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Old 02-03-2007
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jonnyGURU jonnyGURU is offline
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Default The "power supply FAQ"

The following FAQ was written by me many, many years ago and serves as a "generic" FAQ of basic computer power supply knowledge.

For more specific answers to particular questions, please see these additional FAQ's:The power supply is the single most overlooked component in a computer system.

CPU's, RAM, video cards, hard drives... at a performance level, any of these components can easily cost hundreds of dollars each. Yet the one component that powers everything in your machine, and could potentially screw up everything in your machine, is still the one component most people hesitate to spend at least $100 on (there are VERY FEW acceptable exception.)

(Paragraph from A PC power supply is one of the most important components in a computer, yet it is often the least appreciated due to its "low-tech nature". When a power supply is dead, your entire system is dead. A bad computer power supply could also cause other parts of your system to fail. As personal computers become ever more powerful, the importance of a reliable power supply is more than ever before.

A power supply that has fluctuating rails can cause lock ups, crashes, etc. The ATX standard has quite a bit of tolerance; 5%. Regulators on the motherboard and other components have even more tolerance. But when a power supply has weak rails, voltages can easily drop below these tolerances. Sometimes a multimeter can't even pick up these sags in voltage because they happen so fast. But all it takes is a sudden dip to lock up your system.

MOST power supplies have protection that prevents overvoltages, undervoltages, short circuits and high temperatures from killing other components. But just like any other component in your PC, failures are unpredictable and these protection circuits can fail. When this happens, damage can occur to components.

Picking the right power supply


Common sense is going to tell you that the best way to determine the quality of a power supply is to stick with a brand that’s known for quality products. Also, COMPETENT reviews help as well.

One indicator I’ve used is the UL number of a power supply. UL actually certifies that a product is "safe" to use within a determined operating range and environment. Although I don't believe they actually load test every power supply, cross referencing a UL number can be handy for many reasons.

You can use a UL number to find out who actually makes a particular power supply. Also, if the UL logo is on the same label as the specs, you can be pretty sure that the UL listing pertains to the specs on that very label. Can you believe that there are a few companies that claim UL listing, but don’t put the logo on their label? That’s because the specs on that label are not the specs given to UL. And it’s a regular Easter egg hunt trying to figure out what the actual specs are using the UL number. So why bother? If the company can’t put a UL logo on the same label as the specs, they have something to hide and not worth doing business with.

It used to be that one could classify the quality of a power supply is just from its weight. Unfortunately, as power supply topolgies change and newer, more efficient designs are introduced, better power supplies become lighter while an old, antiquated design may still be heavy.

Also keep in mind the “get what you pay for” adage. It doesn’t always apply from brand to brand, especially if people take into consideration USEFUL improvements like modular cables, active PFC, etc. all add to the cost of a power supply. But when you see a power supply with a bunch of lights and other pretty things, you have to take into consideration that this added bling isn’t free.

Picking the right power supply

Watts don’t mean squat!! Know how to read the label!

I hope by now that you all know that Amps multiplied by Voltage equals Watts. And since a power supply for a computer puts out multiple voltages, the statement of how many total watts this unit puts out is really subjective.

The wattage rating is on the box of a power supply (like "Billtronic's 500W Mega-kleen-power") is the total capability of ALL of a power supply's rails COMBINED. The 5V, 12V, 3.3V, -12V, -5V and 5VSB capability are all added up to calculate a power supply’s “total wattage rating.” That total number really tells you nothing about the power supply's actual capability as it pertains to your particular PC.

First you have to ask, "is that wattage continuous power or peak power?" Some power supplies will give you output ratings based on what the power supply can continuously output, while others give you peak power. For you audiophiles, this is similar to the difference between RMS and Peak when looking at amplifiers.

There are also variables that come into play like “what was the temperature at which the testing was performed?” “For what period of time was the testing performed at the specified wattage?” Basically, you should look at the amperage each rail is capable of and then just consider that the power supply's BEST CASE SCENARIO capability.

