Choosing The Right Power Supply December 10, 2002 Alan Dang

Introduction

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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.

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But how much power do you really need, exactly? Most of you probably don’t know the answer to this. In fact, before last month, I didn’t know the correct answer either. Early on, I used to rely on AMD’s power supply list, but AMD stopped updating that. Since then, I’ve just dropped back to my old-school criteria: start with a case/power supply combo that’s in the range of $50, and then put down extra cash for a better design and cooling.

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.

But this is the wrong way to pick out your power supply. You first need to decide what the rest of your system will look like, and figure out how much power that takes. Then you can buy a power supply that meets your budget. Sounds simple, but there’s one little secret…



SIDEBAR: For systems equipped with fast graphics cards and CPUs, manufacturers typically recommend power supplies of at least 300 watts.

Budgeting your power needs

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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?

And therein lies the problem. You see, the Pentium III and all CPUs before it ran on the +5V rail. Since the CPU remains the largest consumer of electricity in an x86 machine, power supplies were engineered to provide significant amounts of current on the +5V rail. Modern CPUs such as the Athlon and Pentium 4 run on the +12V rail. The problem is that many power supplies are still based on older Pentium III-era designs and so even for many mid-range gaming systems, chances are that the +12V component of the power supply is not going to be adequate.

This will all make sense when you actually calculate your power consumption budget. In the following table, you’ll find estimates of power consumption for common devices that we obtained from AMD technical documents; they are a bit on the high-end (but you’re better safe than sorry)

Current Consumption (Estimates)
+3.3V	+5V	+12V	Device
3	2	0.3	Motherboard w/ onboard devices
0	0	0.25	High Performance fans (including power supply fans)
0	2	0	Memory (128MB DDR)
3	0	0	VGA
0.5	0.5	0	PCI Sound
0.4	0.4	0	PCI Network Card
0	0.8	2	High performance hard drive
0	1.2	0.8	CD-RW
0	1.2	1.1	DVD
0	0.8	0	Floppy
0	0.5	0	USB devices
0	0.25	0	Keyboard
0	0.25	0	Mouse
0	0.5	0	PCI Modem
0	1.6	0	FireWire

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:

Power budget (cont’d)

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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
Four fans
- 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
CD-RW
DVD-ROM
USB printer
Keyboard
Mouse

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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Discussion

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…

Well not exactly.

At FiringSquad, we don’t think you should expect to sacrifice stability when your system is under full load and so for the rest of this article, we’ll continue to assume 100% power consumption on all devices. That said, because it is rare to have every component active, a reasonable approximation is to assume 80% of your components are active at any given instant and that your CPU is always drawing full power.

Manufacturers can bend the truth when it comes to power supply ratings and you never want to be so close to maximum capacity all the time. At FiringSquad we like for power consumption to be at 80% of advertised maximum and so we need to alter our numbers to account for that. Before I go on, let me say that this is personal preference. We are willing to pay the extra cash for the extra peace of mind, but you don’t need to do this step. But for now, our power demands are:

9.125 amps on the +3.3V rail
20.375 amps on the +5V rail
18.125 amps on the +12V rail
349 W total

So a 350W would be just fine right? We’ve added a fudge factor for misadvertised products right? No, you need to look at the individual rails, remember?



SIDEBAR: While CPU, memory, and video card prices always fluctuate, power supply prices in general tend to be much more static.

Test cases

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A Straightforward Example

Antec makes great power supplies, so let’s look at them. The ~$50 Antec 350W (SL350) power supply has

28A on +3.3V
35A on +5V
16A on +12V

Based upon these specifications, the Antec offers more than the minimum spec, and so in most cases the system will be very good. It’s not the “80% of efficiency” that we’d prefer, but it should be all right.

[image]

A Tricky One

What about a 265W Enermax EG265P-VE?

22A on +3.3V
25A on +5V
18A on +12V

This offers plenty of power on all three rails, and in fact it’s even better for the +12V rail, but remember, we need 279W at peak use. Here, our peak is 265W. Not enough. The 300W Enermax has the following spec:

28A on +3.3V
30A on +5V
22A on +12V

Now, the individual rails are good enough for us and the total wattage is sufficient for our minimum. If you didn’t want the extra piece of mind at running 80% maximum efficiency, the Enermax seems to be a good choice. If you wanted the extra piece of mind, the 350W Enermax will give you top-notch performance.

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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.

20A on +3.3V
30A on +5V
10A on +12V

The 3.3V and 5V rails are great, but the 12V rail is too weak for our system! Under heavy processor load, Athlon XP system would be very unstable and likely spontaneously reboot. What about another 300W power supply such as the Codegen for $30?

20A on +3.3V
25A on +5V
10A on +12V

Still not enough. Let’s go up to 350W generic cases. $38 gets you a nice looking Foxconn setup with “350W AMD recommended power supply up to XP2100+”


24A on +3.3V
35A on +5V
12A on +12V

Better, but the 12A still isn’t enough for our system since we have too many drives. What about a $56 400W generic case?

24A on +3.3V
35A on +5V
12A on +12V

What the heck? A 400W and 350W power supply offer the exact same specs? Well no, the individual “power pumps” for the +3.3V and +5V and +12V are the same, but the pump at the power source is bigger.

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.

28A +3.3V
30A +5V
15A +12V

The +12V is just good enough to meet your bare minimum spec, but don’t think about adding extra hard drives. This power supply is capable of providing a lot of juice to the +3.3V and +5V rails, but you really have nothing useful there. It’s the 12V you’re worried about and this 450W only provides a maximum of 180W on +12V.



SIDEBAR: AMD lists 39 power supply manufacturers on its website.

Conclusion

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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!