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

From: Simon Moore simon.moore at ndirect.co.uk
Date: Tue Feb 6 17:14:37 EET 2007
The nokia charger states what the current is on it that it provides 
which is around the current left over after everything else is power 
from the charger I selected.  The nokia appears to have an 
intelligent charger it it which cycles on and off dependent on need, 
not I think the supplied current.


At 19:57 01/02/2007, Larry Battraw wrote:

>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
>  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.
>>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
>   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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