Power supply questions

What would be better in a power supply.... for example: PSU #1 has a single 12v rail rated at 26a. PSU #2 has dual rails rated at 20a each. PSU #3 has four 12v rails rated at 15a each. Obviously the price on these is not going to be the same, and presumably #3 is going to be considered the best because of the 4 rails. But what about #1 and #2?? Is a single high-amp rail preferable to dual lower-amp rails? What if #1 had 32a on the 12v, and #2 stayed the same at dual rails at 20a?
I'm not really looking to upgrade or anything, I'm just a bit curious and I want to learn seomthing about PSUs.

(Yeah, I know, get a life dlb...)

 

This should help a few review about PSU http://www.guru3d.com/index.php?page=psu

 

PC Power & Cooling has a pretty good rep as a PS manufacturer I believe. I'm no expert on the questions you're asking but here's PC Power & Cooling's take on it: (Look at question/answer #8)

http://www.pcpower.com/technology/myths/

PC P&C says that 1 rail is better and that PS design is shifting away from multiple rails back to single rail design for the reasons they cite on that web page. Is this true? Well, some of what they say makes sense to me.

For example the bit about some of the rated power being unavailable if it isn't used. The argument is if one rail in a multi-rail design can provide, say, 16 amps, but only 10 amps are being drawn from that rail, the unused 6 amps from that rail can't be shifted to another rail -- they're just unused and not available and similarly for additional unused power on other rails. Whereas, by funneling all the power/total rated amp capacity through a single rail it's all available.

Apparently, though, there is generally a safety factor in multiple rail designs that will shut down the PS if more than 20 amps are being drawn from any one rail. One supposed advantage of a single rail design is that 20 amp limit doesn't apply to a single rail design. UL still gives their sticker to such single rail designs, though I presume there are, conceivably, some single rail designs that might not be UL rated. What sort of built-in safety factors are there in single rail designs, such as PC P&C's or other companies' designs that replace or make unnecessary the 20 amp limit common to multiple rail PS design? I don't know. If you find out, please post back here dlb. I only found that PC P&C web page above today, so that's all I know about the topic.

I haven't had time to check Tom's Hardware to see what, if anything, they have to say there about this question of single vs. multiple rail power supplies, but I suspect it would be worth reading if they've weighed in on the subject.

 

Hi, DLB

As you repair pcs, I assume you have an understanding of the electrical side of Volts, Amps, Watts and Resistance?

Even the (relatively) heavy duty wires and circuit board tracks have resistance so all currents produce some voltage drop when passing along them.

If two circuit modules are connected to the same power supply wire, a sudden surge in demand by one module can adversely affect the other because the surge from the first module increases the voltage dropped along the common wire or track. The second module sees the effect as a sudden change in its own power supply. To counter this separate wires/tracks are often run to different modules from a single point in the main psu. All the various 12 volt etc wires go to a common point inside the power supply, there are not several 12 volts supplies.

Computer power supplies produce all the various rails from one convertor which changes the AC input to suitable DC. Sub circuits change this to 12volts, 5 volts 3.3volts etc.

The total power that this single convertor can supply is the rated output of the psu.

However you will note that this is less than the sum of the rated current on each rail times its respective voltage.
This allows trade off between the various modules in the pc which draw current on different rails.
For example you can have a heavy demand graphics sub system or lots of drives that load the 12volt rail. Or you can run a P4 prestcott processor that draws heavily on the 3 volt rail. Attempts to do both may overload a given psu or require a bigger one.

There is one exception to this. The 5volts 'permanent' supply to the motherboard is given a wire of its own and is independant of the main convertor. This is to make the motherboard always partly live, so that it can exchange control signals with the psu unit and execute remote wake functions.

 

I took a quick look at the Tom's Hardware website to see if I could find an article there about single 12v rail power supplies, but didn't find any detailed discussion. But I note that in a high end ($3500) gaming machine build they did last fall they did incorporate a PS with a single 12v rail rated for 70 amps on that 12v rail alone!

http://www.tomshardware.com/2007/09/19/sbm_high_end_system/page9.html

 

'Single Rail'

I think we are talking advertising speak.

