Standard resistor values

- the E series for preferred resistor values

The values given to resistors (and capacitor) fall into a number of standard or preferred values. They have a logarithmic sequence related to the component accuracy, enabling the standard values to be values to be spaced according to the tolerance on the component.

Component values such as resistor values can never be manufactured exactly, each resistor has a tolerance associated with it. These may typically be ±20%, ± 10% and ±5%. Other tolerances such as ±2% are also available.

In order to enable the standard resistor values to be spaced apart according to their tolerance, a series known as the E series for standard or preferred values is used. Resistors are spaced apart so that the top of the tolerance band of one value and the bottom of the tolerance band of the next one do not overlap. Take for example a resistor with a value of 1 ohm and a tolerance of 20%. If the actual resistance of the component falls at the top of the tolerance band then it will have a value of 1.2 ohms. Take then a resistor with a value of 1.5 ohms. Again it is found that the value at the bottom of its tolerance band is 1.2 ohms. By calculating a range of values in this way a series can be built up. This is repeated for each decade.

The series generated in this way is known as the E series and these are preferred values. If a ±40% tolerance is used there will only be 3 values in each decade (1, 2.2. and 4.7), and the series is known as the E3 series as there are three values in each decade. There is also an E6 series with six values in each decade for a ±20% tolerance, E12 series with 12 values in each decade for a ±10%, E24 series with 24 values in each decade for a ±5% tolerance. Values for resistors in these series are given below. Further series (E48 and E96) are available, but are not as common as the ones given below.

The E6 and E12 resistors are available in virtually all types of resistor. However the E24 series, being a much closer tolerance series is only available in the higher tolerance types. Metal oxide film resistors that are in common use today are available in the E24 series as are several other types. Carbon types are only available in the lower tolerance ranges as their values cannot be guaranteed to such a close tolerance.