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From: Kevin McMurtrie
Subject: Re: What is maximum current for LED?
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Thu, 03 Oct 2002 21:18:50 GMT
NNTP-Posting-Date: Thu, 03 Oct 2002 14:18:50 PDT
In article ,
"R. Lewis" wrote:
>"markp" wrote in message
>> "John Woodgate" wrote in message
>> > 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 will
>> > >flash on and off at a frequency of a few hertz, 50% DC, for a duration
>> > >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 AND
>> > (more important) no excessive heating effect.
>> > --
>> > Regards, John Woodgate, OOO - Own Opinions Only.
>> > you need to go to 40 mA to produce the same visible
>> Not strictly true John, the eye has significant persistence (or rather the
>> 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.
>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.
It is a myth. I've tried pusling various LEDs to see if it could save
power. There was no advantage at all unless an inductor was added to
make it a switching power supply.
A better idea is to try various ultra-brights. Fry's sells a green
10mcd LED that puts out a strong amount of light even using a pair of
fingers as a dropping resistor. Go up to 1mA and it's hard to look at.
The full 20mA makes it a flashlight.
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