Power Supply FAQ

Discussion in 'General Hardware' started by Makalu, Feb 10, 2009.

  1. Makalu

    Makalu Guest

    All questions, comments or suggestions are welcome as always. :)

    Power Supply FAQ​

    FAQ #1: How much power do I need?

    Surely the most frequently asked power supply related questions are those related to the power requirements of a specific system. A good place to start estimating power requirements is with a couple of the PSU wattage calculators on the internet. The calculator by Snoop-WiggleS at Overclockers Australia gives the most detailed results but doesn't offer as many component entries as the one at eXtreme outervision. The free "Lite" version will suffice. Note that the results obtained at eXtreme Outervision are the recommended PSU wattage, not the estimated power consumption. I recommend you keep the default settings for CPU TDP at 85% and load at 90% and use 10% capacitor aging for PSU's over three years old.

    Here's another method for estimating your systems power requirements that I use:

    What we are going to do is estimate the 12V power consumption only instead of total power consumption. 12V loads will be at least 80% of the total load in a modern system and it's the 12V load that will determine what PSU is suitable. Loads on the 3.3V and 5V rails are generally low while PSU capacity on the 3.3V and 5V rails is generally high. 12V capacity of various PSU's of the same total wattage varies greatly and if you have a PSU's whose 12V capacity includes a comfortable amount of headroom over the 12V load then it will also have good headroom between the total wattage capacity and total wattage load.

    First find the estimated peak CPU load with the CPU section of the eXtreme outervision calculator. Select Lite(free) version and enter your CPU model only. Then check the "overclock my CPU!" box and enter any OC or just reenter the stock values if you aren't overclocking. Press the "Overclock" button and write down the Overclocked CPU Wattage result in the box there.

    Now go to the list of estimated GPU wattages at AtomicPC forums and find your graphic card/s wattage, increase by 10% for GPU overclocking and add that to the CPU watts.

    Now add 12 watts for every hard drive and optical disc drive in the system. Add 3 watts for every 80mm fan and 5W for every 120mm fan. If you have a water pump and don't know it's power consumption add 20 watts and another 60 watts on average for a thermoelectric(TEC) CPU cooling kit.

    The sum of all the above is the approximate peak 12 volt load if every component is stressed at once with benchmarking programs. Since games don't stress everything at once the system load is generally 20% less than peak loads. I recommend a minimum 25% overhead between expected loads and the 12V capacity of the PSU. Obviously there can be a large difference in the power requirements of identical systems depending on the amount of CPU overclocking and software loads. If you use a system for Folding@Home or benchmarking or any high duty cycle applications then use some common sense and don't skimp. Also be aware that high case temperatures above 40°C or less will lower the output capacity of many PSU's.

    Divide the wattage sum by 12 to find the number of amps required. Many individual power supply 12V amp ratings can be found here.

    There are other ways to determine power supply needs. Most graphics cards come with a retailer PSU recommendation. Be aware that these recommendations (ex: 450W unit and 12V@28A) refer to PSU capacity, not PSU loads. They are the recommended PSU rating for powering the complete system and includes some overhead. The amperage given refers to the combined 12V rating in multi-rail designed units not to individual rail ratings (ex: 18A) nor does it refer to the sum of individual rail ratings. A PSU with four 18A rails does not have a 12V capacity of 72A (4x18).

    Also the ATI single card and Crossfire and Nvidia SLI certified PSU lists can be used to help determine your power needs and any of the specific models listed there have passed strenuous testing. Many online review sites are giving system power consumption measurements along with their GPU and CPU reviews now too. Most of these measurements are taken between the computer and the AC mains wall outlet therefor the DC load on the PSU is generally 75-85% of the measurements given depending on the PSU's energy efficiency. The test system configurations and software loads vary a lot between the reviews so weigh that in when comparing their results.

    Besides insuring that a PSU is big enough for a system I also recommend that it has the required PCIe connectors for the card/s. Using adapters instead of native PCIe connectors may overload an individual rail on an otherwise big enough PSU.

    FAQ #2: Is this a good PSU?

    It's been said that any power supply that's working is a good power supply. Certainly it's better than one that isn't working but power supplies do have vast differences in quality and each individual has to decide what is "good enough" for them. However, some PSU's out there are shockingly bad, in more ways than one. Following is a list of some of the known very low quality brands that I recommend should be avoided.

