For brevity, the notation often does not specify the unit (ohm orfarad) explicitly and instead relies on implicit knowledge raised from the usage of specific letters either only for resistors or for capacitors,[nb 2] the case used (uppercase letters are typically used for resistors, lowercase letters for capacitors),[nb 3] a part's appearance, and the context.
The notation also avoids using adecimal separator and replaces it by a letter associated with the prefix symbol for the particular value.[nb 4]
This is not only for brevity (for example when printed on the part or PCB), but also to circumvent the problem that decimal separators tend to "disappear" whenphotocopying printed circuit diagrams.
Another advantage is the easier sortability of values which helps to optimize thebill of materials by combining similar part values to improve maintainability and reduce costs.[nb 5]
The code letters are loosely related to the correspondingSI prefix, but there are several exceptions, where the capitalization differs or alternative letters are used.
For example,8K2 indicates a resistor value of 8.2 kΩ. Additional zeros imply tighter tolerance, for example15M0.[12]
When the value can be expressed without the need for a prefix, anR orF is used instead of the decimal separator. For example,1R2 indicates1.2 Ω, and18R indicates18 Ω.
Forresistances, the standard dictates the use of the uppercase lettersL (for 10−3),R (for 100 = 1),K (for 103),M (for 106), andG (for 109) to be used instead of the decimal point.[12]
The usage of the letterR instead of the SI unit symbol Ω for ohms stems from the fact that the Greek letter Ω is absent from most oldercharacter encodings (though it is present in the now-ubiquitousUnicode) and therefore is sometimes impossible to reproduce, in particular in some CAD/CAM environments. The letterR was chosen because visually it loosely resembles the Ω glyph, and also because it works nicely as amnemonic forresistance in many languages.[citation needed]
The lettersG andT weren't part of the first issue of the standard, which pre-dates the introduction of theSI system (hence the name "RKM code"), but were added after the adoption of the corresponding SI prefixes.
The introduction of the letterL in more recent issues of the standard (instead of anSI prefixm formilli) is justified to maintain the rule of only using uppercase letters for resistances (the otherwise resultingM was already in use formega).
Similar, the standard prescribes the following lowercase letters forcapacitances to be used instead of the decimal point:p (for 10−12),n (for 10−9),μ (for 10−6),m (for 10−3), but uppercaseF (for 100 = 1) forfarad.
The lettersp andn weren't part of the first issue of the standard, but were added after the adoption of the corresponding SI prefixes.
In cases where the Greek letterμ is not available, the standard allows it to be replaced byu (orU, when only uppercase letters are available). This usage ofu instead ofμ is also in line withISO 2955 (1974,[14] 1983[15]),DIN 66030 (Vornorm 1973;[16] 1980,[17][18] 2002[19]),BS 6430 (1983) andHealth Level 7 (HL7),[20] which allow the prefixμ to be substituted by the letteru (orU) in circumstances in which only theLatin alphabet is available.
Though non-standard, some manufacturers also use the RKM code to markinductors withR indicating the decimal point inmicrohenry (e.g.4R7 for4.7 μH).[23][24]
A similar non-standard notation using the unit symbol instead of a decimal separator is sometimes used to indicatevoltages (i.e.0V8 for0.8 V,1V8 for1.8 V,3V3 for3.3 V or5V0 for5.0 V[25][26][27][28][29][30]) in contexts where a decimal separator would be impossible to use or inappropriate (e.g. in signal or pin names, invariable names, infile names, or inlabels orsubscripts). Alternatively, letterP (presumably standing for "positive voltage" or "power supply rail")[nb 6] is seen being used instead of theV sometimes in device models and netnames (i.e.1P8 for1.8 V,3P3 for3.3 V).[31][32][33][34][35][36] Respectively, both variants are also used as part of the MPN codes ofzener diodes[27][37] andvoltage regulators[36] by some manufacturers.
