Serial interfaces (was Re: Any former Psion 5 owners out there?)

[Eric Smith]

Tony Duell wrote:
> The most stupid one I saw was a coffee cup warmer. Stupid because IIRC
> the maximum current a USB device can draw is 1A (after negotiation with
> the host). And the supply is 5V./ A 5W heater is not going to do a lot in
> keeping coffee warm.
>    

It's worse than that.  USB through 2.x only gives you 500mA maximum.  
Anything beyond that is out of spec, and you might or might not trip an 
overcurrent shutdown on the port, depending on the USB host circuit 
design.  Even if the port voltage is on the high side, the most  you can 
count on getting is 2.75W.

USB 3 ups the max current to 0.9A, so you might be able to get nearly 
5W, which is still, as you say, not really going to keep coffee warm.

Eric