# 2.4 Colour coding and standard resistor values

Conventional fixed resistors have coloured bands showing their value. These can be decoded using Table 3. For example, a resistor with four bands where the coding is yellow (4), violet (7), red (× 100) and brown (1%) has a resistance of

(47 Ω × 100) ± 1% = 4700 Ω ± 47 Ω

There is a similar five-band scheme where there are three instead of two digits in the resistance value, and a six-band scheme that also gives temperature information. Sometimes it can be hard to see these colour values on the resistors and a multimeter is used to check them.

Table 3 The four-band colour-coding scheme for resistors

Resistors are required across a range of millions of ohms. To keep costs low, manufacturers have defined ‘standard’ sets of resistor values such as those shown in Table 4 for resistors with a tolerance of ±5%.

Table 4 Standard resistor values in ohms (±5%)

The standard or preferred values shown in Table 4 are chosen such that whatever value resistance is required, there is one within 5% of the specified value. Fixed resistors’ costs are usually very low.

OpenLearn - An introduction to electronics
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