Thread: Solution: N900 Charging from dumb power sources *UPDATED*
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shadowjk is offline shadowjk
2010-01-04 , 08:29
Posts: 1,258 | Thanked: 672 times | Joined on Mar 2009
#59
Originally Posted by jeffsf View Post
As an EE, I find that hard to swallow.

As long as, within reason, the proper voltage is being supplied to the USB port, then it is up to the charging circuit to determine how much current it will draw. If you hook a 5V supply to a 25 ohm load, you'll consume 1 Watt from any supply can source 200 mA or more.
The USB standard allows devices to draw 100mA. A full 500mA can be provided if the device negotiates this with the connected host. The host's usb ports should be overcurrent protected and shut down the power if the device consumes too much power.

This is why you can't just use the logical method of draw power until voltage drops.

A new charging profile was added to the usb standard, which includes signaling for letting the device know when it can behave as if it was connected to a charger. For cheap/dumb chargers, the simple signaling of d+ and d- shorted together is used.

The N900 complies with the USB charging standard, but the problems come from everyone using random nonstandardized charging schemes before the USB charging standard was finalized.

It's worth noting that there's no usb charging over miniusb, for example..
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That the N900 allows "shorted data" USB supplies to bypass the handshake contraindicates the hypothesis that this is somehow supposed to "protect the charger."
The usb charging supplement is available to the public. Might contain some clues to the design decisions made

I find it reasonable that Nokia is worried about the case where
I hooked my N900 to my laptop and it fried my $3000 laptop.
iirc there already was a case where someone hooked it up to a freerunner and nearly fried it. A prototype n900 I think. Sadly, in practice it seems many computers lack effective overcurrent protection on USB and will happily shove 20 amps over it until something fries and voltage drops from 5 to 0 in one go

In neither case, if the charger had the data pins shorted, would the apparent N900 behavior, have saved the charger or the N900.
I think the datapins shorted together decision might have been to make the most unambigous way of distinguishing between a computer's usb power and a charger, some computers give power on usb when off/suspended, but have nothing on the datapins.

To me it looks like they (the usb standards body) picked the most reliable and inexpensiv way of distinguishing between a real usb host and a charger.
Last edited by shadowjk; 2010-01-04 at 08:43. Reason: qupte/snip fail
 

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