safer place: • No horseplay in workshop.
• No eating and drinking in workshop.
• Wear protective clothing and equipment when using dangerous tools and machines.
• NEVER operate machines without supervision and permission.
• Never use any tools or machines unless you have been properly instructed by the teacher.
• Tools that have sharp edges should be carried with sharp edges pointing downwards.
• Always clean the work area aft er the completion of a task. • Make sure all machine guards are kept in place.
• No bags allowed the workshop, as people can trip over them. • Keep hands away from moving/rotating machinery.
• No unauthorised persons allowed in workshops. • No smoking in workshop.
Th e OHS Act compels the employer to ensure that the workplace meets safety standards by appointing health and safety representatives. Th e main task of such representatives it is to monitor the safety in the workplace. Th ese health and safety representatives need to
• regularly review health and safety measures in the workplace; • identify dangerous and hazardous conditions;
• investigate incidents in the workplace in partnership with the employer; • investigate complaints of employees regarding health and safety issues; and • do regular inspection of the workplace, including tools and equipment. Such health and safety representatives can be held responsible and be punished if machinery or working conditions are found to be unsafe. To maintain high safety standards in the workplace we can all ensure that:
• the workshop remains clean and neat.
• equipment and materials are stored in their proper places.
• the fi rst-aid kit is easily accessible and contains the necessary medical items. • fi re extinguishers are maintained and in good working order.
• walking paths are clearly indicated and obstacle-free. • poisonous materials are safely stored and used. • sharp tools are used with caution.
• games and jokes have no place in the workplace. • smoking and drinking are prohibited in the workplace.
• any materials or liquids that are spilled are immediately cleared. • any damaged or broken tools or machinery are immediately reported.
Unsafe acts
As mentioned earlier, most accidents are caused by human carelessness. However, accidents also occur when workers are too complacent and then take risks, because they think nothing will happen to them.
Below are some of the unsafe acts responsible for most accidents in workshops: • Failure to wear protective clothing and eye wear
• Th e unsafe placement of tools
• Horseplay in the workshop (running around and playing the fool) • Th e unsafe use of equipment or incorrect use of equipment • Trying to do adjustments or working on moving equipment • Taking up unsafe positions
• Working at an unsafe speed.
Unsafe conditions
Th is is also a major contributor to many accidents in the workplace. Th e following are unsafe conditions:
• Inadequate guarding
• Bad ventilation – high temperatures and a lack of clean fresh air can lead to tiredness and breathing problems
• Rough or slippery fl oors
• No personal protective equipment • A disorganised workshop
• Overcrowding in a workshop • Badly planned workshop
• Loose-hanging clothing and long hair • Insuffi cient light in workplace.
Ergonomics
“Ergonomics” means using scientifi c information regarding humans to design
objects, systems and the environment for human use in order to optimise human comfort and overall system performance. Ergonomics is about improving employee comfort, reducing chances for occupational injuries, improving productivity, and improving employee job satisfaction. Th e goal of ergonomics is to design jobs to fi t people. Something is ergonomically designed if it is optimised to fi t people. Th is means taking into account diff erences such as size, strength and ability to handle information for a wide range of users.
Ergonomics is concerned with the reduction of one or more of the following risk factors:
• Uncomfortable posture: If a job/task looks uncomfortable, it probably is and this increases the chances of injury. Whenever possible, strive to arrange the work environment or work processes to allow employees to work from a comfortable, neutral posture. Excessive bending, reaching and awkward neck,
Figure 1.2: Safety
• High repetition: Repetition can be controlled by using equipment that reduces repetition, allowing employees to rotate tasks, assuring adequate staffi ng, and ensuring that employees take regular breaks away from highly repetitive tasks. • Extreme force: Th e need to exert too much force should be controlled through
use of proper equipment, ensuring that equipment is operating properly, and getting adequate help when needed.
• Contact stresses: Contact with sharp, abrupt edges, whether from a fi xed piece of furniture or from a tool, should be avoided.
• Vibration: Vibration can be reduced at the source through tool or equipment
selection or by padding the body against vibration, e.g. wearing padded gloves.
• Excessive temperatures: Th e temperature in the workplace should be controlled whenever possible. Th e improvement of the thermal environment – air
circulation, ventilation, room temperature and humidity – can ensure a healthier place to work.
Remember, the way one works and the conditions in which one works will defi nitely aff ect one’s eff ectiveness and effi ciency as an employee. Controlling stress, both physical and psychological, following a balanced daily diet, resting and “recharging batteries” and working in a well-organised offi ce or workshop environment are major factors in a person’s work life.
Workplaces may take either the reactive or proactive approach when applying ergonomics practices. Reactive ergonomics is when something needs to be fi xed, and corrective action is taken. Proactive ergonomics is the process of looking for areas that could be improved and fi xing them before they become a bigger problem. Problems may be fi xed through equipment design, task design, or environmental design. Equipment design changes the devices used by people, whereas task design changes what people do with the equipment. Environmental design changes the environment in which people work, and not the equipment they use.
In conclusion, ergonomics can help reduce expenses of companies by improving safety. Th is would decrease the money paid out in workers’ compensation.
Housekeeping principles
Good housekeeping in a workshop simply means an orderly arrangement of tools, equipment, operations, storage facilities and materials. To put it in simpler words, housekeeping can be defi ned as everything in its place and a place for everything. Th e simple principle of housekeeping is that an orderly, clean, tidy and bright workshop is far safer than one that is none of these things. One of the key factors to good housekeeping is making sure it involves everybody and that everybody understands its necessity – from the factory owner to the cleaner. It is, therefore, important to educate the entire workforce about the methods and desirability of the highest standards of housekeeping, which includes fostering an awareness of danger and having a code of safety practices in place. Organisation is one of the keys to an eff ective workplace. It seems like such a simple thing, but the fact is that when one
A number of years ago, a major vehicle manufacturing company identifi ed fi ve housekeeping principles for ensuring an organised work area. Th ese principles are simply good housekeeping practices, but they can have a major impact on any organisation. Th e fi ve principles are:
• Sort: Remove all items that are not immediately needed for the job at hand from
the work area. Th is includes tools, documents, containers and equipment.
• Set in order: Everything left in the work area should have a designated place.
Th ere are many ways to designate locations, including marked lines on the fl oor, signs hung from above and labels on all storage devices.
