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Understanding Electronics

Reading Circuit Diagrams

A regular column that explains the electronics of music

The ability to look at a circuit diagram and fully understand each part of the circuit, plus the overall working of the unit, is something that can only be achieved after a great deal of experience and knowledge have been attained. Even then there are so many areas of specialist interest that it is unlikely that anyone can comprehend every circuit. However, translating a circuit diagram just in terms of how the various components are interconnected is a much more simple process, but is nevertheless a very useful asset for the musician using electronic equipment. Furthermore, many of the articles in E&MM have circuit diagrams which can give you a practical insight into the workings of instruments and projects.

In essence, circuit diagrams are very straightforward and simple to understand with lines being used to represent wires that connect the various components together, and the components being represented by symbols. The main difficulty in translating the diagram into actual wires and components is that there is no real standardisation of the symbols used, although if a little common sense is employed this should not be a great problem in practice. Another problem is merely that two components that are physically quite different can have the same circuit symbol.

Figure 1.

Where two lines on a circuit diagram meet and there is a dot at the junction of the two, as shown in Figure 1(a), this means that the two wires connect together. If the two lines cross over and there is no dot, or one line is looped over the other, as shown in Figure 1(b), the two wires do not connect to one another. Note that dots are only used to show connections at "T" junctions on the diagram, and in Figure 1(c) for example, where no dots are used, the capacitor is in fact connected across the resistor.

Some of the more common circuit symbols appear in Figure 1. Here symbols (a) to (e) are all capacitors of various types and are respectively an ordinary capacitor, an electrolytic capacitor, a polarised non-electrolytic capacitor (such as a tantalum type), a trimmer capacitor, and a variable capacitor. Electrolytic and other polarised capacitors normally have a "+" sign marked on the body of the component to show which lead is which, and electrolytic capacitors sometimes have an indentation around one end of the component's body to indicate that the positive leadout emanates from this end of the component.

The symbols shown in (f) to (h) are for various types of resistor, and respectively represent an ordinary (fixed value) resistor, a preset resistor, and a variable resistor or potentiometer. The alternative symbols shown in (i) to (k) are often used instead of those shown in (f) to (h).

Some circuit diagrams use a rectangle in the capacitor symbol instead of the two parallel lines, and also use the resistor symbols of (f) to (h). Such diagrams are usually rather slow and difficult to follow since the resistor and capacitor symbols are similar, and in the case of a fixed resistor and a fixed non-polarised capacitor they are identical! In order to distinguish between the two, it is necessary to look at the identification legend for the component in question: it will start with a "C" in the case of a capacitor, or an "R" if the component is a resistor. Also, the value will be shown in farads for capacitors and ohms for resistors. Fortunately most circuit diagrams do not use this system, and it is usually only found in manufacturers handbooks, service manuals, and similar publications.

Most resistors and some capacitors have their value marked using a colour code, and this was covered in detail in last month's "Understanding Electronics".

Bipolar transistors use the circuit symbols of (l) and (m); these being n.p.n. and p.n.p. types respectively. Transistors can be a bit awkward when it comes to dealing with the actual components since there are a large number of different case styles in use, although there are relatively few that are in common use. Another area of confusion is that of identifying the leadout wires, and the difficulty arises due to some case styles (such as the popular TO-92 plastic encapsulation) having more than one leadout configuration. Thus, although the 2N3702, 2N3904, and BC650 all have the same basic encapsulation, the leadout configuration is different in each case! It is a good idea to equip yourself with a transistor data book or one of the large mail order component catalogues that give transistor case and leadout information, and to always check the leadout details of unfamiliar devices.

Figure 2.

Integrated circuits are normally represented by a large rectangle or triangle from which wires emanate, with the pin numbers being marked around the circuit symbol as shown in (n). There are only a few integrated circuit case styles in common use these days, but one point to bear in mind is that whereas a transistor leadout diagram such as that given in Figure 2(a) is a base view, an integrated circuit pinout diagram such as the one shown in Figure 2(b) is a top view. In other words the transistor is as it would be seen with the leadouts pointing towards the viewer whereas the integrated circuit is as it would be seen if viewed looking at the side having the type number, and with the pins pointing away from the viewer.

The circuit symbol for a diode or rectifier is shown in (o), and (p) is the symbol for a light emitting diode (LED). The alternative symbols of (q) and (r) are often used. The cathode leadout of a diode is normally indicated by a band marked around the appropriate end of the component's body. This is by no means always the case though, and many diodes have a number of bands. In such cases the bands are normally offset slightly towards one end of the component, and are offset towards cathode leadout wire. Some diodes, for no apparent reason, have the band marked around the wrong end of the component! It is a good idea to check which leadout is which using a test meter, continuity tester, or semiconductor tester, when dealing with unfamiliar diodes. LEDs often have the leadouts identified by having one slightly shorter than the other, but unfortunately there is no standardisation as to which is the shorter, and other systems are sometimes used. It is therefore necessary to either refer to the manufacturers (or retailers) data, or to check using a suitable piece of test equipment.

The circuit symbol of (s) is the one used for a ferrite cored inductor (R.F. choke). This is another component which in some circuit diagrams is represented by a rectangle, together with resistors and capacitors.

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Apr 1982

Scanned by: Stewart Lawler

Feature by Robert Penfold

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