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Thread: why is +5vSB so low considering the # of USB devices people use?

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    Question why is +5vSB so low considering the # of USB devices people use?

    Hello,

    I'm shopping for a new psu and just discovered that USB devices draw power from the +5vSB connection.
    Frankly, I'm kind of surprised that there aren't more problems as a result of this! I don't need a psu for gaming so I have no reason to spend more than say $65 on a decent psu. In fact, one of the main reason I was shopping for a nicer psu in the first place is so that I wouldn't have to worry about plugging as many usb devices as needed.
    If I spend $80 I can get one with 3 amps on the +5vSB, otherwise it's going to be 2.5 amps. This just seems extremely low safety margin considering the proliferation of usb devices that people use, especially with high performance external drives and enclosures (why would anyone use a 5400rpm external drive anyway unless they are broke?).

    I also read that older mobo's used to have a jumper to divert +5vSB power to the +5v connection...why the hell would this feature be discontinued now, shouldn't it be more common with all the usb devices people use?

    Is the easiest solution to just buy a usb hub with external power? I certainly don't want another external thing around the desktop but like I said, I'm tired of worrying about whether or not my "modern" computer can actually handle the number of usb ports the *@#!!% mobo came with.

    I'd appreciate any feedback on this subject...sorry it's a bit of a rant but I was set to buy a psu & then made this discovery :-(

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    5vSB is the 5v Stand By rail; it's only used when the PSU is off.

    Normally the, while the computer is on, USB devices draw power from the normal 5v rail.

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    Quote Originally Posted by Tator Tot View Post
    5vSB is the 5v Stand By rail; it's only used when the PSU is off.

    Normally the, while the computer is on, USB devices draw power from the normal 5v rail.
    Hey thanks for getting back to me so quickly. But I thought I specifically read that USB used 5vSB all the time, in fact I thought it was Jonny that said this? Sorry I don't have a link, but I hope you are right!

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    Most boards are using the 5v Rail for power for the USB Ports; Gigabyte does this specifically (which is part of their 3:3:3 Marketing.)

    Most of the time, the easiest test of this is trying to charge two devices at once. Like an iPhone or iPad and a PSP, etc.

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    The maximum amount of power supported by a USB port is 500mA, or 0.5A. Most USB ports actually support less, around 250-300mA--motherboard USB ports are the only ones, generally, that can provide the full 500mA. Most devices draw 30-250mA. The only device I've ever found that can't run on a 250mA port was a laptop cooler; my laptop has two ~250mA ports and one 500mA port, and the fan doesn't spin as fast as it should on the lower amperage ports.

    What I'm trying to say is, it's not an issue. Mouse, keyboard, flash drives, USB hard drive (w/ indy power plug), mic, headphones, wireless NIC, etc etc etc... You could have twenty such devices and not have any issues with a 2.5A +5VSB rail. If you really need more, get a powered USB hub.
    It's my PSU in a box!
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    Quote Originally Posted by Phaedrus2129 View Post
    The maximum amount of power supported by a USB port is 500mA, or 0.5A. Most USB ports actually support less, around 250-300mA--motherboard USB ports are the only ones, generally, that can provide the full 500mA. Most devices draw 30-250mA. The only device I've ever found that can't run on a 250mA port was a laptop cooler; my laptop has two ~250mA ports and one 500mA port, and the fan doesn't spin as fast as it should on the lower amperage ports.

    What I'm trying to say is, it's not an issue. Mouse, keyboard, flash drives, USB hard drive (w/ indy power plug), mic, headphones, wireless NIC, etc etc etc... You could have twenty such devices and not have any issues with a 2.5A +5VSB rail. If you really need more, get a powered USB hub.
    OK thank you, my confusion comes from the fact that my external power supplies I use for hdd enclosures (on my laptop) are rated as 2 amps each. I still don't understand completely for that reason, but you sound like you know what you are talking about.
    Most boards are using the 5v Rail for power for the USB Ports; Gigabyte does this specifically (which is part of their 3:3:3 Marketing.)
    thanks tater, the mobo is a 2 yr. old gigabyte so that's good news regardless.

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    If it's not a Laptop drive, that's why.
    3.5" Drives consume more power than a 2.5" drive will. Some 3.5" drives say on them what the power draw (in amps) is on the 5v and 12v rails.

    Also, what Gigabyte board is it?

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    Quote Originally Posted by Tator Tot View Post
    If it's not a Laptop drive, that's why.
    3.5" Drives consume more power than a 2.5" drive will. Some 3.5" drives say on them what the power draw (in amps) is on the 5v and 12v rails.
    Gotcha, however Scorpio Black WD3200BEKT 2.5" supposedly requires 1amp
    Quote Originally Posted by Tator Tot View Post
    Also, what Gigabyte board is it?
    GA-MA785GPMT-UD2H

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    The external power supplies for HDDs mean that the 2A ISN'T being pulled from your +5VSB. The +5VSB provides ~200-300mA for the HDD's logic circuits, while the external PSU provides +12V current for the HDD motor, and possibly additional +5V current for the read/write head.
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    Quote Originally Posted by Phaedrus2129 View Post
    The external power supplies for HDDs mean that the 2A ISN'T being pulled from your +5VSB. The +5VSB provides ~200-300mA for the HDD's logic circuits, while the external PSU provides +12V current for the HDD motor, and possibly additional +5V current for the read/write head.
    I know that using the external power supply means power won't be pulled from 5vSB. I mentioned that to say what it seems it would require if it was using usb for power.

    The power requirements manufacturers give for hdd's is the main reason I thought +5vSB doesn't provide a lot of power. If you simultaneously use 3 external drives that are using usb ports to supply power, and each requires 1amp, seems like it's too much total. There are other usb devices being used e.g. mouse, keyboard, card reader, but external drives are my main reason for concern.
    Sorry if I'm still misunderstanding something...

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