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On circuit diagrams, C stands for Capacitor, R for Resistor, L for inductor (for Lenz) as explained here, but why is U used for ICs on circuit diagrams?

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  • \$\begingroup\$ ANSI/IEEE Std 315 (1975) has that information \$\endgroup\$ Commented Jan 25, 2012 at 17:19
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    \$\begingroup\$ This question is not answered by the question that it is marked as a duplicate of. That asks what symbols means, this is one particular why question. I hope I've now made this clear by editing the question. \$\endgroup\$ Commented Jan 20, 2017 at 18:18

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U was originally the designator for "unspecified". I suppose that made sense when integrated circuits were new. There was no existing category for them, so they went into the none-of-the-above category. I use "IC" as a designator for ICs because I think it makes more sense, especially nowadays when ICs are common.

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    \$\begingroup\$ Whoever downvoted this, it would be useful to know what you disagree with. \$\endgroup\$ Commented Jan 22, 2017 at 12:56
  • \$\begingroup\$ I downvoted because this Altium article states 'For trivia fans, the origin of the reference designator "U" meant "Unrepairable"', not "unspecified". Is there a source you can provide that contradicts that? \$\endgroup\$ Commented Jan 21, 2020 at 22:09
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Good question - "U" stands for:

inseparable assembly
integrated-circuit package
microcircuit
micromodule
photon-coupled isolator  

I think the U was picked because these are all classed as "Unrepairable", since they are all in sealed packages.
FWIW, the designator for separable assemblies is "A"

Note that it was actually "IC" (and other things - for instance I've seen Z used) before it changed to U. If you look at schematics from pre ~1980 you will see a lot of different (to todays standard) designators used, e.g. CR or V for diode, G or X for crystal, etc

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  • \$\begingroup\$ Not debating you or anything, but even back before integrated circuits existed there were other components that were sealed and pretty much unrepairable: Transistors, vacuum tubes, electrolytic capacitors, carbon resistors, diodes, ... I haven't heard of anybody trying to repair any of those things either. \$\endgroup\$ Commented Jan 29, 2019 at 22:07
  • \$\begingroup\$ I believe this is the correct answer. This Altium article states 'For trivia fans, the origin of the reference designator "U" meant "Unrepairable"'. \$\endgroup\$ Commented Dec 10, 2019 at 20:12

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