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[OM] Re: OT - the latest ? USB plug ins

Subject: [OM] Re: OT - the latest ? USB plug ins
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
Date: Thu, 13 Jul 2006 13:43:24 -0400
Since the max you can get out of a USB port is 500ma at 5 volts (and 
that may be questionable too) you've only got 2.5 watts to heat your 
coffee.  Prepare to wait a long time for that hot cup of java.

Here's data on power from the USB spec on Wikipedia.  If you read 
nothing else be sure to read the last paragraph on non-standard uage
-----------------------------------------------------

Power supply

Standard

The USB specification provides a single 5 volt wire from which connected 
USB devices may power themselves. The specification allows no more than 
5.25 V and no less than 4.375 V.

Initially a device is only allowed to draw 100ma, additional units of 
100ma may be requested from the upstream device in units of 100ma up to 
a maximum of 500ma (in practice most ports will deliver the full 500ma 
or more before shutting down power even if the device hasn't requested 
it or even identified itself as a valid usb device). If a (compliant) 
device requires more power than is availible then it cannot operate 
until the user changes the network (either by rearranging USB 
connections or by adding external power) to supply the power required.

If a bus powered hub is used then the devices downstream may only use a 
total of 4 units of power (this limits compliant bus powered hubs to 4 
port among other things). Equipment requiring more than 500ma, hubs with 
more than 4 ports and hubs with downstream devices using more than 4 
100ma units total must provide thier own power. The host operating 
system typically keeps track of the power requirements of the USB 
network and may warn the computer's operator when a given segment 
requires more power than is available.

Non-standard

A number of devices use this power supply without participating in a 
proper USB network. The typical example is a USB-powered reading light, 
but fans, battery chargers (particularly for mobile telephones) and even 
miniature vacuum cleaners are available. In most cases, these items 
contain no electronic circuitry, and thus are not proper USB devices at 
all. This can cause problems with some computers—the USB specification 
requires that devices connect in a low-power mode (100 mA maximum) and 
state how much current they need, before switching, with the host's 
permission, into high-power mode.

Some USB devices draw more power than is permitted by the specification 
for a single port. This is a common requirement of external hard and 
optical disc drives and other devices with motors or lamps. Such devices 
can be used with an external power supply of adequate rating; some 
external hubs may, in practice, supply sufficient power. For portable 
devices where external power is not available, but not more than 1 A is 
required at 5 V, devices may have connectors to allow the use of two USB 
cables, doubling available power but reducing the number of USB ports 
available to other devices. Amongst others, a number of peripherals for 
IBM laptops (now made by Lenovo) are designed to use dual USB connections.

USB-powered devices attempting to draw large currents without requesting 
the power will not work with certain USB controllers, and will either 
disrupt other devices on the bus or fail to work themselves (or both). 
Those problems with the abuse of the USB power supply have inspired a 
number of April Fool hoaxes, like the introduction of a USB-powered 
George Foreman iGrill [2] and a desktop USB Fondue Set [3].
----------------------------------------------------------

Chuck Norcutt





John Hudson wrote:

> While going through the check out of a local Canadian Tire store [a
> country wide chain of automotive / handyman stores] I was interested
> to see two little gizzmos that would help ease the druggery of
> working at one's computer screen:
> 
> - a small, cigarette packet sized, coffee cup warmer that plugged
> into a computer's USB port, and
> 
> - a small, Mars Bar [candy bar] sized ozone generator / air freshener
> device that plugged into a computer's USB port
> 
> Neither device was wired for any other input source and cost about
> $10 each.
> 
> What else is a USB port good for ?
> 
> jh
> 
> 
> 
> 
> ============================================== List usage info:
> http://www.zuikoholic.com List nannies:
> olympusadmin@xxxxxxxxxx 
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> 
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