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View Full Version : What's the -5V rail for?



qazwsxokmijn
04-12-2006, 12:04 AM
Well, I saw an SLi capable PSU, but it doesn't have a -5V rail. What is the -5V rail for? And for my specs in my sig (change the vid card to X850XT PE), will the PSU support my system? It's 600W, with dual 12V rails.

I just don't know if I'll need the -5V rail.

Can anyone please help? Googling -5V rail was useless.

trinsic
04-12-2006, 01:09 AM
I run a X800 GTO2 with a 400W. 600W is way overkill. And why get a SLi PSU when you are using non-capable video cards :)

Stick to something like a Enermax 400W or TASK or even Silverstone.

qazwsxokmijn
04-12-2006, 07:40 AM
Future-proofing. I hate having to buy a new PSU everytime I upgrade. ;)

Terry Porritt
04-12-2006, 07:48 AM
If present, a -5v rail is quite unnecessary, it is not used these days. It is a legacy from when it was seldom used on ISA cards, and for backwards compatibility.

pctek
04-12-2006, 09:10 AM
More importnat is the +12vs. You want high amps on it.

qazwsxokmijn
04-12-2006, 10:10 AM
OK, so -5V is OK if it's out. Yeah, I understand I need heaps of amps on the 12V rail. But what I don't understand is, if the 12 volt rail is so important, why is it that many PSUs don't have a big 12 volt rail??

trinsic
04-12-2006, 02:16 PM
You can't really future proof PSU's because they degrade over time :)

Graham L
04-12-2006, 04:36 PM
Again we see this curious idea that "big amps" are necessary. "amps" are just amps. There's only one size of amp, and in DC there's no form factor scaling to do. :D

Enough current from each of the supply rails is an ample sufficiency. If you think you might add another disk or two at some time, allow a few more amps for each one to the current requirement at 12V of the current hardware.


The -5V rail has been provided for historical reasons. It was necessary on the original IBM PC, because most NMOS chips (especially memory) used a negative bias on the substrate. I doubt if any such chips are used in modern computers. (The -12V rail is used only for the RS232 serial port ... that will probably be the next to go.)

pctek
04-12-2006, 05:00 PM
Again we see this curious idea that "big amps" are necessary. "amps" are just amps.

Yes, yes. Most people don't know how to calculate the requirements so telling them to look for plenty of amps is easiest.