Letter code for resistance and capacitance tolerances:
Code letter
Tolerance
Resistance
Capacitance
Relative
Absolute
Symmetrical
Asymmetrical
C <10 pF only
A
A
variable (±0.05%)
variable
variable
B
B
±0.1%
—N/a
C
C
±0.25%
—N/a
±0.25 pF
D
D
±0.5%
—N/a
±0.5 pF
E
±0.005%
—N/a
—N/a
F
F
±1.0%
—N/a
±1.0 pF
G
G
±2.0%
—N/a
±2.0 pF
H
H
±3.0%
—N/a
—N/a
J
J
±5.0%
—N/a
—N/a
K
K
±10%
—N/a
—N/a
L
±0.01%
—N/a
—N/a
M
M
±20%
—N/a
—N/a
N
±30%
—N/a
—N/a
P
±0.02%
—N/a
—N/a
Q
—N/a
−10/+30%
—N/a
S
—N/a
−20/+50%
—N/a
T
—N/a
−10/+50%
—N/a
W
±0.05%
—N/a
—N/a
Z
—N/a
−20/+80%
—N/a
Before the introduction of the RKM code, some of the letters for symmetrical tolerances (viz. G, J, K, M) were already used in US military contexts following theAmerican War Standard (AWS) andJoint Army-Navy Specifications (JAN) since the mid-1940s.[38]
For resistances following the (E48 or)E96 series of preferred values, the former EIA-96 as well as IEC 60062:2016 define aspecial three-character marking code for resistors to be used on small parts. The code consists of two digits denoting one of the "positions" in the series of E96 values followed by a letter indicating the multiplier.[12]
For capacitances following the (E3,E6,E12 or)E24 series of preferred values, the former ANSI/EIA-198-D:1991, ANSI/EIA-198-1-E:1998 and ANSI/EIA-198-1-F:2002 as well as the amendment IEC 60062:2016/AMD1:2019 to IEC 60062 define aspecial two-character marking code for capacitors for very small parts which leave no room to print any longer codes onto them. The code consists of an uppercase letter denoting the two significant digits of the value followed by a digit indicating the multiplier. The EIA standard also defines a number of lowercase letters to specify a number of values not found in E24.[46]
^In case a RKM code starting with letterR could be confused with a symbolic designator for a particular resistor, the "R" could be replaced by0R.
^abcThe letterM was an exception to the rule that all different letters are supposed to be used for resistances and capacitances. Today, a lowercase letterm should be used for capacitances whenever possible to avoid confusion.
^abcdeIn old issues of the IEC 60062 standard, uppercase Latin letters were not only used for resistances, but also for capacitance values, whereas newer issues specifically use lowercase letters for capacitors (except for the special case ofF).
^As differentdecimal separators are used depending on the locale (most commonly. and,), and these characters are also used asthousands separators in some areas, avoiding to use decimal separators also has the advantage of not risking to become ambiguous in an international context.
^Alphanumerically sorting part values in RKM notation results in sorted groups of nearby values. Within some limits, this makes it easier to identify and combine similar values within these groups in preparation of abill of materials in order to rationalize part inventory, ease part procurement and safe costs. For example, sorting the following random part values (3.3 kΩ, 4.7 kΩ, 4.7 MΩ, 3.6 kΩ, 5.1 kΩ, 3.3 Ω, 1.0 Ω, 5.6 MΩ, 9.1 kΩ) would conventionally result in a list like 1.0 Ω, 3.3 Ω, 3.3 kΩ, 3.6 kΩ, 4.7 kΩ, 4.7 MΩ, 5.1 kΩ, 5.6 MΩ, 9.1 kΩ, but would result in 3K3, 3K6, 4K7, 5K1, 9K1, 4M7, 5M6, 1R0, 3R3 in RKM code, where it is easier to spot that the values 3.3 kΩ and 3.6 kΩ as well as 4.7 kΩ and 5.1 kΩ are, depending on application, close enough to be potentially subjects for optimization.
