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I have seen this multiple times in schematics with LEDs. Why is the symbol for the cathode a 'K'? Is there any specific reason for this?

Example A, example B

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I would think that it's because of the origin of the word: kathodos

Greek \$\kappa\alpha\theta\omicron\delta\omicron\varsigma{}\hspace{7pt}\$ (kathodos), descent : kat-, kata-, cata- + hodos, way, path.

That and to avoid confusion with C which is already taken by capacitors.

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    \$\begingroup\$ Also, in other languages (such as German), it is spelled with a K. \$\endgroup\$ – Dave Tweed Apr 17 '13 at 16:08
  • \$\begingroup\$ I'm not quite sure, but I remember old English texts (old... from the 1800s) spelling words quite differently and generally more etymologically close to the originals. It wouldn't surprise me if the spelling in the late 1800s/early 1900s would be 'Kathode' in English. EDIT: Nope, the earliest mentions in dictionaries (late 1800s) are all spelled with a C. \$\endgroup\$ – user36129 May 11 '15 at 15:30
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The "K" was introduced into CAD library schematic symbols and PCB footprints by CADPRO, Inc. around 1990 in San Diego, CA. As the CADPRO PCB library team was cataloging all of the alpha-numeric pin letters for Transistors, Power FETS and Diodes, the philosophy was to list all the letters in a table:

  • C = Collector
  • E = Emitter
  • B = Base
  • S = Source
  • G = Gate
  • D = Drain
  • A = Anode
  • K = Cathode (because "C" was reserved for Collector)

It was forecasted that someday in the future a Transistor and a Diode or LED would reside in the same package and the alphanumeric pins would be assigned logically. However, after 20 years there is no electronic package that contains both a Transistor and a Diode.

The CADPRO library became the default footprint library for some CAD tools such as OrCAD Layout, PADS Layout and Protel. And those libraries were translated to other CAD/CAE tools.

So today, we are free to use "C" for Cathode for a Diode or LED footprint pin names.

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  • \$\begingroup\$ You're unlikely to convince anyone that the first instance of "K" being used for the cathode occurred as late as 1990. \$\endgroup\$ – Ignacio Vazquez-Abrams Feb 25 '15 at 4:03
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    \$\begingroup\$ Very unlikely. I actually have a book, Physics, Principles and Applications, by Margenau, Watson, Montgomery that has a cathode ray tube schematic with cathode marked as K. (Fig 35-24, pg 519). This book was published in 1953. So, not only does it predate CAD, it likely predates silicon diodes. \$\endgroup\$ – caveman Feb 25 '15 at 4:08
  • \$\begingroup\$ Also, ON semi makes a power mosFET / schottky diode pair in a package: NTLJF4156N \$\endgroup\$ – caveman Feb 25 '15 at 4:13
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    \$\begingroup\$ Cathode being marked with a K started in tube electronics and goes back to the 1800's when the first tube or valve invented was the Fleming valve also known as a diode. \$\endgroup\$ – tubes11792 May 11 '15 at 15:05

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