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PSU efficiency & amps questions

post #1 of 12
Thread Starter 
Hello I like to know a lot about the computers and whats best for them but one thing I know barley enough about is PSU's. I've looked at guides but none of them are very easy to understand.

My first question is about PSU efficiency. So if a PSU is rated at 600w and at max load runs at 75% efficiency does that mean that it only pumps out 450w's really? How does this work..

Then theres the amps questions. What uses what kind of amps? I know GPU's use 12v's but does it use any 3.3v or 5v? What about hard drives, optical drives and CPU's? I also know that there are 12w in one amp so does that mean a PSU that pumps out say 35amps on the 12's that just means it's putting out 420w's? I'm just a little confused on how to rate what a system needs on my own rather than using a calculator every time.

Any information would much appreciated and rep given for things that help me understand how to see what kind of watts a PSU really puts out. Either way, thanks
post #2 of 12
Quote:
Originally Posted by Sora1421 View Post
My first question is about PSU efficiency. So if a PSU is rated at 600w and at max load runs at 75% efficiency does that mean that it only pumps out 450w's really? How does this work..
A PSU is rated at the output side... Thus, it will deliver 600W on the 12V/5V/3.3V etc. rails. When it has 75% efficiency, that means it will draw 600/0.75 = 800W from the wall outlet when it's at full load (which likely doesn't happen often).

Quote:
Originally Posted by Sora1421
Then theres the amps questions. What uses what kind of amps? I know GPU's use 12v's but does it use any 3.3v or 5v? What about hard drives, optical drives and CPU's? I also know that there are 12w in one amp so does that mean a PSU that pumps out say 35amps on the 12's that just means it's putting out 420w's? I'm just a little confused on how to rate what a system needs on my own rather than using a calculator every time.
CPU and GPU use 12V only. That's why the 12V rail is the most important to pay attention to. Hard drives and optical drives use a mix of 12V and 5V (not too sure about optical, but hdd's yep, for sure). RAM uses the 3.3V rail.

The 12V rail is the most important, and if a PSU that delivers 35A on the 12V rail, that meanst that the PSU is capable of pushing out 420W on the 12V rail. However, it also delivers power on the 5V, 3.3V and other rails. Thus, that PSU can easily have a 500W rating (like the 500W SIlverstone Element.)

Note that any good quality PSU with 30A or more can run a rig with an 8800GTX.
    
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post #3 of 12
Thread Starter 
Ok so your saying that if the PSU is rated at 600w the efficiency has already been taken into consideration and it really runs at 800w?

Do you know much any chance a SATA drive takes off the 12's?

Imaginary rep for you +1
lmao
post #4 of 12
Quote:
Originally Posted by Sora1421 View Post
Ok so your saying that if the PSU is rated at 600w the efficiency has already been taken into consideration and it really runs at 800w?

Do you know much any chance a SATA drive takes off the 12's?

Imaginary rep for you +1
lmao
yeah, something like that

A SATA drive takes only about 1A of the 12V rails. Check the label of the hard drive, and you should be able to see the specs.
    
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post #5 of 12
Thread Starter 
Oh hard drive have specs on them too?? No way lol thanks heh
post #6 of 12
Quote:
Originally Posted by Chozart View Post
CPU and GPU use 12V only. That's why the 12V rail is the most important to pay attention to. Hard drives and optical drives use a mix of 12V and 5V (not too sure about optical, but hdd's yep, for sure). RAM uses the 3.3V rail.
Actually, SATA (and SATA II) drives do not use their 5V rail yet, which is why we can still use Molex connectors to power them.
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post #7 of 12
off topic kinda but what was the point of the 5v rail?
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post #8 of 12
Quote:
Originally Posted by rx7speed View Post
off topic kinda but what was the point of the 5v rail?
USB ports are 5V, but I'm not sure if that means anything. SATA specifications call for a 5V lead to be on the SATA power plug, but as I stated in my previous post, this has yet to be utilized.
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post #9 of 12
Sora -- be careful w/ looking at the wattage.

Reputable PSU makers, like silverstone, will say 600 watts when they mean 600 watts come out on the output side.

With shoddy PSU makers, you never know. For example, with this piece of crap: http://www.newegg.com/Product/Produc...82E16817708001

There's absolutely no way that 500 watts comes out on the output of that thing.

The brand is VERY important, much more important then the wattage rating. Since some companies straight up lie, or don't follow standards when rating it.
post #10 of 12
Quote:
Originally Posted by demoship View Post
With shoddy PSU makers, you never know. For example, with this piece of crap: http://www.newegg.com/Product/Produc...82E16817708001

There's absolutely no way that 500 watts comes out on the output of that thing.
You are right and wrong at the same time. If you look at the specifications, the 5V rail has a 50A limit, which will never be reached by the average user but makes the 500 W rating technically correct (though I have yet to see this tested). The 12V rail can only hand 18A, so that is why the PSU is so cheap.
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