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Simple question about rails - Page 3

post #21 of 23
Quote:
Originally Posted by ghell View Post
So single rail is no better or worse than multi rail? If this is the case, why are both sold but single rail becoming increasingly popular in very high end PSUs?
Three reasons:

1. Single +12V rail is cheaper to build. You completely omit the sub-circuitry that divides and puts limits on each group of connectors.

2. Easier to engineer. I'm not talking about the EE guy that actually designs the PCB and specs the components. I'm talking about the guy that takes the platform and has to determine how to group the connectors in such a way to not trip the OCP (Over Current Protection) if the +12V rail is split. If you omit the OCP's altoghether, guess what? You don't have to worry about what connectors go where.

3. Marketability. With folks like Nvidia saying "this graphics card needs 30A" and the average customer not knowing this means for the whole system and that this 30A can be spread across multiple +12V rails, it's just easier to market a PSU with a single +12V rail that is > whatever Nvidia says whatever graphics card needs.

That's not to say there isn't marketing on the other side as well. Some companies are actually trying to sell single +12V rail PSU's (no split, no limits) as multiple +12V rail PSU's because some customers see that +12V1 = 18A, +12V2 = 18A, +12V3 = 18A, +12V4 = 18A and just adds the four rails up. 72A FTW! Never mind the actual +12V capability should be somewhere else on the label and is a much lower number than the sum of the limits on all of the +12V rails.

Quote:
Originally Posted by smokinbonz View Post
From what i can understand the multilpe rail psu is a marketing gimmic. #1 there is only 1 rectifier in the psu giving you the 12volts. so electrically there is 1 rail no matter how many 12v leads come out of it. They use sensors to know when 1 rail is pulling too much current.
Although this is true 95% of the time, that doesn't make it ALL marketing.

Short circuit protection pretty much only works if there's no resistance at the short. Like if you shoved a nice metal paper clip across the +12V and a ground, or cut a live power supply's wires with a pair of metal cutters. But most circuit boards, like a sound card or the PCB on a drive, has enough resistance to not trip SSP. What happens instead? The load increases to the point where the wire heats up, insulation melts off and you have a blob of molten plastic on the bottom of your case. Splitting and capping +12V rails DOES have it's purpose.

Quote:
Originally Posted by smokinbonz View Post
Basically if you buy ba single rail psu = no worries
buy a multiple rail psu you may find yourself in a predicament, lets say your vid card needs 18A and your rails supply 60 amps over 4 rails, guess what yoyu cant run that card cause no single rail is capable of delevering over 15 amps soo youre stuck.

another way is to say you need 5 amps for the vid card ok well guess what you just lost 15 amps cause you cant split them up 1 rail feeds the vid card then you lose the 1 rail.

single= simple

this would never happen with a single rail
Sorry. All of that information in that part of your post is almost completely wrong. But it's not your fault. It just sounds like "single +12V rail marketing" has done it's job on you.

The only time a customer has to worry about multiple +12V rails not being "enough" is if the PSU wasn't designed for the application in the first place.

Example: Let's say a graphics card needs up to 18A via two PCIe connectors. Those two PCIe connectors will likely be on their own rail and the OCP on that rail set to 18A, or possibly a little more (like 20A or 22A because overhead is always good.) Therefore, the customer need not "worry" about anything.

Now let's flip that around: The OCZ GameXStream has two PCIe connectors on two +12V rails capable of 18A each. That's THREE TIMES MORE power than a PCIe connector would ever need, right? A third rail powers the CPU and a fourth rail provides power to the +12V on the 24-pin connector and all of the Molex and SATA connectors. So, this power supply should have NO PROBLEM powering ANY SLI system... unless....

Let's say the graphics cards are 8800 GTX cards with two PCIe power connectors each. Hmm... the PSU only has two PCIe connectors. So you use the Molex to PCIe adapters. Fire up a game and the PSU shuts down. But why?!?! It's a 700W PSU! More than enough! Well, the Molex to PCIe adapters put half the graphics card's load on a rail that wasn't meant to support graphics cards. You just split the power with a rail that's already trying to support a bunch of drives, fans, etc. AND provide some power to the graphics card via the slot. Aint gonna happen. But again, that PSU wasn't designed to run those cards.

As far as "trapped power" goes, that's usually a myth. Certainly is completely false with any PSU with three or more +12V rails. Just do the math. Add up the rating of each +12V rail. Then look at the combined +12V rail rating. The sum is always greater than the actual. Why? That overlap prevents any "trapped power". If you have 18A capabilty on a rail and you're only using 9A, the other 9A can go somewhere else! It's not like each rail is it's own little regulated DC output. It's just a group of connectors with a limit as to how much power can go down the wire. All the power still comes from the same source.

Quote:
Originally Posted by ghell View Post
Ok, Thanks all. I'll probably either wait and see what new single or dual high current rail PSUs come out by the time I get any new hardware that might push my existing 600W too far or just get impatient and get the multi rail flagship enermax.
Yeah, if you're talking about high capacity PSU's, you're typically not talking about dual +12V rail (although I've seen some dual +12V rail units with something like 32A per rail. Guess they figure the insulation still won't melt at that point). You're usually talking about four or five or even six +12V rails, and at that point you're definitely talking about proper rail distribution and a full complement of connectors.

Quote:
Originally Posted by rx7speed View Post
one simple word sums it up

marketing.


you make your product sound good people will buy it. you tell them single rails are best people buy single rails. you tell them multi rail is best they will then buy multi rail.
Exactly. Don't get sucked in by the marketing. Single vs. multiple should be the last concern on your list. Does the PSU have enough power (and look at the total combined for the +12V and not the individual rails) and does it have the full compliment of connectors you need. Then go look for some reviews and real world experiences.

Quote:
Originally Posted by slytown View Post
Only real disadvantage, at least with quality PSUs, for multi rails is you have to calculate a little to find the actual amperage.
Right. The OCP is always rated in Amps, but the total combined is rated in Watts. But if you divide Watts by Volts, you get Amps.
post #22 of 23
I've always heard that a single rail is better when the specs are the same. For example: 1 12v rail @ 60amps is better than 2 12v rails @ 30amps. No time for an explanation, but that's what my digging turned up.
DankoniQuad
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DankoniQuad
(13 items)
 
  
CPUMotherboardGraphicsRAM
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Hard DriveOptical DriveOSMonitor
2xWD6400AAKS RAID0 + Samsung F3EG 2TB + 500GB Ext. Samsung SH-S203B Vista Ultimate 64-bit Acer B233HUbmidhz 
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post #23 of 23
Quote:
Originally Posted by dankoni View Post
I've always heard that a single rail is better when the specs are the same. For example: 1 12v rail @ 60amps is better than 2 12v rails @ 30amps. No time for an explanation, but that's what my digging turned up.
Then you heard wrong.

First off.... you wouldn't have two PSU's with the same specs where one had 60A on one +12V rail and the other has 30A across two. Because there's no overlap in the power. If you really had a PSU with 60A capability on the +12V, it would be more likely to have about 35A per +12V rail or, more likely, split that power up across four +12V rails rated at 20A each.

Read my post above. It should clear things up and show you how whatever you may have heard is actually quite wrong.
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