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Thread: PSU rails and component

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    Default PSU rails and component

    Hello guys,
    I am trying to write a little article about choosing the right PSU.
    I am trying to locate every component that draw watts from the psu and find from which rails it draws what.

    correct if I am wrong, CPU's and Graphic cards draws only from 12V rail?

    I am not sure if the DDR and DDR2 draws from 5V rail, are they?

    Thanks.

    btw. if there is an article that already present all the numbers that I seek, please let me know. thanks.

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    Thanks. is it somthing but I still have lots of Q's.

    I understand that graphic cards draw power from 3.3V as well?

    what about the motherboards. how much power does they draw and from what rails?

    Thanks

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    If this is for an article, it might be worth while to get a clamping DC ammeter and clamp the wires feeding power to the components and motherboard.

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    I would if I knew how.
    To see the Amp I need to clamp the wires of the DC ammeter serial to the cirucit, not parallel. How do I do it without damaging the PSU or its wires? Where exactly I have to clamp the ammeter? How do I know what goes to the motherboard and what goes to the memory moduls or CPU ect'

    If the information was on the net it would be easier.... I need the numbers in general.
    I want to make a guide "how to pick the right PSU". I can't see people throw things that they don't have a clue what they are talking about all over my website.
    you know it's very easy to recommand on 650W TT toughpower or corsair 520W for everyone who buy new PC. but I it's not right. there are lot of cheaper PSU on the market that can do the job just the same. not everyone is looking for OC and SLI/CF.

    Thank you!!

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    CLAMPING ammeter. You just clamp over the insulated wires. That simple. If the meter has logging capability (most DC clam, you can pull a max figure.

    On the 24-pin, you're just going to have to guestimate what power goes where. It's not so hard when you have redundant components as you can collect comparitive values. For example: One stick of RAM vs. two. On board graphics vs. PCIe card. Of course, you can also look up power consumption numbers from the manufacturers of RAM, hard drives, etc. but that's an even more exhaustive search.

    Of course, I must say I don't exactly support your efforts. Even good meters can't capture sudden transient loads. And although someone with a simple 7600 card or 8500 card, one hard drive and a dual core can run all day with a 300W power supply, you have to quality that it should be a GOOD 300W power supply. You can attempt to educate people all day long about rated temperatures, de-rating curve, etc. But it still doesn't always sink in. An average 300W might run the PC today, but what about three years from now?

    Also, you have to keep in mind that the best quality power supplies are simply not made in lower wattage. With the component selection used it's simply cost prohibitive to make the unit lower wattage and still be price competitive. Sure, not everyone want's "the best". If that were true, Kia wouldn't sell cars. But do you realize that the difference in BOM cost to make a PSU only 100W less is typically only about $2 to $5? They widen this gap by using cheaper caps, group regulation, cheaper fans, cheaper housings, lower gage wire, no sleeves on the cables, single layer PCB's.... a power supply's worth is NOT just about the amount of wattage they put out. So people need to be aware that they're not just "settling" for a lower wattage unit... they're settling for a cheaper quality unit as well.

    I agree that there is NO NEED for a power supply on the market > 800W. Even three way SLI doesn't need 800W unless you have dual procs, but on the flipside I don't think it's wise to recommend the smallest of power supplies when a quality, ample unit can cost as little as $50 to $75.

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    Quote Originally Posted by jonnyGURU View Post
    They widen this gap by using cheaper caps, group regulation, cheaper fans, cheaper housings, lower gage wire, no sleeves on the cables, single layer PCB's.... a power supply's worth is NOT just about the amount of wattage they put out. So people need to be aware that they're not just "settling" for a lower watt
    Out of curiosity, what's the problem with single layer PCBs? I see how it makes the design harder, but as long as you keep the switching loop small, a good design should still be possible.

    I see how it's associated with more heavily cost-optimized PSUs, but does using a single-layer PCB cause low quality in some way?

    (I usually work in the 2-6 layer range, but I did design one simple one-layer relay board. The fun part was not using a single jumper!)

