Talk:Power supply rail

Improvement needed: Im sure this article is great for the computer term, however I need information on the electric engineering term.

72.201.102.241 (talk) 01:49, 9 January 2010 (UTC)

Some discrepancies, but I would like a second opinion (or third, etc)
I am apparently starting this thread for discussions, so if you want to clean me up, then by all means do so, I don't know enough about posting to new Wikipedia discussions.

On the main page of this article, it starts off with "and as many as six +12V rails." and then the bullet point for +12V rails says "As such, modern ATX 12V PSUs may have as many as five separate". So obviously there is a flaw in this logic somewhere.

In addition, this part needs cleaning up: Note: Most PSUs create their 3.3V output by regulating down their 5V rail. As such, 3.3V and 5V typically have a shared rating. For example, a 3.3V rail hov.may have a 10 amp rating by itself (33 watts), and the 5V/12/24 rail may have a 20 amp rating (100 watts) by itself, but the two together may be able to output 110 watts. In this case, to load the 3.3V rail to maximum (33 watts), the 5V rail will only be able to output 77/42 watts. Should read more like this: Note: Most PC power supplies create their 3.3V output by regulating their 5V rail to the lower voltage; 3.3V and 5V rails typically share ratings for this reason. For example, a 3.3V rail may have a 10 amp rating by itself (approximately 33 watts), and the 5V rail may have a 20 amp rating (approximately 100 watts) by itself, but the two together may be able to output 110 watts total. In this case, to load the 3.3V rail to its maximum of 33 watts, the 5V rail will only be able to output 77 watts. In addition, I think that last block should mention that most people tend to over "supply" their systems by purchasing a 450W PS where a 400W PS would be sufficient.

Thank you for reading my changes, and if you decide to make them, if you'll drop me a line at cole/talon/biz or if you need to ask me questions, I'll be able to find a source to back myself up if need be.

In addition, the PSU link at the top should refer to http://en.wikipedia.org/wiki/Computer_power_supply instead of where it does point to, and that page should have a link back to this one at some point, perhaps under this bullet about half way down the page: In computer power supplies that have more than one 12V power rail, it is preferable for stability reasons to spread the power load over the 12V rails evenly to help avoid overloading one of the rails on the power supply. 74.7.17.26 22:27, 18 October 2007 (UTC)

Some ideas for a rewrite
I think a historical timeline/context would be useful. Basically, it went:

71.41.210.146 (talk) 21:01, 27 January 2008 (UTC)
 * 1) AT motherboards had +12V supplies (mostly for motors) and +5 supplies (which all the TTL logic ran on).  There were also −5V and −12V supplies for minor purposes like serial ports.
 * 2) The ATX standard added a 3.3V output, because electronics had moved to the 3.3V level.  It also added +5Vsb.  At this time, most of a computer's demand was for 5V and 3.3V power, and typical PSUs could supply about half their rated capacity in the form of +12V.
 * 3) Then, as logic supply voltages went ever-lower, came the development of on-board VRMs that supplied the processor with huge amounts of very-low-voltage power.  This was impractical to supply over long wires, so the processor supply was generated on the motherboard, powered by +12V, and a separate "P4 connector" was added to supply it.
 * 4) That is now the standard, and all of the high-power loads are on the +12V rail.  Modern ATX power supplies can supply 80–90% of their total capacity in the form of +12V.  Popularly advocated by google, some modern power supplies (I'm thinking of Antec's "signature series") generate only +12V directly from the AC input, and generate other voltages from that.
 * 5) It was this development which led to the need to have a safety standard, and multiple +12V rails.

