A question about the negative 5 volt rail on my power supply.

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jaypee68

I have an older power supply with a -5v rail and an MSI K7N2-6570
motherboard. With this combination, the BIOS shows voltage on the -5v
rail, but if I put in a different power supply that doesn't even
support a -5v rail, I get 0.00 voltage in the BIOS. I know what you
are probably thinking, "duh! of course you won't see voltage if you
don't have the -5v rail.".

My question here is, if my newer power supply doesn't support the -5v
rail, and the motherboard is expecting it, will I have any problems?
The system ran fine through POST, and memtest x86, but it wasn't kept
on for much longer after that. What would the motherboard be
utilizing a-5v rail for? Will the motherboard malfunction without
this -5v rail? Any chance of damaging anything by not supply this -5v
rail?

TIA :)
 
If I recall correctly some early memory chips required -5 volts. It has been a few years since any chips required it and most power supplies now do not have -5 volts.
 
I have an older power supply with a -5v rail and an MSI K7N2-6570
motherboard. With this combination, the BIOS shows voltage on the -5v
rail, but if I put in a different power supply that doesn't even
support a -5v rail, I get 0.00 voltage in the BIOS. I know what you
are probably thinking, "duh! of course you won't see voltage if you
don't have the -5v rail.".

My question here is, if my newer power supply doesn't support the -5v
rail, and the motherboard is expecting it, will I have any problems?
The system ran fine through POST, and memtest x86, but it wasn't kept
on for much longer after that. What would the motherboard be
utilizing a-5v rail for? Will the motherboard malfunction without
this -5v rail? Any chance of damaging anything by not supply this -5v
rail?
Click to expand...

Some old MBs or ISA cards required -5v. It was removed from the ATX PSU
specification in 2002. Read:

http://www.playtool.com/pages/psurailhistory/rails.html
 
I have an older power supply with a -5v rail and an MSI K7N2-6570
motherboard. With this combination, the BIOS shows voltage on the -5v
rail, but if I put in a different power supply that doesn't even
support a -5v rail, I get 0.00 voltage in the BIOS. I know what you
are probably thinking, "duh! of course you won't see voltage if you
don't have the -5v rail.".

My question here is, if my newer power supply doesn't support the -5v
rail, and the motherboard is expecting it, will I have any problems?
The system ran fine through POST, and memtest x86, but it wasn't kept
on for much longer after that. What would the motherboard be
utilizing a-5v rail for? Will the motherboard malfunction without
this -5v rail? Any chance of damaging anything by not supply this -5v
rail?

TIA :)
Click to expand...

As the other replies indicated, you don't need -5V, and it
was probably poor judgement for the bios to have a
monitoring/alarm function for it when it isn't used.

No chance of damage, providing the PSU is decent quality and
appropriate for the system otherwise, basically meaning it
would be new enough to have sufficient current on 12V rail
for CPU VRM supply plus the hard drives and video card (if a
power hungry gamer model).
 
kony said:
As the other replies indicated, you don't need -5V, and it
was probably poor judgement for the bios to have a
monitoring/alarm function for it when it isn't used.

No chance of damage, providing the PSU is decent quality and
appropriate for the system otherwise, basically meaning it
would be new enough to have sufficient current on 12V rail
for CPU VRM supply plus the hard drives and video card (if a
power hungry gamer model).
Click to expand...

Don't the hard drives use the 5v rail? They have wiring for it.
 
GT said:
Don't the hard drives use the 5v rail? They have wiring for it.
Click to expand...


1.2A off the 5V rail and 2.5A off the 12V is about maximum.
http://www.storagereview.com/Testbed4Compare.sr

These numbers become more significant as the number of hard drives grows and
when staggered spinup is not possible.

Example: 10 drives x [(2.5Ax12V) + (1.2x5V)] = 360W
Click to expand...

While it's true that some drives may peak up to 1.2/2.5A, it
would be unlikely to cause that peak simultaneously except
the initial surge current when the system is first turned
on. For example I've an old fileserver which continually
has 6-8 drives and has been running off a 240W (Delta) PSU
for several years. It just happens to have a good match for
system 5V/12V power usage vs PSU 5V/12V power capability.

Regardless, if the particular PSU is not known to be very
high quality, better safe than sorry with so many drives, so
much data, and spend more to ensure a PSU won't take the box
down. There aren't many 300W PSU I would have trusted for
the above system, Delta/Zippy/PC P&C/LiteOn-Server are about
the only ones, everything else seems to have inferior
capacitors until you get into the much higher priced (and
higher capacity) models which makes them unsuited for long
term use.
 
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