The first thing you can do is to try to figure out your computer's WORST CASE SCENARIO load. There are several calculators on-line that allow you to "add up" your computer's power. Unfortunately, the bulk of these give you a final calculation in wattage. But if you can figure out what your +12V load needs to be, which is going to be the bulk of the wattage of a modern PC, by adding up your drive motors, fan motors, lights, pumps, video cards and CPU's, and add them all up, you'll be pretty close to figuring out what your PC needs.

You’ll soon enough figure out how important the way the manufacturer distributes power across the rails really is. If you have a 500W power supply with 40A available on the 5V line and you're using a Prescott with SLI video cards, you might be in trouble because the 5V line alone is using up 200W of that power supply's total power not leaving much else for other rails! Given that most power supplies give you 20 to 30A on the 3.3V (which is way high by today's standards, but even 30A on the 3.3V is only 100W) and split up about 20W for negative voltage and stand by, you're only left with 180W for the 12V rail. That's only 15A! Mind you, we're talking maximum combined peak power, but better safe than sorry, right?

If you don't have the time or resources to do this, then just do this instead: Try to figure out if your PC is going to be 5V heavy or 12V heavy, and then buy the biggest, best quality power supply you can afford with the load balanced most appropriately for your PC. For example: If you have a Pentium III or an Athlon XP board without an ATX12V connector (like Biostar Socket A motherboards never have the 2x2 connector) then something with a relatively high 5V is most suitable for you. If you have a Prescott or an AMD64, consider something with a high 12V rail or rails (combined wattage) like a Silverstone ST56ZF or an OCZ 520ADJSLI. If you have PCI Express video card or cards, consider something with a really, really high 12V rail or rails (combined wattage,) like one of the SLi approved power supplies on nVidia's website.

So now back to helping you guys and gals read a PSU label. Use this as a reference:

This one is very simple. This power supply gives us 30A on the +3.3V rail and 30A on the +5V rail. Underneath these two, you’ll see where the maximum combined capability of these two rails is 170W. That means, you can load up the +3.3V to 30A by itself, and you can load the +5V rail up to 30A by itself, but you can’t load them both up to their maximum simultaneously. The rails are not additive. You’ll also see 0.5A on the –12V and 3A on the +5VSB. I’ll get back to the 12V rails in a minute.

Note that the total power of this particular power supply is 800W.

Ok… Now look at the 12V rails. There’s four 12V rails (I’ll explain why later) and they are rated at 22A and 22A, 36A and 36A. The maximum combined wattage of those two rails is 65A (780W / 12V = 65A).

What?!? But 22A + 22A + 36A + 36A is 116A? Like the +3.3V and +5V rails I just mentioned, +12V rails are not additive. You can load each one up to 22A or 36A, but you can’t load them all up to their max.

Now let’s go back to the “watts don’t mean squat” phrase. I don’t want to slam any brands, but take a look around at some 500W and 600W units. You’ll actually find that even though they may have more “total wattage” than this particular unit, this unit actually has more USABLE power on the 12V rails. Pretty interesting, right?

Picking the right power supply

So why do they split up 12V rails?

With the demand on +12V becoming greater and greater, Intel decided it would be "safer" to split the duty of supplying +12V across two rails. It's "safer" because less amperage can get to the end of a connector, therefore being less of a burn or a shock hazard or cause for melted wires, connectors, etc. Furthermore, with the isolation of rails, shorts on one rail can be prevented from damaging components on another rail.

Typically, a rail is split by simply taking a single +12V source and breaking up the capability with a series of over current protection; essentially a "logic" that prevents a certain amount of current from going to a wire, or group of wires.

Some people have questioned the principle of multiple 12V rails and for good reason. But I don’t think multiple 12V rails in general should be shunned. It’s best to know how many +12V rails there are and what rails go where when considering using a multi-12V rail power supply with a high end system.

