[maemo-users] [maemo-users] Re: Charge from a USB port.

From: Larry Battraw lbattraw at gmail.com
Date: Thu Feb 1 21:57:28 EET 2007
On 2/1/07, Paul Klapperich <maemo.org at bobpaul.org> wrote:
> Why would it need a current limiter? V=I*R. If the voltage is right, the
> current is completely dependent on the equivalent resistance of the device.
> If the device is designed to pull 500mA and I give it a supply capable of
> 1A, it will only pull 500mA. If I give it a supply capable of only 250mA,
> though, then it won't pull 500mA and the voltage will dip as well. Power
> supplies don't supply current, they supply voltage and have a max current
> possible.

  The equivalent resistance of the device is very low-- it relies on
the power supply to limit current flow.  It's essentially a direct
connection to the 3.7V Li-ion battery, cycled on and off.  I tried
providing a regulated (and high-current-capable) 5V power to the 770
and the less resistance I provided in series with the power line the
more power it drew, up to a point where it started glitching and
charging irregularly, and then finally refused to charge at all.  I
hooked a scope and DVM inline with the power supply Nokia provides and
what happens is the 770 will draw large amounts of power in pulses,
dragging the voltage fed in down to around 4.2 - 4.5V.  If you have a
solid, high-current supply you could potentially destroy the 770's
charging circuitry if you didn't limit the current somehow.

> For example, this guy has a power injector he's used for USB Host mode that
> simply uses a 7805 regulator (pumps up to 1A) and a 9V battery. USB is
> supposed to have a current limiting power source controlling the power sent
> to a device based on the devices requests. That 7805 doesn't do this and
> simply follows ohms law above, yet it doesn't tend to break things because
> it keeps the 5V steady enough.

 I'm not sure how this is relevant since this power injector is meant
to provide a tiny amount of current at 5V, just enough to tell the 770
something is plugged in.  It doesn't actually power the 770.

> (Link:
> http://thoughtfix.blogspot.com/2006/01/usb-power-injector-2.html) This is
> also the same method used by a whole slew of home made ipod and phone
> chargers.

   All the phone chargers I've seen have a significant internal
resistance-- they are _not_ designed the same way as devices that
charge directly from USB.  All the USB phone chargers I've seen have a
inline adapter of some sort (including Nokia's new USB charger) to
boost the voltage and limit the current.  As mentioned, Thoughtfix's
injector tells the 770 it has a USB device plugged in and does not
power the 770.

> Simon's solution sounds like what I would do.

  Simon's solution scares me :-)  I messed around enough to see how
easy it is to start feeding far too much current in very quickly.  USB
ports are typically current-limited, but it still concerns me.


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