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Dual Rails vs. Single Rail?

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February 16, 2005 10:20:31 AM

Hi, all.

Ok, I am getting a bit frustrated trying to figure out which power supply setup is better and I'm hoping someone on this forum can help.

I am looking for a new 400W-500W power supply for a P4 system that has 2 CD-ROMs, an IDE hard drive, an internal zip drive, and a video card that requires power (this alternates between the Radeon 9700 and the GeForce 6600). However, I cannot decide if a 12V dual rail or single rail power supply is better.

Researching this has been difficult because just when I think I have the answer, I read something (mostly Anandtech posts) that points to the opposite of what I was thinking. Does anyone have a definitive answer as to which is the better choice at this time? I can't find any articles on THG that speak to this issue.

From what I understand, the 12V dual rail is part of the new standard and is here to stay. The purpose of the dual rail is supposedly cleaner power to the chip and MB, which sounds good to me. Additionally, I have read that dual rails split up power between the MB/chip and other devices like PCI cards or hard drives. If one rail starts to get overloaded, the other rail can compensate. From what I gather, the 12V requirements have skyrocketed in the recent year due to the P4, serial hard drives, and power-hungry video cards. So, everyone says you have to have very good 12V rails and many claim that the dual rail does a better job of balancing voltages. And yet...

Some people (who sound like they know what they are talking about) are saying dual rail is at best an uneccessary improvement and, at worst, a danger to components. The reasons vary, but chief among them are the worry that video cards requiring power defeat the purpose of separating the chip/MB from the rest of the components and also that two smaller rails (say 14A and 16A) are not as good as one 30A rail. The latter argument claims that, although each rail is supposed to help out the other when approaching overload, in fact they do so rather poorly.

I really don't know what to believe. Further complicating this is information I have read that states many power supplies convert much of their 12V power from the 5V rail. I don't know how that would happen, but it makes me wonder, then, if a very strong 5V rail can make up for a weaker 12V rail.

Basically, I want to buy a PSU for $100 or less that has strong rail ratings in the appropriate places and will last me for some time to come. I just don't know what the appropriate ratings are anymore nor how important the dual rail specification is. Any thoughts on this matter are greatly appreciated.

More about : dual rails single rail

February 16, 2005 6:16:44 PM

What I think:

Normally dual rails are supposed to be split rails. This means that one rail doesn't help the other rail. It means that if the mainboard rail has a huge load it won't affect the videocard and other 12V components. That way there will be more stability because a voltage drop on one rail won't have influence on devices connected to the other rail.
February 16, 2005 6:24:50 PM

Don't have the technical answers your looking for. Sorry.
But...
the Fortron 530watt will be way more power than your system you listed requires, and will only cost around $70.


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And the sign says "You got to have a membership card to get inside" Huh
So I got me a pen and paper And I made up my own little sign</pre><p></font color=red>
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February 16, 2005 8:48:11 PM

Thanks for responding. What do you think about this idea of a powered video card interfering with the split rail idea? Does it seem logical to you that the power going to the card can also make its way into the MB during an overload? Is that a real concern or would it just damage the videocard?
February 16, 2005 10:49:16 PM

There is an article on THG about the dual rail PSU. It looks to be the way to go for sure. I think that your getting way to hung up on the wrong thing. You need a quality PSU that is somewhat more powerful than what your system will need. I will say this. I have one of the split rail PSU's and in fact it has 3 +12V rails instead of just 2. I have never seen a PSU that will hold a more stable rail than this one. Ive been recording a graph of the voltages since the day I put the PSU in my system. The lines are almost perfectly straight. None of the rails has ever once dipped under the voltage that it is rated for.