• Shine: Consistently clean up the area to make it look good. Everyone prefers
to work in a clean area, and this will lead to improved morale and better productivity.
• Standardise: Develop procedures that will keep things like new. Formal
procedures for housekeeping and preventative maintenance are essential components of this principle.
• Sustain: Follow the procedures developed for maintaining an orderly workplace.
Th is requires discipline to ensure that corners are not cut.
Th ese principles are not complicated, but they require commitment. To avoid a position in which one has to decide between production and housekeeping, one should clean and organise as one works. Do not wait until a job is fi nished before cleaning up chips or sweeping around a machine. Practicing housekeeping principles on an ongoing basis means the tasks are easier and less time-consuming. It is, therefore, clear that a neat and organised workplace is a safer workplace, and a safer working environment allows for better profi ts, a happier workforce and saves time.
Signs in the workshop
Th e use of signage in workshops and laboratories has become a legal requirement and it acts as a universal language when it comes to promoting safety. Safety signs and signals are the main means of communicating health and safety information. Proper signage will help reduce accidents and will help make the workplace safer. In view of the importance of signs, it is critical that all safety signs and signals be easily understood. Safety signs should be provided where necessary to warn of hazards, to prevent dangerous practices, and to indicate safe exit routes and safe practices. All safety signs must be placed where they can be easily seen and provide the best warning of the presence of a hazard. Generally, they are best placed above eye level at a height above two metres. If the natural light is not enough to light up the sign properly, artifi cial light must be used. It is important to note that any defective or faded sign must be replaced as soon as possible and unfamiliar signs must be explained to employees, including the action to be taken in connection with them. It is important that everybody using the workshop knows the diff erent types of signs that are being used and their meanings. It must also be noted that diff erent types of signs are printed in diff erent colours. Th e International Standards
Organisation, of which South Africa is a member, has published recommendations on the use of symbolic safety signs. With this information as guide, the South African Bureau of Standards (SABS) has designed the range of safety signs. Th ese signs are recommended for use throughout South Africa.
Figure 1.4: Signs found in
Safety signs
Safety signs should be provided where necessary to warn of hazards, to prevent dangerous practices, and to indicate safe exit routes and safe practices. Specifi c types of signs should be used in dangerous locations, e.g. where there is a risk of slipping, falling from heights, or where there is low headroom.
Figure 1.5: Safety signs
Th e signs are divided into fi ve categories. Each type is recognisable by colour and, with one exception, also by shape.
Colours
Category Colour – Border/oblique diagonal
Background Symbol
P – Prohibitive Signal Red (All) White Black M – Mandatory --- Ultramarine (type of blue) White I – Informative --- Emerald green White F – Fire --- White Signal red (All) W – Warning Black Golden yellow Black
Figure 1.6: The fi ve categories of safety signs
I nformation signs
An information sign informs the user of a workshop or area of certain facts or knowledge or conditions. A sign indicating an emergency exit in a room can be referred to as an information sign. An information sign is a rectangular or square sign with a white picture on a green background. Th e colour green indicates ‘access’ or ‘permission’.
Prohibition signs
A prohibition sign is a sign that orders or forbids something. A sign indicating no eating or drinking is a prohibition sign.
Prohibition signs have a red, circular outline and crossbar running from top left to bottom right on a white background. Th e symbol displayed on the sign must be black and placed centrally on the background, without obliterating the crossbar. Th e colour red is associated with ‘stop ’or ‘do not’.
Figure 1.8: Prohibition signs
Warning signs
Warning signs are signs that warn of a hazardous condition that may exist and that special care must be taken when entering a specifi c area or working with specifi c equipment.
Warning signs are normally triangular with a black picture on a yellow background and black edging. Th is combination of black and yellow signifi es ‘caution’.
Figure 1.9: Warning signs
Mandatory signs
Mandatory signs indicate that a specifi c course of action is to be taken, or that a specifi c behaviour is required. Mandatory signs are round with a blue background and a white picture. An example of a mandatory sign is: wear protective headgear.
Designated areas
Th e risk of accidents happening in workshops can also be reduced by focusing on non-obvious risk areas such as walkways, storage areas and other related areas. In all workshops there are special designated areas where machines are kept, materials stored and where the person using the workshop can walk freely. It is, therefore, important that these areas are clearly marked on the workshop fl oor and with special signs. Normally the walkways are painted green with yellow lines on both sides, indicating the width of the walkway. Work/machinery areas and storage areas are also indicated by special fl oor markings and are sometimes segregated by barriers. For these walkways and designated areas to be eff ective in contributing to keeping the workshop safe, all walkways and designated areas must be kept clean and clutter-free. Th ese areas must be kept dry and free from spillage and trip hazards. It is, therefore, important that these surfaces be cleaned regularly. Care must be taken that these fl oor surfaces are sound and have an easily maintainable, non-slip fi nish.
Th e storage and work areas must be kept separate. All storage areas must be kept neat and tidy, and boxes must not be stored too high as this may pose a risk of falling.
Other areas of special interest in any workshop that need to be highlighted are emergency exit doors and pathways, fi re extinguishers and fi rst-aid boxes. Emergency doors, fi re extinguishers and fi rst-aid boxes must be clearly marked. Emergency doors must be clear of obstacles, both inside and outside, for free movement in and out. Exit doors must be able to open from the inside. Fire extinguishers and fi rst-aid boxes must be clearly marked and easy accessible.
Personal safety
Eye protection
Eye injuries are very common in workshop environments where hand tools and machinery are used. Many eye injuries occur because of workers forgetting or just not using the protective gear given to them. Injuries also occur when people are wearing the wrong eye protection for the job they are doing (e.g. wearing safety glasses instead of goggles or a full face shield when pouring chemicals, or wearing low-impact resistance glasses where a high-impact design is best). Eye wear that does not fi t the contours of the face is also a source of injury. Workers select and use the recommended design but, because it does not fi t the facial contours, particles get into the eye. It is, therefore, of the utmost importance that the correct eye protection be used depending on the tool or machine being used. In the electrical workshop, good quality safety goggles are needed specifi cally for grinding and drilling.
