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Thread: please define the concept of a "rail" (and misc. q's)

  1. #1
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    misc. questions relating to rails and PSU's

    i thought this all made sense, until i was reading on how to tell how much power you're going to need, where it said something about power consumption (with links to sites that will tell you how big of a PSU you'll need (ones I can't find now of course) .....


    i think i kinda get the concept.... (at least I hope i do)

    the 12V rail is the ATX connector? and the 5V rail is some extra connector that i guess should have some molex connectors on it? and the 1V rail is for some kind of LED's (like the status and HDD LED's on my computer now)?

    and if this is correct, you probably should run like the mobo and whatever USB stuff that's plugged in to it on the 12V rail, then use the 5V rail (with molex connectors somehow) to power the HDD and the dvd-rom drive? so in that way it splits the power consumption more evenly (cuz apparently the 12V rail prolly can't handle mobo, processor, HDD, lilliput, USB devices, DVD-rom, ect. all at once)

    another thing is, is there a repository for information on power consumption for different stuff...

    each USB device = ?
    lilliput 6w (max) (http://www.carnetix.com/additional_i...llyCurrent.htm)
    via M10000: 30W (VIA is no longer king)
    HDD(2.5): ????W
    DVD-ROM: ????W

    if we can help that'd be awesome.

  2. #2
    Raw Wave Rob Withey's Avatar
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    FAQ: Power Supplies Explained (Part 1)
    Post 6 should answer the rail question.

    See, the answer was here all along.
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  3. #3
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    that post was far from a definitive answer.... and i did read that section but it missed what I was asking....i'm aware of the hypothetical setup of the rail.... but now i'm looking at the instruction manual for the M2-ATX

    http://www.digitalww.com/M2-ATX-manual.pdf

    and trying to play "find the rail"

    J5 (to LED).. that's the 1V rail? (maybe it's the 3.3V rail?)
    J7 (atx power connector)... that's the 5V rail?
    J2 (optional P4 - 12V power)... that's the 12v rail?

    or does the PSU regulate the 5V and 3.3V and such and just output it to the correct pin in the ATX power connector? if THAT's the case, then it's impossible to choose what runs off what rail? i wanted to run the mobo and USB(GPS unit + wifi) off the 12V rail, and the HDD and DVD-ROM (and lilliput)off the 5V rail.....

    (maybe) anything that runs on a molex plug will run on the 5V rail and anything plugged into the USB port will run on the 12V rail along with the mobo itself and processor.... and that 3.3V... still dunno what that does... and anyway, if that's the case, where on the PSU does the 5V line come from? the picture on digitalww shows the molex connectors by themselves, and the PSU that supplies my regular computer has it coming out of the power supply... but i don't see anything on that m2-atx that molex connectors go to....


    btw, what part of your PC runs on 3.3V?

  4. #4
    Raw Wave Rob Withey's Avatar
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    Rail carries a single voltage. Connectors can not be equated to rails. ATX connector carries many rails (3.3v, 5v, 12v). Several pins in the connector may be the same voltage (rail) to spread the current load across the pins.

    You don't get to choose what runs on each rail the way you want to - devices take from each rail what thye need. Different types of devices use different rails (so you can kind of spread the load to a degree by choosing different products). Fro example desktop hard drives take 12v and 5v. Laptop hard drives take 5v.

    As to what runs off 3.3v, that depends on what motherboard, processor and devices you have. Typically processor and memory take from the 3.3v rail.

    And I have never seen a 1v rail in a PC power supply - where on earth did you get that from?
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  5. #5
    Neither darque nor pervert DarquePervert's Avatar
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    Actually, RAM and the PCI bus pull 3.3v.
    Sometimes other motherboard components do, as well, like the Northbridge.

    CPU's pull from 5v and/or 12v rails.
    Have you looked in the FAQ yet?
    How about the Wiki?



    Under normal circumstances, a signature would go here.

  6. #6
    FLAC is for flaccid parksgm's Avatar
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    I think the original poster is just confused by terms that are commonly bantered about but not often defined:

    "Molex": "Molex" is a company name/trademark name. Their website is here. Because plastic connectors of that type are so widely used, "Molex" has come to mean "a plastic wire connector" in the same way that "Kleenex" has come to mean " a tissue" (or how "asprin" has come to mean "acetosalicylic acid", which is the chemical name of the drug).

    ATX connector: A 20 pin molex (see above) connector. It is the largest plug from the power supply to the motherboard and the 20 wires it connects supply the vast majority of the power the motherboard/processor uses. Each pin/wire on the ATX connector supplies electricity at one of three voltages: 12V, 5V or 3.3V. Several of the wires in the ATX connector supply 12V, several supply 5V, several supply 3.3V, and several are "ground" wires that supply zero volts. No other voltages are supplied or carried by the ATX connector. The ATX connector has more than one wire supplying the same voltage because one wire of the size used in the ATX connector is not large enough to supply all of the 12V current from the power supply to the motherboard (one wire would overheat and melt).

    Think about it this way: if you used lamp cord to connect your house to your power company's utility pole, and then turned on your oven, the small lamp cord would quickly overheat and melt because it is not capable of carring the amount of current your oven requires. One solution would be to use a much larger cable to connect your house to the utility pole. Another solution would be to use tens or hundreds of lamp cords to make the connection. Combined, those multiple wires would be able to carry the current required by your oven. The analogy applies directly to your PC power supply and motherboard. Instead of using just four large cables to carry the current required by your motherboard, the ATX connector uses multiple, smaller, wires.

    P4 connector: This is a smaller, squarish, 4 pin/wire molex connector that supplies EXTRA power to run motherboards with Pentium 4 processors(specifically) because the ATX connector (even with all 20 wires) cannot supply enough. If you don't have a Pentium 4 processor, this connector is likely not used.

