31

Sometimes I see references to "R" resistors. For example:

enter image description here

Obviously the 100 refers to 100 Ohms. What does 100R mean?

Always Confused
  • 1,591
  • 6
  • 25
  • 49
Tyler Durden
  • 2,186
  • 4
  • 34
  • 49
  • 24
    The R is sometimes used as a decimal point. So 100R is 100 Ohms, 4R7 is 4.7 Ohms, etc. – user57037 Jul 20 '16 at 05:09
  • 5
    " Radix point " – Ignacio Vazquez-Abrams Jul 20 '16 at 05:16
  • 17
    Traditional ascii fonts do not have an omega symbol, so 'R' is commonly used instead. – pjc50 Jul 20 '16 at 08:28
  • 7
    @pjc50 Once I was working with a US student and a Canadian postdoc. The postdoc kept saying "zed" instead of "z", and eventually the student asked "What's a zed?" I told her it was a backwards z, but since we didn't have those on our standard keyboards, we would just type "z" – Scott Seidman Jul 20 '16 at 16:32
  • 1
    @pjc50 Technically the ohm sign, Ω (U+2126) is not a greek capital omega, Ω (U+03A9), though in many fonts they are represented by identical glyphs – bcrist Jul 21 '16 at 03:39
  • 5
    @bcrist: Just because Unicode has a separate code point for "Ohm" (which is silly) does NOT mean that it is not a capital Omega. – Dave Tweed Jul 21 '16 at 16:16
  • 4
    @bcrist Note that Unicode 4.0 states the Ohm symbol is only for backwards compatibility, and explicitly discourages its use in favor of Greek capital omega (see bottom of p. 176 http://www.unicode.org/versions/Unicode4.0.0/ch07.pdf) – Jason C Jul 21 '16 at 16:43
  • 3
    Not mentioned in any of the answers below but the use of R is standardized in BS 1852 and it's successors. (https://en.m.wikipedia.org/wiki/BS_1852) – Jason C Jul 21 '16 at 16:50
  • 1
    @JasonC You should really add that to the accepted answer. It is useful information and here in the comments it will get lost (comments are transient). – Fritz Jul 22 '16 at 07:53
  • 1
    On a tiny SMD part, try to not confuse "4.7" with "47", and "4Ω7" not with "407". – rexkogitans Jul 22 '16 at 10:05
  • @ScottSeidman "Z" ("Zee"?) is called "Zed" in UK English, and Some of its derivatives such as in India. So-far I know, Zed and Zee are the same thing. Btw I never seen use of Z in resistor value. – Always Confused Aug 08 '16 at 16:27
  • 3
    Wikipedia tells : The European notation BS 1852 avoids using a decimal separator ... problem that decimal separators tend to **"disappear" when photocopying** a printed circuit diagram. https://en.wikipedia.org/wiki/Resistor#Electronic_symbols_and_notation – Always Confused Aug 08 '16 at 16:29
  • 1
    Have seen use of E for ohm also . such as 5E6, 56E, 47E etc. – Always Confused Aug 08 '16 at 16:31
  • so-many opinion-based (at-least partly) informations are possible. – Always Confused Aug 08 '16 at 16:34
  • @AlwaysConfused -- yes -- I was joking around with the student – Scott Seidman Aug 08 '16 at 16:48
  • @ScottSeidman ok however I couldn't found relevance of "Z" to the OP. Looks off-topic to me. – Always Confused Aug 08 '16 at 17:01
  • There are many possible duplicates of this question that proves it is an wide problem. – Always Confused Aug 09 '16 at 11:27
  • http://electronics.stackexchange.com/questions/166100/what-does-0-025r-or-30r-mean-here – Always Confused Aug 09 '16 at 11:27
  • http://electronics.stackexchange.com/questions/45272/resistor-value-labeling-what-does-the-second-number-refer-to – Always Confused Aug 09 '16 at 11:28
  • http://electronics.stackexchange.com/questions/107698/what-does-5k1-in-the-given-schematic-mean – Always Confused Aug 09 '16 at 11:28
  • http://electronics.stackexchange.com/questions/28053/what-does-3v3-or-1v8-mean – Always Confused Aug 09 '16 at 11:28

