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PSU Guide

Discussion in 'General Hardware' started by panchoman, Sep 13, 2007.

  1. erocker

    erocker Super Moderator Staff Member

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    Plus, you are posting fact from PC&P's marketing. Kenny is absolutely right, I dab into electronics being a mechanic, go run some tests then post your psu facts.
  2. panchoman

    panchoman Sold my stars!

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    yes, i realized that i shouldn't have pulled the info from a psu company's website, but still. kenny, i think you're trying to fight me instead of contributing to the guide lol. i didn't understand half the stuff you said lol. i'm just a kid that wrote this guide in hopes that it helps people, i'm going to let you guys fill in the technical stuff, but it would help if you backed up what you're saying with some evidence that way i can put it up without being being criticized the way you're crticizing me. i'll try and read this tommorow with a fresher mind, but if i understsand correctly, with multi rail psu's, its capped so that theres a limited number of amps going through each wire or "rail". so once you're rail is being used at full capacity (say 18a) you cant put more, whereas in a single rail, the device can draw more then 18a because theres no cap to limit it, thats greater stablility, and since theres 18a going to the rail with the cpu and the cpu is using 8a, what happens to the other 10a? you cant reroute that cause the other rails already have 18a. okay, i'm very very drowsy so yeah sorry if i sound a lil drunk, i'll try figuring out everything tommorow, good night.
  3. keakar

    keakar

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    well as i stated before all he asked for was some data to back it up, this thread is supposed to educate people with facts as best as we can provide so he didnt want to print things that dont have data to back them up is all.

    as for knowing the engineers, i know a few and they cant even agree on lunch lol. but thats how engineers are.

    i agree that the rails each have individual limits on them but as far as the stated amps on the rails they should add up to 72amps not 57 or 54. it is my understanding that the new standard required actual amp outputs to be stated on the label for all rails running together at the same time not what one rail is limited to by itself. if they are all are listed as 18 amps then they should all be able to put out a maximum of 18 amps on all four rails at the same time or it is labeled wrong and should say 54 amps limited to 18 amp per rail.

    on a personal note

    you said "Why is that? Because it doesn't matter. You don't believe me so go try it yourself."

    convince me with an explanation of why your point is valid and why i am not understandind it correctly, dont just give up and say im right and thats it so dont believe me.

    you sound like you are smart and you know what you are talking about but when anyone asks too many questions you give up on discussion to quickly. cant you have a discussion with someone without saying screw it you dont believe me?
    Last edited: Sep 17, 2007
  4. keakar

    keakar

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    you are totally incorrect here, anytime a wire has been cut and reconnected by a connector of any kind you will experience a "small" voltage and amperage drop due to the fact that the connection is not solid but plugs and unplugs. corrosion and oxidation on these pins and sockets further increases the voltage and amperage drop through this connection. a solid wire unbroken and solered to the board is always better than a plug in type connection.

    the higher the quality of the connector and the type of metal it is made from will determine how much drop occurs but it is a guaranteed fact you will experience voltage and amperage drop.

    any basic electronics book will tell you that.
  5. keakar

    keakar

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    how does this in any way have a bearing on what he said?

    his example was a video card needs 20 amps and you have a 2 rail psu putting out 17amps per rail. what he said, granted it could be worded better but, a single rail psu could give you a full 20 amps or more to run that card stable where most of the 2 rail psu doesnt have the necessary amps on a single rail to handle it. what he did fail to mention though was the high end psu do have the amps if you can afford them.
  6. keakar

    keakar

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    whoever suggested using yellow is stupid, what a dumb color.

    everyone knows you cant see yellow very good on a white background! :slap:
  7. KennyT772

    KennyT772 New Member

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    Keakar if you look at the sticker on my power supply it shows a maximum of 680w (57*12=684w thats where I got the 57amps from). If I were to try and run all four rails at 18amps I would be at 72amps and my psu would be overloaded seeing as that would be 864w (12*72) coming from a 700w psu.

