From: "R. Lewis"
Subject: Re: current source for array of infrared leds
Date: Mon, 2 Dec 2002 22:13:50 -0000
Organization: Nextra UK
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NNTP-Posting-Date: Mon, 2 Dec 2002 22:12:21 +0000 (UTC)
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"ånønÿmøu§" wrote in message
> On Mon, 02 Dec 2002 17:32:48 GMT, "Sir Charles W. Shults III"
> >You are missing some important factors here. Not meaning to be pushy or
> >anything, but if you did not understand the physics "stuff", perhaps you
> >missed something. The LED does not have a constant internal resistance
> >be used as a current limiter. Batteries often do this in small
> >torch applications due to their internal resistance.
> Maybe you can help me out by answering this question about an LED.
> Will an LEDs' - Vf (forward voltage drop) continue to drop when its
> allowed too much current to flow through it? If so, just how small a
> voltage could you expect it to drop to?
> Would you expect to see: 10%, 20% or more?
Generally an leds resistance (dV/dI) reduces with increasing current, its
forward voltage (Vf) reduces as a function of junction temperature.
Theta junction to ambient, for 5mm leds, can be large. If on any length of
lead (stand off) it can be huge.