Sweet!
http://www.solarbotics.net/library/circuits/misc_switching.html
is quite brilliant...
only concern is is 3.3 volt enough gate voltage for the FET (2n7000)?

I wonder if using a transistor (advantage: always available) would it, when
saturated to low colector emiter voltage, conduct current both directions:
charge and discharge?



On Fri, Dec 4, 2009 at 8:57 AM, Michael Rigby-Jones <
Michael.Rigby-Jones@oclaro.com> wrote:

> Reposted with correct tag!
>
> > -----Original Message-----
> > From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
> Behalf
> > Of Tobias Gogolin
> > Sent: 03 December 2009 03:26
> > To: Microcontroller discussion list - Public.
> > Subject: Lower voltage drop than diode?
> >
> > Hi Folks!
> > I have some solar cells that almost appear to be made to charge Lion
> > batteries from discarded cell phones! I use these battery solar cell
> > combos
> > to rig little LED flashlights!
> > If I connect the PV directly charging the battery I get almost 40 mA
> > charging current, however of course in shade or darkness it discharges
> the
> > battery, so i always use a diode!
> > However that bit of voltage drop is critical enough to then only get
> 12mA
> > charge current!
> > I have been thinking about using a transistor voltage drop (<.3V) or
> maybe
> > a
> > FET, but I'm not sure I am imagining correctly that the circits I'm
> > thinking
> > off would indeed prevent reverse currents!
> >
> > So if anybody could point me to some well established circuits for
> this
> > purpose I'd be very grateful!
>
> This is a fairly simple (and quite cunning IMO) circuit that does what
> you want:
>
> http://www.solarbotics.net/library/circuits/misc_switching.html
>
> Note that there will be a small quiescent current drawn from the battery
> (mainly through R3), the value of which will depend on your battery
> voltage which you don't seem to have stated.  At 4.2v (fully charge
> lithium polymer voltage) LTSpice says it will be of the order of 45uA
> with the component values shown.  You can probably reduce this somewhat
> by increasing the value R3 since the circuit is designed for lower
> voltages.
>
> Regards
>
> Mike
>
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-- 
Tobias Gogolin
Tel. Movistar (646) 124 32 82
Tel. Telcel (646) 160 58 99
skype: moontogo
messenger: usertogo@hotmail.com

You develop Sustainable Ranch Technology at
http://tech.groups.yahoo.com/group/SURA-TECH
an Open Source Electric Motor/Alternator at
http://groups.yahoo.com/group/Performance_Axial_Flux
and an Open Source Motor Controller at http://groups.yahoo.com/group/GoBox
-- 
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View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist