Questions about PSUs

[thilanliyan]

Can someone explain this to me?

http://www.xbitlabs.com/articles/other/display/atx-psu5_14.html
"The PSU is declared to have three +12V lines with a max combined current of 50A and a limitation of 18A on each line. However, the PSU manual informs that the lines are united into one when the limitation is exceeded. In other words, there is no actual division of the lines inside the PSU, but the user has already got used to the idea that there must be several 12V outputs, so the manufacturer couldn?t but specify several such lines on the PSU label. Of course, the lack of a ?virtual? splitting of the +12V power rail into several output lines has no effect at all on the PSU?s voltage stability or output power."

read more about the fake rails here:
http://www.xbitlabs.com/articles/other/display/atx-psu5.html

So does this mean that all the stuff about multiple rails is BS??

So we don't have to be careful about putting different components (CPU and GPU(s)) on different 12V lines?

 

[AbydosDallas]

The PSU you describe sounds like it uses a series-parallel design when considering Ohm's Law. You would still draw upon the same power the circuit provides for all 12 volt rails, however, the reactive capacitance might play a factor into how a single rail might perform in terms of resistance. So in "real-world" terms, unless each rail was obviously wired to its own respective bundle of connectors, the average user might not find the performance that the manufacturer is claiming. In all actuality, I prefer a single rail for each distinct voltage in PSU design due to the elimination of having to worry about what rail a wire bundle is connected to. I know what components are hungry for 12 volts and expect that rail to be the most robust in my PSU. So in answer to your question, I'd say yes, multiple rails are BS, but they do have their benefits depending on what is in your system if their performance is up to par for what they are connected to. Your basic AC power supply starts with your plug coming from the wall to your computer, passes through a transformer, then diodes help to rectify the AC waveform into DC, then capacitors help to smooth those bumps in the waveform into a more pure DC that your system can use. At the transformer, you can have different voltages coming off it, but the final stages between that and output to your components should be roughly the same. In a multiple rail system, a possible simple design would be having multiple 12V outputs from your transformer (keep in mind this is a simple circuit description and other options are out there). In such a system, you would individualize each rail a bit more with respect to the current it can pull for your components (i.e. the transformer can pull V/A/Watts but it divides them among different outputs). This means that hooking your spiffy SLI/Cross-Fire cards up to the wrong rail could make them underpowered or cause other anomalies within your system. Until such a time comes that all PSUs have their wire bundles clearly labeled with respect to what rail they are attached to (and that rail's true output in some cases), your safest bet is to go with a an efficient single-rail design. (But I've been wrong before and full of BS on more than one occasion) Hope that helps I'm gonna go grab a beer now.

 

[thilanliyan]

Thanks for the response.

However, what I'd really like to know if, in practical applications would a PSU claiming to have triple 12v rails (but that combine when needed, like from what I quoted in my original post) with a combined current of say 50A be almost the same as a PSU with an advertised single 50A 12V rail.

I would also prefer a single rail (my OCZ is a single 30A 12v rail) but if there is no practical difference if the muli rail PSUs become single rail PSUs when pushed, then we needn't worry about putting which components where correct?

Also, is there any way to tell in the rating if a particular PSU with advertised multiple rails is in fact one with a single rail? What about if there are 2 12v transformers (does that mean it's a TRUE dual rail PSU)??

 

[AbydosDallas]

In short, you have no idea how a PSU is built until you A) look at the schematic diagram of the circuit layout or B) see enough tests that confirm how you believe it to behave. Since most of us fall under B, all I can suggest is read the small print. I don't want to plug PC Power & Cooling's, but they do have some pertinent information relative to your concerns on their site. In contrast, most of today's 600W+ PSUs can power most modern setups, if you figure out through trial and error what power connectors you should connect to your devices. I had a few bad experiences and RMAs, but when I sat back and thought about how a PSU works, I tried different setups until I got it right. As far as the rating goes, most PSUs are not single rail unless explicitly advertised as such. If they lie, you have the freedom to file action against the company when your purchase does not perform as advertised if the warrantee doesn't cover them first. Albeit I wouldn't, go that route until talking on the phone with a tech rep from the company and trying to work out whatever issue I'm having. Most manufacturers with decent tech support are more than willing to help you out, it's just good for business.

 

[Gstanfor]

No consumer class PSU that I'm aware of actually uses phsically separated 12V rails with independent transformers, regulators & coils, despite the grandiose advertising. Why not? Simple. Cost.

So does this mean that all the stuff about multiple rails is BS??

At the consumer level, yes. At the enterprise level its true for some PSU's.

 

[thilanliyan]

Originally posted by: Gstanfor
No consumer class PSU that I'm aware of actually uses phsically separated 12V rails with independent transformers, regulators & coils, despite the grandiose advertising. Why not? Simple. Cost.

I saw the inside of I think it was a Silverstone PSU and it had 2 12v transformers...at least that's what the reviewer stated.

 

[craftech]

Although it isn't foolproof, an indicator if how a PSU is built is the weight. Usually it is listed by various vendors and you can lift them at Computer Shows. Heavier is usually better.

John

 

[cubeless]

i believe that a review of the antec 'triple' showed that all are on one rail...

watch the cheap psu makers add some lead to their units to meet the 'heavy is good' rule...

the review of the antec 'eco' psu showed small (light) components but supposedly this was because of the efficiency and airflow ( i think tha this was jonnyguru and he couldn't burn the thing up...)...

u just never know from what the box (or the newegg reviews) says...

i think that the most honest gauge right now is price (after rebates)... the $50 rosewill that jonnyguru looked at being the starting point...

 

[Gstanfor]

Originally posted by: thilan29

Originally posted by: Gstanfor
No consumer class PSU that I'm aware of actually uses phsically separated 12V rails with independent transformers, regulators & coils, despite the grandiose advertising. Why not? Simple. Cost.

I saw the inside of I think it was a Silverstone PSU and it had 2 12v transformers...at least that's what the reviewer stated.

Yes, most (but not all) Silverstone PSU's are consumer versions a server PSU's.

 

[apoppin]

Originally posted by: craftech
Although it isn't foolproof, an indicator if how a PSU is built is the weight. Usually it is listed by various vendors and you can lift them at Computer Shows. Heavier is usually better.

John

i dunno mine is relatively light ... and quiet ... and powerful ...
a little "ripple" ... that's all
:wine: