A sub-system is a part of a system which performs an identified function within the whole system; the amplifier in Fig. 2.1 is an example of a sub-system. A component or element is usually the simplest part of a system which has a specific and well-defined function – for example, the microphone in Fig. 2.1. The illustration in Fig. 2.1 is called a block diagram and electrical/electronic systems, which can often be quite complicated, can be better understood when broken down in this way. It is not always necessary to know precisely what is inside each sub-system in order to know how the whole system functions. As another example of an engineering system, Fig. 2.2 illustrates a temperature control system containing a heat source (such as a gas boiler), a fuel controller (such as an electrical solenoid valve), a thermostat and a source of electrical energy. The system of Fig. 2.2 can be shown in block diagram form as in Fig. 2.3; the thermostat compares the actual room temperature with the desired temperature and switches the heating on or off.
There are many types of engineering systems.Acommunications system is an example, where a local area
network could comprise a file server, coaxial cable, network adapters, several computers and a laser printer; an electromechanical system is another example, where a car electrical system could comprise a battery, a starter motor, an ignition coil, a contact breaker and a distributor. All such systems as these may be represented by block diagrams.
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