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Resistor values are organised into a set of different series of preferred values or standard values.
These standard resistor values have a logarithmically based sequence and this enables the different values to be spaced in such a way that they relate to the component tolerance or accuracy.
Resistor tolerances are generally ±20%, ±10% ±5%, ±2% and ±1%. More accurate tolerances are available for some resistors, but these are not as widely available and costs are higher.
By having these standard resistor values, components from a variety of manufacturers can be chosen, making sourcing much easier and the cost of the components much less.
E series of standard resistor values
The standard resistor values are organised into a set of series of values known as the E-series. The different values are spaced such 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 as an example a resistor that has a value of 1 ohm and a tolerance of ±20%. The actual resistance at the top of the tolerance band is 1.2 ohms. Take then a resistor with a value of 1.5 ohms. The resistance of this component at the bottom of its tolerance band is 1.2 ohms. This process is followed through for all the values in a decade, creating a set of standard resistor values for each tolerance.
The different sets of standard resistor values are known by their E-series numbers: E3 has three resistors in each decade, E6 has six, E12 has twelve, and so forth.
The E series preferred or standard resistor value ranges are internationally accepted and have been adopted by international standards organisations. The EIA (Electrical Industries Association) which is based in North America is one organisation that has adopted the system and as a result the resistor value series are often referred to as the EIA standard resistor values.
|Summary of EIA Preferred or Standard Resistor value Series|
|Number of values in each decade|
|E192||0.5%, 0.25% and higher tolerances||192|
Note: The metal film resistors now widely used for axial resistors and the surface mount resistors are normally available in 1% and 2% tolerance ratings even when included in the E24, E12, E6 and E3 ranges.
Resistor E series
The EIA preferred values or standard resistor values can be summarised in a tabular form to give the different values within each decade.
Current resistor technology enables very close tolerance levels to be achieved, yet there is still a great benefit in using resistors even from the E3 series. It reduces the number of different types of resistor used in a design and this simplifies purchasing and manufacturing processes. Often designs try to keep to the E3 or E6 standard resistor values, only using those in the E12, E24, E48 or E96 if absolutely necessary.
One example where values can be kept within the E3 series occurs with digital design where a pull up or pull down resistor is needed. The exact value is of little is of little consequence and the value can be maintained within the E3 series. For analogue designs it is a little more flexibility is often needed, but even E6 or E12 standard resistor values can be used without difficulty in the design. Occasionally E24, E48, E96 or even E192 series values may needed for high accuracy and close tolerance requirements.
Resistor E series tables of values
Below are the common resistor values. They are the standard E3, E6, E12, E24, E48 and E96 resistor values.
|E3 Standard Resistor Series|
The E3 series resistors are the most widely used and hence these values will be the most common resistor values used within the electronics industry.
|E6 Standard Resistor Series|
The E6 series resistor values are also widely used within the industry. They provide a wider range of common resistor values that can be used.
|E12 Standard Resistor Series|
|E24 Standard Resistor Series|
|E48 Standard Resistor Series|
The E96 and E192 series of standard resistor values also exist, but their use is much smaller than those in the other ranges given above.
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