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Multiple 12v rails VS single 12v rails

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June 29, 2009 9:21:24 PM

This seems like an easy question to me but I cant seem to find the answer anywhere. Is there any real benefit to having a power supply with say 4 12v rails rated at 25amps, instead of one that has 12v rail rated at 100 amps(I only use this as an example probably more power than i need). I ask because i have heard lots of great things about PC Power and Cooling PSU's and from what i see they only use single rail setups, while i see most manufacturers going the other route and using 2-4 rails. If anyone can answer this i would greatly appreciate it.
a c 83 ) Power supply
June 29, 2009 9:27:45 PM

After reading jonnyguru, there isn't a difference. "multi rail" PSUs are infact single rail PSUs that have been "split" into multi rails. All of the "seperate" rails are feed by the same source at the same time, just like a single rail. (You would never find a PSU with 25A rails, the thin wires can only handle 18-20A max. Any more, and the insulation would melt right off due to the heat.)

PCP&C makes good PSUs because they put good parts inside their PSUs. The same goes for Corsair, Seasonic, and many Antec PSUs. Other companies such as Athena power, Silverstone, etc don't.
June 29, 2009 9:31:10 PM

sometimes manufacturers will split the rails in a very bad way, so the user can not receive anywhere near the stated power. Also, if you have 1 rail, as long as you don't exceed that amount you will be good. But, if on a multirail system, you have say a graphics card or something that pulls more current than what is stated for the rail, you can't run it.
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a c 83 ) Power supply
June 29, 2009 9:43:23 PM

Sorry resort, but neither case will happen.

Quote:
sometimes manufacturers will split the rails in a very bad way, so the user can not receive anywhere near the stated power.


I believe you are referring to whats called trapped power. Lets pretend you have a two rail PSU with 18A on both rails, 22A total. (this is similar to the Dell 305W PSU) 12V1 powers the CPU and motherboard, while 12V2 powers everything else. Some people believe that if you plug in a CPU and motherboard that only uses 8A, you will use the entire 18A that rail can provide. They think that 10A is "trapped" from the psu because the mobo/CPU isn't using it, and it can't get to 12V2. Remember however that BOTH 12v1 and 12v2 are feed from the same source. If 12v1 is only drawing 8A, that leaves (22-8) 14A for 12v2.

Quote:
But, if on a multirail system, you have say a graphics card or something that pulls more current than what is stated for the rail, you can't run it.


This isn't likely to happen either. If it did, the device would violate the ATX specs. The specs say 18A per rail, so no device can draw more. This is why you see video cards with 2 PCIe connectors on them. If they had included just one, it would have pulled to much power from the connector.
June 29, 2009 9:50:14 PM

tywilke2 said:
Is there any real benefit to having a power supply with say 4 12v rails rated at 25amps, instead of one that has 12v rail rated at 100 amps(I only use this as an example probably more power than i need).
Me personally, I am fan of the single 12v rail but only because it's just easier to know what the psu can deliver.

4745454b said:
After reading jonnyguru, there isn't a difference. "multi rail" PSUs are infact single rail PSUs that have been "split" into multi rails. All of the "seperate" rails are feed by the same source at the same time, just like a single rail. (You would never find a PSU with 25A rails, the thin wires can only handle 18-20A max. Any more, and the insulation would melt right off due to the heat.)
This is not entirely true. Some psu makers include limiters on the separate 12v rails. If a particular psu maker labels the 12v rails at 25a and they have put limiters on the 12v rails, the rail can only output 25a, no matter how many rails there are. Conversely, if the psu maker does not use limiters and the label states 25a on the 12v rails and there are 4 rails, then you could in theory get 100a all on one rail; but then that would defeat the purpose of multiple rails and load balancing your psu.

I recommend PCP&C, Corsair, Enermax, and Seasonic psu's.

Good luck!
June 29, 2009 10:18:15 PM

Thanks for the info and really fast replies. I really appreciate it.


4745454b,
Its funny that you say that, cause its an athena power PSU that im gonna replace in the next week or so. Since i upgraded my sytem and started paying attention to voltages, I noticed that this PSU has major fluctations.
a c 83 ) Power supply
June 30, 2009 12:40:33 AM

Quote:
This is not entirely true. Some psu makers include limiters on the separate 12v rails. If a particular psu maker labels the 12v rails at 25a and they have put limiters on the 12v rails, the rail can only output 25a, no matter how many rails there are. Conversely, if the psu maker does not use limiters and the label states 25a on the 12v rails and there are 4 rails, then you could in theory get 100a all on one rail; but then that would defeat the purpose of multiple rails and load balancing your psu.


