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. Regards Simon 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 >>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. > >Larry >_______________________________________________ >maemo-users mailing list >maemo-users at maemo.org >https://maemo.org/mailman/listinfo/maemo-users > > > >-- >No virus found in this incoming message. >Checked by AVG Free Edition. >Version: 7.5.432 / Virus Database: 268.17.18/662 - Release Date: >31/01/2007 15:16 -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.432 / Virus Database: 268.17.27/671 - Release Date: 05/02/2007 16:48
