Any PC power supply experts out there? I'm just trying to repair a
friend's PC with a dead PSU. Well I assume it's dead - it does produce
the +5V standby rail OK and when the PC's front panel power button is
pressed the PS_ON signal line is pulled low, but nothing else happens -
none of the main power rails come up. IOW the PSU is getting the
command to turn on, but appears to be failing to do so.
Anyway, a nice simple job I thought, trotting down to the local Chinese
computer take-away (MKC) to buy a new PSU. "Oh they all come with a
24-pin motherboard connector now, not 20-pin like that" (pointing at the
old one I'd taken along). "Don't worry, it'll be compatible, you'll
just have 4 unconnected pins on the connector. Line the latching tabs
up and it'll be OK."
Tending not to trust anything anyone in a shop says, ever, I have
carefully verified that if the new PSU connector is inserted into the
mobo connector pin 1 - pin 1 then all rails and signals do correspond,
and the four new pins only duplicate the +3.3, +5 and 0V rails. The
latching tabs don't align in this position, but that's immaterial, other
than confirming my mistrust...
But there is one possibly significant difference. The old PSU provides
a -5V rail on pin 12, but the new one doesn't - there's no connection to
the corresponding connector position (now pin 14 in 24-pin land).
Hmm, time to RTFM. Does the motherboard - an Asus A7V8X - actually need
a -5V rail? The manual (user guide) from their web site shows the
20-pin connector pin-outs, including -5V, but says nothing about what
it's for or whether it's needed. Only slightly more helpfully, the new
PSU's installation guide - it's a CoolerMaster RS-380, by the way - says
"This power supply does not support ?5V for the motherboard for new ATX
form factor operation. Please update the BIOS setting of the motherboard."
Now the questions:
1. DoeS aNyBody know whether the Asus A7V8X motherboard needs a
-5V rail? Will it be harmed if powered-up with this rail
missing (o/c). What is/was this rail used for anyway?
2. What have BIOS settings got to do with this? How can changing a
software (firmware) setting compensate for the lack of a power rail?
3. What is the line labelled PG (pin 8 on both version) for? (This
question is for interest only.)
4. Can you still get ATX PSUs with 20-pin mobo connectors and -5V
outputs, if this turns out to be needed?
5. Is there anything else I should know, before powering-up.
The Joy of Standards? - There are just sooo many to choose from...
- posted 14 years ago