The standard connectors ( 4, 6 and 20/24 way) have several independent pins connected to the same voltage rail (12v, 5v etc). If these are not all present and supplied the pc will not work.
There are, in truth, lots of 12 volt rails within the pc. But they are all connected to the same star point within the supply case for good electronic reasons.

So perhaps the advertisers have picked up on a technical point, 5% understood it, and given it a glamorous name?

 

I recently read something online about how there are very few PSUs with 'true' separate rails for the 12v line. They are 'virtual rails' and as studiot stated above, they all originate from the same place. So it would seem that the dual/multiple rail issue is more advertising hype than anything. So, I guess it comes down to this: get the most amps on the 12v line (or combine the rails) as possible for the amount of money you have to spend. It seems as though if you want more than 30a or 35a, you need to spend over $80 on a PSU.... again, I'm not shopping, I'm just trying to figure this out for the knowledge of it....
Thanks....

 

These considerations (from the PC Power & Cooling link I posted earlier), while they certainly wouldn't apply to most hardware configurations, might be critical, it seems to me, with a really demanding setup:

"Here are the facts: A large, single 12-volt rail (without a 240VA limit) can transfer 100% of the 12-volt output from the PSU to the computer, while a multi-rail 12-volt design has distribution losses of up to 30% of the power supply's rating. Those losses occur because power literally gets "trapped" on under-utilized rails. For example, if the 12-volt rail that powers the CPU is rated for 17 amps and the CPU only uses 7A, the remaining 10A is unusable, since it is isolated from the rest of the system.

Since the maximum current from any one 12-volt rail of a multiple-rail PSU is limited to 20 amps (240VA / 12 volts = 20 amps), PCs with high-performance components that draw over 20 amps from the same rail are subject to over-current shutdowns. With power requirements for multiple processors and graphics cards continuing to grow, the multiple-rail design, with its 240VA limit per rail, is basically obsolete."

It will be interesting to see what actually happens to PS design as it appears that more powerful (in terms of processing power) CPU's seem to be using less actual electrical power than older CPU's while graphics cards seem to be sucking power through a larger and larger straw for even the most powerful single cards and then there are the dual card setups, Crossfire and SLI, that demand even more juice.

 

I went with a pc power and cooling psu for that reason. Plus pc p and c gives their psu's a SUSTAINED rating unlike most that post their peak capabilities as their rating. I run the silencer 610 and my voltage fluctuates from 12.09v at idle to 12.03 under load running 8800gt 2 dvd burners 3 500g sata drives and a quad with 8 gigs ddr2800 ram.

P.S. 12v rail has 49 amps on this psu and all other rails also rock solid

 

So, following the Eezak's info above... that means a 680w PSU with a 'single' rail at 22a is 'better' than a 550w PSU with dual rails at 20a each.... right?

 

a 680 watt would probably have higher amperage on the 12v rail then 22 amps but in essence yes. it would probably be somewhere in the 30's.

 

I think it's difficult to generalize, dlb. I think it would depend on the actual current demands of the hardware. I think it would be a rare case where you had a current demand of more than 20A but less than 22A, but in such a case then certainly a single rail capable of supplying up to 22A would be better than a multi-rail that can only supply a max of 20A on any 12v rail.

But speaking more generally it's better to have a 12v single rail that can supply up to 40A total than to have a two 12v rails that can supply a max of 20A on each rail. But even that is a theoretical advantage that might not apply if the hardware doesn't actually need more than 20A from a given 12v rail (which is still the case on many, maybe even most, computers I think).

On the other hand with continuing increases in GPU capabilities and more and faster RAM on graphics cards and, in addition, multi-graphics card setups, and more system RAM plus increasing numbers of cores in CPU's added to the power needs of DVD burners, sound cards, and, say, a large RAID setup or just 3 or 4 non-RAID hard drives and I'd guess that exceeding a 20A limit on a given single rail in a mult-rail 12v setup is going to happen more and more often. Hence the PC Power & Cooling argument for the superiority of a single 12v rail PSU, all other things being equal (which in the case of power supplies is hardly ever the case of course).