^abcIn the non-standard voltage notation the usage of letterP (and potentiallyN) instead of letterV may be ambiguous in some contexts because in old issues of the IEC 60062 standard uppercase lettersP (andN) were used for capacitance values, whereas newer issues of the standard specifically prescribe the usage of lowercase lettersp (andn) for capacitors.
^The usage of the Latin letterE instead ofR is not standardized in IEC 60062, but nevertheless sometimes seen in practice. It stems from the fact, thatR is used in symbolic names for resistors as well, and it is also used in a similar fashion but with incompatible meaning in other part marking codes. It may therefore cause confusion in some contexts. Visually, the letterE loosely resembles a small Greek letteromega (ω) turned sideways. Historically (i.e. in pre-WWII documents), beforeohms were denoted using the uppercase Greek omega (Ω), a small omega (ω) was sometimes used for this purpose as well, as in 56ω for 56 Ω. However, the letterE is conflictive with the similar looking but incompatibleE notation in engineering, and it may therefore cause considerable confusion as well.
^The IEC 60062 standard prescribes the usage of an uppercase Latin letterK only, however, a lowercasek is nevertheless often seen inschematics andbills of materials probably because the correspondingSI prefix is defined as a lowercasek.
^In order to reduce the risk for read errors, the lettersG (6),I (J,1),O (0,Q,D),Q (O,D,0),Y,Z (2) are not used as their glyphs look similar to other letters and digits.
^Due to the ambiguity of many month initials (A,J,M) the code for the most part uses digits. Since letterO is easily confused with digit0, the code is arranged so that the letterO is used for October, the tenth month, rather than for January.
^In order to reduce the risk for read errors, the lettersI/i andO/o are not used as their glyphs look similar to other letters and digits.
^Tooley, Mike (2011-07-19)."BS1852 Resistor Coding".Matrix - Electronic circuits and components. Archived fromthe original on 2016-12-20. Retrieved2020-05-14.
^ISO 2955-1974: lnformation processing - Representations of SI and other units for use in systems with limited character sets (1 ed.). 1974.
^"Why are the characters such as "4R7" or "100" printed on some products?".FAQ.TDK Corporation. 2022.Archived from the original on 2022-12-09. Retrieved2023-12-06.These are the inductances expressed in a unit of microhenry (uH). The first two digits indicate significant figures and the third digit a multiplier. When there is an "R", it indicates a decimal point, and all numbers are significant figures.
^xtal (2007-12-29)."Re: Question about Mo[n]te Carlo mismatch model file - Reply #20".The Designer's Guide Community Forum. Designer's Guide Consulting, Inc.Archived from the original on 2025-03-26. Retrieved2025-03-26.// model = bsim3 // NewModel = 3 // MetaSoftware Compatibility Mode // These are BSIM3v3 Model Parameters // LotName=LOT_NAME UserName=USER Date=03-01-2001 // Typical Models nmos_1p8 ***
^Yong, Jestine, ed. (2014) [2006]."[Zener diode labelled 2V4]".Electronic Repair Guide.Archived from the original on 2025-03-17. Retrieved2025-03-17.
^Buttner, Harold H.; Kohlhaas, H. T.; Mann, F. J., eds. (1946). "Chapter 3: Audio and radio design".Reference Data for Radio Engineers(PDF) (2 ed.).Federal Telephone and Radio Corporation (FTR). pp. 52, 55.Archived(PDF) from the original on 2018-05-16. Retrieved2020-01-03. (NB. While the tolerance codes according to AWS/JAN are listed in this second edition of the book, they are not listed in the 1943 original edition.)
^Kurth, Rüdiger; Groß, Martin; Hunger, Henry, eds. (2021-09-27) [2011]."Integrierte Schaltkreise".Robotron Technik (in German). Beschriftung der Schaltkreise.Archived from the original on 2021-12-03. Retrieved2021-12-06.
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