Newcomers to electronics could be forgiven for thinking that colour codes are used on certain electronic components purely to make life difficult. In fact, there are a couple of good reasons for using colour codes, and one of these is simply that a value marked in this way is less likely to be obliterated than one marked in figures and numbers. Even if the colour coding should become partially removed it is still likely that easy and accurate decoding would be possible. Partially obliterated letters and numbers are almost certain to be meaningless, or worse still, misleading with the value being read incorrectly. Another point in favour of colour codes is that they enable experienced users to quickly pick out a component of the desired value from a large batch of components, or quickly locate a component of a certain value on a large circuit board. With experience you learn to read colour codes without really having to consciously think about it.

Resistors

Resistors usually have their value marked by means of four coloured bands around the body of the component, and very few types have the value marked by other means. The first band, which is the one nearest to one end of the component (see Figure 1) gives the first number of the value, the second band gives the second digit, and the third band gives the multiplication factor needed to bring this two digit number to the correct figure. The fourth digit gives the tolerance of the component's value. The power rating of the component is not indicated by the colour coding.

The colours used in the resistor colour code and the figures they represent are given in Table 1.

It we now take a few examples it should make the code perfectly clear. A component having red, violet, orange, gold as its markings would have 2 as the first digit of the value and 7 as the second digit. This gives 27 which must be multiplied by orange, which represents 1000 when it appears in the third band, and this gives a value of 27000 ohms (27 kilohms or 27k in other words). The gold fourth band indicates that the tolerance of the component is ± 5%, by which we simply mean that the actual value of the component is within 5% of its marked value.

A resistor having bands one to four with the code brown, black, blue, silver would have a value of 10000000 ohms, or 10 megohms (10M as this would normally be written). The brown and black bands give the first two digits of 1 and 0 respectively, and then the third band gives the multiplier value of one million. Ten multiplied by a million is obviously ten million ohms, or ten megohms. The silver fourth band indicates a value which is within 10% of the marked value, so that the actual value of the component would be between nine and eleven megohms.

A resistor having a coding of red, red, gold, red would have a value of 22 x 0.1 which equals 2.2 ohms. The tolerance would be ± 2%.

Capacitors

Not many capacitors use colour coding these days, but one common range of capacitors that do is the C280 range of printed circuit mounting polyester capacitors. The first three bands indicate the value in the same way as the first three bands in the resistor colour code, but the value is in picofarads (pF) not ohms, of course. The figure indicated should be divided by 1000 to give the value in nanofarads (nF) or 1000000 to give the value in microfarads (uF). Green is used in the third band of the C280 colour code, unlike the resistor colour code. The first band of a C280 colour code is the one at the top of the component, opposite the leadout wires, as shown in Figure 2.

The fourth band indicates the tolerance of the component, but the code here is different to that used for resistors. Only two colours are commonly used in the fourth band: black which indicates a tolerance of ± 20%, and white which indicates a ± 10% tolerance.

The C280 colour code has a fifth band and this indicates the maximum DC working voltage of the component. Again only two colours are normally used here; red for 250 volt components, and yellow for 400 volt types.

For example, consider a capacitor with the marking yellow, violet, orange, black and red. The first three bands give 4, 7 and a multiplier of 1000 as with the resistor colour code. Thus the value of the capacitor is 47000pF or 47nF. The fourth black band indicates that the tolerance is ± 20% and the fifth red band a maximum DC working voltage of 250V.