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Silicon Chip's Guide to Discrete Electronic Devices With this first issue of "Silicon Chip" magazine, we begin a series of articles on discrete electronic components. These embrace virtually all electronic components except for integrated circuits. And if you want to understand how integrated circuits work, it is essential to have a prior understanding of discrete components. Electronic circuit components can be divided into two broad categories: discrete and integrated. Discrete components are those which usually only have two or three terminals and provide only 88 SILICON CHIP one circuit function within their package. Integrated components are these days mostly integrated circuits. These usually contain thousands or even millions of internal circuit components which together may provide many functions. Discrete components can be regarded as the basic building blocks for all electronic circuits. They are individually soldered or otherwise connected into circuit and they are made in an almost countless variety of types and values. Passive & Active Devices Within the broad range of discrete devices there is a further division into Passive and Active devices. These terms need defining although as we will ultimately see, the dividing line between active and passive devices is a blurred one. According to The New Penguin Dictionary of Electronics an active device is one which "introduces gain or has a directional function". More generally, an active device Left: all the components in this photograph are discrete devices except for the integrated circuit. Most discrete devices have only two or three connections, but some (such as valves) have many more. Our series will look at the majority of devices. can be one which amplifies, detects or otherwise changes the shape of a signal waveform. Within that definition we can include transistors, diodes and a whole host of semiconductor devices. Similarly, passive devices are those which do not amplify or change the shape of a signal waveform. They include resistors, capacitors, inductors and variations and combinations of these devices. Active & Non-linear Where the distinction between active and passive devices becomes blurred is with the terms linear and non-linear. Most passive components are said to be linear which means that their output is more or less similar, or in direct proportion, to their input. Under that definition most active components can be said to be non-linear. The tricky part comes when considering components such as thermistors which are a special type of resistor. Thermistors are certainly not linear but they are not active either. Fuses, another type of resistor, are passive but also highly non-linear. Incandescent lamps (the familiar light globes), which are resistive components, are again highly non-linear, because their light output and resistance is not at all proportional to the amount of current fed through them. The more you look into electronic components the more you will find that none of them can be simply defined as active or passive, linear or non-linear. Bear with us though, because even though the distinctions are blurred, the scene will become a lot less confusing as you read on. For the moment then, let us blithely assume that everything is black and white and carry on with the thought that there really are these two broad divisions of discrete electronic devices. In fact, TABLE 1: ACTIVE AND PASSIVE DEVICES Active Diodes Transistors Silicon controlled rectifiers Triacs Light emitters Light detectors Trigger devices Hall effect devices Thermionic devices Gas discharge devices Cathode ray tubes X-ray tubes Camera tubes Magnetrons Travelling wave tubes let us list a whole gamut of components under those two broad headings, as shown in Table 1. We could go on but we have probably already started arguments among the academics as to whether some of those devices listed above are legitimate or whether they are merely variants of others. For example, relays and solenoids might be regarded as special types of inductors, while light dependant resistors, thermistors and strain gauges might all be regarded as variants of the broad category of resistors. Well so be it. We have started a few arguments. But we have also left out many devices which are crucial to electronics such as the many different types of transducers, motors, power sources, and peripheral components such as switches, connectors and heatsinks. While we are in the process of making definitions, let us comment on the word discrete. Some people take this to mean small and low power, especially when applied to semiconductor components such as diodes and transistors, which tend to be very small. It is true that most discrete electronic components are very small and there is a continuing trend to make them much smaller. But in using the word discrete to describe the whole gamut of electronic devices, we are embracing some very large and high power components indeed. These can be the very large transformers, Passive Resistors Capacitors Inductors Transformers Relays Solenoids Light dependant resistors Strain gauges Piezoelectric devices Quartz crystals Thermistors Ferroelectric components Incandescent lamps Fuses Antennas capacitors, reactors, rectifiers and circuit breakers used in electrical power distribution. And let us not forget the very large valves used in radio and television transmitters and the even larger antennas used to broadcast the signal. In fact, we could point to specific examples of all the classes of components listed above which are very large and rated for very high power, as well as examples at the other extreme which are highly miniaturised and operate at very low powers. The smallest of these are called surface mount components and these are quite difficult to handle, simply because they are so small. They are specifically designed to be handled and assembled into electronic equipment by machine. Finally, we made the comment above that most discrete components have only two or three connecting terminals and this is indeed true. But it is possible to point to devices which have many terminals, even though they truly come under the heading of discrete devices. Some examples of these are · valves, transformers and cathode ray tubes, all of which can have six or many more terminals. This is just another case where the general rule can be shown not to apply universally. Here then is the end of the beginning. In the next chapter we shall look at the most simple and perhaps the most commonplace discrete component, the resistor. ~ NOVEMBER1987 89