Power Supply Questions

Thermaltake / Tr2 w0070ruc
December 23, 2009 at 05:21:39
Specs: Windows XP
I have a few questions about PC Power Supplies. Perhaps someone here can help me out.

Does a power supply use all of the Watts all the time? For instance, if I buy a 400 watt psu does it use a full 400 watts all the time? Is it going to be like running a 400 watt bulb all the time and send my electric bill through the roof?

Also, I'm considering putting a new video card in my old computer, which would probably require a new power supply. I've had several cards and psu units suggested to me on other forums, but I'm still wondering about the numbers.

For instance, someone recommended this card-


and told me that this psu would be good for it-


I'm not sure I understand. The card says it needs a psu with a 12 volt current rating of 20 amps or more, but the psu has, what I assume are "dual rail" with amps of 14 and 15 on each 12 volt rail.

Another was this card-


and this psu-


Again, the card has recommended 18 amp on the 12 volt (20 on the EVGA site) while the PSU has 10 and 15 amp on the 12 volt rails.

If someone can explain how to square these numbers up in a way I can better understand, I would GREATLY appreciate it!

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December 23, 2009 at 05:59:56
OK, I think I can clear up the confusion.

The PSU will only use the power required to get the job done. Kind of like the circuit your computer is probably connected to right now. It is probably 15A @120VAC. You are only pulling a fraction of that with your computer.

The requirements you are finding are TOTAL 12V amperages, not just for the card.

Graphics cards primarily use 12V power but so does other hardware. The combined 12V load needs to be less than what the PSU can produce steadily.

The card makers are estimating what your total 12V amperage requirements will be, or worse yet, guessing at what the rating will be on a PSU with a 400W rating.

The two big power draws are the CPU and the graphics card. You need to add them together and add in for the remaining hardware to calculate total requirements.

To determine the current draw of the card you are considering. That number is usually given in Watts. Divide the watts by 12 to get the amperage draw of ONLY the card.

Do the same for the CPU.

There are other items that also need 12V current. The drives, MBoard.

You will find PSUs with as many a 6 +12V rails. Most experts recommend buying a PSU with 1 +12V rail. The reason for this is that when splitting the available 12V current between multiple rails you can end up with not enough power available on some rails while having surplus on others.

My recommendation is to look for a PSU with a single +12V rail. Start the calculations at about 15A and then add the current draw for the CPU and each of the graphics cards.

Below is a chart showing wattage of many current graphics cards.


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December 23, 2009 at 06:13:13
In answer to your first question, No. That is the rated output. If your hardware only requires 300 watts then that is all that will be used. You could put a 700 watt PSU in and it would draw the same current as the 400.

In answer to your second question, neither of the PSUs you mention would be considered ideal. Latest opinion in PSU technology is to select one with a single 12V rail with a high AMP rating on that rail. Say 40-60amps.

Think about it. With multi rails you may end up with several devices connected to one rail drawing up to that rail's capacity, while the other rail is carrying a lighter load.

If one of the first rail's devices needs to draw more current under peak conditions you risk overloading that rail while the other rail's spare capacity remains unused and unavailable to meet that peak load. If all devices run off a single high capacity rail then all the output of the PSU is available to meet peak load.

Whichever graphics card you choose, just make sure you get a good quality PSU with single 12V rail. Expect to pay $50+ for a decent quality unit.

Goin' Fishin' (Some day)

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December 28, 2009 at 01:41:33
Thank you very much! This information helps me a lot. I do have a few more questions though. I should have given you guys a little more info on my PC. I have an older system so I want to make absolutely sure I don't get the wrong kind of psu.

Here is what I have right now-

HP Pavilion a320n computer.

My current psu is this one-


Along with the following-

A7N8X-LA (explorer 2) Motherboard



AMD Athlon XP 2.08ghz.
2 GB DDR3200 memory.
120 GB hard drive.
Windows XP Home Edition.
Nvidia GeForce 4MX w/64 MB shared video memory (Current Video Card).
DVD Rom drive
CD-RW drive
3 1/2 floppy drive
7 card reader
Occasional USB powered devices (not all the time)

What I want to know for sure, is-

1. Will ANY 20 pin ATX power supply work with my PC? I have read articles that some companies make it so you must use OEM power supples. How do I tell?

2. All those cables and plugs coming out of a psu, do all power supplies come with the appropriate cables? Is it possible to use cables from my current psu if a new one lacks an essential cable? Also, what about EXTRA cables? Some units have PCI-E cables, which my computer has no use for.

3. Fans. Some units have 120mm fans or bigger. Can any type of fan be used with my current HP factory computer case? My current PSU has a small fan (80mm?) with the vent on the back. I have seen 120mm fans that have the whole top (or bottom?) of the psu open. My current psu is located in the top-rear area of the case. Will those units with 120mm open top (or bottom?) fans work properly in my case?

4. The 12 volt rail again. I have read and been told how important this is, but I must ask, since my older computer probably relies more heavily on the 3.3 and 5v parts of the psu, is a very powerful 12 volt rail really necessary for JUST a new Video Card, like the two I suggested at the beginning of this thread?

5. Does any company make a psu with a single 12 volt rail that has about 30 amps on 3.3, 5, and 12? That would probably be ideal, if all my research is correct.

If anyone can help me answer these questions, or point me to info that has the answers, I would greatly appreciate it!

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