What I'm unclear about is where that 20A limit for each 12v rail in a multi-rail system comes from. Is that an Underwriters Labs requirement? And why doesn't it apply to a single rail system? Sure, 240/12 = 20 .... but why doesn't that same limit apply to a single rail system? I think I read an explanation somewhere the same evening I was surfing for power supply info and found that PC Power & Cooling link, but now I can't recall what it was or where I read it.

While I hope I understand more than just 5% (see studiot's post above) of what I read about power supplies and electricity, I certainly am nowhere near expert when it comes to understanding electricity, much less how it all interacts as it gets routed to the various transformers and other devices in a computer power supply -- all the changing resistances and other electrical and magnetic effects.

 

I just spent the past 45min to an hour looking at power supplies. I'm almost cross-eyed, but it seems that the so called 'quality' brands like SeaSonic, FSP, ThermalTake, and RaidMax aren't all that special. I found several other makes with equal or better specs, with equal or better warranties, for up to $40 less for almost identical items.... but then again, maybe it's not all about straight specs. Maybe the name brands use better components, but I saw a ThermalTake PSU (I don't remember the exact wattage but it wasn't over 550w) with dual 12v rails, rated at 16a and 14a, and only 28a on the 5v line, and it was like $75.00.... that seems a bit high for such low numbers...

 

I think it's not all about specs or at least not about manufacturers' claimed specs. I think there's not much regulation about how PSU companies have to measure and publish their specs so that off brand PSU's can and do appear, on the surface, to be equal to more expensive name brand units.

Brand name PSU's, according to what I've read, really do typically have better components -- for example larger and better quality capacitors. In addition a brand name PSU may rate their power output more conservatively, giving a figure that the unit can sustain over time whereas a cheaper PSU may say it can produce the same number of watts, but can't sustain that level of power, just put it out momentarily as peak power output.

You can often judge the quality of what appear from the specs to be similar power supplies by comparing their weight. A quality brand name power supply will typically weigh more than an off brand with similar specs and that increased weight is, according to what I've read, a good indication that the heavier PSU is made from better quality parts.

In addition there may be differences in the design/engineering of the PCF (power correction factor -- and there are passive and active PCF designs), the efficiency of a PSU, and the ability of a PSU to overcome limitations and fluctuations in the quality of the incoming electricity supply that powers it. And the safety circuitry in a brand name PSU may be better designed than an off brand unit.

Then there are things like how many and what types of connectors (beyond what the ATX spec requires, for example), quality of the insulation, how quiet and effective the fan or fans are, how fan speed is regulated (some PSU's regulate fan speed automatically based on temp sensors and some may only provide a manual switch that the user must remember to change if he thinks he needs more PSU cooling).

I found this article about PSU's some time back and it addresses a few of the quality issues I've mentioned above such as inflated and unrealistic power output ratings (see "page 6" of the article linked below for example):

http://www.hardwaresecrets.com/article/181/1

One example alone doesn't prove the general point, but here's a story from my own experience. When I first moved to Connecticut and was building a new computer I hadn't learned where the good local computer shops were. Just driving around I spotted a small shop and went in to buy a power supply for my new build. All they had were some off brand PSU's. I was dubious about buying one of them, but the fellow behind the counter in the small store assured me they were good power supplies (what else was he going to say, right?). Well, I was in a hurry and hoping to get my new computer up and running by the next day. As it happened, I was too busy for the next several days with other things and, in the meantime, found a chain store with name brand PSU's and picked up an Antec with the same watts rating as the off brand I'd already purchased.

I used the Antec in the machine I was building and never got around to returning the off brand PSU I'd bought. I stuck it away in the closet until the PSU in another computer I'd built failed. I replaced it with the off brand PSU and it failed after less than 6 months use. That was over three years ago and the Antec, whose power output spec was the same as the off brand, is still running fine. The real clincher by the way, once I'd bought the Antec PSU, in my deciding to install it for that first build and put the off brand PSU aside, was the difference in the weight between the two units. The Antec, which admittedly cost more, also weighed more -- in fact, I'd guess it weighed at least twice as much as the off brand unit.

 

That's a good link, Eezak.
Here are some more.