ATX specifications only say that the CPU (the 2x2 4-pin connector) is put on a separate rail from the ATX connector (the 20 or 24-pin) and the drive (also used for fans, lights, etc.) power connectors. They also specify that no one rail should have more than 20A available on it (that’s their “safe” limit, so to speak.) This rule is often broken with higher performance units, like the one in our example above with both 22A and 36A +12V rails.

So if you breeze through reading that, you would say “Ok. The CPU gets it’s power from the 12V2 and everything else gets it’s power from the 12V1.” But then you realize there’s a problem with that. 20A for just a CPU, even a dual core or even a dual CPU, is overkill. And 20A may be enough for some drives, lights, fans, etc. But what about PCI express video cards that regulate their voltage from the 12V rail via an auxiliary 6-pin connector? High-end video cards can easily tax 7A or more EACH off of the 12V rail. 20A leaves zero overhead.

Unfortunately, some power supplies adhere to the “quick read” version of the ATX standard and put everything but the CPU on one rail. This is where everyone seems to be running into problems. Fortunately, some other power supply companies have gotten creative with rail distribution. I’ve seen power supplies with the PCI express connectors on 12V2 and even some with one PCI-e connector on each of the two 12V rails. THESE are the kind of dual rail power supplies you need to look for.

Some power supplies have more than two rails. The Antec NeoHE, for example, has three. Two modular connectors are labeled for 12V3 use. These are the two ports one should plug their PCI-e connectors into. Other power supplies have four 12V rails. These typically adhere to a standard other than ATX called “SSI” but PCI-e is taken into consideration by keeping the PCI-e off of the same rail as all of the drives. Even if a PCI-e is plugged in using a typical drive Molex, that rail is still separate from the ATX connector, and the 2x2 4-pin connector.

Picking the right power supply


The calculation for efficiency is DC Output divided by AC Input.

When a power supply is more efficient, it will use less power from the wall than one that is less efficient even if it produces the same amount of DC power.

The obvious upshot of this is a lower power bill. But also, the difference in wattage is dissipated in heat. So a more efficient power supply runs cooler. This can be used to an advantage one of two ways. One: A power supply will last longer if it’s not exposed to prolonged temperatures. Two: A quiet fan can be installed because not as much air flow is required to cool a more efficient power supply.

Efficiency ratings are very subjective, though. First off, no power supply is going to have the same percentage of efficieny all across the board. One may be 75% efficient at 200W, but drop down to 70% under a 300W load. Some power supply companies only tell you what the best efficiency is, but not at what load that power supply obtains that efficiency. Other power supply companies only tell you worst-case scenario, like "70% nominal at full load." Some power supply companies may be rating their efficiency at 230V instead of 115V. A PSU that runs at 230V is often more efficient because higher voltage means lower amperage and lower amperage means less resistance and less resistance means less heat. Q.E.D. Furthermore, a power supply may be more efficient at a lower ambient operating temperature than another. If a power supply is 80% efficient at 20C, that doesn't mean it's 80% efficient at 50C. This will lead me to explaining "de-rating curves" in my next post.

Picking the right power supply

De-rating Curves:

A de-rating curve is something every power supply is subject to. As a power supply gets hotter, it’s ability to output power is reduced. This relationship is called a de-rating curve.

For example; most power supplies are rated at 20C and have a de-rating curve of -1W per +1°C. That means for every degree over 20°C, your maximum sustained output is reduced by 1W. So in a more typical ambient temperature of 50°C, a 500W power supply may only be able to output 440W. It’s not a substantial loss of power, but not all power supplies have this de-rating curve and not all environments are at or below 50°C and typically even a power supply with a good de-rating curve can drop exponentially when temperatures exceed 50°C.

