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On PSU Voltages

post #1 of 15
Thread Starter 
I'm going to clear up a bit of confusion on what voltages a power supply outputs and what they do and how important they are.


First off, a mistake many people (generally non-enthusiasts) make about power supplies is they think that a power supply changes its voltage to meet the computer's needs. This is not the case. The voltages a power supply provides are constant, or nominally so. When a component demands more power, the voltage remains the same. That's the pressure in the pipe. To meet the increased power needs, the PSU delivers more current, or "increases the flow". So voltage remains constant (or nearly so) while current is the variable.

However, voltage isn't always absolutely constant. First off, the voltages rarely achieve their exact nominal voltage (for instance, the +12V rarely runs exactly 12.00V). Also, a PSU's voltage changes depending on the load put on it; a rail's voltage usually drops with additional load. Also, rapidly changing load levels can cause dips and spikes. Because of this each voltage rail is allowed a certain margin of error.

The allowed voltage ranges for each rail are given below. If they are outside of the range allowed by the ATX spec, then you will probably suffer instability and maybe even hardware damaged. Outside of what I list as "preferred" you may see minor instability or other issues.

To test your voltage, DO NOT use software voltage monitoring like SpeedFan or Everest. They are unreliable, inaccurate, and imprecise. They read sensors off your motherboard, and these sensors are not precise to being with, and are never perfectly calibrated. Furthermore, sometimes the software will read the wrong sensor. You can often see this with the -5V rail, since most PSUs don't have it. Since the software can't find the sensor reading, they'll just pick a random sensor and display that. The readings from BIOS are a bit better since you know it's the correct sensor, but the sensor itself is still imprecise and inaccurate.

For a rough idea of your voltage, you should use a digital multimeter. Set it to read DC voltage in the 0-20V range, and take a molex or other connector. Touch one probe to a "hot" wire (colors listed below), and the other probe to a ground wire, and you should get a reading. Note that each wire has its own variations in voltage due to resistance, so this is only a rough guess. However, it is generally more accurate than software readings.

The color codes are:
Black - GROUND
Yellow - +12V
Red - +5V
Orange - +3.3V
Purple - +5VSB
Blue - -12V
Green or other - Special

DO NOT trust color codes. Always double check and make sure of the connector's pinout before testing.



So now that we've got that out of the way, let's get to the meat of this article and discuss what voltages a power supply delivers and what they do. What voltages do what has changed over the years; I'll be focusing on modern (post-2002) power supplies and computers.



+12V
Nominal value: 12.00V
Allowed voltage variation: +/-5%, 11.40V - 12.60V
Preferred voltage variation: +/3%, 11.64V - 12.36V
Allowed ripple: <120mV
Preferred ripple: <80mV
Info: The +12V rail is the main power rail in modern computers. Modern computers draw 70%-95% of their power from the +12V rail, and the power available at +12V should make up at least 80% of the total wattage, preferably 90% in enthusiast units.
It powers:
CPU
Graphics card
Main motherboard chipsets
Hard drive motors
Optical drive motor
Most fans
Water cooling pumps
PCIe cards
Most PCI cards


+5V
Nominal value: 5.00V
Allowed voltage variation: +/-5%, 4.75V - 5.25V
Preferred voltage variation: +/-3%, 4.85V - 5.15V
Allowed ripple: <50mV
Preferred ripple: <30mV
Info: The +5V rail used to be the primary power source, but has since fallen out of major use. Even though it isn't a major power source, many of the things it powers are vital, so it must be kept tightly regulated. It also powers external USB devices like keyboards and flash drives and such.
It powers:
Some motherboard chipsets
Hard drive circuitry
Optical drive circuitry
Some fans
Some PCI cards
USB devices


+3.3V
Nominal value: 3.30V
Allowed voltage variation: +/-5%, 3.14V - 3.46V
Preferred voltage variation: +/-3%, 3.20V - 3.40V
Allowed ripple: <50mV
Preferred ripple: <30mV
Info: The 3.3V was also a major power source in old power supplies, though not today. It plays many minor roles, and should still be just as tightly regulated as the +12V and +5V.
It powers:
Some motherboard chipsets
RAM modules
Graphics card onboard control circuitry
Some PCI cards