Coveralls/Overalls
As with eye protection, the overall/coverall/apron is used to protect the user from harm that may be caused by sparks or spillage of toxic or harmful liquids. Th e aim is to protect the clothes and body. Many expensive school uniforms have been damaged by sparks or chemical spills because of learners neglecting to put on their overalls/coveralls/aprons. For general grinding and drilling work in the electrical workshop, a woven cotton overall/coverall or apron is required, but when working with chemicals during PCB manufacturing, a rubber apron is required. It is important to note that specifi c overalls/coveralls/aprons must be used for specifi c jobs: it is not a one fi t for all.
Figure 1.11: Safety
goggles
Hearing protection
In today’s busy and noisy world one simply cannot aff ord not to safeguard one of our most delicate senses. Once one has lost one’s hearing, it is gone forever. It is, therefore, very important that one protect one’s hearing whenever exposed to high noise levels. Regular use of hearing protection will help protect against long-term hearing loss. In electrical workshops, one of two types of hearing protection can be used: earmuff s and earplugs. Either will provide an acceptable level of hearing protection and, depending on the noise level, some people may even use both types at the same time.
Earmuff s
Earmuff s look a lot like a large pair of headphones. Typically, the earpieces completely cover the ears to try to form a tight seal and keep out as much sound as possible. While they tend to provide a bit better sound reduction than earplugs, they also are quite bulky and cumbersome.
Earplugs
Ear plugs are much smaller and less cumbersome than their earmuff counterparts, but do not provide the same level of protection. Earplugs are typically made of a type of memory foam that the user compresses, inserts into the outer ear cavity and waits for the foam to expand to form a tight fi t. Many users fi nd these more comfortable than earmuff s, particularly when working in warm weather.
Always remember, no matter which type of hearing protection you prefer to be sure to use hearing protection when using power tools. Your ears will thank you for your foresight in future years. Below is a table indicating the estimated noise level from diff erent types of sources:
Noise Source Db
Watch ticking 20 Quiet street noise 40 Normal conversation 60 Ringing telephone 80 Aircat impact wrench 82 Motorcycle 85-90 Rock concert 80-100 Hairdryer 90 Hand drill 95-105 Chainsaw 110 Jackhammer (3’) 120 Jet engine (100’) 130 Shotgun blast 140
Figure 1.13: Ear muff s
Figure 1.14: Ear plugs
Figure 1.15: Diff erent
noise sources
Did you know?
It is believed that exposure to noise levels at 85 decibels (db) or higher for eight hours or more per day puts your hearing at risk.
Protective gear for machinery
Besides the user of machines wearing protective clothing and all sorts of other protection, industrial machines are big and can be hazardous to those working around them if they need protective gear too. Th ese machines have not been made safe for the user. Blades, fl ying objects and loose belts are all items that could present a safety risk to the persons working around these types of industrial machinery. Some of the protective gear for machinery can include the following safety requirements:
Anchors
It is important for all industrial machines used in a workshop or factory to be fastened securely to the ground, preventing them from moving during operation. It also prevents them from being placed in prohibited areas, such as in front of a fi re escape, etc. when they are not in use.
Protective shields/guards
Protective guards/shields are required on all pieces of equipment and machinery. Th ese guards/shields will prevent pieces of the machinery from coming loose and hitting the workers who are near the machines. Protective guards/shields can also prevent workers from falling into the machines or getting too close to dangerous blades and belts that could injure them severely.
Guarding/shielding is commonly used with machinery and equipment to prevent access to
• rotating end drums of belt conveyors;
• moving augers (a corkscrew-shaped thread), or auger conveyors and rotating shaft s;
• moving parts that do not require regular adjustment;
• machine transmissions, such as pulley and belt drives, chain drives, exposed drive gears; and
• any dangerous moving parts, machines or equipment.
Th ere are three types of guarding/shielding: fi xed, removable or adjustable guarding/shielding. If no access is required to parts of a machine, the guarding/shield can be permanently fi xed. Removable guards/shields are used when parts of the machine can be removed for a short period to do some repairs to the machine. It is important that the guard/shield can only be removed with a special tool which is not normally available to the operator. Adjustable guarding/shields incorporate movable sections or panels of the guard and allow for material or parts to be fed into the guarded area while still preventing bodily contact.
Interlock guarding
Interlock guarding occurs when the act of moving the guard (opening, sliding or removing) to allow access stops the action of the dangerous mechanism. Interlock guarding works by
• mechanically disconnecting the drive mechanism,
• isolating the power source of the drive mechanism (stops the motor), or • a combination of mechanical and power disconnection.
Interlock guarding is generally achieved via mechanical or electrical means, but may also include hydraulic or pneumatic control systems.
Figure 1.16: Machine
Stopping mechanism
Some industrial machines have areas or parts that can easily grip hands and fi ngers. If the machine has such an area, it must be shielded. However, it must also have a stopping mechanism so that workers can safely stop the machines should any part of the worker be caught in the machine.
Simultaneous two-handed operation
Where a machine has only one operator, the use of simultaneous two-handed operation buttons can serve as a risk control. Th is ensures that operation of the dangerous mechanism cannot occur until both hands are clear of the danger area. Th e two buttons must be pushed at the same time and are located at a distance from each other, which prevents simultaneous operation using one hand.
Th e operation should be designed so that if either or both of the buttons are released, the hazardous action of the machine or equipment cannot be reached; or when it can be reached, the mechanism has returned to a safe state.
A two-handed control option may be suitable for ensuring that a machine cannot be operated until both hands of the operator are clear of the hazardous area.
Presence sensing systems
If physical guards/shields are not reasonably practicable, then a presence sensing system can be used as a control to reduce risk. Presence sensing systems can be used where people enter areas shared by moving production equipment. Presence sensing systems are capable of providing a high degree of fl exibility with regard to access. Th ese systems detect when a person is in the identifi ed danger area, and stop or reduce the power or speed of the mechanism at the time of entry to provide for safe access. Presence sensing systems can rely on foot pressure pads, infra-red sensing, light beams or laser scanning.
Activity 1
1. What do you understand by the concept of safety?
2. What is the signifi cance of the OHSA with regard to safety?
3. Name THREE things that the employer must put in place to maintain safe working conditions.
4. Safety in the electrical workshop must receive the highest priority. Write down at least fi ve rules that will make the workshop a safer place.
5. Unsafe conditions are the cause of many accidents. Name FIVE unsafe conditions in an electrical workshop.
6. Briefl y explain, in your own words, what you understand by the term ergonomics, with reference to the electrical workshop.
