A Couple Questions About Power Supplies

joh06937

joh06937

New Member
i want to get better at judging power supplies and such (and i always love learning new things). so i have a couple questions:

if you have a single 12v rail power supply that can put out say 70 amps max and a gpu that has 2 pci-e connections and requires (for example) 25 amps, does each connection pull a portion of that, both adding up to the 25 amps required? so if a power supply has say 6 12v rails, each rated at say 20 amps, would the example gpu be able to run even though it might require more than the amps available on one of the rails?
 
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A 6 pin PCIe power connector can only pull 75W and a 8 pin 150W. Who ever told you that you could pull 25 amps through a PCIe power connector. A 6 pin would be around 6.5 amps. and a 8 pin would be around 12 amps.
 
StrangleHold said:
A 6 pin PCIe power connector can only pull 75W and a 8 pin 150W. Who ever told you that you could pull 25 amps through a PCIe power connector. A 6 pin would be around 6.5 amps. and a 8 pin would be around 12 amps.
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nobody ever told me that (and that's not even what i said in the first place). like i said, i am not very familiar with the real workings of power supplies and would love to learn. so if a card required something like 20 amps (which seems more realistic than a 25 amp one), how exactly does it get ALL of its power? does it only draw from the pci-e connections or does it use some from the pci-e 16x connection as well? i only ask since i know some cards don't use even a 6 pin pci-e connection.
 
It can get 75W from each 6 pin and 150W from each 8 pin. Plus a PCIe 1.0 slot gives out 75W and PCIe 2.0 is suppost to be up to 150W but it varies by board.
 
ah ok. thanks.

i think i only have two question left now :P

what kind of amperages do other components like hard drives and fans pull? what would would be a good amount to add for other components (other than cpu and gpu) when estimating a systems usage?

what exactly do the 5.5v and 3.3 v rails power? doesn't the 5.5.v handle things like keeping the power supply "on" when the computer is off?
 
Harddrives/Fans/CD/DVD/fans dont really pull that much. The CPU/Video card pull the most.

PCI cards use the 3.3/5V rail

Motherboard chipsets use the 3.3/5V rail

CPU/Video cards use the 12V rail

CD/DVD/Harddrives use the 5/12V rail

Like your power supply has 6 rails, but its not a true multi rail. They all pull off a single 12V rail divided. The amp rating on each rail is just the max. that (one) rail can pull without tripping it. It really has a total of 68 amps on the 12V.
 
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As stranglehold said, most multiple rail units are in fact just glorified single rail units. You can liken a power supply with multiple rails to household circuit breaker almost, where a single source supplies the power but is then divided. Generally speaking, single rail units are a better option.

Also, stranglehold, 75W and 150W are the just the ATX Specs for pcie connectors, in reality though you usually end up drawing more than that from them by the time you overclock and such.
 
And finally, on many domestic PSUs you cannot add up each of the single rails (on 'multi rail' designs) and say oh it has 4 x 25A 12V rails therefore it can provide 100A. It is the total wattage on the 12V rail divided by 12 to get the max amps. Even then on older and cheaper units, 12V rail 1 will be dedicated to the CPU, that is it cannot 'share' its power at all. Meaning for many systems the 12V rail wattage / 12 is not going to give you the right answer. Then look for active power factor control (PFC), high efficiency (80% plus), good warrantly, MTFB rated at 40oC rather than 25oC and so on.

read the multi rail/single rail link in the sig
 
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Also, don't just go off of the label when trying to figure out amperage. Cheaper off brand units won't output what the label says, and most won't provide clean power. Http://www.jonnyguru.com is about the most thorough review website for power supplies.
 
Another precautionary rule should be never run your PSU more than 75% of power capacity as this can result in early failure of the supply.
 
MMM said:
Another precautionary rule should be never run your PSU more than 75% of power capacity as this can result in early failure of the supply.
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Not true,quality power supplies are designed with a rated continuous capacity for a reason, cheaper units may be rated at peak power vs. continuous power, which is why this myth has been constantly propagated. Capacitor aging and other factors will result in slow losses in overall output of a unit over multiple years, however that will occur whether or not you are running a unit at or near its rated maximum.
 
bomberboysk said:
Not true,quality power supplies are designed with a rated continuous capacity for a reason, cheaper units may be rated at peak power vs. continuous power, which is why this myth has been constantly propagated. Capacitor aging and other factors will result in slow losses in overall output of a unit over multiple years, however that will occur whether or not you are running a unit at or near its rated maximum.
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Yes we all know anything will age and result in failure eventually.
As you stated it depends on quality and build, plus theory and pratical is two different things.
I'm stating from experience and a lot of PSU's specs are overrated
 
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MMM said:
Yes we all know anything will age and result in failure eventually.
As you stated it depends on quality and build, plus theory and pratical is two different things.
I'm stating from experience and a lot of PSU's specs are overrated
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And alot of PSU's are cheap pieces of garbage that You shouldn't be purchasing in the first place. Any somewhat reputable unit will be rated for its maximum continuous load capacity. Cheap off brand units won't deliver clean power, nor will they deliver anywhere close to what their label says, case and point right here:
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=187
 
Some PSU's can even be safely run OVER their rated limit. Antec's TP-750 supposedly could be sold as a 900W product, if they didn't want the 80+ rating. I wish I could find a review like that for my TP-550...see what it can do, you know? (not that I'll ever own equipment powerful enough to do that...my current rig would struggle to hit 175W)
 
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