--- William Chops Westfield <westfw@mac.com> wrote:

> 
> On Sep 21, 2004, at 11:15 AM, Marcel Duchamp wrote:
> 
> >> I would like to drive a white LED for as long as possible at a
> roughly
> >> constant current from the stored charge in a goldcap or other
> >> large-value capacitor, eg 1F/5.5V.  It's for a one-off personal 
> >> project.
> >
> > Check this page out:
> > http://www.emanator.demon.co.uk/bigclive/joule.htm
> >
> The Joule thief circuit is neat, and runs down to a pretty low
> voltage,
> but I don't think it is very efficient...  Being efficient over the
> range you're talking about is pretty tough...  Boost part of the
> time,
> buck part of the time :-(  I wouldn't expect that you could easily do
> much better than the white-LED driver chips that have been coming
> out,
> and not very many of them have spectacular efficiencies...

I would look at SEPIC convertors. I believe many are good down to 2.5V.
National semi has a few and Linear Tech does too (I think).  He might
need to make his own from discrete though.

Cyb


		
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