7. Why is it so important to have good housekeeping in place?
8. Th e South African Bureau of Standards (SABS) has designed the range of safety signs. Name the fi ve main categories of these safety signs.
9. Identify the following safety signs.
10. Why is it important that emergency exits be clearly marked? 11. Name the type of protection you will use when doing the following:
• Working with a grinding wheel. • Drilling a hole.
• Etching of a PCB.
12. Name TWO types of hearing protection that can be used in a workshop and give the main diff erence between them.
13. What do you understand by a pro-active approach to ergonomics? 14. Name at least THREE aspects of good housekeeping and give a brief
explanation of each.
15. What do you understand to be the main reason why safety signs are used? 16. What is the best height at which safety signs should be displayed in workshops?
17. What is the signifi cance of warning signs?
18. Mandatory signs indicate that a specifi c course of action is to be taken, or that a specifi c behaviour is required. Name THREE mandatory signs. 19. What is the importance of using protective guards/shields for big machines? 20. Name THREE types of guarding/shielding used on machines.
21. Name THREE safety features used on big machinery to help prevent injuries.
Tools and measuring instruments
Housekeeping
Soldering and
de-soldering
A
B
Safe use of
power tools
Oscilloscope
A
B
Introduction
Th is chapter deals with the basic test instruments used in an electronics workshop: a digital multimeter, an insulation tester, a function generator and the oscilloscope. It will also deal with the safe use and care of special tools such as the crimping tool and bending spring, as well as the safe use of power tools such as the drill, grinder and jigsaw. Aft er the principles of soldering, the chapter provides a step-by-step guide to designing and making a PCB and ends with a section on housekeeping.
Digital multimeter
One of the most common and versatile measuring instrument used in all electrical/ electronic workshops is the electronic multimeter – more specifi cally the digital multimeter. Th is is a multifunctional measuring device that is most commonly used for the measurement of voltage, current and resistance. However, as technology has advanced, digital multimeters have been developed that can even measure capacitance, frequency and temperature, as well as test semiconductor devices such as transistors and diodes. Th ere is such a wide range of multimeters that it is important to read the operating instructions before using some of the more sophisticated digital multimeters. Th e pictures below show examples of simple digital multimeters: one with a manual ranging and the other with an auto ranging selection. With the manual ranging multimeter it is always important to select the correct range within a selected function before taking any measurement, while with the auto ranging multimeter, you can simply select the function and the multimeter will automatically select the appropriate range to take the most accurate reading. Th is means that once the function is selected (volts, amps, ohms, etc.), the meter will select the range or scale automatically.
Figure 2.1: A manual ranging multimeter Figure 2.2: Auto ranging multimeter
1. Digital display: Th e digital display displays the reading of whatever measurement is being taken.
2. Selector switch: Th is switch is used to select the correct function for the task to be performed, e.g. measuring volts, measuring resistance, or even testing a diode. One must always be sure that the multimeter is on the correct setting before taking any measurements. Selecting an incorrect setting can damage the multimeter.
1 2 3 4 3 2 1
For standard use of the multimeter, the red lead is put into the socket marked “V/Ω/mA” and the black lead is put into the socket marked “Com”. Only for the measurement of very high currents is the red lead put into the socket marked “10A/DC”.
4. Transistor tester: Th is is a socket for connecting a transistor (NPN or PNP) in order to measure the hFE (gain) of the transistor.
Principle of operation
Unlike the analog multimeter that makes use of a moving coil meter to display any measurements taken, the digital multimeter makes use of a liquid crystal digital (LCD) display to display the information measured. Th e basic operating principle is that the multimeter makes use of a simple digital voltmeter to take the basic measurements. Th is digital voltmeter is made up of three major parts: • an unknown input voltage,
• an analog to digital converter (A/D converter), and • a digital output display.
For voltage measurements, the unknown analog signal is measured and converted to a digital signal by the A/D converter. Th is digital signal is then displayed on the digital output display of the multimeter. Th e digital output display will constantly change with the change in input voltage being measured.
For current measurements, a precision-fi xed, low-value resistor is connected across the input leads. Th e unknown current measured will thus fl ow through the resistor, creating a voltage drop across the resistor which, in turn, is measured by the digital voltmeter, as explained in the previous paragraph. Th is voltage measured would be proportional to the current being measured.
For resistance measurement, a known amount of current is sent through the unknown resistor, once again creating a voltage drop across the unknown resistor. Th is voltage drop is measured by the digital voltmeter and displayed on the digital output display. Th is voltage measured would be proportional to the resistance of the unknown resistor being measured.
Applications
Digital multimeters are very versatile measuring instruments and are widely used in electrical/electronic workshops. As mentioned previously, digital multimeters can be used for measuring voltage, current, resistance, frequency and temperature, as well as for testing diodes and transistors.
Always remember that when one does a voltage measurement, the probes must be across the points to be measured (in parallel). Th e red lead must be connected to the positive side of the circuit and the black lead to the negative side of the circuit. For current measurements, the probes must be in series in the circuit. (Th e circuit must be broken and the digital multimeter must complete the circuit.) Th e red lead must be connected to the positive side of the circuit and the black lead to the negative side of the circuit.
For resistance measurement, the circuit must fi rst be switched off . Th e resistor must then be removed completely, or one lead must be removed completely, before a resistance reading can be taken. Th e probes must be connected across the resistor (in parallel).
Care and maintenance
Digital multimeters are very sensitive electronic devices and must always be handled with care. Th e following points are very important:
• Always put the multimeter in a protective cover if possible. • Store the multimeter in a dust-free area.
• Never allow the meter to be dropped on the fl oor. Th is can damage the display. • Ensure that the leads are connected to the right input sockets (red and black). • Always select the highest range on the function select input if you are not certain
about the quantity being measured.
• Always make sure that the multimeter is on the correct setting before taking any measurements. Selecting an incorrect setting can damage the multimeter. • Never roll up the test leads tightly, as this may cause a break in the leads. Rather
let them hang loosely in the storeroom.
• Always keep the terminals and terminal posts clean, as dirt on the posts may lead to incorrect readings.
• Follow the correct operational instructions/procedures for the specifi c instrument.
• Always use the right equipment for the right job. • Equipment must always be stored in a secure place.
