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From: "Harry Dellamano"
Subject: Re: current source for array of infrared leds
Date: Mon, 2 Dec 2002 08:59:31 -0800
Organization: Posted via Supernews, http://www.supernews.com
X-Newsreader: Microsoft Outlook Express 6.00.2720.3000
> It's hard to answer something that makes little sense. LEDs are not
> devices- as you apply voltage, their apparent resistance changes. The
> method of driving a LED properly is to set a current - that would be a
> number of electrons moving through the device.
Snipped physics stuff that I did not understand......................
> Chip Shults
> My robotics, space and CGI web page - http://home.cfl.rr.com/aichip
LEDs are non linear but can be modeled as a diode with a larger threshold
voltage and a series resistor. The apparent resistance does not change but
the diode does have a negative tempco. Constant current implies adding a
large impedance in series to ballast the current and not allow it to change.
The LED already has internal resistance which can be used to control
current if the voltage is set properly and give the added benefit of
constant intensity over temperature.
Good by Mr. Chips
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