But thats exactly how multi rail PSUs are done these days, limiters. If PSU maker XYZ puts out a PSU with 4 rails, each with a limit of 18A (or 25A using your guys suggestion) then 18A is the limit. Notice I said limit, and not output. The rail is LIMITED to 18/25A, not REQUIRED to output 18/25A. You said "the rail can only output 25a, no matter how many rails there are." making me think you believe in the trapped power belief. If the limit is 18/25A, then it can provide power up to that amount.

tywilke2, I'm not surprised. There are many brands of PSUs, not all of them are good. Theres a reason why when someone says they have a 500W PSU so they should have enough power I still ask who made it. Not all 500W PSUs are created equal. For example I run an Antec Earthwatts 500W. Active PFC, 80%+, 32A (?) on the 12V. Someone else in a thread was running an "off brand" 500W, no PFC, 70%+, and only 22A. Big difference in quality and power output.
a b ) Power supply
June 30, 2009 3:11:41 AM

Do not worry about single vs muti-rail as there are advantages/dissadvantages to both when comparing. Be more concerned about the "Quality" of the PSU.

Just to correct a minor point. The wires are not the problem, it is the connector/pins on the GPU and the PSU. A single pair (4 wires) of 20 ga wire could handle most graphic cards (up to 264Watts), but the connector on the graphics card would probably melt the plastic shell and/or melt the solder. This is the reason they use multiple connections.
a c 144 ) Power supply
June 30, 2009 4:10:35 PM

I have three PSU's in systems. I have a single rail Corsair 750TX and a couple of older "three rail" Antec TP3's (550 watt and 650 watts). On the Antecs, I measure a short between any two yellow wires - not electrically separate and no limiters present.

I therefore conclude that I have three single rail PSU's.

I have come to favor a single large rail.
a b ) Power supply
June 30, 2009 5:42:15 PM

4745454b said:
After reading jonnyguru, there isn't a difference. "multi rail" PSUs are infact single rail PSUs that have been "split" into multi rails. All of the "seperate" rails are feed by the same source at the same time, just like a single rail. (You would never find a PSU with 25A rails, the thin wires can only handle 18-20A max. Any more, and the insulation would melt right off due to the heat.)

PCP&C makes good PSUs because they put good parts inside their PSUs. The same goes for Corsair, Seasonic, and many Antec PSUs. Other companies such as Athena power, Silverstone, etc don't.


Your Silverstone good parts statemant is big time in error, I own a Silverstone OP1000W P/S Single 82A 12V rail, the wiring harness is some of the heaviest guage wire I've ever seen on a P/S, I also own a PCP&C 750W Single 12V rail P/S, which by the way is the second, as the first died shortly after powering it up, the Silverstone has never had any problems period, and is one definite high end product.
a c 83 ) Power supply
June 30, 2009 6:30:29 PM

Yeah, I almost edited my post. I'm talking about the "classic" silverstone units, the ones from many years ago. They've come a long way since then, I really need to think better of them.

Fine then, Athena power and sparkle? (why would you name your company Sparkle? Is it supposed to sparkle when you plug it in?)
a b ) Power supply
June 30, 2009 6:50:14 PM

jsc
I'm making a guess. You measured all the "yellow" wires, probably off of the molex connector. I believe you will find that they are all on same rail. Most PSU's use a tracer color to differiate between V2, V3, (and V4 on 4 rail PSU's). Look at your 4/6 pin connector for GPU and/or the 4/8 pin MB power connector. Look to see if they have tracer color and measure your molex to that connector.

Repeat not for, or against ethier Single rail vs multirail PSU. I look for "quality"

The biggest advantage of a single rail PSU is that you DO NOT have to worry about which rails are connected. Also most (NOT ALL) single rail PSUs are of the higher quallity. But this is also its biggest single disadvantage - fault tolerence/safety. Shoud your system develop a "short Circuit" of 0.2 ohms, your PSU would not shut down ( only 60A ). the power dissipated at the "short" would be 720 Watts, all in the form of Heat
a c 121 ) Power supply
June 30, 2009 7:03:43 PM

+1 @4745454b & RetiredChief.

