Re: LED Driver delivers constant luminosity

From: Tam/WB2TT (t-tammaru_at_c0mca$t.net)
Date: 06/18/04 

Date: Thu, 17 Jun 2004 20:24:35 -0400

"R.Lewis" <h.lewis-not this bit-@connect-2.co.uk> wrote in message
news:casqkc$em2$1@thorium.cix.co.uk...
>
> "Ian Stirling" <root@mauve.demon.co.uk> wrote in message
> news:40d0961e$0$527$ed2619ec@ptn-nntp-reader01.plus.net...
> > Alt.electronics deleted, it's a bogus group, and my server won't let me
> > post to it.
> >
> > Activ8 <reply2group@ndbbm.net> wrote:
> > > On Wed, 16 Jun 2004 21:47:09 +1200, Simon Dyer wrote:
> > >
> > >> I very much like the circuit by Israel Schleicher in EDN Jun 12 2003
> for its
> > >> ability to drive a 3.6 volt Superbright LED off a 1.2 to 1.5 volt
cell
> with
> > >> very minimal and inexpensive components. Since it pulses the LED,
> greater
> > >> perceived brightness can be acheived at great efficiency. See
> > >>
>
http://www.reed-electronics.com/ednmag/article/CA302239?pubdate=6%2F12%2F200
> > >> 3&spacedesc=designideas
> > >>
> > >> According to the formula, changes to R, L or C could thoretically
> provide
> > >> more current to the bright white LED - I am looking for 30ma average,
> but in
> > >> practice I have not been able to acheive this.
> > >>
> > >> Can anyone tell me where I am going wrong? I would be happy to use
> three
> > >> cells if it helped.
> > >
> > > By believing that pulsing an LED produces greater perceived
> > > brightness.
> >
> > It can, but only if the flicker can be percieved.
> > (this may be at a slightly higher frequency than (say) 100Hz, as with
> > short pulses, the eye when scanning will tend to see spots at the
> > peak brightness, rather than streaks at the average brightness, which
> > may be less visible.
>
> Every few weeks it is proposed that modulating a led gives an increased
> perception of 'brightness' due to some magical property of the eye.
> Whilst there is some evidence that, specific to the individual, a flash
rate
> somewhere between about 8Hz and 16Hz may give a peak in stimulation
> response, otherwise the eye integrates exceptionally well the photons it
> receives.
> At 100Hz the perceived 'brightness' will be an almost negligible amount
> *less* than the true integrated value - not more.
>
>
Actually, it depends on the type of LED. On some, the efficiency increases
as you increase the current - up to a point. I did not calculate this out,
but a Fairchild high efficiency LED that I looked at would probably be about
twice as bright at 30ma and .2 duty cycle as it would be at 6ma, This is
real brightness, not perceived. On the other hand, going beyond 30 ma did
not increase the efficiency.

Tam

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