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The value or the resistance of a resistor is obviously very important. There are a number of ways in which this can be indicated. On some of the larger wire-wound resistors the value can be printed onto it in the normal way. However the smaller resistors like those most commonly encountered are colour coded. The reason for this is that any numbers printed on the component would very easily be removed and they would not be easy to read. To overcome this coloured rings are used.
Resistor colour code or coding scheme
There may be three four or sometimes even five rings. These rings have values as shown in table 1. The first two indicate the significant figures in the value. The second is a multiplier and shows the power of ten to which the significant figures must be multiplied. As an example if a resistor had the rings yellow, purple, red these would correspond to the values 4, 7, 2, indicating a resistance of 47 x 10^2 or 4700 ohms. It can be seen from this that the third ring corresponds to the number of zeros after the significant figures.
The fourth ring if it is present shows the tolerance, i.e. how accurate the resistor is. This is indicated as a percentage. Many resistors today are either 2% or 5%. This means that their value will be within 2% or 5% of the stated value. Years ago most resistors were only 20%, although even today tolerances of this order are quite acceptable for many situations.
The fifth ring, again if it is present, indicates the temperature coefficient. As the value of a resistor will change with temperature this may be important in some situations. This information may be added in the fifth ring on the resistor. These figures are quoted in ppm/C i.e. parts per million per degree C. In other words a 1 kohm resistor with a 100 ppm temperature coefficient would change by 0.1 ohms for every degree Celsius it changes.
Resistor Colour Coding Scheme
| Band 2
| Band 3
| Band 4
Marking scheme for Surface mount resistors
Surface mount resistors are only sometimes marked with their values. When a value is marked it will be printed on the component as three figures - the two significant figures and the multiplier. No further figures are given - the components are too small to print any more figures on them.
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