Articles :: Power Supplies 101: A comprehensive guide :: Motherboards.org

Jon Gerow · 06-26-2006 · Category: Guides

The PC power supply label:


The specs on the label are important beyond what the total "wattage" written on the box is and since it’s going to be the way most of you are going to get the information you need to know about a particular power supply, you need to understand what all of the writing on that label means. I will use five different examples. All five power supplies are 500W, but are very different from one another. The brands of these power supplies are being kept anonymous so we don’t offend anyone. ;-)


Table 1: A typical dual 12V rail performance power supply

Max Current
Output
+5V
+3.3V
+12V1
+12V2
-12V
+5VSB
30A
30A
16A
18A
0.6A
2.5A
Max Combined
Output
160W
384W
7.2W
12.5W
481W
19.7W
500W

“Table 1” is a fairly typical dual 12V rail 500W power supply. It’s called a “dual rail” because there are two +12V rails. What the purpose of this?

A couple years ago, Intel’s ATX specifications stated that 240VA (which is equal to 240W DC) was too high for a single DC output rail to be considered "safe." 240VA on a +12V rail is 20A. So it was determined that if more than 20A was required for a +12V rail, the +12V load could be split up across multiple +12V rails.

The introduction of the multiple +12V rail to the ATX specification occurred in revision 2.0 of the ATX12V spec: http://www.formfactors.org/developer/specs/ATX12V%20PSDG2.0%20Ratified.pdf

This solution actually does more than make the power supply “safer.” Intel also specified that this extra +12V rail should be used for the CPU and be kept separate from the peripherals on +12V1, typically drive motors, fan motors, lights and other such devices that can add noise to the DC output of a power supply. So by isolating the +12V2 from the +12V1, the CPU gets “cleaner” power.

A funny thing about dual 12V rails that you need to be aware of is that the capability of the two +12V rails combined is rarely the same as the sum of the two +12V rails individual capabilities. Using our “Table 1” example, you can see we have a +12V rail that can do 16A and a +12V rail that can do 18A. 16A + 18A is 34A, but the combined wattage for both of the 12V rails is 384W, which is 32A.

All that really means is you can’t max out both rails at the same time. One or the other is fine, but never both… at least you can’t do it and expect good performance from those rails.


Table 2: A "low end" dual 12V rail power supply

Max Current
Output
+5V
+3.3V
+12V1
+12V2
-12V
+5VSB
30A
35A
14A
12A
1.5A
2.5A
Max Combined
Output
200W
180W
18W
12.5W
500W

Now let’s look at “Table 2.” This is still supposedly a 500W power supply, but you’ll notice there’s less amperage available on the two +12V rails. Now, there might not be anything wrong with a +12V1 that can do 14A and a +12V2 that can do 12A if you’re using the power supply in a smaller, lower performance machine. But take a look at the combined output rating for the +12V rails: It’s only 180W. That’s only 15A and less than half the capability of the first 500W! A better power supply could still have two +12V rails rated at 14A and 12A like this one, but may have a combined output of 22A or 24A on the combined +12V rails.

Another thing about this label is how the numbers simply do not add up. It’s not unusual for the total wattage to be less than the sum of the individual rails. I haven’t met a power supply yet that will actually do the max load on all of the rails all at the same time. But this power supply is the exact opposite! 200W + 180W + 18W + 12.5W is 410.5, not 500W.

How can this be? Hard to say because it seems that every power supply manufacturer uses a different methodology to rate their power supplies (which is too bad,) but I’m quite sure the combined wattage ratings for the +3.3V, +5V and +12V rails are sustained output and the 500W total is a peak rating. And on that note, it must be said that peak to some manufacturers may be a reading at a completely different interval as peak by some others. While sustained means "continuous," peak means that the power supply can only put out a certain amount of power for only a brief interval, but it is rarely disclosed how brief that interval is.


Table 3: A performance power supply with a single 12V rail.

Max Current
Output
+5V
+3.3V
+12V
-12V
+5VSB
30A
45A
26A
1A
2.5A
Max Combined
Output
275W
312W
12W
12.5W
473W
24.5W
500W

“Table 3” is your classic, single +12V rail power supply. It is NOT compliant with the ATX12V standards because the +12V rail is capable of more than 20A. Again, we’ll see that this power supply, despite being a 500W like our first two examples, it does not put out as much juice on the 12V rail as our first example.

