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Usually when I design a PCB, I name the LEDs as LED1, LED2, ... , but now I'm working on a set of boards where I do not have much space for the component names on the silkscreen.

The boards are divided in four functional blocks.

In order to be able to better identify which parts belongs to which functional block, I'm naming R101, R102, ..., for functional block 1, R201, R202, ..., for functional block 2, and so on.

The problem is that it generates very large names for LEDs, LED101, LED102, ...

So in order to solve this problem I replaced LED101 with D101.

It was ok, since at the time, I didn't had diodes on the board, but now I need to use diodes and I want to easily distinguish which components are LEDs and which are diodes, but I don't have space to have LED101 on the silkscreen.

Is there any convention for one letter prefix to use for LEDs that is different for diodes?

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  • \$\begingroup\$ I have seen LD used for this purpose, and also L (where there weren't any inductors!). But what's wrong with D? \$\endgroup\$ – MikeJ-UK Jul 18 '12 at 11:53
  • \$\begingroup\$ @MikeJ-UK I want to avoid using D just to have different prefixes for LEDs and diodes. \$\endgroup\$ – Bruno Ferreira Jul 18 '12 at 11:59
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    \$\begingroup\$ @Bruno - Why do they have to be different? See my answer \$\endgroup\$ – stevenvh Jul 18 '12 at 12:03
  • \$\begingroup\$ @stevenvh It may seem a bit odd, but usually I number the components relative to as close they are from the source of the signal the functional block deals with. So, for example, on a regulated power supply the input capacitors would be C1 and C2 and the capacitors on the output would be C3 and C4. But for this particular circuit the diode is close to the source of the signal on its functional block so should be D101, but the LEDs are connected to the output of a microcontroller and in the code they are named as LED1, LED2, etc. I was just trying to maintain the numbering system. \$\endgroup\$ – Bruno Ferreira Jul 18 '12 at 12:21
  • \$\begingroup\$ @stevenvh (Ran out of space :-) The goal was to have the same number in code and on the board without having to change the way I number the components. \$\endgroup\$ – Bruno Ferreira Jul 18 '12 at 12:26
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added IEC allows Stainless steel screen printing down to 1mm (but I know where you can get legible screen and printing at 0.5mm

Q Is there any convention for one letter prefix to use for LEDs that is different for diodes?

A Yes.

DS for Display lampS (LEDs) and CR for current rectifiers (diodes)

I wonder if any international standards committee members who stay awake at night solving these issues are wondering, why don't they teach standards in Engineering at University?

IPC-2581 for Commercial Reference Designations

Light Emitting Diodes< REFDES = Reference Designation

ANSI/IEEE >>>> DS (preferred)

IEC >>>>>>>>>>> E (preferred in EU)

Other>>>>>>>>>> LED,DIS,CR,D

I have always used DS for LED's since my military training days after university in Aerospace R&D according to Mil-Std Hdbk and my draftsman. Standards permit universal schematic interpretation in any language.. It has worked for me. I can read any Japanese or Chinese or Russian schematic instantly.

Added .. "In 2005 IPC and IEC (International Electrotechnical Commission) established a joint standard for land pattern geometries (IPC-7351/IEC 61188-5-1)

Table 1 contains list of the standard reference designators from the IPC-2612 standard for schematic symbol generation.

enter image description here

Table 1: Standard Reference Designators for Schematic Symbols

*These class letters would not appear in a parts list as they are part of a PCB and not an active electronic component.

**Not a class letter, but commonly used to designate test points for maintenance purposes.

Note: The above list is not exhaustive. See the standard list of class designation letters in ANSI Y32.2/IEEE Std 315, Section 22 and the Index."