    Yes you may have used one of these brands for years without problems but bear in mind that no PSU lasts forever. Some PSU's die gracefully, quietly and alone while others commit murder, arson and suicide in that order. The shut off protection circuitry in case of a fault is one area where they cut corners in these cheap units. It's ofen non-existent or non-functional. Using a PSU with a high failure rate that's apt to damage other components when it fails is a risk no one should take. A poor quality PSU will also have poor AC filtering which can damage RAM and other sensitive chips over time while the PSU itself appears to be functioning just fine. Only an oscilloscope will disclose the excessive amount of ripple & noise in the outputs.

    The following is not a complete list of every "Yum Cha" PSU in the world. There are many other low quality brands, often bundled with a computer case, that should be considered disposable junk. A well built PSU that can deliver 300 actual watts costs about US$30 to manufacture so that "Golden Deluxe 600W gamers" power supply selling for $30 is probably a very low quality 300-350W unit at best. Poor quality and false wattage claims usually go hand in hand.

    Another thing to be on the watch for is there are still many shiny new PSU's being sold that are built around a late 1990's design. They may have modern features like 120mm fans, PCIe connectors and claims about multiple 12V rails but the internal design predates the types of loads in a modern system. These units won't follow the Intel ATX and SSI EPS PSU guidelines for modern 12V heavy systems and won't have the same performance and output quality as a PSU with a modern design.

    Some of the external signs that a PSU is of an outdated design are: A 5V rail rating higher than 35A, has a -5V rail in the specs and/or the 20/24pin connector has a white colored wire.

    Brands To Avoid
    Apex *SL model numbers are acceptable low quality
    Blue Star
    Broadway Com Corp
    Clipper Pro
    Delta Power
    Future Power
    Great Wall
    JPAC Computer
    Key Mouse
    King Star
    Lead Power
    Power Age
    Prudent Way
    Qtec *not to be confused with Qtech
    Rite Star
    Real PC Power
    Sun Pro

    FAQ #3: What are some good power supplies?

    What follows is not a list of the best power supplies nor the only good power supplies. These are merely some enthusiast grade power supplies built around good designs with a proven track record that are available across much of the planet under one brand name or another.

    As you see many different brands will sell models made by one PSU manufacturer and all based around a single platform. Be aware though that these are not always identical units. The cabling, connectors, cooling scheme, capacitor brand and quality control can differ between brands. Also warranty length and availability of local customer service in case of a problem are justifiable considerations when making a purchasing decision.

    Model numbers and 12V ratings for these units can be found here.

    Channel Well Tech PUC platform
    Note: Rating temperatures are 50°C. These are all quad rail design with the exception of the Corsair which is dual rail with very high current limits.

    Thermaltake Toughpower 1000W/1200W/1500W
    Corsair HX1000
    Gigabyte Odin Pro 1200W
    LC Power Legion 1000W
    LC Power Monolith 1200W
    Istar PD1 1200W
    InWin Commander 1200W/1500W
    Ikonic Vulcan 1000W/1200W
    Xclio GreatPower 1000W/1200W

    Enhance ENS-05xx platform
    Note: Rating temperatures are as noted. Comes in quad rail or dual rail with very high current limits.

    Akasa PowerMax 850W/1000W 50°C
    Antec TruePower Quatro 850W/1000W 50°C
    Zalman 850W/1000W 45°C

    Enhance EPS-0300 platform:
    Note: Rating temperature is 50°C. Six rails.

    CoolerMaster Real Power Pro 850W/1000W/1250W

    Seasonic M12 platform:
    Note: Rating temperatures are as given below. The Corsair versions are all single rail despite the claims on the label. The Seasonic versions are also effectively single rail (possibly a dual rail with 30A or more current limits).

    Seasonic M12 500HM, 600HM, 700HM 50°C
    Seasonic X900 SS-900HP 25°C
    Corsair HX520/HX620/TX650 50°C
    PCPower & Cooling Silencer 610W/750W 40°C

    Channel Well Tech PSH platform:
    Note: Rating temperatures are 50°C. Different brands and wattage levels feature single, triple or quad rails with good connector distribution across the multiple rail versions. Actual multiple 12V rail OCP levels are somewhat higher than as rated.

