Questions regarding 12V rails

[MilkmanAl]

I tried to search for these questions and could not find what I was looking for, but forgive me if they've been answered explicitly before.

I've seen several people mention that a PSU with a single 12V rail is desirable. Why? What are the advantages of having only 1 12 V rail? More wattage? Better efficiency? Are there any disadvantages to only having 1 12V rail?

On the other side of the coin, what are the advantages and disadvantages of having multiple 12V rails? Is there a danger of a multi-rail PSU not pushing sufficient wattage to run a system even though the PSU has a much higher wattage rating than would be necessary for said system?

This might be a stupid question, but how do you determine from a PSU's specifications how many 12V rails it has? That seems like info that would be readily available, but I asked about this power supply and was told it was great for its single 12V rail. Being one who enjoys researching products thoroughly before buying, I checked out Tagan's site which claims that PSU pushes 70A across 6 rails. Is this merely a case of Microcenter giving misleading specifications?

On a related note, what is the load range I want my PSU to operate under? 50%? 75%? The lowest percentage my bank account will allow? I'm certainly not averse to buying something crazy like a 1300W PSU to power a 400W system if that's the best way to go and will make my system last. By the way, I plan to build a system that will last as long as possible (target life is ~5 years, ideally), so reliability is key.

Finally, if having a single 12V rail really is significantly better than multiple rails, could I get some links to nice single-rail PSU's? I'll be trying to find some myself in the mean time, but I can use the help I can get. Someone mentioned that Silvertone offers a nice single-rail PSU. Is it worth checking out?

 

[MarcVenice]

Wow lots of a questions and misconceptions. They started building multiple 12v rails as a safety measure, so you don't overload a certain rail. But in the end it turned out to be unnecesary, long story short. Reason why some people prefer single 12v rail psu's is because if you have multiple rails, you will lose some amps on each rail. Imagine having 3 rails for example, 1 dedicated to the CPU, the other spread over the PCI-e, sata and molex connectors. Let's say the 12v1 rail has 20a, all for the CPU, but the CPU only really needs 10a, that's 10a you can't use. Same goes for the other rails. If you have one BIG 12v rail, you have no such problems. Capice ?

There is no real minimum load you want your PSU to operate under, although at some loads they are more efficient then others, and the lower the load, the less stress is being put on the PSU, and the less warmth and noise it will produce. There is a maximum 'adviced' load though, which is around 70-80%, you don't want to go above that, and some highly regarded people usually can agree on 50-60% being the ideal maximum, but less is always better. But, less is also more expensive, and usually pure overkill. Now, depending on the rig you are building, you won't need more then X amount of wattage. If you are building a high-end rig, with a SINGLE videocard solution, excluding the HD3870 X2 and 9800GX2, a 500w PSU will easily suffice, like a earthwatts 500w, a corsair 450/550vx, or corsair 520hx, the latter is modular too which I like myself.

Just to give you an example, a typical PC, like a q6600 or E8400, 2/4gb of ram, 8800GTS 512mb, 2 HD's, dvd drive, few case fans, p35 mobo and a soundcard won't draw more then 300w during it's peak. Give or take 50w when you overclock or not. Meaning, a 500w PSU will be more then enough. As for the Tagan PSU, it sais 70a on 1 rail, but I don't know for sure. In the end it matters little though, with such high wattage units. You could look around for some hot deals, not long ago there were some cooler master real power pro PSU's, 650/750w going for as little as 50-70$, which is AMAZING value for money, since it's a pretty good qaulity unit.

 

[MilkmanAl]

Wow lots of a questions and misconceptions.

That's why I ask before I buy.

Reason why some people prefer single 12v rail psu's is because if you have multiple rails, you will lose some amps on each rail.

Okay, so it is an efficiency issue. Good deal.

Thanks for the info.

 

[Nathelion]

It bears mention that with multiple rails, you may end up overdrawing one particular rail even though you are far below the total wattage advertised for the unit.

I don't know if this is actually a concern in real life or not.

 

[nefariouscaine]

Originally posted by: Nathelion
It bears mention that with multiple rails, you may end up overdrawing one particular rail even though you are far below the total wattage advertised for the unit.

I don't know if this is actually a concern in real life or not.

It really depends on the amperage and the use you're giving to each rail. If you're running a 2 +12v rail psu with a heavy OC'd quadcore + a 8800GT/GTS (high end single PCI-e connector card). More than likely *if* that PSU is running with 2 6 pin PCI-e connectors 1 of them is on the same rail as the CPU - not a good idea to run the video card on the PCI-e connector on that same rail.

doing research is the best way to find out stuff - see if someone has tortured your intended PSU & taken it apart - some MFG's are putting out "multi rail" units that are actually large single rail PSU's in disguise - not too too common but not unheard of

 

[MarcVenice]

The CPU is on a seperate 12v rail, it has to be, because that's how it's specced out by Intel, and that's what manufacturers usually 'blindly' follow. It's really hard to overdraw a 12v rail on a high wattage unit btw, you'd have to get really unlucky and put all your high wattage components on 1 rail, instead of distributing them over several rails.