Insulation tester (Megger)
Th e insulation tester is a portable instrument used to measure the insulation resistance of electrical machinery or systems. It is an instrument that tests the safe operation of electrical installations, motors, transformers, generators and other electrical appliances. Regular insulation testing is one of the most cost-eff ective methods of identifying aging in any type of electrical equipment. Th e insulation and continuity tester is also commonly known as a ‘Megger’. It is a tester that must be able to measure very high resistances of up to 200 megohm at test voltages of up to ±1 000 V (DC), as well as very low resistance, as low as 0,01 ohm.
Th e device must be very accurate, especially at very low resistance readings. Th e older type of meters comprised two parts: a hand-driven direct current (DC) generator capable of producing diff erent test voltages, such as 6 V for the testing of continuity and ±500 V DC for the testing of insulation resistance; and a direct driven ohmmeter. Currently more and more people are using the more modern digital insulation and continuity tester because it is easier to operate and more accurate.
According to Regulation 8.7.8: Insulation resistance in SANS 10142-1 2003 Wiring code as amended, a single phase installation must be tested with an insulation tester (500 V DC or AC) at twice the operating voltage. When this test is carried out, the reading should not be less than 1 megaohm.
Did you know?
Th is famous brand name MEGGER dates back to 1889, when the fi rst portable insulation tester was introduced with the MEGGER brand name on it.
1. Measuring lead connector sockets (on top of Megger): Th ese are the inputs for the measuring leads (red and black). It is important to put the leads into the correct sockets.
2. Digital/Analog display: Th e digital/analog display displays the reading of whatever measurement is being taken. Th e digital display gives a numeric representation of the reading, while the analog reading is given by means of a pointer moving across a calibrated scale.
3. Selector switch: Th is switch is used to select the correct function for the task to be performed, e.g. measuring high or low resistances. One must always ensure that the insulation tester is on the correct setting before taking any measurements.
4. Test button: Press TEST to initiate the selected test or to repeat the selected test.
Th e test button is used when insulation testing is done.
Zero adjust: (Analog meters only) To ensure that readings are accurate, the analog multimeter must always be zero-adjusted manually. Th is is done by turning the selector switch to the ohm scale, shorting the two test leads and adjusting the zero adjust setting on the front of meter until the pointer is exactly on zero ohms on the analog scale.
Principle of operation
Figure 2.4: Circuit representing a Megger
An insulation tester (Megger) consists of the following basic parts:
1. Control and defl ection coil: Th ese are normally mounted at a right angle to each other and connected parallel to the generator. Th e polarities are such that the torque produced by them is in opposite directions.
2. Permanent magnet: Th ese permanent magnets with north and south poles produce a magnetic eff ect for the defl ection of the pointer.
3. Pointer and scale: A pointer is attached to the coils and the end of the pointer
fl oats on a scale which is in the range from “zero” to “infi nity”. Th e unit for this is “ohms”.
4. DC generator or battery connection: Th e testing voltage is supplied by a hand-operated DC generator for a manually hand-operated Megger and a set of batteries and electronic voltage charger for an automatic Megger.
1 2
3
4
• Th e current carrying coil (defl ection coil) is connected in series and carries the current taken by the circuit under test. Th e pressure coil (control coil) is connected across the circuit.
• Current limiting resistors are connected in series with the pressure coil (control coil) and the current carrying coil (defl ection coil) to prevent damage in case of a low resistance in the external circuit.
• In hand-generator units, the armature moves in the fi eld of the permanent magnet or vice versa, to generate a test voltage by electromagnetic induction eff ect.
• As the potential increases across the outside circuit, the defl ection of the pointer will increase; and as the current increases, the defl ection of the pointer decreases so that the resultant torque on the movement is directly proportional to the potential diff erence and inversely proportional to the resistance.
• When the external resistance is infi nity, i.e. the insulation measured is very good, the pointer will read infi nity (α). When there is a short circuit in the circuit, the pointer will read 0 ohm.
Applications
Th e insulation and continuity tester can be used to test • for open circuits.
• for short circuits. • earth continuity.
• insulation resistances between conductors.
• insulation resistances between conductors and earth. • the polarity of switches.
Care and maintenance
All electronic test instruments are very sensitive devices and must always be handled with care. Th e following points are very important to remember when using an insulation tester:
• If battery operated, always ensure batteries are in good order (fully charged). • Keep the insulation tester in a protective cover if possible.
• Always make sure the test leads are in good condition.
• If the defl ection type insulation tester is used, be careful not to damage the defl ection coil as this may lead to incorrect readings.
• Always store in a dust-free area, because dust can be harmful to the instrument. • Follow the correct operational instructions/procedures for the specifi c
instrument.
• Th e circuit connections and exposed metalwork of an installation or the equipment being tested must not be touched.
• Do not allow anybody to touch the leads while a test for insulation is in progress (they will get a shock).
• Do not move the rotary selector switch position while a test is in progress. • Replacement fuses must be of the correct type and rating.
• Equipment must always be stored in a secure place.
Function generator
Th e function generator is a waveform generating instrument that is also widely used within the electronics industries. Th e function generator is an electronic instrument responsible for generating diff erent types of signals or waveforms (sine, square and triangular (sawtooth) waves) to help with the design, testing, or troubleshooting of electrical devices. It also allows the user to change the frequency and the amplitude of such waveforms. Th e frequency can range from a few Hz into the MHz scale, and the amplitude can vary from a few mV up to about 10 V, depending on the type of function generator. As with multimeters, there is also a huge variety of function generators, from very simple ones to very complicated and sophisticated ones. Some function generators have a digital display to display the frequency being generated. For the purposes of this book we will focus on the very simple, basic function generator.
Figure 2.5: Function generator
1. Digital output display: Th e digital display provides a numeric display of the frequency being generated by the function generator. Th is make for easy reading and accurate setting of the frequency needed.
2. Function selector: Th is facility allows one to select the type of waveform required (square, sine or triangular (sawtooth)).
3. Frequency range selector: Th e frequency selection function enables one to select the frequency range to be generated, e.g. 100 Hz, 1 kHz or 10 kHz.
4. Amplitude selector: Th is function is used to set or adjust the amplitude of the waveform that is being generated. Th is can range from a few mV to about 10 V, depending on what amplitude value is required.
