Screaming Circuits: Reference Designators


Reference Designators

Not a subject I give much thought to.

For one, we here at Screaming Circuits don't really care too much what convention you use for your components. We want them to match and be properly formatted when in your BOM, of course. But because we program our SMT machines electronically, we don't really care if you mix things up. i.e. O for resistor instead of R or F for capacitor. It's not a good idea to do that, but we can still build it.

But, if we can build it, shouldn't anybody be able to build it? And, if anyone can build it, why should it matter? Well, in theory, it shouldn't matter at all. In practice though, people tend to be human and humans tend to be error prone. That's why we have standards, conventions and test procedures - to reduce the chances for errors. We also have conventions for the purpose of distributing bad, overpriced food and educational sessions, but that's probably a different convention.

It would be kind of like if you drove into a small town and there was a sign at the city limits indicating that in this town, red means go and green means stop. You would have all of the information needed safely traverse the town, but you would still be very prone to go with the green light.

I just recently saw a design where the connectors were labeled U1, U2... Again, we can build this and we did. But, if it comes time to do any rework, or if you want to make some design mods in-house, of if someone else needs to work with the board, they'll see "U something" and think you're talking about an IC instead of some sort of connector.

There are some specific industry standard documents covering the reference designator conventions, but I bet it's one of those things that most people just sort of know, but don't have the official document to go with it. Wikipedia has a list and a lot of companies probably have their own conventions.

It is easy enough to find these lists of conventions, but it does leave me wondering how some of them came to be. I get "R" for resistor and "C" for capacitor. "Q" for transistor even makes sense, although it's derived from a property of the device instead of the name as are R and C. But, why "U" for integrated circuit? It used to be "IC", but that's fallen out of favor now. Really weird is the inductor. It starts with "I", it's inductance value is measured in "henries" and henries are indicated by "L." Go figure.

Duane Benson
U take the high road and L take the low road



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How have you known that and/or where is the source regarding your description that the "Q" the reference designator of a transistor had been derived from its properties?

The "official" list of class designation letters (class letters) is in ANSI/IEEE 315, clause 22. "A" is for a separable assembly, "U" is for an inseparable assembly, "Y" is for a crystal or crystal oscillator, "X" is for a socket, "P" is for the most movable part of a connector pair, "J" is for the most fixed part of a connector pair (this is how you would ref des a sexless connector), and there is no "JP". The Wikipedia list says "does not comply with the standard".

We need more letters; 26 ain't enough

IIRC, U comes from the letter mu (µ) for micro-... as in microchips.

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