    Panchoman the reason for the 18a is the amount of current going over the wire. Most power supply's are wired with 16ga to 18ga wire. 16ga can carry 22a without overloading and 18ga can carry 16a without overloading (http://www.powerstream.com/Wire_Size.htm). The 18a limit is a safety precaution so you don't draw 72amp over one wire and start an electrical fire.

    On Keakar Personal Note.
    I said prove me wrong considering I seem to be very argumenitive towards panchoman as far as you guys are saying and I would like to only contribute useful and correct information to this guide, not senseless marketing garb trying to sell PCP&C's single rails over multi rail designs ETC.

    I know quite a large amount about hardware design as that is what I'm going to college for next year. My main approach to computer hardware is by logic and explanation, this clears away any marketing ploy by providing only the cold hard truth, much in the way a CSI solves a crime.

    In terms of this failing me such as the pci-e 2,0 connector, which i thought was 4x 12v and 4x ground, but it is 3 12v and 5 ground, just with higher gauge 12v wires to allow 4.25amp per wire.

    Need more explaination? I seek to serve.

    Edit: Keakar

    If a single video card required 20amps - 6.25amp (75w) would come from the pci-e bus, another 6.25amp (75w) would come from a pci-e 6pin connector, and the remaining power would come from the pci-e 2.0 8pin 12.5amp (150w) giving you a total of 25a (300w) available. Todays PCI-E 2.0 power supply's have each pci-e plug on a seperate rail so no one wire can become over loaded.
    Last edited: Sep 17, 2007
  8. keakar

    keakar

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    so wait a minute

    you have 4 rails putting out 18 amps that adds up to 72 amps yet this doesnt match the stated amps or the rated wattage of the psu. how can then put out these numbers as fact because you still have the 3.3v and 5v rails?

    what happened to them having to be acurate with labeling?
  9. KennyT772

    KennyT772 New Member

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    The transformer has a maximum output of 57amp. To allow flexibility of useage they give you four 18amp (again that is the maximum amperage that is safe on 16ga wire) to prevent unusable power. As sneekypeet just showed me here is a picture of the OCZ GameXstream pcb showing how the single transformer outputs into a common rail, which is then devided and limited to 18amp each.
    [​IMG]

    Keakar if you had a power supply that output 72amp over four rails you would have to use every single individual plug to max out your power supply. The odds of incorrectly loading your psu are much higher than if you had say 6 rails with some room to play with.
  10. sneekypeet

    sneekypeet Unpaid Babysitter Staff Member

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    Well fellas I hate to say it , but Kenny is right on the money here. I did a lot of review shopping befor I purchased my PSU.

    just a couple of points.....The PSU manufacturers dont "have" to put the correct specs on a PSU. Case and point is the OCZ GSX 700 which when sold by FSP is labled as a 600W PSU, but again the same PCB and equiptment is used by Zalman ,once again its labled as 700W.

    Second point and I'm out....most labels are incorrect, all you need to do is google a couple of formulas. They are an Industry standard for any type of electric power. Plug in the numbers and do the math, it's that simple. I'd even be willing to say that if you do the math on your PSU , I bet the math doesn't match the sticker.

    I dont think Kenny is out to get you or anything , he just strongly believes in hat he is saying, because its true. Math doesnt lie, there is only human error, and the blatant abuse of a company to make money on false info.
  11. keakar

    keakar

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    ok so the amps stated for the rails on a atx 2.0 psu mean nothing then, only the total amps mean anything since they are only amp limits for each rail and not total available amps.

    with 57 total available amps on your power supply you have the 3.3v and 5v rails using amps so lets say that adds up to 7 amps, you have 50 left for 4 rails so if you spread your load evenly you only have the ability to get 12.25 amps from all 4 rails at the same time or 17amps from 3 rails if nothing is using the 4th rail.

    now lets assume you want to look at whats available to you in maximum usage limits (which is why you compare psu in the first place and you can really only use 3 of the 4 rails in the real world use of it.
    Last edited: Sep 17, 2007
  12. sneekypeet

    sneekypeet Unpaid Babysitter Staff Member

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    On the chart below we compiled the current values and their respective powers, from a given 300W ATX power supply. As we can notice on the chart below, the value of combined power is 150W (+3.3/+5V). In order to calculate the maximum capacity of an ATX power supply, we add the power value of the +12V output, the combined power (+3,3V/5V), the power of the -5V ouput, the power of the -12V output, and the power of +5V Standby. The result will be the maximum power that the power supply can provide to the computer.