5. BNC output socket(50 Ω): Th e 50 ohm BNC connector is used for connecting the function generator to other equipment. Th e most common connection used is a cable with a BNC connector at one end and two croc clips (red and black) at the other end. Always ensure that the cable is connected correctly to the circuit being tested. Th e function generator is polarity conscious, i.e. the ground lead of the function generator must be connected to the ground of the circuit being tested.
6. Frequency adjustment knob: Th is control allows one to set or adjust the frequency to a specifi c value.
3 4 5 1 6 2
Principle of operation
Figure 2.6: Simple block diagram of a function generator
A function generator generally provides an AC signal of variable amplitude and frequency. Because of the wide range of function generators, each one will have a unique principle of operation. In general, the three basic waveforms can be generated by a simple 14 pin 8038 waveform-generated IC. Th is IC is capable of generating sine, square and triangular (sawtooth) waveforms. With the correct additional circuitry connected to the IC, one can adjust both the frequency and the amplitude of the selected waveform. Th e function switch will allow one to select the desired output waveform, which is then transferred to the BNC output socket.
Applications
Th e function generator can be used to help with the design of electronic equipment and apparatus and with the testing and troubleshooting of electrical and electronic devices.
Care and maintenance
Th e function generator is a very sensitive and delicate piece of electronic equipment and must be handled with care at all times. Haphazard operation or improper setting of the controls can damage the electronic equipment. Th e following points are very important to remember when using a function generator:
• Always store in a dust-free area, because dust can be harmful to the instrument. • Follow the correct operational instructions/procedures for the specifi c
instrument.
• Always make sure the test leads are in good working order.
• Make sure the BNC sockets on the function generator are in good condition and rust-free.
The oscilloscope
Th e oscilloscope is a measuring instrument that is also widely used within the electronics industry. Th e oscilloscope is a tool commonly used by engineers and technicians to analyse and troubleshoot electronic systems.
An oscilloscope is an electronic measuring device which makes use of a cathode-ray tube (CRT) to display a two-dimensional visual representation of a signal. Because the oscilloscope allows the user to see the signal(s), the characteristics of the signal(s) can be easily measured and observed. Th e oscilloscope displays a graph of voltage (on the vertical axis) over time (on the horizontal axis). Most electrical circuits can be connected to the oscilloscope using probes without diffi culty. Most standard oscilloscopes will have the facility to connect two oscilloscope probes. As
Waveform generator Amplitude and frequency adjustment Output Power supply
even see digital oscilloscopes. Instead of displaying a signal by defl ecting an electron beam that is traced across a CRT, the digital oscilloscope uses a processor to sample, digitise and display the incoming analog signal on the display screen. For the purposes of this book we will focus on the very simple, basic analog oscilloscope.
Figure 2.7: An oscilloscope
1. Horizontal position control: Th is is used to move the waveform horizontally (from side to side) across the screen.
2. Time/Div control: Th e time per division setting allows one to select the rate at which the waveform is drawn across the screen (also known as the timebase setting or sweep speed). Th is setting is a scale factor. For example, if the setting is 1 ms, each horizontal division represents 1 ms and the total screen width represents 10 ms (ten divisions). Changing the time/div setting enables one to look at longer or shorter time intervals of the input signal.
3. Trigger mode: Th e trigger mode determines whether or not the oscilloscope will draw a waveform if it does not detect a trigger. Common trigger modes include normal and auto.
In normal mode the oscilloscope only sweeps if the input signal reaches the set trigger level; otherwise (on an analog oscilloscope) the screen is blank. Normal mode may be confusing since one may not see the signal at fi rst if the level control is not adjusted correctly.
Auto mode causes the oscilloscope to sweep, even without a trigger. If no signal is present, a timer in the oscilloscope triggers the sweep. Th is ensures that the display will not disappear if the signal drops to small voltages. It is also the best mode to use if one is looking at many signals and does not wish to have to set the trigger each time.
4. Trigger source: Th is function allows one to select the source that must triger the scope. Several sources can trigger the sweep:
• Any input channel (Chanel 1 or 2)
• An external source, other than the signal applied to an input channel • Th e power source signal
• A signal internally generated by the oscilloscope.
1 2 3 4 5 12 11 10 9 8 7 6
5. Slope adjust: Th is allows the user to invert the waveform and, therefore, have the waveform displayed “upside-down.”
6. Mode select: If one has a dual trace oscilloscope, this function allows one to display one of the waveforms at a time, display two waveforms
simultaneously and even to add the two waveforms.
7. Vertical position control: Th is is used to move the waveform vertically (up or down) across the screen.
8. AC/GND/DC select: Th is selector is used to connect an electrical signal from one circuit to another. Th e input coupling is the connection from the test circuit to the oscilloscope. Th e coupling can be set to DC, AC, or ground. DC coupling shows all of the input signals. AC coupling blocks the DC component of a signal so that one sees the waveform centred at zero volts. Th e ground
setting disconnects the input signal from the vertical system, which enables one to see where zero volts is on the screen.
9. Volts/Div control: Th e volts per division setting varies the size of the waveform on the screen. Th e volts/div setting is a scale factor. For example, if the volts/div setting is 5 volts, then each of the eight vertical divisions represents 5 volts and the entire screen can show 40 volts from bottom to top.
10. Screen: Th e screen will display the waveforms measured. Th e display is usually a CRT or LCD panel which is laid out with both horizontal and vertical reference lines referred to as the graticule.
11. Focus control: Th is is used to adjust the sharpness of the waveform. Digital oscilloscopes may not have a focus control.
12. Intensity control: Th is is used to adjust the brightness of the waveform.
Principle of operation
Did you know?
Th e graticule is a grid of squares that serve as reference marks for measuring the displayed trace.
Th e oscilloscope can basically be divided into four sections: the display, the vertical controls, the horizontal controls and the trigger controls.
Th e display is usually a CRT or LCD panel which is laid out with both horizontal
and vertical reference lines referred to as the graticule. In addition to the screen, most display sections are equipped with three basic controls: a focus knob, an intensity knob and a beam fi nder button.
Th e vertical section controls the amplitude of the displayed signal. Th e signal to be measured is fed via the scope probe to an attenuator and then to the vertical amplifi er to either amplify or attenuate the input signal. From the vertical amplifi er the signal is fed to the vertical defl ection plates (y-plates) of the CRT. Th e vertical plates move the trace from top to bottom. Th is section of the scope contains the volts per division (Volts/Div) selector knob, the AC/DC/Ground selector switch and the vertical (primary) input for the instrument. Additionally, this section is typically equipped with the vertical beam position knob.