If you can get IEC 60617 series that would be a plus which are a subset of the symbols in IEEE 315 marked with IEC

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    \$\begingroup\$ Standards permit universal schematic interpretation in any language. If they're used, yes. In my 22 years as an EE I've never heard this getting mentioned by anyone. \$\endgroup\$ – stevenvh Jul 18 '12 at 17:02
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    \$\begingroup\$ I was asking for a one letter prefix. So I guess the answer is no. \$\endgroup\$ – Bruno Ferreira Jul 18 '12 at 20:42
  • \$\begingroup\$ I like to recommend standards.. I indicated ( pun intended) E, LED,DIS,CR and D are all used on OTHER than standards, so YES to single letter, but actually no to standards. Stevenh have you ever read schematics in Russian, Chinese, Japanese, Korean, British and German? I can read them faster than any comic strip and I only read English. Make sense? Bruno Sorry I had your answer but the site format did not wrap around. I'll edit it. \$\endgroup\$ – Tony Stewart EE75 Jul 18 '12 at 20:48
  • \$\begingroup\$ Bruno if I had re-arranged the table.. you would have seen two single letter solutions one the IEC standard "E" for emitter \$\endgroup\$ – Tony Stewart EE75 Jul 18 '12 at 20:55
  • \$\begingroup\$ @TonyStewart I was also thinking about using E. Do you know wich IEC standard defines this? Is it IEC 60027-2? \$\endgroup\$ – Bruno Ferreira Jul 19 '12 at 0:03
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We had lots of discussions about component prefixes, but the main idea was to use as much single letter prefixes. So diodes are "D", whether they're LEDs or rectifiers. On a schematic the symbol should make the difference clear, on a PCB the silk outline may, though maybe not always for SMDs. Once the board is populated it should be clear in any case.

Sometimes components are so close together that there's simply no room for refdes's. In that case we only have the outline on the silk, and in the documentation have an enlarged layout where the refdes appears inside the outline. This means that you need two silk layers per side, one for the outline, one for the refdes, so that you can selectively disabled the latter, or even both is possible.

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    \$\begingroup\$ Yup, +1, that's pretty much what I do too. I use Eagle, so I use the TDOCU layer for smaller component designators that go on the assmbly drawing, whereas the silkscreen gets component designators only where they fit and won't be confused as to which part they belong to. The silkscreen labels need to be some minium size due to the silkscreen printing process. I usually stick to 8 mil line width to be conservative. Any board house in the world can do that. \$\endgroup\$ – Olin Lathrop Jul 18 '12 at 12:51
  • \$\begingroup\$ @Olin - Not easy getting "R123" inside an 0402 outline :-( \$\endgroup\$ – stevenvh Jul 18 '12 at 12:55
  • \$\begingroup\$ Yeah, I don't like 0402 and that's one reason. Another is that they're smaller than dirt particles on my table. They are fine for automated manufacturing, but rework is a pain. The documentation text can be much smaller than the silkscreen text. It is only used on the board assembly drawing, which is usually several times the size of the actual board. That helps, but 0402 still suck except when you really need the space. \$\endgroup\$ – Olin Lathrop Jul 18 '12 at 13:22
  • \$\begingroup\$ @Olin - an intern was doing rework with a tape and reel. Peeling open caused half the resistor to be upside down. So he turned those, until I explained that it's much cheaper to use the other ones and wipe the rest on the floor. 0402 is what I still just can handle with my Magic Tweezers. \$\endgroup\$ – stevenvh Jul 18 '12 at 13:32
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I primarily use D for LEDs. Sometimes LED but not usually.

For other components I use the following conventions:

(substitute <n> for a cardinal number)

B<n>    | Battery
C<n>    | Capacitor
D<n>    | Diode or LED
FB<n>   | Ferrite Bead
F<n>    | Fuse
I<n>    | Current Source 
J<n>    | Connector
K<n>    | Relay
L<n>    | Inductor
P<n>    | Plug (usually mates with connector (J) having the same <n> value)
PB<n>   | Piezoelectronic Buzzer (there may be something better)  | Pushbutton
Q<n>    | Transistor
R<n>    | Resistor
RN<n>   | Resistor Network
S<n>    | Switch
SPKR<n> | Speaker (there is probably something better)
T<n>    | Transformer
TP<n>   | Testpoint
U<n>    | IC
V<n>    | Voltage Source 
X<n>    | Crystal
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  • \$\begingroup\$ Thanks for sharing. I usually use D too, but in this case I'll choose E if I can confirm it is an IEC standard. \$\endgroup\$ – Bruno Ferreira Jul 19 '12 at 0:22

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