[alex]

The Power Supply Unit is frequently the last thing people think about when building a p.c. This is because it’s one of the less sexy components needed for a build. The problem with this kind of thinking is that the Power Supply Unit (PSU) is pretty much the single most important component in the build because, should it fail, it can take the rest of your build with it.

A poor quality PSU (insert: cheap) can greatly reduce the life span of a good system or cause instability. A good quality PSU tends to defeat these negatives and also provide help in reducing heat inside the case, along with reducing noise.

ATX-12V measure. (Advanced Technology eXtended).

As technology matures and develops and the general public as a whole demands faster and more powerful components you see components getting faster and faster.

Using Intel as an example;

1993: Intel Pentium released. Fastest version released that year was 66mhz with a 16kb cache. And it required 5V to power it’s 3.1million transistors. Manufactured using 800nm process.



1995: Intel Pentium Pro released. Fastest released that year was 200mhz with a 16kb Level2 cache* and it required 3.3V to power it’s 5million transistors.




*Level2 cache is the dedicated bank of memory that acts as a temporary holding place for instructions being sent to the CPU. It works much faster than a hard drive, and the bigger the cache the more instructions can be held.

1997: Intel Pentium 2 released. Fastest released that year was 233mhz with a whopping () 32kb cache. It required 2.8V to power it’s 7.5million transistors.



1999: Intel Pentium 3 released. Fastest released that year was 450mhz which required 2.05V of power for it’s 9million transistors.



2000: Intel Pentium 4 released. Fastest released that year was 1.3ghz (1300mhz) which required 1.7V to power it’s 42million transistors. Manufactured on 180nm process. So, as you can see using this limited list as examples, we’ve gone from a fabrication technique of 800nm and 5v powering 3.1million transistors to 180nm at 1.7V and 42million transistors. All in the space of 7 years.

This list isn’t being used to show performance increases because raw numbers don’t tell the true story about that (in some applications the Pentium 3 could outperform the Pentium 4 simply due to architectural changes in the newer core). No, this list is simply being used to show how newer and improved fab techniques (800nm to 180nm) can reduce the voltage required by a CPU without directly impacting on performance simply because the transistors are that much closer together, and the closer they are the less power is required for them to switch from a state of 0 to 1.

And we’re now at 45nm for Intel’s current crop of CPU’s.

But GPU’s are a different story. Yes, the manufacture of these are following CPU’s (90nm – 65nm – 55nm) But the wattage required, simply due to the amount of transistors resident in a GPU is, generally, on the increase.

And then you’ve got to take into account SLI and Crossfire motherboard chipsets that allow multiple graphics cards to be connected.

The forces driving this trend are;

Monitors. Resolutions of monitors are getting higher and higher. A few years ago the average home pc user had a monitor with a resolution of 1280x1024. This equated to 1,310,720 pixels that all required updating. Now, the standard resolution as screen prices continue to drop is 1680x1050 (1,764,000 pixels) and it’s fast approaching 1920x1200 (2,304,000 pixels). In part this is also due to high-definition video becoming more mainstream and this video type requires more physical screen lines which means higher resolutions. Simply put, the more pixels a screen has the more powerful GPU that is required to update that screen, specifically in gaming applications.

[
(19” 1280x1024 screen side-by-side with a 24” 1920x1200 screen).

Games. As game developers create more impressive titles with continually more impressive graphics attributes (real time lighting, DX7-10 etc) more powerful graphics cards are needed to produce these, on screen, with playable frame rates.

Now I’ve got that out of the way, what’s it all got to do with ATX-12V?

Simply put, with the continual developments in CPU’s, GPU’s and motherboards (AGP – PCI-E) power requirements have increased at least five fold (build dependent) in the past few years. To help provide this power the ATX12V standard was created. This means that PSU’s have to meet certain requirements in stability, load, and voltage provided before they can be used to power a pc.

(The ATX standard extends further than in regards to PSU’s but I won’t be covering that here).

The ATX form factor has had five main designs throughout it’s life:

1: ATX – 20-pin 12v power connector.
2: WTX – 24-pin 12v power connector.
3: AMD GES – 24-pin 12v power connector for some AMD Athlon’s.
4: EPS 12V – 24-pin primary 12v power connector, 8-pin 12v secondary connector and 4-pin 12v tertiary connector.
5: ATX 12V – 24-pin primary 12v power connector, 4-pin 12v secondary connector and 8-pin 12v tertiary connector.
ATX 12V V1.3 – Guidance for -5V feed removed.
ATX 12V V2.0 – 24-pin 12V power connector and 4-pin 12V secondary connector.
ATX 12V V2.2 - One 20/24-pin connector, one ATX12V 4 pin connector. Many power supply manufacturers include a 4 plus 4 pin, or 8 to 4 pin secondary connector instead, which can also be used as the secondary EPS12V connector.

The current revision, ATX 12V V2.2, is backwards compatible with all previous revisions and ATX 12V V2.3 will be out shortly.

Cheap Not Cheerful



Most power supplies that are priced between £20-£30 (generic ones) generally do not meet the power requirements of the latest CPU’s. They tend to be very loud, and run hot due to the cheaper materials used to manufacture them (why do you think they’re so cheap?).