Th e horizontal section controls the time base or “sweep” of the instrument. Th is is the part of the scope responsible for the defl ection of the electron beam horizontally across the face of the oscilloscope. Th is is normally done with the aid of a saw-tooth generator. Th e speed of the movement of the beam across the face of the scope (horizontally) is done by the time-base generator. Before the signal is fed to the x-plates (horizontal defl ection plates), it is fi rst amplifi ed by the horizontal amplifi er. Th e primary control is the time per division (T/Div) selector switch. Also included is a horizontal input for plotting dual X-Y axis signals. Th e horizontal beam position knob is generally located in this section.
Th e trigger section controls the start event of the sweep. Th e trigger stage receives a signal from the vertical amplifi er. Th e trigger can be set to restart automatically aft er each sweep or it can be set to respond to an internal or external signal. Triggering is necessary to provide for a stabilised signal display on the screen. Th e trigger section is responsible for synchronising the input signal and the time base signal. Th e main controls of this section are the source and coupling selector switches. An external trigger input (EXT Input) and level adjustment will also be included.
In addition to the basic instrument, most oscilloscopes are supplied with a probe. Th e probe will connect to any input on the instrument and typically has a resistor of ten times the oscilloscope’s input impedance. Th is results in a X1 or X10 attenuation factor, but helps to isolate the capacitive load presented by the probe cable from the signal being measured.
Applications
In general, oscilloscopes are used for the maintenance of electronic equipment and for laboratory work. Special purpose oscilloscopes may be used for such purposes as analysing an automotive ignition system, or displaying the waveform of the heartbeat of a person. One of the most common uses of scopes is troubleshooting electronic equipment. Th e advantage of using an oscilloscope is that one can literally see the signals at the measured points.
Oscilloscopes can be used to • measure AC and DC voltages. • measure the frequency of waveforms. • analyse waveforms.
• determine phase relationship between waveforms. • measure the generation period of waves.
Care and maintenance
Th e oscilloscope is a very sensitive and delicate piece of electronic equipment and must be handled with care at all times. Haphazard operation or improper setting of the controls can damage the electronic equipment. Th e following points are very important to remember when using an oscilloscope:
• Always store in a dust-free area, because dust can be harmful to the instrument. • Follow correct operational instructions/procedures for specifi c instrument. • Always make sure the test leads are in good working order.
• Make sure the BNC sockets on the oscilloscope are in good condition and rust-free.
Tools
Crimping tool
Figure 2.9: Crimping tool with ferrules and lugs
1. Crimping jaw 2. Insulated handle 3. Lugs and ferrules
Function: Th is is a tool used to crimp ferrules, lugs and plugs onto wires. It can also be used to strip the insulation off wires and to cut wire.
Safe use: Crimping tools come in diff erent shapes and sizes, so be sure to use the
correct size for the ferrules and lugs. Ensure that your fi ngers are not caught in the crimping jaw. Do not apply too much pressure during crimping, because this might damage the crimping tool or even the ferrule or lug.
Maintenance and care: Th e crimping tool must be kept clean, and the moving parts must be oiled regularly.
Function: A bending spring is used for the bending of PVC pipes (conduit).
Bending springs are about 0.5 m long and come in two basic diameter sizes: 20 mm and 25 mm.
Safe use: Ensure the correct size bending spring is used for the correct size PVC
pipe thickness. Th e bending spring is placed inside the PVC pipe, which is then bent around the knee, until the correct angle is obtained. Th e spring prevents the walls of the PVC pipe from collapsing.
Maintenance and care: Always keep the bending spring in a dry environment,
because moisture can cause it to corrode.
Safe use of hand tools
Electric drill
Figure 2.11: Electric drill
1. Chuck 2. Handle 3. Power switch
Function: Th e electric hand drill can be used for drilling holes through metal, plastic, wood or masonry. Some drills with a speed control can even be used as a screwdriver.
Safe use: Always wear protective goggles when using an electric hand drill. All
loose clothing and hair should be tucked away. Always maintain a secure footing and good balance. Make sure the bit is properly secured in the chuck. Always select the correct bit for the job. Never leave the chuck key in the chuck aft er the replacement of a bit. Th is can be very dangerous.
Maintenance and care: Make sure the chuck is always secure and that the electrical
cord of the drill is in good order. Keep the drill clean and store it in a safe place.
Bench grinder
Figure 2.12: Bench grinder
2 3 1 1 2 3 1. Safety shield 2. Grinding wheel 3. Power switch
Function: Bench grinders are used to sharpen chisels, drill bits and other
tools. Th ey can also be used for general grinding applications (metal and plastic).
Safe use: Th e disc grinder must be used with caution, because it is the cause of many injuries. When using a disc grinder, always wear protective gear over your clothes and eyes. Always use a vice-grip when grinding small objects and never use the side of the grinding wheel. Never try to touch the disk of a grinder that is spinning. Switch the grinder off when not in use.
Maintenance and care: You must always ensure that the grinder is well secured
and that the wheel and all the other guards, shields and safety switches are in good condition. Jigsaw Figure 2.13: Jigsaw 1. Handle 2. Power switch 3. Chip cover 4. Base plate 5. Blade
Function: A jigsaw is a tool used for cutting arbitrary (random) curves/shapes
out of wood and perspex. Th e electric portable jigsaw’s blade has an up-and-down motion and can also be used for straight cutting.
Safe use: Always ensure that the chip cover is in place to prevent sawdust and
fragments from fl ying towards the user. Before use, make sure the blade is secured and sharp. Always wear protective glasses and an apron when using a jigsaw. Do not use on metal objects. Always cut away from the body and be sure the power lead is out of the way. Do not cut material that is too thick, because the blade gets very hot, and it also cannot be manoeuvred as well through thick material.
Maintenance and care: All moving parts must be oiled regularly and the machine
must be cleaned aft er use. 2
1
4 5
Safe and correct use of tools
In the electrical technology workshop one encounters many diff erent tools and types of equipment. To prevent injuries and accidents from happening it is important to use the tools correctly and safely. Always remember that no matter how small or big or how quick a job may be, the unsafe use of tools can cause serious injury. Th e following general rules apply to all tools in the workshop: • Keep all tools in good condition through regular maintenance.
