Summary: One of the most overlooked PC components is arguably the power supply, yet it can have a significant impact on your system's stability. In our latest article, Alan looks at many contemporary power supplies and finds that they're inadequate for the task of powering an enthusiast's rig. This is a must read article if you're about to build a new PC. Check it out!
Don’t forget the power
So, you’ve finally decided which CPU, motherboard, and graphics card you want to put into your new computer. But what are you going to do about the power supply? The power supply has always been the most esoteric part of system building. We all have a vague idea that a power supply with more watts is better than one with fewer watts. We also know, and expect different manufacturers to have different interpretations of “peak output.” In other words, you already know that all 300W power supplies aren’t created equal -- you also know that 550W power supplies are overkill for a standard desktop machine.
Why Power Supplies are Important
Everyone keeps talking about watts, and it’s an understandable approach. People always talk about high-end power supplies being one reason why servers and workstations are more reliable than standard desktop machines. For example, SGI Octane workstations have 800W power supplies, even though the highly efficient MIPS architecture CPUs consume only about 17W each, the same as an x86 notebook chip.
First things first: you need to think of your power supply not as a single unit, but a box that contains multiple, independent power supplies. That is, if you look at the label on a power supply, you’ll see that the maximum current is listed independently for +3.3V, +5V, and +12V. Thought of another way, imagine the total wattage being the power of the pump at the water reservoir, and that there is a pump for +3.3V water, +5.5 water, and +12V water. Just be patient with me, it’ll all make sense by the end of this article.
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Determining your Power Budget
Some people tell you to add up the number of watts that your components support. For example, an AMD Athlon XP 2000+ (Palomino) uses 87.5W. Your motherboard is 23.5W, and so your power needs are 111W for the moment. Add up all your components and you’re done. The problem is that because the power supply is not a unified source of energy, the 87.5W from the Athlon XP is meaningless unless you know how those 87.5 watts are distributed; are they on the +5V rail or +12V rail?
Athlon and Pentium 4 CPUs run entirely on the +12V rail and you can determine their power consumption using the following formula that takes into account voltage regular inefficiencies:
The processor’s current can be determined by reading AMD and Intel PDF tech documents, but as a general rule of thumb, the higher the raw megahertz, the higher the current, and the smaller the die process, the lower the current. We’ve listed some common CPUs below:
It’s worth noting that although Pentium 4’s typically run cooler and have lower voltages than AMD CPUs, their higher raw clockspeed means that they draw more power. We’re not sure why the 3GHz P4 draws less power than the 2.8GHz model, but that’s what Intel’s documents list…
SIDEBAR: The majority of the FS test bed power supplies are manufactured by Sparkle.
With those guidelines in place, you can determine your specific power consumption. Let’s take a typical mid-range gaming Palomino Athlon XP 2000+ system:
Athlon XP 2000+ system
- power supply fan
- CPU fan
- Front cooling fan
- Rear exhaust fan
512MB DDR RAM
AGP Graphics card
PCI Sound Card
2 Network cards
2 IDE drives
Sum everything up and that’s
7.3 amps on the +3.3V rail
16.3 amps on the +5V rail
14.5 amps on the +12V rail
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We know Power = Current * Voltage, and so by the math our minimum power spec is 279W. So all 300W power supplies should be good enough right? And if we wanted to account for “overzealous advertising” we’d be “for sure” safe with a 350W power supply right? Not to mention, those numbers assume that every single drive in your system is being used at the same time, and we’ve already said that the individual components err on the high-end side…
A Straightforward Example
Antec makes great power supplies, so let’s look at them. The ~$50 Antec 350W (SL350) power supply has
A Tricky One
What about a 265W Enermax EG265P-VE?
Generic Power Supplies
That said, most of us don’t buy power supplies independently. We get case/powersupply combos – I mean, why spend $50 on a power supply when you can get a case+power supply for $50? Let us take a look at the Foxconn Mid-Tower with “AMD Listed 300W power supply” that a store like newegg has for $30.
What about a generic 450W power supply?
By now, you might think that “buying a generic case is just a bad idea.” In that case, lets look at Newegg’s $68 Chieftec with a 450W “AMD approved” Austin power supply.
So what does this mean? When you shop for power supplies, you should always watch for the +12V lead. The majority of low-cost power supplies you see at computer shows tend to have +12V rails with 10 or 12A. No matter how many total watts the power supply may have, you’re setting yourself up for instability if you buy that power supply.
Your plan should be to first calculate what your power needs are. Use our data to help you, keeping in mind that in all cases, we’ve erred on excess and that there are places to add “extra” headroom 3 different times.
1) The quoted power consumption for components is on the high-end
2) It’s rare to have every component drawing full power
3) We prefer not to run our power supplies at the limit. It reduces the stress on the component and leaves room to upgrade.
Our approach just gives you the absolute piece of mind that your power supply is not going to be the source of any system instability even when consider the power loss that occurs through conversion, and overzealous specifications. Only you can decide which factors you want to take into account. Even then, it’s important to take this step since you’ll find that there will be many power supplies with inadequate power even if you make no adjustments.
@serve_inline_ad( $current_section ); %> There are also some other lessons to be learned. We’ve all been taught that heat is the enemy of stability, and this is true. It’s true not only for CPUs, but for hard drives and graphics cards as well. If you look back at the worksheet, however, cooling fans run on the +12V rail. If you add too many fans, you could in fact be reducing the stability of your overall system unless you have the power supply to back it up.
A CPU running at 46 C versus 40C is not going to be any less stable. However, if it took you plenty of exhaust fans and blowports to get that performance, and you end up with too much demand on the +12V rail, you’ll have an unstable system. This isn’t intuitive at first, but it’s something we’ve experimentally confirmed. You shouldn’t add extra fans if you don’t have the power to support them.
Bottom line: Read the label the next time you pick out a power supply. Retailers that cater to enthusiasts such as Newegg and Googlegear often take the time to provide images and/or the crucial power figures for each of their products. That way you can shop and find the power supply that's appropriate for you.
SIDEBAR: Do you have a few tips of your own for finding the right power supply? Perhaps you could share a few horror stories or other experiences? Chat with others in the FS news comments!
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