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Subject: Re: What is maximum current for LED?
Date: Thu, 3 Oct 2002 16:53:00 +0100
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"markp" wrote in message
> "Steve Taylor" wrote in message
> > Spehro Pefhany wrote:
> > > The renowned John Woodgate wrote:
> > >
> > >
> > >>Beyond 20 mA continuous you won't get significantly more light. But at
> > >>50 % duty-cycle, you need to go to 40 mA to produce the same visible
> > >>(more important) no excessive heating effect.
> > >
> > >
> > > As the I^2R heating increases with the square of the current, and
> > > brightness generally is (roughly) proportional to average current,
> > > is most often a losing game.
> > Apparent brightness to human vision is related to both the average and
> > the peak brightness of the source ( Siemens LED app note.)
> This is true, the eye remembers peak brightness too. By the way I don't
> agree with Spehro here. Let's say you had a 5V source, 330R resistor and
> at 1.8V forward voltage. So 3.2V across the resistor gives 9.6mA, power is
> 9.6e-3 * 3.2 = 31mW from resistor and 9.6e-3 * 1.8 = 17mW across LED. Now
> double the current for 50% of the time. Power is now (19.2e-3 * 3.2)/2 =
> 31mW across resistor and (19.2e-3 * 1.8)/2 = 17mW across LED. I.e. the
> However, the persistence of peak brightness means you can probably get the
> same perceived brightness at lower current, hence less power.
I think I have made a mistake here. This is peak effect is indeed true
(called the Broca-Sulzer effect) but only occurs below the fusion frequency
threshold, and therefore flicker is apparent. Above this frequency the
Talbot-Plateau Law takes over (which states that the light energy is
averaged over the whole cycle).
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