• Use the right tool for the job. • Do not use tools that are damaged.
• Follow the correct procedure for using each tool.
Intermediate soldering/desoldering skills
(Using a solder wick)
Soldering
By now learners will have discovered that soldering is not as easy as it seems. It is an art that must be practised to perfection. In grade 10 learners started doing simple soldering tasks. In grade 11 it is time to continue working on the soldering skills in order to be able to make good, shiny, smooth solder joints. Remember, soldering is about practice, practice and more practice. Before progressing further, here is a recap of a few important rules to remember when soldering:
• Always solder in a room that is well ventilated.
• Never touch the tip of the soldering iron as it gets very hot (± 400˚C). • When a soldering iron is not in use, switch it off .
• Th e tip of the soldering iron must be ‘tinned’(by melting solder onto the hot tip of the iron) for the best transfer of heat.
• Always clean the tip of the soldering iron with a wet cloth or sponge.
• Wash hands aft er soldering because solder contains lead which is a poisonous metal.
Simple soldering steps
When soldering on vero or PC boards, make sure the copper tracks are clean and grease-free before following these steps:
• Push the component leads or wire links through the board to protrude on the copper side of the board.
• Use the soldering iron to heat up the joint to be made.
• Ensure that the soldering iron is in touch with both the component lead and the copper track.
• Apply the solder to the joint and not to the soldering iron. • Take care not to apply too much heat to the joint to be soldered.
• Feed a little solder to the joint. It should fl ow smoothly onto the lead and the track to form a shiny volcano-shaped joint.
• Remove fi rst the solder and then the soldering iron. Allow the joint to cool off for a few seconds.
• Inspect the joint to ensure that a good, shiny volcano-shaped joint has been formed.
• If the joint is not up to standard, apply more heat and a little more solder. • Be careful not to apply too much solder as this may result in a ‘dry’ or gray joint.
Warning
Be careful! Th e tip of a soldering iron can easily exceed 400 degrees Celsius, and the tip of a cool soldering iron looks exactly like the tip of a hot one!
Desoldering using a solder wick
Soldering two electronic components together is an art form in itself. Th ere oft en comes a time, however, when a soldered joint must be taken apart, or components must be replaced or removed from a PC or vero board. Th is requires the use of one of two tools – either a solder sucker or a solder wick. A solder wick is a long spool
of braided, oxygen-free, copper wire. Th e use of the solder wick allows one to desolder a connection without damaging the connected components. As with the solder sucker, the use of the solder wick must be practiced for the best desoldering results.
Figure 2.16: Solder wick
Th e following steps can be followed to desolder a component with the aid of a solder wick.
• Plug in the soldering iron and wait until it reaches operating temperature. • Press the solder wick against the joint to be de-soldered.
• Press the soldering iron against the wick. As the wick heats, it will melt the solder in the joint and soak it up through capillary action, leaving the joint clean and solder-free.
• Always remove the wick fi rst, then the soldering iron.
• Remember not to apply too much heat to the joint as this might also damage the fragile copper tracks.
• Cut off the solder-soaked end of the wick and throw it away, leaving the wick clean for the next job.
Tip of soldering iron
Solder wire
Lead from conponent Circuit
board Copper pad on board
shiny solder BAD JOINT (dry joint) GOOD JOINT (volcano shape) component lead copper tracks component dull solder PCB or stripboard
Figure 2.14: Correct solder process Figure 2.15: Soldering joints
Did you know? Capillary action
is the tendency of a liquid to rise in narrow tubes or to be drawn into small openings such as those between grains of a rock. A familiar example of capillary action is the tendency of a dry paper towel to absorb a liquid by drawing it into the narrow openings between the fi bers.
Printed circuit board manufacturing skills (design
and make)
Design a PCB
Designing a PCB can be done simply by using a permanent marker to manually trace a circuit onto the PCB, or by using a PCB soft ware package which will help to design the PCB. In most cases the more complex the circuit, the more diffi cult it will be to draw it manually. One can use the PCB soft ware packages to draw the schematic diagram and then to create the PCB design. However, some soft ware packages will even allow one to get the PCB design directly from the circuit diagram. In most cases the capability of each design package is directly linked to the cost. For the purpose of this book, reference is made to a PCB package that is available on the Internet free-of-charge, Express PCB. While this package is very user-friendly, the user does have to play around with it to understand all its capabilities. However, once the program has been downloaded, it is possible to design a PCB in a few simple steps, as set out below.
Step 1: Determine size of PCB
Once you have opened the program, you must determine the size of your work area (this is also the size of the PCB). Th is is important because the size of the PCB is determined by where the PCB will be fi tted. If, for example, the PCB must slot into an enclosure of 80 mm × 40 mm, you determine your work area by simply dividing 80 and 40 by 2.54 to give you the number of dots horizontally and vertically across the work area, i.e. 80/2.54 = 31 (dots) horizontally and 40/2.54 = 16 (dots) vertically.
Step 2: Select the components
Begin your layout by adding the components. Select the correct components from the Component Manager dialogue box.
Figure 2.18: Selection of components
Step 3: Position the components
Drag each component to the desired position on your screen (working area).
Step 4: Add the traces
Now add each trace by clicking on the pin of a component and dragging the trace to another pin. Make sure your trace is not too thin, because it will be etched away if etched too long.
Figure 2.20: Add traces
Step 5: Setting item properties
You can set the properties of items in your layout by double-clicking on them. For example, double-click on a trace to change its layer or width.
Figure 2.21: Setting item properties
Place a track
Red tracks
Step 6: Select tracks
Select the tracks that you would like to move to the bottom copper layer of the PCB by simply clicking on the icon with the green line and the two arrows on top.
Figure 2.22: Selection of tracks
Step 7: Printing
Once you have moved all tracks to the bottom copper layer, you are ready to print your design. Th is program allows you to print diff erent options, e.g.
• Th e top copper layer only • Th e bottom copper layer only
• Silkscreen layer (component outlines), or • Silkscreen, pads and text on top layer.
Figure 2.23: Printing
Once you have printed your desired copper layer, you are now ready to transfer your design to the PCB. Th e transfer of your design to the PCB is described in the section on the making of a PCB.
Move to bottom copper layer icon
Green tracks