    2the Max GPX 600W/750W/850W
    Aerocool Horsepower 650W/750W
    Chieftec Super 600W/650W/700W/750W
    Chieftec Turbo 600W/650W/700W/750W
    Coba Nitrox 500W/600W/650W/750W
    Corsair VX550/TX750/TX850
    Gigabyte Odin Pro 550W/680W/800W
    Gigabyte Odin GT 550W/680W/800W
    Hiper Type-M 630W/780W/880W
    Hiper Type-R MkII 680W/880W
    Ikonik Vulcan 650W/850W
    In-Win Commander 650W/750W/850W
    Istar PD1 750W/850W
    LC Power Hyperion 700W
    *LC Power Archangel "850W"
    Mtek PSH 550W/650W/750W
    Thermaltake Toughpower 600W/650W/700W/750W/850W
    Thermaltake Purepower RX 450/500W/550W/600W
    Thermaltake Toughpower XT 650W/750W/850W
    Xclio GreatPower 500W/550W/600W/650W/700W/750W/850W
    Xigmatek NRP 650W/750W/850W

    *This is really a 750W model, not 850W as claimed by LC Power

    FAQ #4: What are some decent yet cheap power supplies?

    Seasonic S12II platform
    Note: Budget platform with lower tier build quality and mid grade output quality but still acceptable for most applications. Rating temperatures are as given. They are all single rail design despite the labeling.

    Antec Earthwatts 380W 50°C
    *Antec 'Original OEM' Earthwatts 430W/500W 50°C
    Arctic Cooling Fusion 550W 50°C
    Corsair CX400/VX450/HX450 50°C
    PCPower & Cooling silencer 310W/360W/370W/420W/470W/500W 40°C
    Seasonic S12II 330W/380W/430W/500W 50°C
    Silver Power Gorilla 400W/500W 50°C

    *The original Seasonic made Earthwatts EA430 and EA500 can be identified by their two 12V rails each with 17A current limits stated on the label.

    FAQ #5: Are PSU's with a single 12V rail better than those with multiple 12V rails?

    The short answer is "no". You may need to examine the issue on a case by case basis and know which connector is on which 12V rail in a specific PSU model. But for the most part engineers make good decisions about the rail current limits and connector distribution in multirail PSU's and the typical gaming or benchmarking system configuration won't come close to exceeding a typical 18A-20A rail limit. If however you want to build a system with an extraordinary amount of 12V components like 15 hard drives or 30 high performance fans daisy chained you should probably stick with single rail units or multi rail units with higher than typical 18A-20A ratings.

    In practice only certain dual rail units have much risk of exceeding an individual rails' limits. A PSU with three or more 12V rails will have much lower currents across its' rails then a dual rail PSU powering the same system. However a PSU with two 18A-20A rails that puts the CPU connector/s on one rail and all of the other connectors on the other rail can easily overload that other rail with dual graphics cards or a single very high wattage card like the GTX 295. But since PSU's with two 18A-20A rails naturally top out at around 550W most users will recognize to begin with that the PSU is too small for such a high end system.

    If your PSU is not undersized for the total system load and you haven't added a large extra load with connector adapters then you won't need to micromanage your connector/rail distribution. If you are exceeding an individual rail limit you are probably exceeding the 25% total overhead recommendation too and need to switch to a bigger PSU.

    For more information regarding the differences between single and multi rail designs I recommend reading jonnyGURU's article here.

    FAQ #6: Which PSU's are the quietest?

    Bear in mind that the term "quiet" is subjective and relative to other noises in the area and some people simply have better hearing than others. Noises from the PSU are usually just the fan but sometimes coil whine or switching frequency buzz can be heard. Fanless units do exist but are limited to ~300W. SilentpcReview.com maintains a list of the quietest PSU's that they've reviewed here.

    FAQ #7: Are my PSU's voltages OK?