5 Answers5

51

The idea is that the multiplier replaces the decimal point. This dates back to pre-CAD schematics which were hand drawn and then photocopied and reduced. A decimal point could easily get lost during the copying process. By writing 4k7 rather than 4.7k the risk of these errors was greatly reduced. R was used for a multiplyer of 1 because omega could easily be mistaken for a 0. So ... 4R7, 47R, 470R, 4k7, 47k, 470k, 4M7, 47M.

The same approach is used with capacitors: 2p2, 22p, 220p, 2n2, 22n, 220n, 2u2, 22u, 220u. In the old days larger values were still marked µF so the next decade was marked 2200u but with large capacitor values common now we're seeing 2m2, 22m, etc. I've never seen an equivalent of the 'R' as in 2C2 for a 2.2 F - yet! 2F2 may be more sensible. The current use of 'R' would then be excused (4R7 instead of 4Ω7) on the basis that Ω isn't readily available on most keyboards.

This system may be more popular in Europe.

Thanks to @JasonC for pointing out that the 'R' notation is covered by British Standard BS 1852.

Transistor
  • 168,990
  • 12
  • 186
  • 385
  • 13
    And if you stare at the R for long, it sort of looks like someone got a stretchy omega and pinched its center to join the two legs, and moved it up and to the left a bit. – This company is turning evil. Jul 20 '16 at 11:55
  • Ω not being available cannot be an excuse. There is always copy & paste. – glglgl Jul 20 '16 at 14:03
  • 12
    @glglgl There are file formats and programs that can only store 8bit characters in strings, and don't support code tables apart from the system code table. Copy & paste into these file formats or programs will produce anything but legible greek characters (except maybe on a greek computer system). – Alexander Jul 20 '16 at 14:06
  • @Alexander That's another thing than not having it on the keyboard. Although, in 2016, programs behaving this way should be considered buggy and incomplete. – glglgl Jul 20 '16 at 14:09
  • 2
    @glglgl As programmers, we consider them "legacy" like two-digit year numbers. – Alexander Jul 20 '16 at 14:22
  • 3
    @glglgl you would be surprised how much very old legacy code is running in current (and very expensive) CAD software... – alex.forencich Jul 20 '16 at 14:43
  • 3
    @glglgl: Incomplete yes, buggy no. When one considers the cost of support for non-ASCII characters in a schematic entry program, compared to features that are directly related to circuit design, one can see why such a thing may be way down the priority list. – Ben Voigt Jul 20 '16 at 14:44
  • 5
    @Alexander: Not only that, but many CAD programs use one or more sets of hard-coded stroked-character shapes, rather than using system fonts, and thus cannot use any characters which are not in their hard-coded set regardless of how many system fonts include it. – supercat Jul 20 '16 at 14:58
  • @supercat Ok, that's indeed a valid point. – glglgl Jul 20 '16 at 15:20
  • @glglgl The only somewhat widely used 7/8-bit character set that [defines a capital omega](http://www.fileformat.info/info/unicode/char/03a9/charset_support.htm) is [ISO-8859-7](https://en.wikipedia.org/wiki/ISO/IEC_8859-7), and even that is relatively uncommon compared to US-ASCII, ISO-8859-1, Windows-1252, Mac OS Roman, etc. You would certainly encounter rendering issues in software without Unicode support. In 8859-1 0xD9 isn't defined, in Windows-1252 it's a Ù, in Mac OS Roman it's Ÿ. – Jason C Jul 20 '16 at 17:39