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February 17, 2005 12:27:56 AM

You may be right. What power supply do you have? Also, I cannot find the article you refer to. Everything I find is either too old or doesn't mention the dual rail specifically. Thanks.
Anonymous
a b ) Power supply
February 17, 2005 6:15:05 AM

<A HREF="http://www.newegg.com/app/ViewProductDesc.asp?descripti..." target="_new"> This one </A> isn't bad either, decent price also. You definitely want the newt atx 2.0 standard, it just seems better all around, and its nice with the newer high end vid cards also

<font color=green> Woohoo!! I am officially a <b> Newbie </b>!! </font color=green>
February 18, 2005 8:37:29 AM

After much more reading, I still am not sure if dual rails are worthwhile, but I am fairly sure it won't matter for my system now. I just worry about the long run...

However, it does seem that if one is running an SLI configuration, dual rails are probably not the way to go unless you get a very powerful (expensive)power supply where the dual rails are individually very strong. Since I don't intend to run SLI, I will try not to worry about it.

Probably, I will go for a strong single rail PSU with the ATX 12v 2.0 standard. It may well end up being that price is the deciding factor for me. Thanks to all who participated.

One thing of note - this board doesn't seem to care about dual rail that much (judging by the posts I have read) while other boards like Anandtech have much to say on the matter. No slight intended here, rather I am just wondering if those who use THG have already made up their minds on the matter or if the issue simply hasn't come up.
Anonymous
a b ) Power supply
February 18, 2005 9:31:57 AM

Hold up, I thought part of the 2.0 standard was that an extra 12v rail is included for the high end graphics cards such as geforce 6800GTs and ATI x850's. Isn't that right??

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February 18, 2005 12:20:33 PM

I think that dual rail psu's are not that good... Since the seperate rails cant combine under any circumstances, at times the system could be instable, i mean if you have a very powerful videocard and bunch of other things, the supporting rail could break, because it has only 12-15A or so... I think that dual rail psus which are said to be designed for sli are nothing but scam.. One rail should be the only way for the sli.
for 80-100$ price range, i would by the tagan t480-u01. It really is a psu beyond its price range.
February 18, 2005 5:24:33 PM

The 2.0 standard only specifies 22A minimal on the 12V rail, nothing about dual rails afaik.
Anonymous
a b ) Power supply
February 18, 2005 11:49:23 PM

OIC But isn't better to have separate rails for those high end GPU's? I understand people who used one rail have had problems when the line is too taxed, and the vid card can't get enough power.

<font color=green> Woohoo!! I am officially a <b> Journeyman </b>!! </font color=green>
February 19, 2005 1:01:28 AM

Ive got the Thermaltake 680W with 3 seperate 12V rails.
<A HREF="http://www.newegg.com/app/viewProductDesc.asp?descripti..." target="_new">http://www.newegg.com/app/viewProductDesc.asp?descripti...;/A>

Here is the Toms article
<A HREF="http:///www.tomshardware.com/howto/20041223/index.html" target="_new">http:///www.tomshardware.com/howto/20041223/index.html&lt;/A>

Im not sure why you dont see the advantage. Yousay you worry about the future. Well if this is true then you should get one. Its the way to go if your buying a new one, and whats the big deal anyway, the price difference on the ones that I have been looking at isnt much at all. The one I bought was way over the top, but there are some good prices on the Fortrons for one.


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February 19, 2005 1:05:01 AM

You need to do some better research. Im not even going to begin pointing out the inaccuaricies in your reply.

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February 19, 2005 1:07:42 AM

Mine says 2.01 standard. Not sure about the numbers though.

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February 19, 2005 1:14:51 AM

I just looked at the three Fortrons on Newegg
Three split rails that have a total of at least 30AMPS on the 12V rails. The single rails are somewhere in the 15 18 AMP range. Which would you rather have?


