Light Current Electrical Applications III

Although the term ‘regulated power supply’ could cover a whole range of types of power supplies, in the context of that used to supply electronic equipment it is usually taken to mean those types supplied from the 240 V 50 Hz mains and providing a d.c. output. Furthermore, although two basic types exist, constant voltage and constant current, by far the most usual and hence the most important is that of the constant-voltage type, the basic purpose of which is that it should provide a constant output voltage irrespective of the input conditions to or the current being drawn from the unit.

An electronic system requires amplifiers in order to produce, from low-level signals often of the order of microvolts, signals of sufficient level to drive an output amplifier stage; the output stage in turn produces signal power to drive a transducer of one sort or another. As an example, a television receiver amplifies the weak aerial signal of some 50 to 300 μV and ultimately uses the result to provide sound and picture information. In one case the speaker system associated with the sound channel may require several watts of signal power to provide sufficient sound level and in the other case over 100 V of signal is needed to provide the picture information via the cathode-ray tube. Multiple-stage amplifiers with gains in excess of 10⁵ are therefore normal and the problems associated with such amplifiers are the maintenance of stability and constant gain of the amplifier. In general in the explanations that follow concerning amplifiers npn transistors are considered but identical circuits may be used with pnp types the only difference being that the supply potentials are reversed from those of npn transistors.

The previous chapters have indicated how electronic circuits may be used to amplify and provide signal power to a load. However, a complete electronic system invariably requires some sort of waveform to be generated as part of its function; examples of such waveforms are shown in figure 3.1. All these waveforms are capable of being produced by electronic circuits known as oscillator circuits and since the waveform is generated completely within the circuit itself this implies that the circuit supplies its own input.

Most problems are not capable of simple solution in the sense that a yes/no decision cannot be made until several factors are considered. In arriving at a solution to a problem the process is one of a series of yes/no decisions taken in sequence to arrive at an overall result. Electronic circuits may be used to form a ‘model’ of a particular problem and by the use of simple ON/OFF circuits simulating the alternatives, the solutions to complex problems may be obtained.

Although the semiconductor device is generally thought of as a low-power component, some semiconductor devices are used at very high power, examples being the reverse-blocking thyristor (usually referred to as the thyristor) and the bidirectional thyristor (usually referred to as the triac). Both these devices are widely used in industrial applications where they may be used, for example, to control the speed of rotating machinery or to determine the amount of current supplied to an electric furnace in order to control the furnace temperature. At the other end of the scale they are used in the domestic field in such items as washing machines to control motor speed and in television sets where they may be used to stabilise d.c. supplies against mains variation or changes in load current.

The natural outcome of the quest for smaller components since the introduction of the transistor in 1948 is the monolithic integrated circuit, the word ‘monolithic’ implying that the circuit is produced within a single crystal of semiconductor material.