    Output Voltage Electric Current Maximum Power
    +12V 8A 12 * 8 = 96W
    +5V 30A 5 * 30 = 150W
    +3.3V 14A 3.3 * 14 = 46.2W
    +3.3V/+5V 150W
    -5V 0.5A 5 * 0.5 = 2.5W
    -12V 0.5A 12 * 0.5 = 6W
    Standby 1.5A 5 * 1.5 = 7.5W
    Power Supply Total Capacity 96 + 150 + 2.5 + 6 + 7.5 = 262W


    As we can notice the power supply which we have used in our example is, in fact, a 262W power supply and not a 300W one, as it's been advertised! Unfortunately, this is a common practice among some power supply manufactures, who inform the maximum power value, incorrectly. The easiest way of discovering the real maximum power provided by the power supply is calculating it, like we've shown.

    directly taken from hardwaresecrets website on how to get a true read of your PSU
    http://www.hardwaresecrets.com/article/181
    Namslas90 says thanks.
  13. KennyT772

    KennyT772 New Member

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    Correct. All rails do is place limits to prevent damange to the computer or the power supply. Depending on the power supply there could be limits placed in many ways including 3.3+5 output and then 12v output. Or grouping in any combination. All that matters is the amount of total 12v amps available, as it is very difficult to overload a psu rail with a properly designed psu.

    The 57amp number you quoted is total available from all positive outputs so that you cant overload the unit. I misread the label (human error..what a shock) and was thinking that was the total 12v output not 12v + 5v + 3.3v. I wish I had a load tester as mentioned in this article to figure the total 12v amperage. http://www.hardwaresecrets.com/article/410/1
  14. Namslas90 New Member

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    Thanks for pointing that out Sneekypeet, I've been telling people for ever, to check the Specs and do some SIMPLE MATH, to reveal the truth.

    :toast:
  15. sneekypeet

    sneekypeet Unpaid Babysitter Staff Member

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    I suggest looking at the link by everyone with an intrest in PSU's ....great read there for sure.
  16. Wile E

    Wile E Power User

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    Right, but with a high quality PSU, the amount is ridiculously small. Not big enough to say that modular is a bad design in every instance. That's the point Kenny is making.

    By the modular=bad logic, my Corsair HX620 is a bad psu.

    About the multi-rail vs single rail debate, Kenny is right, period. Neither is better than the other. Both have distinct advantages and disadvantages. Most multi rail psus are a single rail, split "after the fact" (transformer), and just have limiters placed on the "rails" to prevent overloading a single wire.

    More about my PSU. My psu is rated at 620w total. The 12v rails are rated at 600w. 600w/12v=50A. Now, there are 3 rails, and they're rated at 18a each. 18a*3=54A. I could run 18a thru 2 of them for 36a, but I'd only be able to run 14a thru the third, any higher and I'd trip the power supply. Or conversely, I could put nothing on the first 2 rails, then try to run 20a thru the third, but my psu would still trip, even tho I'm at less than half of it's rated capacity. It's purely for safety reasons.

    Link to my psu, for reference: http://www.corsair.com/products/hx.aspx
  17. keakar

    keakar

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    ok so lets take this example of a two rail psu which would probably be the more popular choice for the average buyer.


