Preferred Values.
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 any 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. We simply select a 100KΩ resistor with the exact value we need.
In practice it is very rare that we need to be so precise. We can usually select a resistor with a suitable tolerance whose value will be close enough to the value we need for any particular circuit. The diagram below shows how these tolerance ranges are used in the E12 series of resistors to cover all 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; 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 resitors 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 accurate (less tolerance) we need in resistors chosen for a particular purpose, the more values must be in the range we choose to use (and generally the more expensive each resistor will be).
E12 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 sequence can be scaled up by adding zeros, K (Kilohms) or M (Megohms) after the value. Equally they can be scaled down by adding a decimal point to give fractions of ohms.Thus we can have preferred values based for example on the number 47 including;
.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;
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.
- 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.