Standard resistor values
- standard resistor values given in the EIA E series - with explanations of the system & tables of these common resistor values.
The values given to resistors fall into a number of preferred or standard resistor values. These standard resistor values have a logarithmic sequence related to the component accuracy, enabling the standard resistor values to be spaced according to the tolerance on the component. These standard resistor values are also applicable for capacitors and other components as well as resistors.
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 ensure that standard values can be chosen from one of a variety of manufacturers, list of preferred values or standard resistor values have been devised. By using these preferred resistor value lists, common resistor values can be chosen from available components. This not only makes manufacture easier, but it enables stock holdings of manufacturers to be reduced by having a preferred resistor value range. As most component values need not be high precision special values, this is a particularly attractive idea.
E series of standard resistor values
In order to enable the common 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 for standard resistor values is known as the E series and these are preferred values. The most basic series within the E range is the E3 series which has just three values: 1, 2.2 and 4.7. This is seldom used as such because the associated tolerance is too wide for most of today's applications, although the basic values themselves may be used more widely to reduce stock holding.
The next is the 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 rarely available these days and in any case would only available in the lower tolerance ranges as their values cannot be guaranteed to such a close tolerance.
The E series preferred or standard resistor ranges are widely used and have been adopted by standards organisations. The EIA (Electrical Industries Association) based in North America has, for example adopted the system and as such the values included are often referred to as the EIA preferred values.
|Summary of EIA Preferred or Standard Resistor Series|
|Number of values in each decade|
[normally also available in 2% tolerance]
|E192||0.5%, 0.25% and higher tolerances||192|
Preferred and standard values of other components
The system for adopting standard component values works very well for resistors. It is also equally applicable for other components. The same concept of using values in a standard list that are determined by the tolerance of the components is equally applicable.
Accordingly the E series preferred values are also widely used for capacitors, where some of the lower order series are used - E3, E6 as the values on many capacitors do not have a high tolerance. Electrolytic capacitors typically have a very wide tolerance, although others such as many ceramic types have a much tighter tolerance and many be available in ranges conforming to the E12 or even E24 values.
Another example of components that follow the EIA E series preferred values is Zener diodes for their breakdown voltages. The Zener diode standard voltages typically conform to the E12 values although E24 series voltage values are also available - especially 5.1 volt Zener diode for 5 volt rails.
By Ian Poole
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