piclist 2002\12\27\094704a >
Thread: Pushing LEDs to the limit
www.piclist.com/techref/displays.htm?key=leds
picon face BY : Spehro Pefhany email (remove spam text)

part 1 1144 bytes content-type:text/plain; charset="us-ascii"; format=flowedAt 01:29 PM 12/27/02 +0000, you wrote:
>I suspect it might be  different with white LEDs, as the phosphor may have
>some time-dependent
>characteristics.

It's actually worse because white (blue) LEDs have relatively high internal
resistance, so the heating increases greatly at high peak currents
(Vf is not constant)

See the attached graphs of luminosity vs. current and max current vs.
duty cycle for a high-quality white LED. At 20mA luminosity is 1.0
(normalized),
at 100mA it is only 3.5, not 5 or greater.

Lower perceived light output at the same average current, and lower
maximum permissible current due to the heating (10% maximum at 100mA
compared to 100% maximum at 30mA (Ta = 25'C)- it's *clearly* a losing game.
Not even close to break even.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
@spam@speffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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Subject (change) Pushing LEDs to the limit

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