So how can you “tame the curve?” There’s lots of ways. One is to simply buy a lot bigger power supply then you actually need. That’s a no-brainer. The other thing you can do is make sure that your power supply is not solely responsible for evacuating heat out of your chassis. If the power supply's air intake is < 40°C, it's very likely your power supply can put out as much as 85% of it advertised capability. Make sure you have a fan in the back below the power supply to exhaust heat, but also make sure you have an intake fan for positive pressure, because it the pressure inside the chassis is less than the pressure inside the power supply, you can actually DEFEAT the airflow of the power supply! I’ve seen a few instances where a user had a 120MM in the back and no intake. The PSU was trying to also suck air into it’s housing, but the vacuum caused by the rear exhaust fan actually reversed the airflow going through the power supply! Very not good.

Of course, one could just buy a PSU that is rated at 40 or 50°C. Then, instead of having to do math to determine how much power your power supply can provide under it's current conditions, you know that your power supply can put out just as much as is stated on the label as long as the temperatures inside of the computer is within operating range.

UPDATE: In PC Power and Cooling's "Power Supply Myths exposed" they show a "500W" with a de-rating curve of -4W/1°C. This isn't completely fictitious, but I don't think any of you guys are using the kinds of power supplies that have a -4W/1°C de-rating curve.

Picking the right power supply

Resistance: Modular connectors, adapters and splitters.

Years ago, there was this cat named Ohm and he explained to us that resistance sucks.

Ohm’s law as it pertains to resistance in electrical current is R (resistance) X I (current) = V (voltage.) So you can see, the greater the resistance, caused by either length of wire, gage of wire or having to go through connectors and/or the greater the current, the less voltage you get.

In simple terms, having a modular power supply may drop your voltage a little because of the resistance between the modular interface and the cable. And using a 20-to-24 pin adapter or any kind of splitter can cause a slight drop in voltage because of the resistance caused by any imperfect contact between the pins of such an adapter or splitter. But on that same note, every single connection you make (PSU to drive, or motherboard, or video card) is another connector that is going to create a little more resistance.

There’s been a lot of scare tactics used to convince people to not go with a modular power supply. But the reality is, even at high loads the resistance is quite minimal if the correct measures are taken. For example: A PCI-e cable is going to have less resistance if there’s 3 12V leads on each side of the cable and 3 grounds on each side of the cable. Unfortunately, some modular power supplies may only have one or two wires split into three for each row for a PCI-e connector. Some homework needs to be done on how the cables are constructed when considering a modular power supply.

And when using a modular power supply, adapters or splitter, make very certain that the connection between both interfaces is secure, firm and flush. Make sure all of your connectors are fully seated. This goes for standard power supplies and the connections you make to the motherboard, your drives, etc. as well. Because if you have a connector that is not fully seated, you create resistance. That resistance not only can cause a drop in voltage at the end of that particular wire, but also create heat. I’ve actually seen BURNT connectors from cables not being plugged all of the way into their sockets.

One last thing; Gold plated contacts. They don't do any good unless they're interfaced with gold plated connectors. In fact, the mating of dissimilar metals is actually more prone to corrosion than if both connectors were tin. So if you get a modular power supply with gold connectors, keep in mind that it may be better to have gold only on the power supply side where the modular interfaces are also gold plated, but not on the component side. I haven't seen hard drives and motherboards with gold plated power connectors.

UPDATE: In PC Power and Cooling's "Power Supply Myths exposed" they state that "the voltage drop can be as much as would occur in 2 feet of standard wire." Actually, two feet of wire don't present much resistance. But they do make the point that they may "can easily loosen, corrode, and burn." That should read, "corrode or loosen and burn." Fears of corrosion are rather unrealistic. A power supply connector has as much chance of corrosion as any other contact point in your PC. Your video card? Your RAM? Even the connectors to your drives, motherboard, etc. Obviously, when you double the number of connectors you double the chance of corrosion, but unless you live on a House board, corrosion is rare. The loosen and burn I explain. Solution: There's no reason to keep unplugging and re-plugging your power connectors. Make sure they're in tight and leave 'em alone.

Picking the right power supply

Power Factor Correction:

The Power Factor of an AC electric power system is the ratio of the “real power” to the "apparent power."