At least the numbers pretty much jive on this power supply. 473W + 24.5W is 497.5W, which is close enough. Besides, there may be an undocumented –5V rail capable of .5A or .6A that could easily compensate for the minor discrepancy.

Ironically, as video card GPUs require more and more juice, we're actually starting to see more and more power supply manufacturers that are saying "ATX12V specifications be damned!" CPU and are putting out new power supplies with one large, single 12V rail. Because by splitting the +12V rail up into two separate rails, each with a limiter set to prevent the rail from sustaining more than 20A each, there's hardly enough power to run a pair of high end video cards, never mind whatever else needs to get power from that same rail.

Some companies will split up two PCI-e connectors across two different +12V rails. This works, but you're still left with a "rob Peter to pay Paul" type of scenario. Therefore many are finding the solution is to just allow all devices to pull all of their power from a single, large +12V rail.

It is becoming more and more apparent in the power supply industry that if the demand for power from high end video cards increase, there is no way dual +12V rails with a 20A limit on each is going to properly work. I believe we're going to start seeing more power supplies with very large +12V rails or three or more +12V rails.


Table 4: An older 500W power supply

Max Current
Output
+5V
+3.3V
+12V
-12V
+5VSB
50A
45A
18A
1A
2.0A
Max Combined
Output
260W
216W
12W
10W
476W
22W
500W

"Table 4" is an example of an "old school" 500W power supply. You'll note that there's considerably less amperage on the +12V rail. I'm not saying that this power supply isn't top notch quality and it might work perfectly for even a high end system. But there's a lot of power on the 3.3V and 5V rail where you simply do not need it in a modern day computer.

Before the introduction of the 4-pin 2x2 connector, CPU's would regulate their core voltage off of the +5V rail. We've since moved away from that because CPU's were getting to the point where they needed too much amperage. Now, the same CPU that was drawing 12A from the +5V rail only requires 5A from the +12V rail. Less amperage, less resistance.

There are still a lot of peripherals that use the +5V rail for power. The logic for all of your drives uses +5V and some boards use the +5V rail for memory. +3.3V is typically used for memory and most PCI cards are also powered by +3.3V. But current machines typically have very high +12V rail demands because of faster CPU's and higher end PCI-e graphics. So we're going to want to pass on a power supply with a lable like this and consider a power supply with more amperage available on +12V rail.


Table 5: A "tri-12V-rail" power supply

Max Current
Output
+5V
+3.3V
+12V1
+12V2
+12V3
-12V
+5VSB
17A
23A
17A
17A
17A
0.8A
2.5A
Max Combined
Output
85W
75.9W
456W
7.2W
12.5W
500W

“Table 5” is a tri 12V rail power supply. There are three +12V rails. Now you may think, if this thing has three +12V rails and each one is capable of 17A, how come it’s only a 500W like the others? Again, we need to take a look at the maximum combined wattage rating for all three +12V rails. It’s 456W. That’s 38A, which is good. But hardly 17A times three!

This design puts the PCI-e connectors on their own +12V rail. So tri-rails really gives you the best of both worlds. There's plenty of power for the PCI-e cards because there's nothing else robbing power from the rail that they're on. And there's still separate rails between the CPU and the rest of the peripherals, so there's no need to exceed 20A on a +12V rail and the CPU gets the "clean" power it needs to perform, and perhaps overclock, well.

There are also quad-12V rail power supplies. The lay out of the rails are very much like our tri-rail example above. These tend to be 600W and above and were first introduced for use in servers where the first two +12V rails typically power each of two CPU's in an SMP computer. When a "modern day" quad rail power supply is used in a "gaming machine," the second CPU's rail may be used for the second video card.

Some quad-rail power supplies have been known to be problematic with gaming systems because you'll sometimes find that both PCI-e connectors are on the same rail and the demand of two high end video cards is too much for a single, smaller +12V rail. Usually, the solution to this is to move one of the PCI-e cards off of that rail and onto another by using a 4-pin Molex with an adapter.

Contents

  1. Introduction
  2. The PC power supply:
  3. The PC power supply label:
  4. Defining the connectors of an ATX power supply
  5. ATX power supplies DO NOT turn on at the flip of a switch
  6. Testing your power supply's voltage: Software vs. Multimeter
  7. Power Supply Efficiency
  8. The Derating Curve
  9. Power Factor Correction
  10. The resistance of modular connectors, adapters and splitters
  11. Conclusion

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