Purpose of -5V and -12V rails

[phoenixlament]

-------------- Background Info.. (skip if you like) --------

I recently found an abandoned low-end P4 computer with a generic power supply inside. The computer was able to boot properly. Suspecting power supply malfunction as a possible cause of the abandonment, I opened the PSU (Y2K YoungYear PS-IV 350-1, actually 250W) and discovered a leaky fuhjyyu and a burnt out resistor, adjacent to a mildy burnt patch on the pcb.

I replaced the leaky fuhjyyu and the other identical fuhjyyu next to it with two working caps (can't remember brands) in an attempt to raise the +12V,-12V, and -5V rails which were registering at 11.12,-10.92,-4.27 volts respectively. All other rails are working within the acceptable +/- 5% range.

After the two cap replacements, I'm getting 11.4/-11.08/-4.37 volts respectively. The improvement wasn't significant probably because of the burnt out resistor. I was unable to replace it because the color coding has burnt off and I don't have a schematic for the psu. However, the computer still boots as expected. (I plan to move the psu onto a P3 1ghz psu-less computer lying around)

----------------End background info --------------



So question is: What is the purpose of the -5V and the -12V rails? Do they do anything at all these days?

[bluto]

Re: Purpose of -5V and -12V rails

For legacy stuff(not really doing much for you these days), specifically:

-12 V: This voltage is used on some types of serial port circuits, whose amplifier circuits require both -12V and +12V. It is not needed on some newer systems, and even on older ones not very much is used, because the serial ports require little power. Most power supplies provide it for compatibility with older hardware, but usually with a current limit of less than 1 A.

-5 V: A now archaic voltage, -5 V was used on some of the earliest PCs for floppy controllers and other circuits used by ISA bus cards. It is usually provided, in small quantity (generally less than 1A), for compatibility with older hardware. Some form factor power supplies such as the SFX no longer bother to supply it (systems using the SFX power supply are intended not to have ISA bus slots).

http://www.pcguide.com/ref/power/sup...oltages-c.html

[stj]

Re: Purpose of -5V and -12V rails

wrong,
it's used for balanced op-amps in high-end audio circuits.

you wont be likely to see it used on the built-in shit, but high-end cards will probably use it.
you can often remove it, but it will effect the sound quality.

[Wizard]

Re: Purpose of -5V and -12V rails

Atom ITX boards I use do need the -12V. That how I found out because of bad cap in PSU.

Cheers, Wizard

[phoenixlament]

Re: Purpose of -5V and -12V rails

Thanks for the info guys.

[Per Hansson]

Re: Purpose of -5V and -12V rails

The -5v rail was removed from the ATX spec as of ATX12V 1.3

[japlytic]

Re: Purpose of -5V and -12V rails

I wonder when the -12V would be removed from the ATX specification... but it is used in the PCI specification, but not by PCI Express slots.

[Per Hansson]

Re: Purpose of -5V and -12V rails

I wonder when the -12V would be removed from the ATX specification... but it is used in the PCI specification, but not by PCI Express slots.

It is probably not so far off
It took the PCI spec roughly 10 years to completely supersede ISA
And the PCI Express bus was introduced in 2004...

My bet is 2013

[boxy]

Re: Purpose of -5V and -12V rails

wrong,
it's used for balanced op-amps in high-end audio circuits.

you wont be likely to see it used on the built-in shit, but high-end cards will probably use it.
you can often remove it, but it will effect the sound quality.

I'm suprised a high end sound card would use the noisy raw supplies from the main SMPSU. Maybe use +/-12V and then internally regulate it down with linear regs.

[Slakky]

Re: Purpose of -5V and -12V rails

Yeah, I'd be interested to see what cards these are too. Even the old Lynx TWO got by on split 5V for balanced transmission, and running unfiltered SMPS voltage pretty much destroys your noise floor.

[370forlife]

Re: Purpose of -5V and -12V rails

How much ripple is too much? ATX spec for 3.3v and 5v on a computer psu is 50mv, 120mv for 12v and 200mv for -12v.

[PCBONEZ]

Re: Purpose of -5V and -12V rails

-5v was power from some 16-bit IC chips.
Those chips are usually only found on ISA cards but some boards [talking P3 vintage] may have used the same chips directly on the mobo.
-
Some early MSI P4 vintage boards [maybe others too] continued using 16-bit LAN and/or sound chips directly on the mobo long after those chips should have been retired as a cheap-out method.
[Talking about the time of the Intel i845 chipset].
Those boards still need the -5v even though they have no ISA slots.

-12v is still used.
Standard PCI slots and some 32-bit chips use it.

[stj]

Re: Purpose of -5V and -12V rails

a lot of serial ports still use -12 because a charge-pump like a HIN202 costs a few cents more than the old '188 and '189 pair and it needs 4 decent ceramic caps too.

[Slakky]

Re: Purpose of -5V and -12V rails

How much ripple is too much? ATX spec for 3.3v and 5v on a computer psu is 50mv, 120mv for 12v and 200mv for -12v.

If you're talking about audio line driving applications, your dynamic range is 2V RMS absolute max, so 1mV of ripple is a 66dB noise floor, which you might find on the world's crappiest onboard audio. 200mV is hearing damage country.

[PCBONEZ]

Re: Purpose of -5V and -12V rails

We are talking about motherboards, mobo PSUs and the ATX spec.

[PCBONEZ]

Re: Purpose of -5V and -12V rails

I've seen a drawing [mobo schematic] that showed -12v being used as a ref voltage for something like the MCH or the chip that sets CPU voltage but I can't find the drawing to refresh my memory.

[brethin]

Re: Purpose of -5V and -12V rails

-12 for the ATX spec will never go away. Ok never is a long time but I dont think we will see it.