    Since most users asking this question are looking at the voltage readings in a software utility program or perhaps in the BIOS the short answer is that the software and BIOS methods can't be trusted and if you aren't having issues with the system then the PSU is probably just fine too. Sometimes the software readings can yield values that are impossibly out of tolerance range, such as a 12V reading of 8V. If the 12V was really that far out of tolerances the system would'nt even boot and run without errors. Even when not yielding outright crazy numbers the software and BIOS methods are not very accurate and can sometimes show voltage fluctuations when the PSU is actually stable or vice versa. In my experience BIOS and motherboard manufacturer supplied software utilities are more accurate than third party software such as Everest or Speedfan. Also the reading is usually, but not always, a little bit higher than what a multimeter will read.

    A low-cost multimeter can be purchased if you need to be sure about your PSU's output voltages. How-to guides can be found here and here.

    I recommend measuring the 5VSB (back probe the purple wire on the main 20/24 pin connector) as well, especially if the system is having issues and be sure to take measurements under both idle and load conditions. A 3D graphics intense game will give a realistic load or Furmark and Prime95 can be used to put a full stress load on the GPU and/or CPU. For reference your voltages should remain within the following ATX tolerance ranges:

    +5VSB (Min: 4.75v Max: 5.25v)
    +3.3V (Min: 3.14v Max: 3.47v)
    +5V (Min: 4.75v Max: 5.25v)
    +12V (Min: 11.4v Max: 12.6v)

    FAQ #8: My system doesn't power on. How can I test the power supply?

    Disconnect the 20/24 pin connector from the motherboard and use a paper clip or suitable chunk of wire to jump between the pin for the green wire and any of the black wire pins. If your PSU has wires of all one color then refer to this pin-out diagram with the retaining clip on the left side and jump between PS_ON and any of the COM's (common ground).


    Use the external ON/OFF switch on the PSU and if it powers on and spins its fan continously then it's probably good and the problem is elsewhere.

    FAQ #9: Can I use two power supplies to power one system?

    Yes, connect the "master" PSU's 20/24-pin to the motherboard and connect two jumper wires between it and the "slave" PSU. Referring to the pin-out diagram above, connect green to green and any black to any black on the two units and the slave will power on and off when the master does with the front chassis switch. The slave units are usually used for their additional 12V capacity to power graphics cards, fans, water pumps and the like but should also be used to power a couple of drives, either optical or hard, too. This will put a small load on the 5V rail and less stress on the PSU trying to deliver a 12V only load. Instead of using drives for a 5V load you can make a "dummy load" with cheap resistors. Two 10-ohm 10W ceramic resistors or a single 5-ohm 10W aluminum will put a 1A load on a 5V circuit and dissipate ~5W of heat.

    Ready made dual PSU adapters are sold by Ultra and Lian Li and also come bundled with Cooler Master dual PSU capable tower cases. Performance-pcs.com selling the Ultra adapter does ship worldwide.

    FAQ #10: Do modular power supplies limit available power?

    No, the extra resistance through the extra connection is negligible and less resistance than through the lengths of copper wires themselves.

    FAQ #11: Where can I find pin-out diagrams and rating information for all of the PSU connectors?




    FAQ #12: Will a bigger PSU raise my electricty costs?

    No, power supplies output whatever the system load is. A 1200W PSU will still only output 300W if that is what the system demands from it. Only power supply energy efficiency differences will affect your electricity costs.

    FAQ #13: Is it true that a 700W PSU that is 80% efficient can only output 560 watts and an 18 amp rail can only output 14.4 amps in an 80% efficient PSU?

    Absolutely not. A true rated 700W PSU can output 700W and an 18 amp rail can carry 18 amps. Energy efficiency is the amount of energy lost converting AC main power into DC PSU outputs. An 80% efficient 700W PSU would require about 880W from the wall to output a full 700W.

    FAQ #14: How can I obtain 7V from a PSU to power a fan at low speed?

    Connect the positive (usually red, sometimes yellow) fan wire to a 12V yellow PSU wire and connect the negative (black) fan wire to a 5V red PSU wire as shown here:


    CAUTION!: You must connect at least one hard or optical drive to one of the other molex connectors on the same cable from the PSU or risk damaging the PSU. This will put a load on the 5V and prevent the fan mod from sending current into the PSU. A dummy resistor load as described in FAQ #9 will work too.

    FAQ #15: Can I use one power supply to power two systems?

