A series aimed especially at musicians who prefer to build; rather than buy, their gear. Tim Edwards introduces some essentials for beginners

Reliability is often quoted as an advantage of using electronic, rather than mechanical or electro-mechanical devices. Modern electronic circuits can be very reliable indeed, with circuit failure more usually due to a bad connection or a poor joint.

The importance of soldering is highlighted here — so that's where we'll begin the series.

Choose Your Weapon

Start by selecting the right soldering iron. For most work, one rated at 15W to 30W will suffice. Many people prefer the smaller 15W type, but I prefer a 25W iron so that the heat is transferred to the joint as quickly as possible and the components themselves are heated no more than necessary. Size of iron is a matter of personal preference, and perhaps a 17W model is a good compromise. The size of the bit is also very important; a 3mm diameter point is usually large enough. Anything larger is difficult to manoeuvre around IC legs or closely packed components.

Once you've chosen your iron, getting the soldering technique right is just a matter of practice. Like everything else, practice makes perfect, but there are several important points to remember.

Once the iron is hot, the bit must be well tinned, that is, covered with molten solder. The excess solder must be wiped off with a damp sponge or cloth to give a clean tip with a thin coating of solder. This should always be good quality, cored electrical solder, the core containing a resin "flux" which absorbs non-conductive oxides from the joint to ensure a good connection. The metals in the solder will only fuse to other metals such as the conductive copper used on PCBs and 'Vero' board, not to an oxide layer or indeed to certain other metals such as aluminium.

The diameter of solder is given in Standard Wire Gauge (SWG); the smaller the gauge number, the thicker the wire. A reel of 22SWG solder is useful for most jobs; anything larger than 18SWG is too thick for the smaller soldering irons to melt, and can result in too much solder being put on the joint. It's much easier to add a little more of a small gauge solder than to remove solder if you've added too much.

Getting Started

If we assume that you want to solder components onto a circuit board, begin by inserting the components into the pre-drilled holes in the board, and bend the leads slightly to hold them in place (Fig 1). Now press the top of the iron firmly onto the copper track and heat the component lead at the same time. Then quickly melt the solder into the junction between the copper track, the tip of the iron and the component lead — don't put solder straight onto the iron itself. The solder will make a good thermal contact between the parts and allow the temperature of the leads and track to rise quickly, causing the solder to flow around the joint. Make sure that there is sufficient solder to cover the entire joint, no more and no less. Then remove the iron and the solder. When you've got the hang of neat soldering you'll find that the solder itself does most of the work, particularly if the joint is reasonably clean.

Figure 1a shows the appearance of a good solder joint, which should have concave surfaces of shiny solder. A dull joint that appears to have cracked or 'crazed' probably indicates that you have moved the component before the solder has cooled sufficiently. Blobs of solder mean either dirty tracks and components, or insufficient heat applied to the joint.

Commercially produced boards will usually be pre-tinned, that is, covered with a film of solder. With this type of board not much preparation is necessary. A home-produced board is likely to be of bare copper, and in this case it will be necessary to clean off any possible oxide layer with a fine abrasive. This will leave a shiny copper surface which will take solder easily. When soldering a loose component, such as a wire to a potentiometer tag, it is usually best to pre-tin both to ensure a good thermal contact again. As the iron touches the parts, the solder melts and carries the heat around the joint. Try making the joint with and without any pre-tinning and see the difference for yourself.

During the course of soldering you will notice a build-up of solder and dross on the iron. The dross consists of oxides of tin and lead and of resin from the core. Periodically wipe this away with a damp sponge; it's best to keep the bit of the iron clean at all times to prolong its life and efficiency.

In Glorious Colour

Most of the components you'll be working with carry a colour code to indicate their exact value and other information. Usually this is used because there is not enough physical space on a component to print any information; after a while you can become familiar enough with the colour coding system to recognise the commoner values immediately. Let's start by looking at the codes for resistors.

Normally, there will be 4, 5, or 6 colour-code bands as shown in Fig 2. When there are four bands, the first three will denote the value of the resistor, and the last will indicate the tolerance. Tolerance is quoted as a percentage and tells you how close to the stated value the resistor is likely to be: the manufacturers will guarantee that it is within 1%, 5%, 20% or whatever. High precision circuits require close tolerance components, which are more expensive often simply because of the quality control procedures necessary to identify them. The standard values available in resistors are carefully chosen so that if a particular component falls outside the required tolerance for one value, it can be re-coded within the stated tolerance of the next value (up or down). That way there's no waste at the manufacturers' end, and the overall effect is that components stay reasonably priced for the constructor.

Tolerance is given as the last band of a four-band code; a gold band indicates a tolerance of 5% for instance.

The first and second bands of a four-band code indicate the value in two figures, and the third a multiplier as a power of ten. So, for instance, a resistor of value 10k would be coded:

1st band Brown = 1
2nd band Black = 0
3rd band Orange = 3 therefore times 10³(1000)
(A complete list of colour codes is given in Table 1.)

The five band colour code for 10k would read:

1st band Brown = 1
2nd band Black = 0
3rd band Black = 0
4th band Red = 2 therefore times 10² (100)
5th band Red = 2 (two percent tolerance)

Some resistors, notably wire wound types, are marked according to the Standard Code of Resistance Notation. This is given in Table2.

Other Components

Capacitors are not marked in exactly the same way as resistors, in fact they can be marked in several different ways. Polyester types are marked in a fairly similar way to resistors (see Fig 3) with the first three bands indicating the value. The fourth band indicates the tolerance, which is more likely to be Black (20%) or White (10%) for instance, and the fifth band indicates the working voltage (Red 250V DC, Yellow 400V DC). Take note, however, that these bands are not separated by a gap, which can be confusing. If, for example, the value is 33 nanoFarads (33nF) plus or minus 20%, 250V, the code would be Orange, Orange, Orange, Black, Red (note that the value is given as 33000 picoFarads or pF), and since the first three bands are the same colour they will blend and appear as one wide Orange band.

Larger capacitors will be marked clearly in figures. If there is no unit of capacitance included in the marking, the value will usually be in picoFarads. So the figure 1000 on a small ceramic capacitor indicates 1000pF or 1nF.

Electrolytic capacitors are polarised, that is, they have to be connected the right way round to the power in the circuit in order to function correctly. The indentation in the metal can of an electrolytic is the Positive end; alternatively the leads may be marked, usually the negative lead, particularly in the case of radial configuration capacitors where both leads protrude from the same end.

Next month we'll continue with component ID, concentrating on semiconductors — transistors, diodes and integrated circuits.