Preferred Resistor Values
- After studying this section, you should be able to:
- • Describe different classes of resistor according to the EIA Preferred Value System.
- E6, E12, E24, E48, E96 and E192.
- • Describe the BS1852 ‘No decimal point’ notation for resistors.
The BS 1852 Notation System for Resistors
The values of resistors are of course quoted in Ohms (Ω), though with high values, Kilohms (KΩ) or Megohms (MΩ) are common units. With low values a circuit diagram may state a resistance value as for example 15R instead of 15Ω. When a value contains some fraction of a Kilohm or Megohm, such as 4.7KΩ or 5.6MΩ it will often be written 4K7 or 5M6 respectively. This is done for clarity. It avoids using the point (.) or the Omega (Ω) symbol, both of which may be misread when the printing is very small either on printed diagrams, or on actual components.
The EIA Preferred Value System
To manufacture resistors of every possible value would be impractical. Instead Resistors are made in a restricted range of values and each value is quoted as a specific number of ohms plus or minus a percentage of the quoted value, this range of possible values is called the tolerance of the resistor.
A 100KΩ resistor having a +/-10% tolerance might be any value between 90KΩ and 110KΩ. Therefore there is no need to manufacture resistors with values between these upper and lower limits. If a resistor of exactly 100KΩ is needed (an unusual situation) a resistor with the exact value can be selected from within this range or (more likely) a resistor with a closer tolerance can be used.
The E12 Series
Fig. 2.2.1 shows how these tolerance ranges are used in the E12 series of resistors to cover (almost) all possible values of resistance between 10Ω and 100Ω. The E12 series is so called because 12 ‘Preferred Values’ of resistor, each having a tolerance range of +/-10% covers all values from 10Ω to 100Ω. This range of values is called a decade, and the next higher range (decade) in the E12 series covers values between 90Ω (100Ω -10%) and 902Ω (820Ω +10%) and so on. The E12 range with its 10% tolerance therefore has 12 values per decade.
Other ranges of resistors such as the E6 and E24 ranges cover wide ranges of values in a similar way and have tolerance values of 20% and 5% respectively. The E6 range has 6 values, and the E24 range 24 values per decade. The more accuracy (closer tolerance) needed in resistors chosen for a particular purpose, the more values must be in the range chosen (and generally the more expensive each resistor will be).
Fig. 2.2.1 E12 Series of Overlapping Tolerances
The E24 Series
This method can be used of course over a much wider range of values and with different tolerances. The E24 series of values shown below gives a decade of preferred values available. Other decades are available in this series.
10 11 12 13 15 16 18 20 22 24 27 30 33 36 39 43 47 51 56 62 68 75 82 91
The E24 sequence (or any other E sequence) can be scaled up by adding a number of zeros for K (Kilohms) or M (Megohms) after the value. Equally they can be scaled down by adding a decimal point to give fractions of ohms. In this way a preferred value of 47 in the decade range may represent (multplying or dividing by ten);
.47Ω 4.7Ω 47Ω 470Ω 4.7KΩ 47KΩ 470KΩ 4.7MΩ etc.
Note that to avoid misreading of the decimal point, the above values will sometimes be written, both on components and in technical literature, using the BS1852 notation;
R47 4R7 47R 470R 4K7 47K 470K 4M7 etc.
The E ranges of resistors are specified by the Electronic Industries Association (EIA) with each range having its own specified tolerance, which in turn gives the required number of values to cover each decade.
Other EIA Ranges
- E6 20% tolerance. (Very little used).
- E12 10% tolerance.
- E24 5% tolerance. In common use, also made in 2%.
- E48 2% tolerance. In common use, better coverage of the decade range than E24, also used instead of E96 where cost is more important than specific accuracy.
- E96 1% tolerance.
- E192 less than 1% tolerance, used only where great accuracy is important.
Resistors with wider tolerances need less preferred values to cover a given range than close tolerance types. More data can be found in Component Distributors catalogues.