Computer Power Supplies - A Guide

Proximon

Illustrious
Moderator
This Guide
This is a guide intended for anyone who would like to learn enough about computer power supplies to be able to choose one for themselves or others. The language will be as simple as possible but there is a Glossary.

Why?
There are many factors that must be considered when selecting a Power Supply Unit, or PSU for short. You may need one for a new system, or simply because you have heard about the dangers of owning a poor PSU. If you are reading this you probably understand that you want a safe PSU, one that won't fail and damage your components in the process. What you may not understand is the difficulty of diagnosing a PSU that has partly failed.

A poor or slightly damaged PSU can cause a system to display a wide range of symptoms which also resemble other issues. It can be almost impossible to diagnose a PSU problem at home without having another PSU to swap into a build. This is a very strong argument for getting a quality PSU... you need to have as much confidence as possible in this part because of the difficulties caused by a bad one.


What makes a good one?

There are several things to look for in a power supply, but the primary concern is the quality of power it produces. The power from your wall is way too energetic for delicate electronics, and that is why all of your electronics have some sort of power supply that converts the wall power to something useable. This is easy if you are powering one little device that has set needs. It gets a lot harder when you are powering a hungry device like a gaming computer that has very different power needs under variable circumstances.
So, your PSU has to be able to produce clean, adequate power while idling, and has to do the same under full load. But then it also has to do so at any level of power demand in its range.
When we say "clean and adequate power" we mean several things, and it starts to get technical. We can easily see one thing though, and that is that making a device that can supply 45W whenever asked is a lot easier than making a device that can supply 50W to 1200W and any number in between.

80plus - Some food for thought. This is a temporary link until I add a little more data and explanation.
http://www.hardocp.com/article/2011/10/04/80_plus_irrelevant_to_you_when_buying_psu/

If you would like to learn more about power supplies and what makes a good one, here are some links. Be warned, it's not something you'll pick up in 10 minutes.
Hardwaresecrets.com - Everything You Need to Know About Power Supplies
jonnyguru.com - Power Supply FAQ
TechSupportForum.com - Power supply information and selection
A list of all the ATX spec sheets

What wattage?
The total wattage of the PSU is not really the most important. Computer Power Supplies have several different voltages they produce, but the most important one is 12V. That's the type of power that you need most to power a modern computer. So, getting a "750W" power supply that only actually produces 600W of 12V power might not work out for you.
Please see the next post for a detailed description of how to read a PSU label. This will help you figure out how much actual 12V power that PSU can supply.
Almost all quality power supplies are capable of producing MOST of their rated wattage as 12V power. There are a few notable exceptions though.

Calculating total power needs
Here are a few links to help you determine how much power you will NEED in your computer:
Atomic MPC forum - Mark84's extensive graph showing power usage of almost any gaming GPU from the last several years.
As the GPU is the largest single load on your PSU, this table can take you a long ways in determining your needs.
Extreme Outervision power estimator This is the most used and best of these tools, but it uses wattage not 12V amps.
Once you figure out how much power you NEED, you still should determine how much you WANT. Opinions vary on this, and it's a good way to start an argument on the boards... but I will say this: Buy a quality PSU that will last you for many years and cover any eventuality you can reasonably predict.

Glossary
Here is a glossary of terms that may be used in connection with PSUs, here and elsewhere:
http://www.tomshardware.com/forum/279124-28-glossary-common-power-supply-terms

Now that you are armed with some basic knowledge, you might want to make up your own mind about which one to buy. Most websites, and indeed most so-called PSU reviewers lack the knowledge and the equipment to test power supplies correctly though. So where do you turn for expert reviews regarding power supplies?
Here are the top 3 websites that review PSUs
http://www.jonnyguru.com/
http://www.hardwaresecrets.com/
http://www.hardocp.com/

And here is a database of most power supply reviews from ALL the qualified PSU reviewers
http://www.realhardtechx.com/index_archivos/Page541.htm

A List of Recommended PSUs
Were you hoping we would just tell you which PSU to buy? No problem. We have that covered too, with a list of the most recommended PSUs. Please note that this is just a partial list based on frequency of recommendation and such. There are many, many more good PSUs.
http://www.tomshardware.com/forum/322966-28-list-recommended-psus#t2430472

Johnny Lucky, a long time contributor here, has also assembled this large list of PSUs that have reviewed well:
http://www.johnnylucky.org/power-supplies/psu-recommendations.html

As well, Johnny Lucky has assembled a list of known lemon PSUs:
http://www.johnnylucky.org/power-supplies/psu-lemon-list.html

If you have general questions about this guide, or suggestions for it, feel free to make them here. If you want specific help about your build or intended PSU, ask in a new thread please.
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Proximon

Illustrious
Moderator
What are "12V amps"?