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Anonymous
a b ) Power supply
February 19, 2005 4:54:01 AM

That's what I thought. Man you're on a role here lol :smile:

<font color=green> Woohoo!! I am officially a <b> Journeyman </b>!! </font color=green>
February 19, 2005 4:55:53 AM

Thanks for the links. Actually, I had read that article, but glossed over the author's terminology for "dual rail." He used the phrase, "+12V voltage slot for the CPU" and I hadn't realized that meant another +12V rail (this stuff is pretty new to me). It seems the author does believe the two rails are separate and, if I am reading him right, the rail for the CPU will have some "wasted" amps until the CPU happens to need more.

My concern about this has been people who say that 15 amps or so are not enough for the CPU and motherboard combination, especially when one adds PCI-E into the equation. If that is true, it would seem to be a good reason to be wary of dual rails since you apparently cannot combine wattage (again this may just be a problem with SLI configurations, but you never know what technology is coming down the pike and what power demands it might have).

By the way, it seems as if you believe three rails can add up to 30 amps or more. Is that what you are saying? Most of what I have read claims this is not so and that you must treat the rails as totally independent as far as supplying needed power is concerned. Therefore, you could have one +12V rail that is doing fine with a light load on it and another +12V rail that is getting overtaxed, but unable to be partially relieved of its burden by the other +12V rail. Am I getting this right?

Also, I don't know what single rail PSUs you are looking at, but most of the decent ones I have seen are rated for at least 24 amps and much that I have read recommends no less.
February 19, 2005 6:44:42 AM

Dont take this the wrong way, because im not trying to be short with you. Your getting this to a point and then I keep losing you. Im not exactly sure how to expain this so that we can get on the same page. I think that the first thing that you need to do is try and forget what you know about PSU's so far and start your learning process over. I know how that may sound but I am just trying to come up with suggestions. You are letting what you know about a single rail PSU keep you from understanding the differece with a split rail. There both PSU's but they are different so your not exactly comparing apples to apples, however that is exactly what your doing. Also I keep seeing that you make references to what others have told you. Let me tell you about the blind leading the blind. It dont work :smile: There are alot of people that think they know more than they do and most of them find fault in things that are new to them. This is simply foolish, however it is true. Rather to learn something new, most people will talk negatively about something they truely know nothing about. So my second suggestion is that you stop listening to some of the ones that have put you on the path of confusion. Im no expert, I dont want for to think that I am attempting to be a know-it-all. The truth of the matter is that a quality single rail PSU will do fine for your purposes. You seem to want to understand this newer technology and if it may be right for you, and thats why I have taken this side of the argument. It makes no difference to me which you buy but I would much rather know that you bought the best PSU for your situation now and in the future.

To the statement you made about 15AMPS vs 24AMPS. There is not a uniform or set standard that manufactures must adhere to when producing and marketing PSU's. Because of this, there are many ways to legally deceive the consumer. You cant simply go by what you see on the box, as many factors determine the quality of the PSU. The main thing is that you buy from one of the companys that have a proven track record.

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February 19, 2005 7:52:19 AM

Good point about the blind....

It's just hard for those of us unfamiliar with this subject to differentiate between hot air/incorrect information and genuine knowledge. If you have the time and inclination, could you detail how you believe dual rails work? I would be much obliged and will endeavor to follow your reasoning - really :smile: .

I tend to agree with you that I will be well served with most any power supply put out by reputable companies. I am actually looking at Enermax and will probably buy one of theirs. I already have a Fortron 530W in another system I got a year or so ago - I like it, but it's being sent in for service for a faulty fan and I may be upgrading soon. Anyhow, it's been an interesting learning experience and I will likely choose in the next day or so. Thanks to all who posted.
February 19, 2005 9:33:27 AM

I would be interested in finding out how dual rails work as well.
Anonymous
a b ) Power supply
February 19, 2005 6:45:49 PM

I think what he was trying to say is that when you have high end graphics cards (pci express), if you DO NOT have split rails you WILL overload the psu and cause stability problems. By separating the grafix current from the cpu/mainboard current and other systems, you ensure that your power hungry grafix card will always have enough power :smile:

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February 19, 2005 7:00:37 PM

Im putting together something now, its sorta detailed so it may take a little while.

