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The ATX specification includes a -12V (and an optional -5V) rail on the main power connector.

What are these negative voltage rail used for on the connected circuits?

Zulan
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  • Today they have no use. Also -5 V has been banned from power supplies and may be replaced by another pin in newer versions of the standard. I'll let someone with bit better history knowledge explain why they were used in the first place. – AndrejaKo Feb 07 '13 at 17:39
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    rs232 ports perhaps? – John Burton Feb 07 '13 at 17:46
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    According to Wiki, the -5V was part of the ISA bus, which is very, very obsolete now. – dext0rb Feb 07 '13 at 18:03
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    @AndrejaKo not banned, just no longer required to be in spec. Dell still has -5v rails on their non-atx supplies. – Passerby Feb 07 '13 at 18:40
  • @Passerby I claim that it is indeed banned. Documents such I've read such as ATX 2.2 specification, EPS12V Power Supply Design Guide v 2.92, ATX12V Power Supply Design Guide v 2.2 all clearly state that the pin which was previously used for -5 V is now not connected. As you said yourself, Dell (well-known for using non-standard components) uses -5 V in non-ATX power supplies, which are outside of this question's scope. – AndrejaKo Feb 07 '13 at 19:16
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    @AndrejaKo dell aside, there is a difference between not required, and banned/can't have. – Passerby Feb 07 '13 at 19:21
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    @Passerby Well yes. The pin is set to N.C., so it -5 V can't be on it. Maybe I should retract my previous statement. I can't see that a PSU can't have -5 V on a separate connector. The pin 20 of 24 pin main power connector is clearly NC and signal is reserved, so no -5 V there. I invite you to find a document that claims otherwise. – AndrejaKo Feb 07 '13 at 19:25
  • @iFreilicht Thank you for pointing out this very similar question. The answers are also similar, although I prefer the ones in this question. I'm wondering why you tagged this as a duplicate of the other even though this question is older and basically contains the same quality of answers. – Zulan Jun 29 '16 at 12:07
  • @Zulan Not sure, exactly. I think I did that because the other question was voted higher, but now that it is marked as such, the wording "this question has been asked before" seems a little weird. I also like the first answer on this question a lot, which is invisible to everyone finding the other question. I'll keep that in mind next time. EDIT: Ah there is a "Linked" section on the sidebar, so this can still be found, it's just not as prominent. – iFreilicht Jun 30 '16 at 12:28

4 Answers4

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-5V was the back-bias voltage for 3-rail DRAMs, in the era of 16kbit DRAMs. These used 5V for the interface, 12V to power the storage, and -5V to provide negative bias to the chip substrate. Without -5V, all 16384 storage transistors turned on at once, pretty much shorting the 12V supply to ground and destroying the chip.

You needed to ensure the -5V supply was working before starting the 12V supply. However as it supplied a bias voltage, it didn't need to source a great deal of current; 5ma was enough for a whole 64k memory! (the 12V supply was somewhere close to an amp)

Later DRAMs (starting with the 64kbit generation though there were some 16k versions) generated their own -ve supply with an on-board charge pump - and possibly still do, for all I know. These were already coming into fashion as the PC started to take off, but were possibly still more expensive...

-12V was necessary for proper RS232 serial port levels; I can't think of any other purpose for it in a PC.

  • Were those PMOS? I remember the Apple I used 1Kbit PMOS shift registers for its video, and those needed a -5 supply, but my understanding was that the bias voltage was needed so that the parts could pull their outputs all the way to ground (since an enhancement-mode PFET can only pull something down to a voltage which is a fair bit higher than its gate voltage). Am I misremembering? – supercat Feb 07 '13 at 22:53
  • I didn't think so but couldn't remember for sure, so looked up the datasheet : http://www.alldatasheet.com/datasheet-pdf/pdf/103083/MOTOROLA/MCM4116.html and they are NMOS. Now some PMOS chips had *really* weird supplies... –  Feb 07 '13 at 23:51
  • Another thing that uses the -5V rail on the original IBM PC motherboard [is the cassette chip](http://www.vintage-computer.com/vcforum/showthread.php?32459-PC-XT-Power-Supply&p=240320#post240320) – Yuhong Bao Jun 15 '13 at 03:07
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ATX specifications still require a negative 12 volt rail to meet PCI specifications. Regular PCI, not PCI Express. It will probably be phased out in the future as the negative 5 volt rail has been.

Passerby
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I once had a modem which needed -5 volts for its audio circuitry, and I wouldn't be at all surprised if some sound cards needed it as well. It's possible that -12 would have worked just as well (and indeed that some cards that expect -5 there wouldn't mind if the pin carried -12 instead), but if an op amp needs to accommodate inputs and outputs in the range of e.g. +/-1 volt, there would be no reason for the card's designer to favor -12 over -5. The higher voltage would more than double heat dissipation, and even if the extra heat wouldn't pose a problem good engineering practice would suggest that it be avoided.

supercat
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  • By what reasoning would -12v, rather than -5v be bad practice? – Jodes Jan 18 '14 at 18:47
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    @Jodes: For low-current applications, when efficiency isn't important, it was historically significantly easier to convert -12V to -5V than to convert anything else a PC would have to -12V. Since serial cards needed a negative supply with magnitude of at least 9V (and 12V was better), there was a good reason for PC designers to include a -12V supply. The Tandy 1000 personal computer was mostly PC-compatible, but (probably as a cost-saving measure) it omitted the -5V supply [guess how I know that the modem needed it!] – supercat Jan 18 '14 at 19:00
  • Some early sound cards did require -5V, and [this video](https://youtu.be/8QXmxdiNWIs?t=2m24s) demonstrates what it sounds like if you use them when the -5V rail is missing. – Malvineous Dec 13 '15 at 06:50
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Long story short: -12V isn't really needed and i've seen quite a lot of mini PCs and motherboards that run all fine without it. -5V became obsolete with the introduction of ATX 2.0. Full-thrown DB9 RS-232 ports are hard to find on a modern mobo and if you don't need them anyway, you could just omit the -12V. If you need the port later don't forget about that, as the port won't work without it. If you plan to make your own SPS, like i'm about to do, you could use a small isolated converter or even a charge pump. the current requirements are usually less than 10mA.

user36334
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  • Just FYI, a -12V supply is no longer needed to provide RS-232 signal levels. Many RS-232 drivers will generate the negative voltage internally. – Joe Hass Jan 29 '14 at 18:10