  • 1
    *"less easy to misread"* - Adding to this, worth noting that different locale's decimal separators (point vs. comma) could make things very tricky when, say, manufacturing a pcb / publishing schematics then making them available globally. – Jason C Jul 20 '16 at 17:41
  • @JasonC "rendering issues in software without Unicode support" But IF the software has Unicode support, it isn't a point anymore, that was my point. And if it hasn't, it is legacy, broken or both. But there are valid points such as the stroked-character shapes, which have convinced me. (Although there should be more or less complete fonts even for this shape.) – glglgl Jul 21 '16 at 07:13
  • @BenVoigt Even if it is way down the prority list: Unicode exists since 1991, UTF-8 since 1998. That's 25 resp. 18 years. In such a long time, one could have fixed such things. OTOH, people are accustomed now to writing R for Ω and uF fur µF. – glglgl Jul 21 '16 at 07:15
  • @glglgl You appear to be making a point about something very inconsequential, unless you are lobbying for some sort of global movement away from "R" and towards "Ω" (not sure what problem that would solve though) - in which case this is not the most effective place to start. Electronics aren't broken because people are writing Rs everywhere. I think it's going to be OK. – Jason C Jul 21 '16 at 07:33
  • @JasonC I am not lobbying at all – I just am sometimes annoyed by software (in general) which, in 2016, still doesn't cope well with Unicode. That's indeed nothing which can be solved here, was meant as a "side-comment" and has now grown to an extended discussion which essentially serves to nothing. The sentence I was referring to was __The current use of 'R' would then be excused (4R7 instead of 4Ω7) on the basis that Ω isn't readily available on most keyboards.__ which isn't the point – it is not the keyboard whihc is the problem. – glglgl Jul 21 '16 at 07:38
  • @transistor Adding to this answer: The use of R (as well as the other letters) is standardized in BS 1852 and it's successors. (http://en.wikipedia.org/wiki/BS_1852). – Jason C Jul 22 '16 at 15:19
  • 1
    @glglgl: "_... it is not the keyboard **whihc** is the problem._" Ha ha! – Transistor Jul 22 '16 at 18:09
  • Thanks for discussing capacitor-nomenclatures also. some resistors and capacitors are so tiny . Abbreviations probably helpful for alphanumeric coding. Some resistors and most capacitors are alphanumeric coded. however capacitor alphanumeric-codes often contains decimal points I've seen so-many times. – Always Confused Aug 08 '16 at 16:57
  • 1
    @AlwaysConfused: Yes, but none of the others asked specifically about 100R. I might do a question for each of the E12 series to earn some extra rep. What do you think? ;^) – Transistor Aug 09 '16 at 11:15
  • @Transistor sorry I've misplaced the comment . They were to be under the original question. I thought link them up would help the readers find similar-discussion. As well, so-many possible duplicates prove this is a very common problem which is often not explained clearly. – Always Confused Aug 09 '16 at 11:24
16