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February 19, 2005 7:04:28 PM

Thats part of it, another advantage is that splitting the rails cuts down on heat which helps stability. From what I have read this is also an advantage to the manufacture because the split rail is slighty cheaper to produce that a single rail.

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Anonymous
a b ) Power supply
February 19, 2005 7:38:57 PM

In other words, THE SPLIT RAIL IS BETTER!! Now I think that should be easy enough to understand :smile:

<font color=green> Woohoo!! I am officially a <b> Journeyman </b>!! </font color=green>
February 19, 2005 10:14:30 PM

That may be true, but I do not see where Mozzartusm says anything like that regarding the video card. Perhaps he believes it is already understood. I look forward to his next post, as many others I have read believe quite the opposite. That is likely what Beliar was referring to (powerful video cards on the same rail as the chip and MB) and it is not foolish to worry about that when the rail is only rated at 15 or so amps.

However, it could be that this is not how the dual rail is set up. In which case, I would tend to agree that dual rails are great and a smart addition to the ATX standard.
February 20, 2005 2:08:04 AM

I hope this helps. I am going to try to keep this as factual as I can and hopefully keep my bias from clouding the issue.

Lets start out with the basics: Watts, AMPS, VOLTS, RAILS.

A RAIL is going to be a delviery vehicle. A rail is the system that is responsible for delivering or carrying electrons from point A to point B. The ones we are most familiar with are the +12v +3.3V and +5V rails. Each of these are to some extent seperate in that they are each seperate pipelines that deliver their own repective power.

How do AMPS fit in to the Pic? The more AMPS the better your supply or flow of power/energy/electrons. So AMPS are very important due to the fact that when you multiply the number of volts(V) and the number of AMPS(A) you get your number of Watts(W). W = V * A or a 12V rail that has 15AMPS has a total output of 180 Watts. OR 12V*15A=180W this of course would hold true for each seperate rail.

For comparison I will use the ANTEC TRUE550W and the Thermaltake 680W Split rail. I know that there is a difference in total Watts, however the number of watts is not the issue so keep this in mind. The Antec has a single +12V rail with 30AMPS so we plug the formula 12V*30A=360WATTS The Thermaltake split rail has +12V(1)*15A=180W +12V(2)*15A=180W +12V(3)*8A=96W for a total of 456WATTS

Ok guy's im not even close to the end but I need a break so here is the first part. I will post the next part in about 30 mins. Someone double check my math please :wink:

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February 25, 2005 6:15:51 AM

Well, I was looking forward to reading Mozzartusm's post on dual rails, but he must be busy. Anyhow, I decided to go with the Enermax (EG495P-VE SFMA Noisetaker 485W 20+4 PIN ATX +12V ver 2.0) PSU. It is dual rail and they seem rather solid at 18A apiece.
Also, I found this one at 3Gplaza.com for $87. That's a pretty good deal, considering it tends to go for $105-$110. Here's the link: <A HREF="http://www.3gplaza.com/estore/control/Computer3G/produc... " target="_new">http://www.3gplaza.com/estore/control/Computer3G/produc... </A> .

Thanks, again, to all who posted.

<P ID="edit"><FONT SIZE=-1><EM>Edited by mondschatten on 02/25/05 03:26 AM.</EM></FONT></P>
February 25, 2005 12:21:38 PM

I still dont understand exactly how the dual rail bit actually works? Are some molex connectors attached to one rail and others to the second? Is it that motherboard that gets the second rail?
February 25, 2005 9:17:04 PM

You know, that's a good point. I believe that both are true or, at least, are true if you hook up the connectors correctly. I am hoping the PSU's manual gives me more information about that. Some people have posted that the dual rail effect is lost if you aren't careful about which molex connectors go where. Hopefully, someone will post a definitive answer on this.
!