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Posted by Barry Watzman on August 29, 2007, 11:38 pm
Please log in for more thread options Re: "It needs a port that is capable of providing the full 500mA. Any
USB port on the computer will be fine as each USB port can provide a
minimum of 500mA."
That's not always true on laptops, unfortunately.
SMS wrote:
> Al Gillis wrote:
>> Family pressures were brought to bear on me yesterday and I had to go
>> to Costco. So, to do my part to keep America and China strong I
>> bought a SanDisk 2 gigabyte flash drive.
>>
>> The specs on this gizmo claim to need a "high power USB hub port". So
>> I looked briefly at Dell dot com but couldn't learn if my Precision
>> M-60 or my Inspiron 1705 has such a port.
>>
>> Can you please help me? Point me to a reference on the Dell site or
>> on the USB.org site? Or explain that the wording "hub port" means I
>> need an external hub to run this gizmo? (I'll go look at sandisk.com
>> as well).
>>
>> Thanks in advance!
>
> What you probably can't do is plug it into an unpowered hub
> when there are other devices on the hub that also require quite a bit of
> power. There are also some PDAs which can be a USB host that probably
> can't source 500mA.
>
> Some HP machines have a special USB connector that can provide 1.5A and
> it's used for external CD-ROM/DVD drives. You can see a photo of one at
> "http://nordicgroup.us/chargers/" click on "Explanation of USB Current
> Limits".
>
> While the USB specification guarantees only 500mA per USB port, this is
> a minimum, the port doesn't shut down at 501ma! In most cases a USB port
> will supply 750-1000mA before the over-current protection circuit shuts
> it down, especially if not all USB ports on the system are used (there
> are no guarantees, so don't run out and design a device that draws
> 1000mA). For example, the National Semiconductor LM3544, a widely used
> over-current protector, limits the current per port to 1.0A (typical)
> even though the total current limit for four ports is 2.0 amps (to
> protect the power supply). What this means is that if you don't use all
> the USB ports, the ones you do use will be able to supply more than
> 500mA of current. This is why external notebook hard disk drives will
> usually work without using the external power adapter, or Y adapter (two
> USB ports power the device), even though they draw more than 500mA.
> Obviously you shouldn't try to connect four external DVD±R/W drives to
> one computer, without using the external power supplies for the drives.
> While you shouldn't design a USB peripheral that exceeds the guaranteed
> 500mA, the reality is that the limit is often exceeded without any
> consequences.
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