A PSU can deliver different kinds of power, but MOST of the power that a modern computer needs is the 12 volt DC kind.

So when you are looking for a power supply for a modern computer, your main interest is how much 12V power it can deliver. The 12V power comes on either a single rail (channel if you like) or multiple rails. Some major PSUs have just one rail and some have many... there is no clear right and wrong there, despite what the various companies may claim.


Let's look at a fake PSU label, so that we can see what it all means:




You'll notice the part we are really interested in is circled in red.

This particular PSU has two rails, apparently. They could be lying but we'll believe them. 12V1 and 12V2.

12V1, according to the chart, can deliver up to 30 amps. If more than that is drawn from this rail, it's supposed to shut down (If it's a good PSU).

12V2, according to the chart, can deliver up to 28 amps. Any more and it's supposed to shut down.

12V1 and 12V2 CANNOT deliver 58 amps combined. Very important. This is ONLY a statement of the maximum each rail can hit BY ITSELF.

We know that amps x voltage = wattage. 58A x 12V = 696W... but this is only a 640W PSU.

Jimbo has told us what these two rails are actually capable of though, and it's 520W.
All we have to do is apply the formula we just learned in reverse:

520W / 12V = 43.33 so we'll just say 43 amps of 12V power

There you have it, that is how we know where a PSU fits with others. We ignore the claimed total wattage and focus on how many 12V amps a PSU can actually deliver to you computer... because that's what matters most. MOST PSUs that are recommended have 12V amps that are reasonable for the total claimed wattage, but if you are trying to figure out if a particular PSU can power something, this should help.

 

Proximon

Illustrious
Moderator
A list of all the things you might consider when choosing a PSU

In no particular order, as they came out of my head:

- Voltage Stability - Can the PC supply 12V of power when it's supplying 600W of power as well as when it's supplying 100W? Or does it droop to, say, 11.4V? Also called voltage regulation.
- Voltage Ripple - How much "noise" is still present in the direct current after it has been converted from AC.
- Efficiency - A PSU will always pull more power from the wall than it actually supplies, but the closer it gets to zero power loss the more efficient it is. PSUs are certified to varying degrees of efficiency by 80plus.org
- Size - Some cases and hardware configurations require special PSUs designed to take up less space. Also, in some cases a slightly smaller or shorter PSU is needed.
- Fan orientation - There are also rare times when the position of fan(s) within the PSU must be different than normal.
- Cable modularity - the ability to remove and replace all the PSU cables, or the ability to remove the cables from the case that are not needed.
- Cable length - Can the cables all reach the required parts? Will some cables need to be extended?
- Cable sleeving - The color of the cables or cable sleeves can be important to many. Does the sleeving reach all the way to the end of the cable?
- Noise level - How much noise does the PSU fan make? Does the PSU produce coil whine?
- PSU color and style -


 

hunter315

Champion
Moderator
I had thought about doing a post like this but i was afraid it would get too technical very quickly, hopefully you will be able to keep it all in layman's terms. Whats the next stage for this guide?
 

Proximon

Illustrious
Moderator
I would like a list of all the possible criteria for choosing a PSU. And I'll be bringing over the PSU label explanation from the old guide, and an explanation of 12V power as opposed to total wattage.
 

killersquirel11

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Wait, sticky? I don't want to get my computer stuck there? Better just post stupid questions directly to the forum.... :lol:

All right, well stupid attempts at humor aside, this looks pretty good.

You might want to add "What makes a bad PSU" section (with the ones that come with cheap cases being a prime example)

Also, not sure if you'd want to add something about Active vs Passive PFC, and efficiency.
 
I don't see any mention of checking for cable length before making the purchase.

