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Subject: Re: What is maximum current for LED?
Date: Thu, 3 Oct 2002 21:54:18 +0100
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"R. Lewis" wrote in message
> "markp" wrote in message
> > "John Woodgate" wrote in message
> > news:rcAfx3AOgum9EwY2@jmwa.demon.co.uk...
> > > I read in sci.electronics.design that Nelson Win
> > > wrote (in
> > .google.com>) about 'What is maximum current for LED?', on Wed, 2 Oct
> > > 2002:
> > > >I bought a few high brightness 3mm LEDs (various colors: R,G,B,W) and
> > > >the specs mention a forward current (If) of 20mA and peak current (If
> > > >(peak)) of 100mA (160mA for red). The specs don't say anything about
> > > >the current beyond that, nor what the recommended maximum current
> > > >should be. There's not even a brand name on the LEDs so I can
> > > >approach the manufacturer. I want to squeeze the brightest light I
> > > >can from the LEDs without prematurely burning them out. Does anyone
> > > >have any experience here to recommend a maximum current? The LEDs
> > > >flash on and off at a frequency of a few hertz, 50% DC, for a
> > > >of a couple or hours at a time. Any advice appreciated.
> > > >
> > > 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.
> > >
> > > --
> > > Regards, John Woodgate, OOO - Own Opinions Only.
> > http://www.jmwa.demon.co.uk
> > > you need to go to 40 mA to produce the same visible
> > Not strictly true John, the eye has significant persistence (or rather
> > brain has) and tends to 'remember' the peak light output. You may find
> > or even less at 50% duty cycle of a few hundred Hertz will give a
> > brightness equal to the 20mA continuous. Generally PWMing can decrease
> > overall power dissipation for this reason.
> > Mark.
> I think this is the urban myth that sticks around because different people
> concoct different pseudo scientific reasons for it being true - and if
> enough people say it... it must be true.
Yep, I believed this wholeheartedly until I started to look for some real
evidence! If you switch at the right frequency you can get the illusion that
the output is brighter than it actually is, but this is below the 'fusion'
frequency so it will flicker (see Broca-Sulzer effect).
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