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PC Power Supply Discussion Troubleshooting and discussion of computer power supplies

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  #11  
Old 12-07-2012
walterm walterm is offline
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There are reviews where actual power used by the computer while in use is measured.
Usually this is less than expected.
There are various devices for measuring power usage. People have been known to use them and say "ah ha, I AM using 400 watts".
This measurement is the power from the wall.
There is a tendency to say, " I have an 80% efficent power supply" therefore 80% of 400 watts = computer power usage. IE, in this example, 320 watts.
This is true, just simplistic. Load testing shows efficency varies with load. And computer in use does not normally stress all rails at once.
The math is correct but misleading. Using an Optical Drive is unlikely when stressing CPU and Video Card. HDD is the same. Outside of deliberate stress testing, maxing out multiple components all at once is unlikely.

Here is a comparison of power usage HD 6670 vs HD 7750
http://www.hwcompare.com/11734/radeo...adeon-hd-7750/

Without overclocking TDP for OEM card is 63 Watts for an HD 6670 and 55 Watts for an HD 7750. The "official" TDP for your CPU is 65 Watts. Remember AMD has to cover the worst "power hungry, inefficent" chip in this number.

So unless you intend to benchmark while overclocking both CPU and GPU you should be fine. If not the system will demonstrate instability, in which case a replacement power supply is indicated.

If you want absolute reassurance there is none. Have I run similar systems on similar power supplies? Yes. Did I wind up upgrading power supplies, yes. Did I have to?
Probably mostly not.

Age and heat can effect the performance of a power supply over time.
This is a case where if you are a worrier, upgrade now. If you can relax and see what happens, use it.

Given the HD 7750 has a TDP of 55 watts $105 without sale pricing
http://www.amazon.com/Sapphire-Radeo...ywords=hd+7750

I think you are fine.
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ThaWade (12-08-2012)
  #12  
Old 12-08-2012
mariush mariush is offline
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12v hdd motor, cpu, ram (most often) , some mb features, pci express slots on mb

5v sometimes ram, sometimes for onboard stuff like soundcards, network card, in pci, electronics on hdd, everything on standard ssd, it was used for cpu and motherboard in old computers (on and before Pentium 4 computers)
usb

3.3v nowadays mostly in pci express slots but usually motherboards generate 3.3v out of 12v, or the video cards have dc-dc converter on themselves ... sometimes ram, sometimes onboard devices, it was used in agp

-12v it's now mostly unused. It was used on ISA, I think it was on PCI slot as well, nowadays only the serial (COM) port may still use it but most motherboards only provide the connector on the board, not the actual jack.

For video cards, the power consumption is relatively simple to measure:

pci express slot is 75w but in theory only 60 watts on 12v , rest on 3.3v
each 6pin pci-ex is 75w max, 8 pin is 150w max
according to atx standard the maximum a video card is allowed to use is 300w in total.

Watts = volts x amps

Hard drives show the values on them, they usually mean the average over the time they run. For example, a 3.5" sata hdd may have written on it 12v @ 0.3A and 5v @ 0.5A ... that's how much it uses as it works in the computer.
Keep in mind however that when it starts and gets up to its regular rotation speed, the motor of the hdd uses a lot of power, about 2-3A on 12v.

Hard drives are like that, a bit of an exception when you estimate consumption - people were often trying to connect 8-10 drives on a 300w power supply and found out it wouldn't work (at boot 10 drives x 2a = 20a, more than the 14-16a max a 300w would provide)
Some motherboards and controllers have staggered initialization, they power on hdd one after another so as to not load the power supply that much.


Power supplies with older designs have the 5v and 3.3v separated by 12v ... as in a 350w may have 200w reserved for 12v, 150w may be reserved for 3.3v and 5v.

So, even if the label says 3.3v at 25A (82w) and 5v at 20A (100 watts), the power supply may not be capable of doing 182 watts. This is still OK because in practice, most systems nowadays only consume a few amps on 3.3v and 5v, so those limits are rarely reached.
But the bigger issue with these kind of power supplies is the lower number of watts on 12v... in this example 200w is only about 16A. This would be enough for a system with a card that has a single pci express 6pin connector, but may be a bit too little for one with video card that has a 8 pin pci express connector

Modern power supplies (like seasonic g-series for example, or seasonic X series) only generate 12v, at the maximum rating they have, and then use dc-dc converters to create 5v and 3.3v, as much as needed (up to a reasonable limit due to the chips used to generate the voltages).

Power supplies with several 12v will still only be able to output as written on the label in watts. They may list a high number of amps for each rail, but that doesn't mean you can pull so much current from each rail.
It usually means the power supply tolerate having so much current pulled before it turns itself off to protect itself and the system.