    Brand Thermaltake
    Model W0093RU
    Type ATX 12V 2.0 Version
    Max Power 500W
    PFC Passive
    Dual +12V Yes
    PCI-E Con No
    SLI Support No
    Efficiency > 70%
    Over Voltage Protection +5V output 7.0 Vmax
    +3.3V output 4.5 Vmax
    +12V output 15. 6 Vmax

    Input Current 10A @ 115V, 5A @ 230V
    Output +3.3V@30A, +5V@28A, +12V1@14A, +12V2@15A, -12V@0.3A, +5VSB@2A

    i was always told since i was buying single rail psu to look at the +12v amps to get the best psu i need.


    now with 2 or more rails they are all giving wildly different amps for all the different rails.

    i see 3.3v rails as high as 54 amps and some 5v rails i have seen at 45 amps and +12v rails vary from the 18v to 24v to 30v, then some give +12v dual rails as 15/15 or 18/17 or 20/20.

    so how the hell are we supposed to select what we need if they are all so different. if there is a standard why isnt it being followed. there is no guide to say the +12v rail requires this amount.
  18. keakar

    keakar

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    ok so where the old style atx psu had individual dedicated rails that came with different amperages depending on the psu even though they had the same wattage rating. this meant that wattages were not a very good indicator of how many amps you might get available to the +12v rail.

    are you saying that now in the new atx 2.0 psu the wattages are a true and accurate indicator of how much amperages you are going to have available to all the rails and the amperage limits given are all just dependent on the size of the wires used for that rail?
  19. Wile E

    Wile E Power User

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    I'm not saying a psu's wattage rating is a true indicator of potential. That varies by manufacturer. As with almost everything in the computer world, there are no standardized testing or rating methods. Each manufacturer uses their own, some are more honest than others.

    But you're right on the amperage part. It boils down to safety, and how much the wires themselves can handle.
  20. keakar

    keakar

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    well if aperages mean nothing and wattages dont ensure accurate amperages then there is no way to know what you get
  21. KennyT772

    KennyT772 New Member

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    The reasoning behind the 14a/15a max rating is either because of the gauge wire used, or the maximum output by the transformer + some extra allowance for load setup.

    What you should look for in a power supply is the maximum combined 12v output. Some power supplies show this and some do not. You need to simply research enough so you understand the layout of the output maximum for the various rails. There is no one true indicator of which power supply you should buy.

    This is why there is a forum for these kind of questions.

    This uncertianty is the #1 reason I would always buy a psu 50w over your worst case scenario. This way you have plenty of room for overclocking, upgrading, etc.
  22. keakar

    keakar

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    please explain, you say amps dont tell true power and neither do watts so explain what does matter
  23. keakar

    keakar

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    the average computer builder will follow the popular trends when building a system so it should not be hard to come up with a general guide for these types of systems.

    for my system at the time i built it, the general guide said it required a minimum of 300watts for a P4 system, 350 watts for a system with extra fans and lights, 400 watts for a modded system with lights accessaries and extra fans, 500+ watts for anything with water cooling and the psu amperage guide said it was poor if it had 12-14 amps, marginal if it had 15-16 amps, and good if it had 17-18 amps on +12v line.

    what I would like to see is a simular type chart to simplify psu selection in reguards to building a system today.

    IMO power supply calculators are notoriously overestimating the size psu needed so we end up buying the higher priced psu when more economical options can be used. while it doesnt hurt to have too much psu, budget builders have less money and cannot overpay where it is not needed.

    these "calculators" have preset numbers they use to figure your needs so a "general outline guide" made using these numbers can be made. those who build "basic" systems could simply make choices following this chart which IMO applies to most of us and those who are building more complicated systems can stick to the calculators. i cant believe a basic generic guideline chart cannot be done.
    Last edited: Sep 17, 2007
  24. KennyT772

    KennyT772 New Member

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    What you can look for is the amount of 12v rails X the amperage of the smallest rail (say 18a for my ocz) then figure 3/4 of it is useable to prevent over loading. For economical users not overclocking and not upgrading a 450w-500w will give you the best efficiency (25-50 percent load is where max effciciency is reached) and will give you a long lifetime.

    What I was saying in my other post is that you cannot go only by the wattage or amps, you have to take both into consideration for the best choice on output. After you figure what you need in terms of power you then need to find a suitable company to buy a unit from, with the extra features you want such as modular, pfc, low dba, etc.
  25. keakar

    keakar

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    ok so for a four rail psu you only add 3 rails together to get accurate picture of amp supply

    but what do you do if its a two rail psu? is that still 75% of the total?

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