Stephen R Phillips escreveu:
> Next time I suggest you use your web browser and do a bit of research fir=
st.. http://compliance.usb.org/index.asp?UpdateFile=3DBattery%20Charging&Fo=
rmat=3DStandard
>
> What you needed to look for on the web was "USB-IF charging spec" to find=
 it.  It took me about 20 minutes to find it because I had to search using =
the right question. There are also numerous posts in their forums under htt=
ps://www.usb.org/phpbb/viewforum.php?f=3D1 . I might be USB clueless but I =
found out where to look, with a little foot work. I knew about usb.org so I=
 searched using there internal search. From there (me being lazy) I used th=
e search "USB-IF charging spec" after reading a few posts (which used that =
phrase) and found the above link.  =

>
> Essentially it says you need to do certain things to allow charging from =
the USB port. You should always use what the USB.org says for compliance wi=
th the charging spec.. That means you may not draw more than 5 units (0.5A)=
 from the USB port OR that port will be turned off automatically. The USB c=
able wire can't handle that much current even though it looks like USB 3 wi=
ll have 'bigger' specs. I've noticed lots of cheats (the parallel plug tric=
k). I personally think that is likely to get there device USB compliance re=
voked eventually.  USB is a complicated bus, so I suggest you be sure your =
charger draws exactly what it needs to (< 0.5A) and never more.
>
> The power is current limited through a sense FET device (I know this beca=
use I've been looking at power switches and they all said USB that could ha=
ndle .5A hmmm suspicious huh?) In any case these will shut off power if you=
 exceed 0.7A, add in line drop you really can't draw more than 0.5A from th=
e bus without the power going bad on you.
>
> Stephen
>   =



In a perfect world... perhaps.

In reality, most add-on USB boards in the market don't care having
active current limiting circuits and none I can remember have overload
detection. What most use is a poly-fuses, but I have seen some without
any protection at all.

Desktop motherboard native USB ports have poly-fuses and some do have
active current limiting.

My Toshiba laptop do have active current limiting and overload
detection, and it works very well.

Bus-powered USB hubs I have seen have no protection at all. Some
self-powered hubs use poly-fuses and others use active circuits.

Isaac

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