What's the PSU 5v rail used for?

[ecosoft]

I have a current "problem" with one of my older systems: Athlon 2500. It will not boot as of last night ... been running 24/7 for several years. I clean the filters, HSF, and fans about every 3 mo., and have good airflow through the case. I get a Norton GoBack error during boot cycle "system unstable blah blah" Ran Memtest86 for 10 cycles... Micron 2x256 PC2700 just fine, ran Spinrite v6 on Samsung 40GB HD and it's just fine. This appears to leave the mobo (GA-7VT600 Gigabyte) and/or PS (Antec Truepower 380) as the source of the instability. Checking voltages in BIOS I have:
Vcore = 1.67 - 1.69v flip flop
DDR Vtt = 1.328 stable
+3.3v = 3.13 - 3.15 flip flop
+5v = 4.16 - 4.30 flip flop
+12v = 11.9 - 12.0 flip flop
5VSB = 4.9 - 5.1 flip flop
Sys temp = 32 deg C
CPU temp = 46 deg C

Since my 5v rail is way low I'm wondering just what the 5v is used for? Can anyone offer some diagnostic guidance about tracking down the "problem"? Swapping out the mobo is the last resort ... if I can find a socket A that's worthwhile.

 

[pengwin]

dunno anyother use of the 5v rail besides USB ports. i know USB ports are powered by 5v rails.

 

[bmouring]

Logic chips are almost exclusively 5V. So, all those little black chips all over your computer (on the mobo, all cards, all drives, everywhere) that tell the bits where to go, that's what the 5V is for. More info then you'd probably care to know.

 

[maury73]

bmouring is wrong: almost all the logic chips used on mobo and PCI cards work at 3.3V (LVCMOS) since many years.
Anyway the 5V rail supplies power to many things like: USB, PS/2 (mouse & kbd), PCI headers and, very important, to all the storage devices (HDU, floppy, CD/DVD).
4.3V is too low of course and with such a low voltage your HDU will have a lot of troubles, because the minimum acceptable voltage is 4.5V on the +5V rail.
I would try to remove all the USB devices, PS/2 mouse and kbd and any PCI card that runs at 5V (audio or video acquisition cards for example). Remose also any stprage device not needed for booting, like CD-R and DVD.
If the voltage is still low there are many possibilities that your PSU is faulty.
If it works, reconnect the cards and devices one at a time, in order to isolate the one that makes the +5V rail drop.

 

[rantonrave]

5V is now used mostly for 5V interface chips that communicate with USB, parallel IDE drives (PATA), mouse, keyboard, and 5V PCI devices.

I would verify that 4.1V reading with a meter because motherboard readings are unreliable, and I doubt a system will run reliably at all with the 5V that low

 

[bmouring]

Touche', that's what I get for posting without sleeping the night before while working with regular TTL components (and the reason I've not slept in a day and a half) Forgot what I was taught in my intro Computer Architecture course.

 

[amiman]

Just the info I was looking for!