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Power Supply Rails

OrbitzXT

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#1
I'm kind of confused about how Power Supplies work and what is considered desirable. When I was purchasing a PSU months ago, I remember reading on NewEgg all the Pros about the number of rails a certain PSU had. Now I picked out this Silverstone PSU, and all the pros talk about it have 80A on a single rail. So what is good? What are some things to look out for beside a brands reputation for reliability?
 
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#2
efficiency for one.

there are PSU threads here .. just do a lil search.
 

panchoman

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#3
rails are basically a safety feature. rails are there to prevent overloading wires and rails allow psu manufacturers to use lower quality wires, which carry less amps. the power is divided up into different rails.. which is almost synonomous with wires. as the power is divided up, the wires have to carry less charge and are capped off to prevent over a certain amount of power going through the wires, this is the safety feature, as the wires are capped off, this also allows psu manufactureres to naturally use inferior wires as the wires will be carrying less current. single rail psu's have higher quality wires to carry the extra wires and are generally uncapped, and you'll get similiar performance from both types of psus, except for high power devices (excluding video cards) such as thermoelectric elements, compressors, and the like. video cards really like to draw power over either one rail, or in a multi rail configuration, they draw from multiple rails. one rail is the one that supplies the pci e 6/8 pin wire.. and the card also obtained a good amount of its power from the pci bus(motherboard) , which recieves power from the 20/24 pin atx connector. cards such as the 2900xt use 2 connectors so that they can access a third rail of the psu as sometimes 2 rails comes short in a small psu. however with pci 2, the pci bus can carry more power so hopefully we'll be sticking with 1 6/8 connector. um yeah.. so dont worry about the rails unless if you're running a tec,compressor, etc., then you'd need a single rail config.

edit: it can also be argued that psu's with single rail are better because power because power cannot be rerouted on the rails, as they are capped off.. so if you have a rail that is for the cpu and cpu only( as many psus are configed, to make sure the cpu recieves clean seperate power), lets say the cpu is only using 10 of the 20 amps on the rail, well the other 10 amps cant be rerouted to the say the video card rail as the video card rail is capped so that it can only take a certain amount of power, as its capped off to prevent the wire from being overloaded. so those 10 amps would be technically lost, as they have become rendered useless..
 
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#4
Quality of components used is important ie: filtering quality, component protection ect. The average amperage supply is another important factor as some brands boast maximum amperage supply and of course efficiency. Does this help at all?
 
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#5
pancho puts it well.. a safetly feature

the rails have a max of 19 amps normally its an itel standard..

cpu and one high end grafix card two rails.. two 15 amp ones would do..

cpu and two high end grafix cards three rails at 15 amp would do..

cpu and three high end grafix cards.. 4 rails at 15 amp would do..

cpu and 4 high end grafix cards 5 rails at 15 amps would do..

one rail for the cpu and one rail for each high end grafix card..

the average 1200 watt psu would have 6 rails at 19 amp.. somewhat overkill unless u wanna run 5 2900xt cards and two cpus.. he he

rails u dont use are wasted..

for comparisons sake the 3870 draws about 8 amps.. the old 2900xt double that..

trog
 

OrbitzXT

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#6
So...this one I picked a single rail. While everyone on NewEgg reviews is praising it, should I get a different one with more rails? Also how many watts do I need to run two 3870 X2's in Crossfire? I realize its not possible yet, but in the future when it is I will seriously consider adding a second one since I game at high resolution.
 
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#7
So...this one I picked a single rail. While everyone on NewEgg reviews is praising it, should I get a different one with more rails? Also how many watts do I need to run two 3870 X2's in Crossfire? I realize its not possible yet, but in the future when it is I will seriously consider adding a second one since I game at high resolution.
treat the 3870 x 2 the same as the old 2900xt.. 15 or 16 amps per card or per rail.. 19 amp rails would be best..

two 3870 cards would run off one 19 amps rail with some to spare..

do your maths is easy to work out..

trog

ps.. think three 19 amp rails or a 750 watt psu for a pair of x2 cards.. the cpu rail dont need 19 amps but they tend to come that way..
 

OrbitzXT

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#8
All of this is a little confusing to me, will I be alright with the PSU I picked out? It has good reviews, a good brand, and a single 80A rail...and a great price for the amount of power.
 
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#9
I rember reading that a single powerful rail is better than multple weak ones, so I went out and bought a PSU with a sinlge large rail, I think that will do you just fine.
 
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#10
single rails deliver higher amps to single devices but if you hook many items to it you may find you need more connectors.

to put it simple, a psu is a transformer, it takes 120v power in and supply 12v amps to your system

a single rail means all leads come directly from the power supply output and share one connection.

multi rails are split on seperate connections for each rail and each rail is running through a limiting regulator so for safety reasons it cant carry too many amps (usually somewhere from 14-20 amps) the single rail is likewise protected but because it feeds everything this is set rather high and can cause melted wires, smoke, and even possably a small flame in the event of a direct short.

multi rail psu are safer for computers without devices needing over 20 amps and are safer in the event of direct shorting, which is rare but running wires in unintended places for hiding them can result in chaffed wires and cause direct shorts. they are safer because they trip faster in the event of overload or short.

single rail psu are better for higher amp devices that draw over 20 amps but because of this are just a tad slower in tripping in case of shorts. a single rail because it has a slightly higher trip threshold, does not protect your system as good as a multi rail would if the psu blew.
 

Namslas90

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#11
So...this one I picked a single rail. While everyone on NewEgg reviews is praising it, should I get a different one with more rails? Also how many watts do I need to run two 3870 X2's in Crossfire? I realize its not possible yet, but in the future when it is I will seriously consider adding a second one since I game at high resolution.
The Manual for that cards recomends;

A PCIe 750 watt or higher power supply with two 2×3 PCIe power connectors.

Refference; http://support.ati.com/ics/support/default.asp?deptID=894

Load page, click "Radeon Support for PC" on left menu, then select card, Download Manual, read manual, enjoy your new ATI product with no problems.
 

bujeezus

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#12
treat the 3870 x 2 the same as the old 2900xt.. 15 or 16 amps per card or per rail.. 19 amp rails would be best..

two 3870 cards would run off one 19 amps rail with some to spare..

do your maths is easy to work out..

trog

ps.. think three 19 amp rails or a 750 watt psu for a pair of x2 cards.. the cpu rail dont need 19 amps but they tend to come that way..
so you guys are talkin amps an rails an all that other crap. what is the wattage rating on a psu used for? an what am i lookin at here under the output specs for a psu with 2 rails? - "+3.3V@28A,+5V@34A,+12V1@20A,+12V2@17A,-12V@0.8A,+5VSB@2.5A" thanks!