They also tend to be very unstable and not very environmentally friendly because they also tend to be very inefficient. And with the cost of electricity and gas set to rise yet again it's something you may want to take note of.

When a PSU manufacturer refers to the ‘efficiency’ of a particular PSU or range of PSU’s they basically mean how much of the electricity taken from the mains by the PSU is wasted, primarily as waste-heat.

A good PSU will run at around a 80% efficiency. This means that for every 100w of power drawn from the mains 20w is being wasted

A typical generic PSU runs at around an efficiency of 50%...... and as stated earlier this wasted wattage is primarily gotten rid of by heat. When you take into account the heatsinks of cheap, generic PSU’s aren’t of the best quality this becomes more worrisome that troublesome.

What’s the Watt?
Wattage ratings on power supplies can be misleading as this is the total combined wattage of all the voltage lines. With the continually increasing voltage requirement by components, the total required output particularly for the +12V line (CPU, GPU, Mobo etc) has become increasingly important. Ideally a power supply should have at least 18A on the +12V line. The actual load you need will vary depending upon your build.

Easy Listening or Thrash Metal
Power supplies generate a lot of noise from fans used to keep them from overheating. If you don't want a lot of noise, there are a number of options available. The best choice is for a unit that either uses larger fans that move more air through the unit at slower speeds or to get one with temperature controlled fans. Another option is fanless or silent power supplies that generate no noise. Although these generally create more heat build-up.

MTBF: Mean Time Between Failures
MTBF is the rating given by a manufacturer for the typical amount of usage a unit will have before a possible failure. Roughly 50% of units fail before this and 50% live longer. The higher the MTBF rating, the better the quality of the power supply.


Square Peg In The Square Hole
There are a variety of different power connectors that come off a power supply. Some of the different connectors include 20/24-pin power, 4-pin ATX12V, 4-pin Molex, floppy, 6-pin PCI-Express graphics and 8-pin PCI-Express graphics. Take stock of what power connectors your PC components require to ensure you get a power supply with the appropriate connectors.

Spaghetti Junction
Look for a PSU with modular connections.




This basically means that the majority of cables aren’t permanently connected to the PSU so you can attach only the leads you require making cable managing your pc that much simpler and easier.

A tip. It’s a good rule-of-thumb that the heavier a PSU is the better it is. This stems from PSU’s made out of better materials weigh quite a bit more than generic PSU’s.

Seriously, do yourself a favour and stay away from generic PSU’s. The ATX measure also give’s boundary’s on the minimum amount of voltage a PSU’s rail can provide; 5v, 12v etc. This is simply to keep a component,powered by the PSU, stable. If the voltage fluctuates too much to a particular component it could cause a system crash, BSOD or even permanent damage to that component.

Also, there is no industry standard relating to wattage ratings so purchasing a generic psu that shouts out "600w for £20" could well be a downgrade in that there is nothing to say it will provide anywhere near this wattage. It's no unknown for generic PSU's to produce around 30% less peak wattage than the label quotes.

Lastly, on PSU's you'll see marked on the rail's label 'Peak' and 'Maximum' wattage.

Maximum relates to the maximum (funnily enough) sustained wattage the PSU can provide.

Peak refers to the absolute limit of wattage the PSU can provide, but it can only do this for very short time-frames and damage can result if too much load is put on the PSU.

As you'll notice I haven't mentioned amps anywhere. That's intentional and will be covered in the next one.

Hope this helps anyone reading.

Helpful sources: PCPW, Intel archives, AMD, Wiki, Enermax, Coolermaster.

[Hybr!d]

Nice work mate, you gonna stick it?

[alex]

Oops.

Thanks for the heads-up.

[xactly]

Very nice guide, i noticed the part where you said not to get a cheap one, i was looking through eBuyer and came across the Artic Power 700w PSU for £40. I am interested to know weather it could power a 8800 GT and if it's efficient, if not i have found the OCZ StealthXStream 600w, Coolermaster RealPower 520W Modular and Coolermaster eXtreme Power 650W. All of the PSU' are between £45-£60 and are the power for what i will need but which one is the best for Price, Power and Efficiency.

You could also reccomend some PSU' for certain builds ie. Gaming Rig...

Thanks.

[Gazmondo]

Is it possible to find out the efficiency of a PSU if the manufacturer hasn't stated it?

[alex]

I'd be very surprised (aside from generic psu's) if a manufacturer hasn't quoted the efficiency somewhere. It's in their best interests to. If they haven't then that should switch a lightbulb on for you.

But if you can't find it I'd be looking for reviews on that particular model. Some sites/mags do indeed bench test psu's for efficiency.

[alex]

All:

I'm going to be adding to the post with some info on gpu's, didn't quite manage to get it finished yesterday (was at work at the time) but it should be upped in the next couple of days.

[Carbon]

Originally Posted by xactly

Very nice guide, i noticed the part where you said not to get a cheap one, i was looking through eBuyer..

All of those work, but I'd go for the modular one, saves a lot of hassle.

You could also reccomend some PSU' for certain builds ie. Gaming Rig...

Thanks.

Try this. Thing is, there's so many choices that it's hard to recommend anything - prices go up and down very quickly.