True origin of "K" symbol for diodes? - Page 1
Forum section: electronics / beginners

author: Jim from Chicago
author_rank: Regular Contributor
timestamp: May 09, 2025, 08:27:47 pm
content: My manager recently claimed that the origin of the "K" symbol for diodes is because the "K" can be interpreted as forming anincompletediode symbol (see the image I uploaded to understand what I mean).Is this the true origin of why "K" is used instead of "C"? Or is this just a post-hoc explanation of why "K" is used, and the true origin is based on other reasons?

author: TimFox
author_rank: Super Contributor
timestamp: May 09, 2025, 08:46:19 pm
content: I have never seen "K" for diodes: usually "K12", for example, refers to a relay (at least, in US usage).The old-style designator for diodes is "CR" (for crystal rectifier), but I usually see "D" in modern drawings."C" designates a capacitor.

author: CaptDon
author_rank: Super Contributor
timestamp: May 09, 2025, 09:01:44 pm
content: Because most of the world spells Kathode with a K. That has been common in XRay tubes forever.

author: thm_w
author_rank: Super Contributor
timestamp: May 09, 2025, 09:19:11 pm
content: https://electronics.stackexchange.com/questions/65981/why-is-the-symbol-for-cathode-a-k

author: TimFox
author_rank: Super Contributor
timestamp: May 09, 2025, 09:22:35 pm
content: The arrow contained in the standard schematic symbol for a diode indicates the direction in which (conventional) current flows in forward-biased conditions.The letter "K" is often used to denote the cathode connection to diodes (either vacuum or solid-state), and "A" for the other diode connection (anode).For a Zener diode, K and A are used to indicate forward bias condition, but normal use of a Zener diode is in the reverse bias condition (at a higher voltage).The Zener diode still conducts in the normal forward bias condition, at a lower voltage.This can cause confusion with the TL431 and similar devices, where K and A denote the terminals equivalent to a Zener diode, even though the current between them flows in what would be the "reverse" direction.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 09, 2025, 09:33:58 pm
content: So just to be clear, "K" designates the cathode/Kathode, not the diode itself, correct?Never seen this designation on a schematic myself. But then I'm a 'Murican.

author: TimFox
author_rank: Super Contributor
timestamp: May 09, 2025, 09:39:17 pm
content: Look at the pinout on a TL431 data sheet.https://www.ti.com/lit/ds/symlink/tl431.pdf?ts=1746801076775&ref_url=https%253A%252F%252Fwww.ti.com%252FThe ONSemi data sheethttps://www.onsemi.com/download/data-sheet/pdf/tl431-d.pdfspells out "cathode" and "anode", and abbreviates them "K" and "A".The pinouts and schematics in the TI data sheet spell out "cathode" and "anode", but the current is denotedIKA.

author: golden_labels
author_rank: Super Contributor
timestamp: May 09, 2025, 09:49:14 pm
content: Quote from: Jim from Chicago on May 09, 2025, 08:27:47 pmYou know, when you spot a “folk etymology” instance.I never seen ‘K’ used as a designator for a diode. It is, however, used to indicate polarity. Together with ‘A’. As CaptDon noted, it’s derived from anode and cathode. However, not necessarily from “most of the world.” The word is derived from Greek and in English it has been written both ways:(Source: Google N-grams viewer)Here are some examples from the period (via Archive.org):Booth “The encyclopedia of chemistry : practical and theoretical : embracing its application to the arts, metallurgy, mineralogy, geology, medicine, and pharmacy” (page 586)Waller, de Watteville “On the Influence of the Galvanic Current on the Excitability of the Motor Nerves of Man”, published in Philosophical Transactions of the Royal Society of London (page 969)Ernest Merritt “On Kathode Rays and Some Related Phenomena”Longden “Electrical resistance of thin films deposited by kathode discharge”Capstick “On the Kathode Fall of Potential in Gases”

author: Perrold
author_rank: Contributor
timestamp: May 11, 2025, 08:16:43 pm
content: The diode symbol predates diodes being used in polarity critical applications. While not widespread there are plenty of cases of what looks effectively like a modern diode symbol being used in the 20s when a solid-state diode was a point contact stuck against a rock. Not a huge leap to have that symbol point in the direction of conventional current flow as it intuitively lends itself to that interpretation. Back in semiconductor prehistory the cathode was often indicated with a + or red dot instead of being explicitly referred to as a cathode. As far as K indicating cathode, it's a widespread convention across different disciplines of engineering and science that also predates semiconductors. I believe it's the norm in electrochemistry as well and that may be, in terms of chronology, how K got to be the favored choice.