Electronics Servicing

Energy is defined as the ability to do work. There are many different kinds of energy—mechanical, electrical, chemical, heat and nuclear energy as well as electromagnetic energy and the energy of sound (which is a form of mechanical energy). All electronic systems in order to work must have a source of energy, in particular, electrical energy. Isaac Newton, the seventeenth-century English physicist, suggested that there is a fixed amount of energy available in the universe of different kinds which can be converted from one kind to another but never destroyed. Earlier this century Albert Einstein showed that energy can in fact be obtained from mass, a fact shown convincingly in nuclear reactions, which are the basis of electricity generation in nuclear power stations and, of course, nuclear bombs. Energy conversion is taking place all around us all the time. In the internal combustion engine the chemical energy of petrol is converted to the mechanical energy of movement of the car, in the electrical generator mechanical energy of whatever drives the generator is turned into electrical energy (see chapter 4) and in the electric fire or the electric lamp (the incandescent type) electrical energy is converted to heat or light respectively. In the case of the lamp both heat and light energy is available since the lamp gets warm.

An electronic system is made up of electronic units, each unit carrying out a particular job on the electronic signal as it passes through the system. Electronic units, in turn, consist of one or more conductive circuits and these are made up of electrical and electronic components connected together so that when a voltage source is applied to the circuit an electric current flows from the source, to and through the circuit and back to the source. If the source is a direct current source the current flow is in one direction (conventionally from positive to negative—but electrons flow in the opposite direction) and if the source is an alternating current source the current flow changes direction periodically. Components and materials used for connecting them are considered in more detail later; for the moment we must examine the basic ways of connecting components together. The component which will be used as an example is one with which most people are familiar, a small lamp similar to a torch bulb, which could be used for indicating purposes in a circuit.

If the same voltage is applied across pieces of different materials such as copper, iron, Nichrome (a mixture of nickel and chromium) and glass, which are of equal size and at the same temperature, the amounts of electric current which flow in each piece are different. The largest current flows in the copper, less current flows in the iron, less still in the Nichrome and the current in the glass will be so small as to be virtually undetectable. The reason for this is the different atomic structure of the materials. Copper atoms have outer electrons which are so loosely bound to their ‘parent’ atoms that only a small amount of energy per electron is required to free it. Glass, on the other hand, has atoms which contain tightly bound electrons and a great deal of energy per electron is required before electrons can move away and form an electric current. The other materials require varying amounts of energy per electron, Nichrome needing about ten times as much as iron. The same voltage (which is a measure of energy per unit charge and therefore energy per electron) will thus cause more current in the copper than in the iron, even less in Nichrome and least of all in glass. Copper and other similar materials such as silver, gold and aluminium are said to be good conductors glass and materials such as rubber and plastics are good insulators. Materials between these two extremes may be called conductors or insulators depending on their electrical properties and there is a special class of materials called semiconductors which, strictly speaking, are neither conductors nor insulators.

The effects of magnetism, or, to be more precise, electromagnetism, have been known for a long time. In both early Chinese and early Greek civilisations it was known that pieces of certain materials, when allowed to hang freely, always pointed in a certain direction; the fact was used as an aid to the navigation of ships. Electricity and magnetism are in fact related, magnetism being caused by the movement of electric charge (as we shall discuss shortly) but the connection between the two was not suggested before the seventeeenth century and was not completely verified until the twentieth century.

Electrostatics is that branch of electricity concerned with static electric charge. We have seen that it is possible for materials to become electrically charged by gaining or losing electrons, one example being when certain materials are rubbed (glass with silk or ebonite with fur). It is also possible to charge materials using in the first instance moving electric charge, that is electric current, the charge then remaining on the materials.

Part of the definition of energy is that it is the ability to change, convert or modify a body’s state, shape or mass or state of rest. Heat is one form of energy and as such is measured in energy units: joules, the symbol for which is J.

Any electronic system may be considered to be made up of a number of subunits, each one containing a combination of active and/or passive components (discussed in chapter 10) designed to process appropriately the electronic signal as it moves through the system. These different processes include generation, amplification, differentiation, integration, modulation, de-modulation and switching. Also all subunits containing active components require electrical energy and this is provided by a further subunit, the power supply. The power supply does not itself generate or process a signal, but without it no generation or processing using active components can take place.

Chapter 7 described the various kinds of electronic signal and in chapter 9 a number of the more commonly used subunits for signal processing were considered. This chapter is concerned with joining together the various subunits to form complete systems.

Chapter 10 was concerned with the various types of electronic subunits that make up a system. These subunits are themselves made up of a number of electronic components connected together to form electrical current paths called a circuit. A diagram showing how the components are connected is called a wiring diagram if it shows the exact location of the components and the wires connecting them, and a schematic diagram if it shows how the components are connected but does not show the physical location of the components within a subunit. To enable a drawing of this type to be made, symbols are used for components; and, in the interests of general comprehension, these symbols are laid down by the British Standards Institution (BS 3939: Graphical Symbols for Electric Power, Telecommunication and Electronics Diagrams).

The number of basic hand tools in electronics servicing is quite small, although there are a number of additional specialist tools available which should be known and which one should be able to use.

Efficient servicing of electronic equipment requires a good working knowledge of electrical and electronic instruments, together with the ability to select a suitable instrument for a given application and to use it safely and effectively. In this chapter we shall examine the more commonly available instruments used in servicing and how they are used.