+5VSB
Nominal value: 5.00V
Allowed voltage variation: +/-10%, 4.50V - 5.50V
Preferred voltage variation: +/-3%, 4.75V - 5.25V
Allowed ripple: <120mV
Preferred ripple: <50mV
Info: The +5VSB is an often overlooked, but still quite important rail. It doesn't provide much power; rarely more than ~1.5A (7.5W) at the absolute most. However, this is the rail that keeps runs the circuitry that starts your motherboard, and powers the memory that stores the date and time and BIOS settings. Its power rating should not be added to the PSU's total output rating.
It powers:
Motherboard start-up circuitry
BIOS ROM
CMOS RAM
Charges CMOS battery


-12V
Nominal value: -12.00V
Allowed voltage variation: +/-10%, 10.80V - 13.20V
Preferred voltage variation: +/-5%, 11.40V - 12.60V
Allowed ripple: <120mV
Preferred ripple: <80mV
Info: This rail is not used. Your PCI slots have a pin that is intended to deliver -12V power. However, no PCI device has used -12V in over ten years. It is kept on as legacy support, and to keep from "technically" violating PCI specifications. Its power rating should not be added to the PSU's total output rating.
It powers:
Nothing


-5V
Nominal value: -5.00V
Allowed voltage variation: N/A
Preferred voltage variation: N/A
Allowed ripple: N/A
Preferred ripple: N/A
Info: The -5V rail was removed from the ATX specification with revision 2.0, and is not allowed on modern ATX power supplies. If you have a PSU with a -5V rail, it is (or is based on) an antiquated ATX 1.3 era power supply which were designed to deliver most of their power on the +5V and +3.3V rails, and thus are not suited for modern PCs. It may have had the +12V rail beefed up, but that can't compensate for the outdated design. Any power supply with a -5V rail should not be used with modern PCs. In the cases where a PSU does have it, its power rating should not be added to the PSU's total output rating.
It powers:
Nothing


I hope this was informative. Remember, the +12V is the most important in modern PCs and should make up at least 80% of the PSU's available power. When a PSU has a split +12V rail, you cannot just add up the amperages, you have to go by the overall +12V capacity of the +12V transistor, which is usually provided.


Have fun.


Further Reading
Edited by Phaedrus2129 - 12/3/10 at 10:05am
post #2 of 15
Mention that dynamic loads may cause the voltage to change since the PSU basically has to adjust output.

You might want to add the difference between ripple vs regulation (for both your posts on the subjects) plus how to test properly.
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post #3 of 15
Thread Starter 
Quote:
Originally Posted by DuckieHo View Post
Mention that dynamic loads may cause the voltage to change since the PSU basically has to adjust output.

You might want to add the difference between ripple vs regulation (for both your posts on the subjects) plus how to test properly.
Thanks.
post #4 of 15
Very nice guide, I think that this deserves a sticky, that really cleared up some confusion on my part. Thanks a lot, +rep!
    
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post #5 of 15
Hello,
Thanks for the psu information, i am a new system builder,psu selection is now for me more important than it was an hour ago . I have bought a ga-890fxa-ud5 (rev2.1) motherboard and throught an xfx850 pro psu would be a good choice but now i am thinking 1000w. Thanks again and Good Day
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post #6 of 15
What will the PSU be powering?
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post #7 of 15
Hello,wave2.gif
My system requirements are GA 890FX UD5, AMD 965 BE 125W CPU, dual XFX HD 5770 graphics card, four hard drives (two SATA II 500gb Samsung drives, two IDE 20 gb drives raid 0), 16 gb ddr3 ram, dvd-rw, cd-rw, five system fans in a coolmaster storm scout case. Do you think that the XFX850W would be a good choice as this was my original selection? Thanks for the post :thumbsupsand thanks for any help.
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post #8 of 15
Quote:
Originally Posted by adamski;12754664 
Hello,wave2.gif
My system requirements are GA 890FX UD5, AMD 965 BE 125W CPU, dual XFX HD 5770 graphics card, four hard drives (two SATA II 500gb Samsung drives, two IDE 20 gb drives raid 0), 16 gb ddr3 ram, dvd-rw, cd-rw, five system fans in a coolmaster storm scout case. Do you think that the XFX850W would be a good choice as this was my original selection? Thanks for the post :thumbsupsand thanks for any help.

You only need a good 550W unit for this.
post #9 of 15
Hello,wave2.gif
Thanks for the reply sheb, I was very concerned about this part of my first build for many reasons, the correct wattage and quality with a modest amount of over capacity. The XFX850W PSU i think will give me the wattage, quality and over capacity that I desire. I bid you a good day and thanks again for the reply.thumbsupsmiley.png
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post #10 of 15
The XFX850 could power a HD6990 just fine, but if you feel the need to overspec is necessary, then ok. But as mentioned, you won't be loading it much. (Maybe 270-300W).
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