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. 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. (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. Simon's solution sounds like what I would do. -- Paul On 2/1/07, Paul Klapperich <bobpaul at gmail.com> 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. > > 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. (Link: > http://thoughtfix.blogspot.com/2006/01/usb-power-injector-2.html) > > Simon's solution sounds like what I would do. > > -- Paul > > On 2/1/07, Larry Battraw <lbattraw at gmail.com> wrote: > > > > On 2/1/07, Simon Moore <simon.moore at ndirect.co.uk> wrote: > > > No we just chopped the end off the nokia charger and soldered to a > > > USB lead. I was unable to source the correct size power plug from > > > Maplin or anywhere else it is smaller than the smallest hence the > > chopping > > > off. > > > > > > Regards > > > Simon > > > > Does this actually work? The 770/n800 need current-limited power, > > and I would think the direct USB connection would feed it too much > > current. Too much current and the 770 will refuse to charge or > > possibly damage it. I bought a USB phone charger adapter and soldered > > a nokia phone plug onto it. The USB phone adapter boosts the voltage > > to 6 volts and limits the current as well. It looks very similar to > > the Nokia one except it has a female USB plug on the other side (to > > plug phone adapter plugs into). Works pretty well but doesn't seem to > > charge as fast as the standard AC adapter the 770/n800 ships with. > > > > Larry > > _______________________________________________ > > maemo-users mailing list > > maemo-users at maemo.org > > https://maemo.org/mailman/listinfo/maemo-users > > > > -------------- next part -------------- An HTML attachment was scrubbed... URL: http://lists.maemo.org/pipermail/maemo-users/attachments/20070201/897e3488/attachment.htm