    Yes, if it's output capacities are enough then you'll just need two main 20/24-pin connectors and two CPU connectors. 1-into-2 main power Y-splitters are available but uncommon...try here. You could also fashion your own out of two 20/24-pin extension cables. Many PSU's have two CPU connectors and Molex-CPU adapters are readily available also. Each motherboard will still need it's own ON/OFF switch implemented. If you boot one motherboard the other one will receive power and spin it's CPU fan but won't boot until it's switched ON.

    FAQ #16: Which connectors are on which 12V rail in a multirail PSU?

    There's no standard method albeit most 2-rail units put the CPU connector/s on one rail and all of the other connectors on the other rail. The 3,4,5 and 6-rail PSU engineers are free to distribute connectors to the rails as they see fit and modular PCIe cables may be on different rails depending on which connector on the modular interface is used. Sometimes a label on the PSU will have the modular interface information. The manual may list the complete rail information or sometimes the wiring diagram tables may number the 12V rails. Sometimes the yellow 12V wires have different colored stripes on the insulation that you can use to figure it out. If the power supply has been reviewed at jonnyguru.com or hardwaresecrets the review will have the rail/connector information.

    FAQ #17: How do I get the pins out of the PSU connectors?

    For the 20/24-pin, CPU and PCIe connectors use this tool and follow these instructions. For the "molex" peripheral drive connectors use this tool. If you need replacement connector housings or pins find the Molex part number here. Mouser.com stocks all of them and ships worldwide.

    FAQ #18: What is ripple and how can I test for it?

    The term ripple as used in the ATX PSU Guide and by PSU enthusiasts is the AC content that remains in the DC outputs after filtering. It can not be measured without an oscilloscope. Ripple that exceeds the ATX tolerance levels can cause system errors and data corruption, damage to low voltage components like RAM, creates heat in capacitors lowering their lifespan and lowers the voltage thresholds when overclocking.

    The software stressing program OCCT does not measure AC ripple, it measures DC voltages (inaccurately, see FAQ #7) and gives a high/low percentage number mistermed as 'ripple'. It's not a meaningful number alone since a PSU that is rock solid for hours with just one hiccup will have the same OCCT "ripple" results as a PSU that is constantly fluctuating. Just ignore it and interpret the graphs yourself with an eye towards the voltage drop when the test is begun and the rate and amplitude of fluctuations, if any.

    FAQ #19: Which system components use the different PSU voltages?

    Modern CPUs and GPUs, fans, drive motors, DC water pumps, TEC assisted CPU coolers (Freezone et al), and cold cathode lighting all use 12V power. AGP GPUs with an external power connector and PCIe GPUs are powered by the 12V rail and a small amount, typically less than 2A, of 3.3V power. Analog fan voltage controllers use just 12V and digital fan PWM controllers use both 12V and 5V. LED lights use either 12V or 5V. Modern north and south bridge chipsets use 12V power.

    If your motherboard is old and lacks a CPU power connector then the CPU is powered off the 5V rail. AGP GPUs without an external connector are powered by the 5V and 3.3V rails and a small amount of 12V power if it has a fan. Drive circuits and some motherboard circuitry uses 5V power. System RAM, USB devices and LCD panel displays use 5V. PCI cards use 5V and/or 3.3V.

    3.3V is used by many motherboard components and some early SATA drives.

    The 5VSB rail powers wake-on events and any associated USB devices, charges the battery and keeps volatile system memory active in standby mode.

    The -12V rail is used as a bias voltage for regulation through certain components and circuitry on the motherboard and some PCI cards.

    The -5V rail was used to power ISA cards and was phased out in 1997. It's not implemented in modern PSU's but still present in many new PSU's that use an old design.

    Keep in mind that the chips that are powered off a certain voltage rail don't necessarily use that particular voltage internally. In most cases the voltage is regulated down to appropriate levels first by various motherboard voltage regulator circuitry. The higher voltage is used to keep current levels low.
    Last edited by a moderator: Feb 21, 2010
    386SX likes this.
  2. Makalu

    Makalu Guest

  3. smashly

    smashly Guest

    Great work on a nice current date FAQ.
    It is good to see the psu's laid out in the way you've done it in #3 makes it easy to see the type of psu a person would want
    I wish I'd seen it sooner.