It is quite common to see the letter "R" used as a decimal point. As in 47R9 = 47.9 ohms. And likewise, it is common to see the letter "K" or "M". For example 6K81 would be 6,810 ohms and 2M3 would be 2,300,000 ohms.

Richard Crowley
  • 14,382
  • 2
  • 20
  • 37
13

Adding to the other answers, sometimes you may even see E used in place of R. So a 100 ohm resistor would be 100E and a 9.1 ohm resistor would be 9E1 for example.

bitshift
  • 887
  • 1
  • 6
  • 18
  • 4
    I've never seen E used as a place holder for a decimal point, "R", "k", "m" and "M" obviously for resistors and "m", "u", "n", "f"and "p"on capacitors and inductors. Where is "E" used in this way? You some times see E used in scientific notation so for example \$1\text{E}3 = 1 \times 10^3 \$ but not on circuit diagrams. – Warren Hill Jul 20 '16 at 06:32
  • 3
    Here's a link to a component supplier where E is used extensively. http://www.mantech.co.za/Stock.aspx?Query=1Eand – bitshift Jul 20 '16 at 06:36
  • 4
    It may not be very common but if anyone ever comes across a schematic with E instead of R this answer may help them. – bitshift Jul 20 '16 at 07:03
  • Why down vote my answer? You can legitimately come across the E notation in Africa. – bitshift Jul 20 '16 at 07:29
  • 7
    @bitshift, I think the people who downvote you have never heard of this, so they downvote because they think you are wrong. I did not downvote you. But I have never heard of this before. I did click on the link you provided, and, indeed, E seems to be used. Learn something new every day. – user57037 Jul 20 '16 at 08:09
  • 4
    I have heard of it many times and subsequently upvoted it to not drive away users from this site. – winny Jul 20 '16 at 09:16
  • Same here, never seen E-marks, but R-marks are quite common. – Overmind Jul 20 '16 at 11:51
  • 1
    This answer is not helpful unless it clarifies whether `E` is being used for scientific notation in these listings, or as a direct analogue for `R`. I suspect it is the former, that 9E1 is a 90 ohm resistor, while 9R1 is a 9.1 ohm. – Ben Voigt Jul 20 '16 at 14:50
  • 1
    @BenVoigt No 90 ohm would be 90E but I've expanded my answer to try and make it more clear. – bitshift Jul 20 '16 at 16:17
  • 3
    Philips do this extensively. An oscilloscope service manual I have in front of me right now has for example `7E5`, `100E`, `1k`, `7k5`, `51k1`, etc. – pipe Jul 20 '16 at 16:44
  • @bitshift You should add the example link (and pipe's comment above) into the answer in case these comments get cleaned up. – Jason C Jul 20 '16 at 17:49
  • I haven't downvoted, but I would note that perhaps downvoters are doing so because while this is interesting and potentially useful, *it doesn't actually answer the question that was asked*. Perhaps you should post this as a question and then self-answer it, and a comment here could make it a linked question? – Jules Jul 21 '16 at 19:19
  • 1
    Is E the first letter of the word for resistance in some language, perhaps? – Hearth Mar 28 '19 at 12:58
10

Typically, resistor "multipliers" are represented as:

KΩ (thousands of ohms),

MΩ (millions of ohms),

GΩ (thousands of millions of ohms) ...etc.

Since the context usually makes it clear that we're talking about resistor values, it's common short-hand to drop the 'Ω' so that, for example, you can write "39K"* instead of "39KΩ". But, dropping the "Ω" leaves the problem of how to represent a resistor value when the multiplier is 1. So it was decided that "R" would represent a "x1" multiplier. So now you can write "39R" instead of "39Ω".

The multipliers (R, K, M, G... etc) can also be used as shorthand for decimal points.
So, for example, instead of having to write "2.2Ω", you can simply write "2R2". All the multipliers can be used in this way. A final example: "3.3KΩ" can be written as "3K3"

Note that it is common practice to capitalise the "K" multiplier when referring to resistor values. Technically this is incorrect, as "k" is the official '1000' prefix. But it's just a shorthand, limited in its use to resistor values, and the capital K is in common use in this context.

Q''
  • 454
  • 2
  • 9
  • 2
    Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackexchange.com/rooms/42779/discussion-on-answer-by-q-what-is-a-100r-resistor). – Dave Tweed Jul 20 '16 at 15:05
2

Wikipedia tells,

The notation to state a resistor's value in a circuit diagram varies. The European notation BS 1852 avoids using a decimal separator, and replaces the decimal separator with the SI prefix symbol for the particular value. For example, 8k2 in a circuit diagram indicates a resistor value of 8.2 kΩ. Additional zeros imply tighter tolerance, for example 15M0. When the value can be expressed without the need for an SI prefix, an "R" is used instead of the decimal separator. For example, 1R2 indicates 1.2 Ω, and 18R indicates 18 Ω. The use of a SI prefix symbol or the letter "R" circumvents the problem that decimal separators tend to "disappear" when photocopying a printed circuit diagram.

https://en.wikipedia.org/wiki/Resistor#Electronic_symbols_and_notation

Also, I've seen, 1. just-like R, also E is being used such as 4E7 etc. 2. the zero for tighter-tolerance sometimes not given, such as 47K, 56K etc.

Always Confused
  • 1,591
  • 6
  • 25
  • 49