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
>
>
>
>
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