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From: "Spehro Pefhany"
Subject: Re: What is maximum current for LED?
User-Agent: tin/pre-1.4-19990216 ("Styrofoam") (UNIX) (Linux/2.2.14 (i586))
Date: Thu, 03 Oct 2002 13:31:14 GMT
NNTP-Posting-Date: Thu, 03 Oct 2002 09:31:14 EDT
The renowned Steve Taylor wrote:
> Apparent brightness to human vision is related to both the average and
> the peak brightness of the source ( Siemens LED app note.)
At high enough frequencies that the source does not visibly flicker,
"average reading" is a good enough approximation for most purposes. Very
short pulses of light might behave differently, but I doubt they do.
The consequence is that the maximum efficiency of modern LEDs is with DC
drive. Efficiency and brightness per average mA is not much affected by
moderate pulsing or multiplexing, up to maybe 10:1 or so, which is a good
thing for driving displays (it simplifies the drive circuitry).
This whole myth seems to have been started by experience with
old-fashioned GaAs red LED calculator displays which had a "threshold"
current of as much as 1mA. Modern (eg. GaAlAs) LEDs will give off faint
light with microamperes of current, there is no real threshold, and light
output per mA may rise or fall a bit with current but is essentially
The lower efficiency at pulsed currents means more watts dissipated in the
LED and MAYBE less dissipated in the drive circuitry (if you're using a
simple series resistor and constant voltage source). If you're using a
switchmode constant current driver, it just means more current from the
power source, which is probably a battery in that case.
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