Read the jonnyguru PSU reviews, and the ones at HardwareSecrets and HardOCP. There is a lot of good information on how PSUs should be made, and how they ARE made. Writers' comments can be humorous as well as damning when the two aren't the same in a particular model.
I have never read a bad competent technical review of an Antec PSU. I'd buy Corsair, PC Power & Cooling, Seasonic, and Enermax; but Antec is my personal favorite.
a c 83 ) Power supply
June 30, 2009 7:23:17 PM

If you haven't, try reading about the smart power line from Antec. They used at the time "suspect" caps that proved to fail over time. I heard Antec was good, but I wish I had known that when I bought my SP450. I wasn't turned off by Antec however, both my machines are now powered by EA500s. For me, Antec simply offers the best performing PSUs for the lowest price.
June 30, 2009 7:34:44 PM

Quote:
The biggest advantage of a single rail PSU is that you DO NOT have to worry about which rails are connected. Also most (NOT ALL) single rail PSUs are of the higher quallity. But this is also its biggest single disadvantage - fault tolerence/safety. Shoud your system develop a "short Circuit" of 0.2 ohms, your PSU would not shut down ( only 60A ). the power dissipated at the "short" would be 720 Watts, all in the form of Heat


Great discussion men :)  Question. When you say, "should your system develop a short," do you mean specifically the power supply? PCP & C 610w installed recently and the 'system' would not shut down properly. Power on occured randomly after shut down. Maybe immediately or maybe 5-10 minutes later the power would come on. OS would not load, but the power was 'magically' on by itself so to speak. Removed the PCP & C 610w and installed OCZ 400w. System now shuts down AND runs properly. The short was a product of the 610w PCP & C unit?
June 30, 2009 7:37:21 PM

I prefer the single rail in theory, because say you have 2 rails limited to 25A each and 3 devices using 15A each then you can only connect two. But in practice you are quite unlikely to have this problem so I pay little notice to the number of rails, generally as long as it's a decent brand and enough wattage then it shouldn't be a problem.
a c 121 ) Power supply
June 30, 2009 8:29:41 PM

Hmmm, was only thinking of the last five years or so. Someone at Antec must have stood up and declared that they would... not... Make... CRAP (or put their name on someone else's).
June 30, 2009 11:13:57 PM

imho

Rails are nothing more than a marketing gimmick. For the rail theory to be sound there would have to be separate transformer and regulatory circuits on each "rail". In practice there is a singe circuit with separate limited outputs, in my mind defeating the entire purpose.
a b ) Power supply
July 1, 2009 2:51:48 AM

Badge - You have a good point however, the short I was referring to was at the sub-system level, ie at a HDD, DVD Drive, on the GPU board, or the Motherboard. On the single rail there is nothing to limit the current so theoretically the PSU could source up to 1200 Watts (That is one He2l of a heater) where as a 25 Amp rail could only provide up to 300 Watts. From a safety standpoint I would sleep better turning the System off at night or when leaving the House. I know a lot of people leave there systems on 24/7 and I know of no one who has made a bonfire of his/her home.

505090. A rail can be either/or, perhaps the best term is split rail. The use of separate source circuits would be a little on the cost prohibited side for "Us" poor boys. A separate rail only needs to be isolated from one another. This isolation is provided by the "I" regulator

If I was running two/three GPU's in I gaming rig I would go with the single rail, but I'm not so my antec 850, which has seved me well for the past 2 yr, is fine.
April 21, 2012 11:23:25 AM

the last resort said:
sometimes manufacturers will split the rails in a very bad way, so the user can not receive anywhere near the stated power. Also, if you have 1 rail, as long as you don't exceed that amount you will be good. But, if on a multirail system, you have say a graphics card or something that pulls more current than what is stated for the rail, you can't run it.


Correct. I bought a PNY GTX 460 OC card for 110 CAN $, and PNY claimed that it required a 38 AMP rail when configured with a 130 watt CPU. Well I realized when looking at Canada Computers and Newegg, that no other competitor than Corsair had PSU as such for less than a 100$, in fact only the corsair builder series 600 watt, and more expensive variants, met the spec at 40 amps single rail, but those were the requirements for 130 CPU.

Let's do some math,

10-50 watts maximum for motherboard and ram
95 watts for modern 4 core Intel CPU
160 watts TDP for GTX 460 V2
25 watts for other components
-----
330 watts safe, not agressive.

Now if you look at, the 430 watt Corsair builder series, its single rail of 28 amps outs 336 watts.
Fuckin PNY playing it really safe; 38 amps lmao.
All other PSUs i saw under 80 bucks used multi rail systems.
If you think of a rail as an outlet, then on a multi rail psu the problem how i do know the outlet has enough power. To eliminate that headache just stick with single rail psu. 34 amps is ample for ANY single GPU configuration save dual GPU card; 500 watt builder series. For multi GPU systems you need 200 to 250 more watts from the rail. If you look at builders series 600 watts, it only provides 40 amps or 480 watts which is insufficient, take a look at the corsair gaming variant rated at 48 amps or 576 watts and your gold.
a c 243 ) Power supply
April 21, 2012 12:56:13 PM

Woohoo, 3 year old thread
a b ) Power supply
April 21, 2012 3:31:39 PM

+1...

:D 
a c 144 ) Power supply
April 21, 2012 8:37:19 PM

This topic has been closed by Jsc
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