Etymology?
Does anyone know the origin of the term "rail"? It's widely used in electrical engineering. I suspect it may come from old bus bars, but maybe it's something else. 71.41.210.146 (talk) 03:04, 30 January 2008 (UTC)

Yes I was also wondering where the term "rail" came from... Also, we are linking to this article from electrical engineering-related articles, such as Operational amplifier, but this article really falls nowhere in that context. Any ideas? JonathonReinhart (talk) 05:25, 15 July 2008 (UTC)

I too was wondering about the term "rail," including etymology and current usage. Some PSU's advertise more than one 12V rail, and others generate four voltages (+12, -12, +3.3, +5) with only two clearly visible rail-like internal structures. One source seems to describe a "rail" as a physical path including internal circuitry to generate the voltage and outbound connector wires to deliver it:. Another corroborates, stating a belief that the term originated from circuit diagrams that used vertical lines resembling railroad switchyards:. Either way, it seems possible to imagine a particular voltage supply 'riding the rail' from its origin inside the PSU out along the connector wire, and back. I don't know if these sources meet WP:RS so I will just leave the observation here in the Talk page in case anyone wants to add more.TVC 15 (talk) 05:28, 30 October 2008 (UTC)

According to another (Wiki!) article I read tonite, the term originated years ago, in the electrical enginering community. Grndrush (talk) 04:51, 14 April 2009 (UTC)

-5 -12 rail
They are optional, but i am missing details about this. specially in a historic reference why they are/were needed. :15:37, 10 April 2009 (UTC) —Preceding unsigned comment added by Leuk he (talk • contribs)

This is going to sound like a roundabout answer but most likely it's there because ISA requires both and PCI requires -12. -- nicklinn (talk / contribs) 02:49, 27 April 2009 (UTC)

Quality
From the top of the article:

> This article needs additional citations for verification. Please help improve this article by adding reliable references (ideally, using inline citations). Unsourced material may be challenged and removed. (April 2009)

Which would leave one THREE-LINE PARAGRAPH in the ENTIRE article. Not that I disagree in the least with the sentiment - this article is so loaded with PoV/unsourced "opinions", I won't even go far past getting started in debunking this nonsensical article, which, MY PoV says delete this article YESTERDAY!

My CURRENT (*3 year old*) computer P/S has 2, 12+V rails, both rated over 20A. I've seen them on the market (the 650W-range ones, even, not just the 1100W types) w/12+V rails as high as 28V - and then there are a few out there that truly have 4, 12+V rails (which SHOULD be avoided - a 17A 12+V rail isn't much good to a high-end GPU today). Grndrush (talk) 04:59, 14 April 2009 (UTC)<-- you might want to do more current day research before you make claims like that... and back it up with said research.

Rail
Be aware that this term did not originate with PC power supplies nor even today is this the primary use for the term. Electrical engineers are accustom to using the term when discussing circuit design. I believe the term originates from the fact that many schematics are drawn between parallel lines (i.e. rails) representing the power supplies. Here are some examples:, , , ,. --Kvng (talk) 22:24, 11 September 2010 (UTC)

History of the PC PSU?
It is a great shame that this article seems to be the History of the PC PSU presented under another name. It's difficult to find much talk in textbooks specifically about the 'rails', let alone why they're called that, the term's just taken as a given. I think as people have pointed out above, that it's a combination of facts: first, the rails in old vacuum tube and breadboard electronic constructions were often solidly mounted horizontal pieces of heavy bare wire that many other connections could be soldered to, and secondly, they are almost always drawn as distinct horizontal lines across the top and bottom of electronic schematics. Thirdly, conceptually, the rail voltages are considered to be immovable extremes that other, signal, voltages may approach and sometimes rest at or near, but only rarely actually cross. They're considered immovable in the sense that, for most analyses, their voltage does not change as varying loads are placed upon them (i.e. as various currents are drawn from them). The history of the PC PSU has little to do with this, other than it was probably somebody's only experience of power supply rails in electronics when they wrote this article. --Nigelj (talk) 13:21, 1 December 2010 (UTC)

multi-level logic
I don't really understand this, but I have heard that some circuits use multi-level logic. In such circuits, wouldn't there be more rails? Can someone address this? — Preceding unsigned comment added by Skysong263 (talk • contribs) 15:51, 24 October 2011 (UTC)

circuit diagrams needed
Could someone please add a circuit diagram showing some basic circuitry connected to rails? Also it appears that most circuit diagrams show "rails" as a line ending in nowhere but letters next to it, like VDD+ or something like that. Could someone include a diagram like that and explain what the letters mean? Perhaps a link to IC power supply pin would be helpful. — Preceding unsigned comment added by Skysong263 (talk • contribs) 15:54, 24 October 2011 (UTC)