    Maybe a link to the psu amp ratings thread or the amp ratings added to #3 list maybe useful.

    Sticky of the thread would be nice see.
    Keep up the good work please :)
  4. maxfly

    maxfly Ancient Guru

    Likes Received:
    2x GTX 980 TI
    great work man! i especially like the brands to avoid section. this will be very helpful!

  5. DSK

    DSK Guest

    A new PSU faq, Very useful indeed thanks Makalu :)
    Last edited by a moderator: Feb 10, 2009
  6. Makalu

    Makalu Guest

    good idea...I've added that, thanks :)

    EDIT: btw your Cyber-Sport is a decent ATNG unit...no worries
    Last edited by a moderator: Feb 10, 2009
  7. elkosith

    elkosith Maha Guru

    Likes Received:
    PowerColor RX 6600
    Just when I'm thingking about getting a new PSU... :)
    Good help. Thanks
  8. contrvlr

    contrvlr Guest

    Have waited to see you do this for a while, thanks Makalu.

    Let's keep it at the top.
  9. Makalu

    Makalu Guest

    oh you aren't the only one around here who's been waiting a while for me to do this ;)

    I think it's the forum censor that is messing up the URL for the link to the SSI EPS .pdf file at PCP&C. Wants to change a "e-c-2" to "***". Anybody got a work around for this? i'm brain dead...

    EDIT: nevermind...i changed the file name and put it on a file storage site
    Last edited by a moderator: Feb 10, 2009
  10. lepitius

    lepitius Guest

    Nice, you got it finished. :D

  11. lmimmfn

    lmimmfn Ancient Guru

    Likes Received:
    nice FAQ, should be a sticky
  12. Makalu

    Makalu Guest

    well I'm not satisfied with some of it but wanted to get a few of the more frequently AQ posted...it needs more info about 12V rails and ratings...and dunno if I'm happy with the "some good PSU's" idea. I think people expect and want a "good, better, best" type list, but I know I wouldn't be happy with that idea either.
  13. Flopster

    Flopster Guest

    lol at the Chiefmax amd Chiefman. Typical made in China stuff which pretends to be some quality piece of hardware and unexperienced users fall for it. Reminds me of the those Firesox's in the vault of fail..:D
  14. Makalu

    Makalu Guest

    lol yeah they do that...and a lot of these units come from just a couple of OEM's...like they just keep thinking up new brand names to cover their tracks. Chiefman and Chiefmax are probably better known under the Powmax brand. Leadman is like the parent company...Demon, Diablotek and Robantan are the same so that's 7 different brand names. Could be some more Leadmans there I'm not sure.

    Almost all PSU's these days are made in China btw...a few in Taiwan...some of the worst are from Thailand
  15. kRze

    kRze Guest


    Great work Makalu.

  16. maxfly

    maxfly Ancient Guru

    Likes Received:
    2x GTX 980 TI
    stuck ;)
  17. Flopster

    Flopster Guest

    This forum is becoming more and more helpful by the days;)
  18. scipio

    scipio Guest

    Thanks Makulu for redoing the psu fact, now people can point to an updated thread.
  19. branko47049

    branko47049 Guest

    Well my Rasurbo DLP-535 is made in Taiwan , but it is on the black list :D .

    A question ; is there any way for a home user to check it`s 12V readings when PC is at full load. The only true way is with unimeter, or HWMonitor or Everset will do the job?

  20. Makalu

    Makalu Guest

    erm I haven't made any statements about units made in Taiwan being better than those made in China....anyway your PSU is made in China according to both the PDF and the label.
    pdf says country of origin CN: http://www.rasurbo.com/pdf/DLP - englisch.pdf

    and the label has the fake CE logo which is supposed to look like the European Certified logo but actually stands for Chinese Export: http://www.hardware-experten.de/images/stories/rasurbo_dlp535/rasurbo_dlp-535_02_g.jpg

    Some of these brands I deliberated a lot about if they should be on the avoid list or not and yeah Rasurbo was one of them (and yeah I tried to get you to not buy that unit but I recall you didn't have a lot of choices there in Macedonia). It's not complete total junk like some of the others on the list. It's just a very low end quality, out dated design and over rated by a good 100W.

    Anyway yeah the only true way to get a measurement is with a meter.

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