For those that have a large tower chassis, this is very important. You don't want to be cursing yourself for not having taken this into consideration.
 

Proximon

Illustrious
Moderator
Both good points. I have planned:

A list of criteria (cable length would be one)
A discussion about ATX specs as they relate to PSUs
Bad PSUs section
Maybe some mention of accessories? Like extenders?
 

killersquirel11

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I like the accessories idea. (extenders, adapters, etc)

Are modular connectors standardized, or is that something that you'd have to find cables on a per-brand basis?
 

hunter315

Champion
Moderator
Most modular units use mini-fit junior connectors but some units like kingwins use different styles, kingwin uses little crystal cubes that are 9 pin connectors so for most of them the end that goes into the PSU is standard but there are issues with swapping cables between units. One issue with modular cables, especially units that are done up with entirely black wire to look nicer, is that the pin outs on the end of the connector that goes into the PSU can vary between manufacturers or even within a single manufacturer, so you need to make sure that if you are adding a cable from something other than the original packaging that it has the same pin out on the side that goes into the unit as expected, otherwise you can cause a short and damage something.

Extenders are always nice, perhaps there should be a discussion about adaquate cable lengths and how it relates to the size tower a unit is suitable for as some units don't have enough length to make a good fit for a case, for example the 8 pin connector on my EA500 is just barely long enough to make it to the 8 pin port on my motherboard so its very tight across my 4850, something that would have been good to know in the beginning.
 

You have it run over the card ?, not behind it ?
tecmo34's build, from his Step-by-Step guide in the New Build section
http://i964.photobucket.com/albums/ae127/Tecmo34/PC Build/DSC_0580.jpg
 

hunter315

Champion
Moderator
Ah, i didn't think of running it between the card and the motherboard, that likely would have fixed my problem. I have an X750 due to arrive tomorrow that should fix the problem as well but thats a neat trick to keep in mind. Cable management was never one of my strengths, hence no window in my case.
 
Nice upgrade, now you need to get some cards to utilize it ;)
 

Proximon

Illustrious
Moderator
Not too much any more. It's hard to say exactly as it goes through the MB and a particular part might use a minor rail or might convert 12V to the needed voltage.

It's probably safe to say that USB uses the 5V rail. I've heard it said that 3.3V was used by RAM... but would that still be true? Might it not be easier to convert the 12V power for that just like is already happening with the CPU?

If anyone has any current references for a modern MB I would like to see them.
 

Exzess

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What about a PSU tester? I recently bought a new system with a quality PSU...is it common practice to test one before you connect it to all the other components?

Thanks
 

My opinion;
For most people it's a waste of money.
Fact;
They are singular in purpose, test the psu prior to installation to see if it has the proper voltage's, done. ( Most people aren't going to be testing a pile of power supplies )

A digital multimeter can do the same thing and more.
 

hyder13

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all the brand mentioned here not even one of them i available in my country :(

only available brand is thermaltake and i am stuck with it :cry:
 

Proximon

Illustrious
Moderator
The most recommended PSUs are not all of the good PSUs, as I have said. You may have access to PSUs made by FSP or Greatwall, for instance, that don't get recommended here but might be better options where you are.
 

kirkp

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With PSUs mounting these days in the bottom of the case, what happens when the case has no bottom opening?
The stickie makes no mention of the fan flow and the ideal setup.
Is it better to mount the PSU upside down on the small ledge in the case so it draws air in along the bottom of the case?
 

Proximon

Illustrious
Moderator
I don't know that there are any hard and fast rules about that. There are certain cases, all small, where an old style PSU with the fan on an end rather than top or bottom is best.

When dealing with any fan, keeping air flow as unrestricted as possible prolongs fan life. Replacing the fan of a PSU yourself is dangerous and voids the warranty.

MOST cases that allow for air to be pulled in from below for the PSU seem to be restrictive. Several newer cases do accommodate the large fans in modern PSUs better. Obviously pulling cool air from the outside is ideal, but in most such cases the air flow inside the case is so high that the ambient temps are the same inside and out.

If you have a high end PSU, the fan may only rarely come on at all. Getting the air from the case might be better in this instance.

So, I would say this question is very build-specific and will come down to a judgement call sometimes.

 

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