(Paragraph from Dan's Data) Power factor correction (PFC) is, essentially, what you do to complex AC loads (such as PC switchmode power supplies) to make them act more like simple loads (such as toasters).

There are two types of PFC, Active PFC and Passive PFC. Active PFC uses a circuit to correct power factor. Active PFC is able to generate a theoretical power factor of over 95%. Active Power Factor Correction also markedly diminishes total harmonics, automatically corrects for AC input voltage, and is capable of a full range of input voltage. Since Active PFC is the more complex method of Power Factor Correction, it is definitely more expensive to produce an active PFC power supply.

Passive PFC uses a capacitive filter at the AC input to correct poor power factor. Passive PFC may be affected when environmental vibration occurs. Passive PFC requires that the AC input voltage be set manually. Passive PFC also does not use the full energy potential of the AC line.

In some parts of the world, customers of the utility companies are actually charged more for poor power factor. In the EU, you are simply not allowed to use an electronic device with a complex AC load without any kind of correction! So certain inexpensive power supplies are simply not available over in Europe.

Sometimes see certain models of power supplies available in the US with no PFC and available in the EU with PFC, but only capable of accepting a 230V input. Remember what I said about power supplies running more efficiently at 230V than they do at 115V? Same rule still applies here. The power factor correction circuitry isn’t going to get as hot with 230V coursing through it as it would with 115V because there is less current going through the circuit. Because less current is going through the circuitry, cheaper components can be used without any kind of performance or quality in service penalty. Makes you with the U.S. adopted 230V mains, doesn't it?

Despite being more efficient for your electric company, power factor may be less efficient to your power supply! The components used to correct power factor generate heat. Naturally, this heat didn’t come from nowhere. It’s using, and wasting, electricity. Furthermore, the heat being introduced to the other components of the power supply causing them to run hotter and therefore less efficient. Fortunately, power factor correction methods are becoming more and more efficient allowing for super-efficient power supplies WITH power factor correction.
Rest in peace Mike Clements, aka "Yellowbeard"

Rest in peace Joerg Theissen, aka "GI Joe"

Last edited by jonnyGURU; 03-28-2008 at 11:25 AM.
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Old 02-12-2007
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Veno(V) Veno(V) is offline
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Nice post JG~

I can link this to my friends when they have PSU questions hehe

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Old 06-30-2008
arie_coolz arie_coolz is offline
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my friends are going to buy new psu...
it's best to recomend him to check this one
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Old 10-03-2009
richardscott richardscott is offline
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Hi Jonny.

That was really such a useful information. I think people also get the knowledge about the Power supply through this forum. And This is the best way to spread out the information.
Thank you.
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Old 04-03-2011
brett2014 brett2014 is offline
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Gday guys (and lady guys)

In a quick read through on PSU's - I didn't see a mention of ripple, which you will not see under no load, but jumps out and bites you at say 75% lading or more. This matters! Sorry if I missed seeing it elsewhere, but it is an issue and easy to check with a meter, or better, a scope.
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Old 04-03-2011
Zero82z Zero82z is offline
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Originally Posted by brett2014 View Post
Gday guys (and lady guys)

In a quick read through on PSU's - I didn't see a mention of ripple, which you will not see under no load, but jumps out and bites you at say 75% lading or more. This matters! Sorry if I missed seeing it elsewhere, but it is an issue and easy to check with a meter, or better, a scope.
Have you ever read a review on this website?
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Old 04-03-2011
rafal_iB_PL rafal_iB_PL is offline
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Good luck checking ripple with DMM too. Even with scope it isn't exactly straightforward.
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Old 12-13-2012
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Thanks for the information... I hope this information will educate people when they build their next system, I would like to see more generic PSU reviews though, and more silvery explosions!
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Old 07-15-2014
sharif sharif is offline
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Nice post

just made a account just to post this reply
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Old 02-16-2015
jamesd168 jamesd168 is offline
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nice tutorial Johnny, I learned something new today

so where is this cat now? I would like to talk to him.
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