For example, a 450w power supply may have 150w reserved for 5v and 3.3v and 300w for 12v (300w / 12v = 25A). But it may write on the label 2 x 12v @ 18A rails. The psu is not all of the sudden capable to do 36A on 12v (430w), it just means that it may allow you to pull around 18a from a single rail before it turns off or does something similar to that.

In recent power supplies and brand name ones, they're often capable of a bit more than what they're listing on label - for example, a Seasonic x-650 may actually do up to 760w with no problems. But you should not rely on that.
Also, when picking a power supply you want to get one that can do a bit more than the maximum your system would consume, but not by a whole lot.

For example, if your system uses up to 200 watts, you may be fine with a power supply that's rated for 430w. A Seasonic G-Series 360w that outputs all those 360w on 12v will do, but it's an exception. a very good power supply with modern design - most power supplies that are cheap and low wattage and the kind that has 3.3v and 5v separated from the 12v.
That means its 12v output is only about 250-300 watts and from your 200 watts, about 150 watts are 12v only. Therefore, you're already above 50-70% of usage on 12v, which may mean the power supply keeps its fan running at high speed to cool itself down or may not be as efficient as possible.

In this case, even though you don't need it, it may be worth going for a 550w power supply, mainly because it would be less noisy and more efficient at the amount of power consumption your system demands.
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ThaWade (12-08-2012)
  #13  
Old 12-08-2012
ThaWade ThaWade is offline
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Quote:
Originally Posted by walterm View Post
I think you are fine.
Thank you for that information. However, I wasn't really trying to find out if my current PSU could handle adding a GPU. I was trying to figure out my worst case max system power usage. So I could get an appropiate PSU. Since from all the reviews and things I've read here. 80+ certified PSU's are not as energy efficient until they get to 20% load. So if I'm right in my conclusion if I bought a 1000w psu I wouldn't see any energy efficiency unless i was running my current system in high stress/max load(in regards to cpu. etc). (correct me if i'm wrong)

Quote:
Originally Posted by mariush View Post
Hard drives show the values on them, they usually mean the average over the time they run. For example, a 3.5" sata hdd may have written on it 12v @ 0.3A and 5v @ 0.5A ... that's how much it uses as it works in the computer.
Keep in mind however that when it starts and gets up to its regular rotation speed, the motor of the hdd uses a lot of power, about 2-3A on 12v.

In this case, even though you don't need it, it may be worth going for a 550w power supply, mainly because it would be less noisy and more efficient at the amount of power consumption your system demands.
That was very nice info on the hard drive info. I thought that the numbers on it would be the max it was. Thanks for that.

And as for the recommended power supply that is what i was looking for with my "math" equation. But I was trying to do it in a way where I would learn about the power requirements of a PC. So I could get an appropiate PSU. Since from all the reviews and things I've read here. 80+ certified PSU's are not as energy efficient until they get to 20% load. So if I'm right in my conclusion if I bought a 1000w psu I wouldn't see any energy efficiency unless i was running my current system in high stress/maxed load(in regards to cpu. etc). (correct me if i'm wrong)
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  #14  
Old 12-08-2012
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Efficiency curve is different for each PSU. In general, PSUs tend to be most efficient under about 50-70% load. Below ~10% load efficienty drops down significantly.
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  #15  
Old 12-08-2012
walterm walterm is offline
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As usual the devil is in the details.
Some power supplies are highly efficent at 10% load, most are not.
From your description system between 350-550 watts would be reccommended.
In general, using any power supply formula, figure it will reccommend higher than required as a CYA move.

Overclocking can raise requirements considerably. AMD CPUs at extreme overclock and some GPUs are extreme overclock can raise power requirements dramaticly at full stress.

On the other HTPCs can easily live at 10-20% of max power calculations, as can many applications. What you do is as important as "max" capability.
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  #16  
Old 12-09-2012
ThaWade ThaWade is offline
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Quote:
Originally Posted by walterm View Post
As usual the devil is in the details.
Some power supplies are highly efficent at 10% load, most are not.
From your description system between 350-550 watts would be reccommended.
In general, using any power supply formula, figure it will reccommend higher than required as a CYA move.

Overclocking can raise requirements considerably. AMD CPUs at extreme overclock and some GPUs are extreme overclock can raise power requirements dramaticly at full stress.

On the other HTPCs can easily live at 10-20% of max power calculations, as can many applications. What you do is as important as "max" capability.
Thanks again for the reply. Very helpful.. I was debating on a 350 watt psu then thought well what if i want to upgrade the GPU in the future.. So I think I will go with atleast a 550 watt. As for overclocking I don't do it.. I don't really see the point since computers are so fast as it is. Compared to my old P4 with 1GB ram.. This "crappy" Athlon IIx2 is the beez kneez. Thanks again for the advice and information.
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