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From: "Sir Charles W. Shults III"
Subject: Re: current source for array of infrared leds
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Mon, 02 Dec 2002 17:32:48 GMT
NNTP-Posting-Date: Mon, 02 Dec 2002 12:32:48 EST
Organization: RoadRunner - Central Florida
"Harry Dellamano" wrote in message
> > 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......................
> Mr. Chips.
> 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
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 have
missed something. The LED does not have a constant internal resistance that can
be used as a current limiter. Batteries often do this in small flashlight or
torch applications due to their internal resistance.
A model of a LED having an internal series resistor misses some important
factors- for one, LEDs will generate photocurrents, and that will affect the
operation by changing the internal "resistance". For another, that internal
resistance in not a constant by any means. The model is just a model, and would
not work in the real world.
Clearly, you are free to burn out as many as you wish experimenting, and
that might be the best teacher. But you cannot operate a LED from a constant
voltage without somehow limiting the maximum current through it, and expect it
to survive for long.
I have emailed you a more detailed explanation.
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip
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