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    I see What you mean about the effort.
    I come from isreal, I guess the Prices here are bit different from yours.

    for example, you can buy here new PC (no peripherals) for 4000 NIS +-
    CPU Q6600
    MB P35
    Ram 2G
    HD 250GB
    GPU 8800GT
    PSU+Case (HEC 500 TRT)

    This is an example for prices of PSU in israel

    HEC 500 TDT = 260 NIS +-
    Seasonic ss 500 ET = 350 NIS+-
    FSP BLUE STORM 500W = 400 NIS +-
    TT Toughpower 600W = 500 NIS +-
    Corsair HX 520 = 600 NIS +-

    As you can see, there are big differences between them.

    you ask "what about three years from now?"
    To my humble opinion PSU doesn't suppose to go with you for the next PC.
    for example, three years ego there was no SATA, no 6 Pin, no 8 Pin.
    Maybe three years from now the main load will get back to the 5V rail instead of 12V.
    Maybe MB manufactors decide they need 30 pin connectors.
    the Market is so unstable/dynamic that we can't predict that far (though I guess you have more Info about it than me ).

    bottom line, it is quite an effort to do this. I hoped that you guys would pull the numbers out of your sleeves, but I guess its not that easy efter all.

    Thank you mr Jonny for the help...

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    Quote Originally Posted by cypherpunks View Post
    Out of curiosity, what's the problem with single layer PCBs? I see how it makes the design harder, but as long as you keep the switching loop small, a good design should still be possible.

    I see how it's associated with more heavily cost-optimized PSUs, but does using a single-layer PCB cause low quality in some way?
    You're correct. It is possible to make a good single layer design. I'm just pointing out it's one of the aspects of what makes a lower wattage unit cheaper. And in the grand scheme of things, it also causes the OEM to use multiple platform, which is actually more expensive since you have to have a different platform design for the cheaper, lower wattage units than the higher wattage units reducing your per unit volume and lifespan. If they can increase their volume with one platform, by creating a solid platform that supports a price competitive 550W on up to a rock solid 850W, they're golden on all fronts... of course, the "you can do it with a 450W PSU" crowd gets left out.

    Quote Originally Posted by maduser View Post
    you ask "what about three years from now?"
    To my humble opinion PSU doesn't suppose to go with you for the next PC.
    for example, three years ego there was no SATA, no 6 Pin, no 8 Pin.
    Maybe three years from now the main load will get back to the 5V rail instead of 12V.
    I appreciate your humble opinion, but it's not widely shared. Of all of the parts in a computer, it's the PSU that often gets overloooked and is also the most likely to get migrated from PC to PC. Yeah, 20-pin became 24-pin, 4-pin became 8-pin and Molex became SATA on hard drives, and more graphics cards need more PCIe connectors, but that doesn't stop people from putting 20-pin power connectors in 24-pin motherboards (hell, even Gateway does this!), 4-pin power connectors in 8-pin motherboards, use Molex to SATA adapters and Molex to PCIe adapter.

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    The easiest way to figure out how much power a component might use is to look at specs.

    PCIe slot in a mobo for a graphics card - Can draw up to 75W, 6pin ("3x2" in ATI parlance) PCIe power cable? Another 75W. 8pin ("4x2) - 150W.

    AGP? 42W iirc.

    DDR2 DIMM? 4.4W, 1.8V per JEDEC spec. But then you've got manufacturers like Corsair who make the Dominator DDR sticks - It can either run DDR2 5-5-5-18, double drop per cycle at 400 Mhz (i.e. it triggers on rising and falling edge of the clock, so the clock runs at 400Mhz, but the RAM triggers twice per cycle, for an effective rate of 800Mhz, which is why it's called DDR2-800Mhz) at 1.8V, OR it can run at clock-doubled 533Mhz (i.e. DDR2-1066) 5-5-5-15 at 2.1V, which is outside JEDEC standard...

    The CPU is usually stepped down from 12v. The 5v or 3.3v rail is stepped down to provide the voltage for RAM, so you've got individual motherboard MOSFET or stepdown efficiency to consider for each of these (sets of) components.

    Then you've got JEDEC spec which says DDR2 shall use 4.4W per stick, which means if you're using 1.8V or 2.1V, and you still have to run a 4.4W, then you're now varying the amperage....

    And, now everything is wonky, do you really want to drop or raise amperage, or do you need to drop/raise wattage and keep amperage the same, &c.

    Different CPU's may be off the same rail, but take more maximum amps - I.e. Q99xx takes up to 200W, 125A max, wheras a phenom 9700 has a maximum amperage draw of about 115, (accounting for 80% efficiency stepdown for the intel CPU, but not for the AMD.)

    These are very different numbers.

    Write a guide on the specs, the connectors, and the how to do the math. It will be much more effective. Otherwise, you're going to end up with 8 different numbers for different models of Corsair RAM in a specific class line (like Dominator) and your guide is going to get out of date _so_ fast.

    Because a different revision of the same model number device may behave VERY differently and have VERY different power requirements.


    And yes, I buy a PSU anticipating it should be usable for at least five years.
    Last edited by treshix; 03-21-2008 at 03:53 AM. Reason: Data correction!

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