If you want the gory tech details

http://www.efficientpowersupplies.o...er_Supplies_Latest_Developments_APEC 2006.pdf

Other more readables sites include

http://www.tomshardware.com/2004/01/22/getting_the_right_power/

http://www.fonerbooks.com/power.htm

 

Awesome info here guys! I'm glad I started this thread. I think lots of folks can learn something from all this....

This is true. I have found that the less a PSU costs, the less it weighs too... It's interesting that you mention Antec as that is one of the few PSUs I've used that actually didn't work as I had hoped. The fan didn't work correctly, and therefore the unit would overheat in under 2 minutes. The first time I installed it was on a new build; I had just done a complete overhaul and upgraded my personal home PC. Imagine this: you have just spent over $300 for new hardware (CPU, mobo, PSU, etc) and you installed your OS using some off-brand PSU because the 'name brand' one was out of stock and will be arriving 3 days later. So you happily use your PC until the new PSU arrives. You install it, the PC boots up, you're all smiles and then BAM!!! The screen goes black, PC shtus off, no warning, nothing. You wait a minute and check all your connections and then power up again, and this time after maybe 60 seconds it shuts off. So I took it all apart and just happen to set my hand on the PSU and noticed how hot it was. And this was an Antec (470w I think) and it did weigh about 3 times more than the generic one (which I have since replaced with mid-grade CoolerMaster), but the fan (when it spun at all) would spin VERY slowly.....
Anyway, thanks again for all the excellent info and links. Very interesting, and very informative. There's enough info here to keep me reading for quite some time. Thanks guys.
:dood You rock! :dood
:major [dlb]

 

studiot, I also appreciated those power supply links you provided. Gave them a quick read earlier today and will look them over again in another day or two and absorb a bit more I hope.

dlb, sorry to hear about your horrible experience with Antec. I hope none of your other hardware was damaged by the overheating power supply? I suppose it's like automobiles or TV sets or whatever. The odds are generally in your favor if you buy higher quality brand name stuff, but even Lexus and Sony occasionally produce "lemons". And that applies to Antec too, as well as PC Power & Cooling, Corsair, Thermaltake, etc etc.

I'm glad you started this thread also, dlb, as I'm interested in understanding power supplies better also. One thing I'm still confused about is why that 20 amp limit on a single rail in a multi-rail design PSU doesn't necessarily seem to apply to a power supply that has only one +12V rail. Maybe studiot can tell us.

 

Just for fun (yeah, get a life) I was looking at the new Geforce 9800GX2 video card. It requires 580w with a minimum of 40a on the 12v rail. With all the power supply research I've been doing, I don't think I've seen ANY with over 36a, never mind 40a. I did see some with 3 and 4 rails rated at 18a or 16a which (combined) would be over 40a....

 

pc power and cooling silencer 610 watt has 49 amps on the 12v rail.

 

True, but it's like $120.00 (which actually isn't bad for a high end PSU) and that's a bit out of my price range. It's a good thing that I'm not really in the market to buy a high end PSU or I'd be all stressed out right about now. And that's one... ONE power supply that meets the requirements for a video card..... if anyone can post another, I'd be happy to check it out.

 

here's another
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139004
yes they cost more but if you run a high end card like that then these are worth it

 

How about 70A/840 watts on the +12V rail?
Uh, how about a $300 price tag to go with it?

http://www.tigerdirect.com/applications/searchtools/item-details.asp?EdpNo=2700093

http://www.tomshardware.com/2007/09/19/sbm_high_end_system/page9.html

That second link is the same one I posted earlier as an example of a PSU with lots of amps on one rail -- the Tom's Hardware article about building a $4000 game machine.

 

http://www.tigerdirect.com/applications/searchtools/item-details.asp?EdpNo=2937371
How 'bout this one??? Only $500 for 117a!!!!!!!!!!!!
:foolish That's just freekin' crazy!!!
But I guess if you're running SLI video cards that cost over $300 each, then it's worth it....
Crazy... totally freekin' crazy.....

 

Small wonder there's East Coast power shortages and blackouts.



Seriously though, this supply is useless. 70 amps at 12 volts =840 Watts.

If the above wattage spec is for the whole supply total there is nothing, zilch, nada left for the other rails.