    J1, J2, J3, etc.: "J" stands for "jumper pin". These are pins on the power supply or motherboard that are used to enable/disable optional features on the motherboard/power supply by connecting them together with a short, metal or plastic and metal part called a jumper that is specifically made to bridge/connect two adjacent jumper pins. On the M2-ATX, for example, the time the computer remains on after the car's ignition is switched off is determined by changing which jumper pins are connected to each other. These pins DO NOT supply power to run the motherboard or the computer.

  7. #7
    FLAC is for flaccid parksgm's Avatar
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    Now, to answer your specific questions (and appreciate my nice HTML list work, too!):
    Quote Originally Posted by CTdubbin View Post
    1. the 12V rail is the ATX connector?
    2. and the 5V rail is some extra connector that i guess should have some molex connectors on it?
    3. and the 1V rail is for some kind of LED's (like the status and HDD LED's on my computer now)?
    4. you probably should run like the mobo and whatever USB stuff that's plugged in to it on the 12V rail, then use the 5V rail (with molex connectors somehow) to power the HDD and the dvd-rom drive?
    5. so in that way it splits the power consumption more evenly (cuz apparently the 12V rail prolly can't handle mobo, processor, HDD, lilliput, USB devices, DVD-rom, ect. all at once)
    6. another thing is, is there a repository for information on power consumption for different stuff?
    1. The ATX connector has three rails, 12V, 5V, 3.3V, and ground (0 volts).
    2. No. See above. The 5V rail is supplied by several wires in the ATX connector.
    3. There is no 1V rail.
    4. The connector that connects the HDD, CD/DVD drive, and other peripherals to the power supply has four wires: 12V, 5V, and two ground wires. Each device uses the power it needs from the connector. All you have to do is plug it in.
    5. The motherboard, processor, and other devices require different voltages, so they must be supplied with different voltages to work correctly.
    6. Yes. There are great posts by users "bugbyte" and "darquepervert" that have lots of good information. Do an advanced search, and search for all posts by "bugbute" with the word "power" (for example).

  8. #8
    High Voltage blk02si's Avatar
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    You kind of have the term 'rail' misunderstood. These are the output circuits of the power supply. Rails has nothing to do with the type of connectors. In fact the 'Molex' connector has 2 different rails going to it. The yellow wires are on the 12volt rail and the Red wires are on the 5 volt rail.you can test any of the wires with a multimeter against a ground to see what voltage its putting out and thus which rail its on. The 3.3 volt rail is used for the MoBo and USB type devices that connect directly to the MoBo. Again 'Rails" has nothing to do with the physical layout of the connectors coming out of the power supply.

    When you hear of overloading a rail this means that too many devices are drawing from one rail. Which is why some people have enough watts in theory total but their PS still does not work. The most common overloaded rail is the 12v rail.

    BTW a m2-ATX PSU will run all the hardware you listed fine. I've loaded up a M2 with even more than that and it ran good.
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  9. #9
    FLAC is for flaccid parksgm's Avatar
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    Quote Originally Posted by blk02si View Post
    . The 3.3 volt rail is used for the MoBo and USB type devices that connect directly to the MoBo.
    That's not entirely correct. From here:
    Voltage:

    Supplied voltage by a host or a powered hub ports is between 4.75 V and 5.25 V.
    Maximum voltage drop for bus-powered hubs is 0.35 V from it's host or hub to the hubs output port.
    Normal operational voltage for functions is minimum 4.75 V.

    Power usage:
    Bus-powered hubs: Draw Max 100 mA at power up and 500 mA normally.
    Self-powered hubs: Draw Max 100 mA, must supply 500 mA to each port.
    Low power, bus-powered functions: Draw Max 100 mA.
    High power, bus-powered functions: Self-powered hubs: Draw Max 100 mA, must supply 500 mA to each port.
    Self-powered functions: Draw Max 100 mA.
    Suspended device: Max 0.5 mA
    SUPER DANGEROUS ADVICE BELOW!

    Quote Originally Posted by blk02si View Post
    BTW a m2-ATX PSU will run all the hardware you listed fine. I've loaded up a M2 with even more than that and it ran good.
    "Loading up" an M2 (in particular) is a really bad idea, as it has very little, if any, overvoltage/overcurrent protection, and frequently destroys the motherboard, CPU, and other connected devices when it fails. A quick search of "M2-ATX" will reveal the multiple users here who have had that happen (including myself). If you value your equipment, DO NOT overload your M2.

  10. #10
    High Voltage blk02si's Avatar
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    Quote Originally Posted by parksgm View Post
    "Loading up" an M2 (in particular) is a really bad idea, as it has very little, if any, overvoltage/overcurrent protection, and frequently destroys the motherboard, CPU, and other connected devices when it fails. A quick search of "M2-ATX" will reveal the multiple users here who have had that happen (including myself). If you value your equipment, DO NOT overload your M2.
    Well another idiotic post. No wonder people are confused. Its funny to me how the people who fry their power supplies are the one who think they know the most about them. There is overload protection on the M2, its called a FUSE buddy, and its rated at 15 amps. The M2 is a very good, very reliable PSU. I have 2 of them in 2 different setups, both over 1 year old. Neither have ever failed me or caused other componets to fail. One setup has a full ATX dell P4 running off it. I did the MATH on it and it is within acceptable draw for each rail. Then again I do hook up my componets the RIGHT way. The hardware CT mentioned is fine for an M2 to run.

    So CT, the starter of this thread, you can believe who you want here. The guy who fries his PSU's and then gives advice about them, or someone who has 6 years of schooling on the subject, and is offering insight on your question.
    Build Things, it keeps your brain busy.

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