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On a printed circuit board, I see lots of tiny letters and numbers. Is there some kind of standard that dictates what letter indicates what type of component?

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4 Answers 4

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The technical term for the markings is "reference designators" (aka "refdes") and there are a few standards can define them. Take a look at this wikipedia page for a quick overview. http://en.wikipedia.org/wiki/Electronic_symbol

http://blogs.mentor.com/tom-hausherr/blog/tag/reference-designator/

enter image description here

For schematic components, most EDA tools start off with one or few alphabets and then a sequential number. For example, R1 for the first resistor, C1 for the first capacitor, IC1 for the first IC and so on. You can download a free EDA tool such as Eagle to play around. Also, see the wikipedia page for a few more examples.

For PCB footprints, different vendors do make naming convention suggestions. See Altium's suggestions here, for example.

Edit: I do NOT know anyone personally that refers to this as a strict standard or a standard at all. It's mostly what you are used to and familiar with.

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The standard which I think is most commonly used for symbols/reference designators is ANSI/IEEE Std 315 (1975). It has been revised a couple of times since but the basics have remained pretty much the same.
You need to be a subscriber to download it.

I have a copy here on my machine, here is an example of the first few letters:

A
*†
(see also U and 22.2.4)
electronic divider
electronic function generator (other 
than rotating)
electronic multiplier
facsimile set 
field-polarization amplitude 
modulator
field-polarization rotator
general circuit element
gyroscope
integrator
positional servomechanism
sensor (transducer to electric 
power)
separable assembly
‡
separable subassembly
telephone set
telephone station
teleprinter 
teletypewriter 

AR 
amplifier (other than rotating) 
repeater

AT 
bolometer
capacitive termination
fixed attenuator 
inductive termination
isolator (nonreciprocal device)
pad
resistive termination

B
blowermotor 
synchro 

BT 
barrier photocell
battery 
battery cell
blocking layer cell
photovoltaic transducer
solar cell

C 
capacitor bushing
capacitor
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    \$\begingroup\$ A flag states that the answer has been downvoted because it's incomplete. I understand that you've not completed it because you're quoting this copyrighted work under the Amount and Substantiality Fair Use exception. I'd upvoted you before the downvote or flag, your net rep is +18 for this answer. This, in my opinion, is the correct answer to the question. You might mention IEC 60617, but they're basically the same AFAIK. \$\endgroup\$
    – Kevin Vermeer
    Jan 19, 2012 at 23:34
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    \$\begingroup\$ Ah, I see. As you guessed it's intentionally incomplete - I just quoted a small part to give an idea of the detail the standard goes into. \$\endgroup\$
    – Oli Glaser
    Jan 19, 2012 at 23:49
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In addition to that, you will also find other markings on the PCB. These are done by the fab house and are used to show UL certification numbers, UL standards that the PCB conforms to, sometimes showing RoHS compliance, and sometimes even a logo of the fab house. These can be done in silkscreening process, or anti-soldermask processes.

You can look up UL cert numbers here: http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/index.htm Fill in the UL file number with the ~7 digit number on the PCB to find who actually fabricated it.

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    \$\begingroup\$ -1 I was talking about the markings next to the components \$\endgroup\$
    – kinokijuf
    Mar 13, 2012 at 17:43
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    \$\begingroup\$ @kinokijuf, this is an equally valid answer to the possible source of markings on a PCB and I could see a marking like this show up next to a component. Why the downvote for a valid yet less directly correct to your question answer? \$\endgroup\$
    – Kortuk
    Mar 13, 2012 at 18:20
  • \$\begingroup\$ @kinokijuf, note I said "In addition to that..." as an extension of other markings you find on a PCB. \$\endgroup\$
    – Kris Bahnsen
    Mar 13, 2012 at 19:43
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Yes, there is, but its not really a standard, everybody simply does it more or less the same way.

  • IC? stands for an IC
  • R? stands for a resistor
  • C? stands for a capacitor

these are the "names" of the component. the boardmaker then has a kind of list where is written what name stands for what component, e.g. R1 - Resistor, 100Ohms
here is a more complete list: http://www.electro-tech-online.com/circuit-simulation-pcb-design/112835-component-designators-pcb-design.html

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    \$\begingroup\$ On all PCB I’ve seen, ICs are marked as U. \$\endgroup\$
    – kinokijuf
    Jan 19, 2012 at 14:37
  • \$\begingroup\$ well, as I said, there is no real standard. but yes, U is much more common \$\endgroup\$
    – milch
    Jan 19, 2012 at 14:42
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    \$\begingroup\$ I can confirm, I hav a board in front on me with "IC"notation \$\endgroup\$
    – clabacchio
    Jan 19, 2012 at 14:45
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    \$\begingroup\$ I think that historically, 'IC' was/is found on British PCBs whilst 'U' is found on US boards. Likewise for transistors, 'Tr' used to be common in Britain but the American 'Q' designator has largely replaced it. \$\endgroup\$
    – MikeJ-UK
    Jan 19, 2012 at 16:45
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    \$\begingroup\$ U is "undefined", which what got used for ICs when they first became available. They are common now, so it makes sense to give them their own designator. I use IC in all my schematics. \$\endgroup\$
    – Olin Lathrop
    Jan 19, 2012 at 17:44

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