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Re: [ATM] LED's and Diode Electronics
Richard,
I've already been there, and wrote about it in a previous post. My lamp, in
my testor, was so bright, I COULD NOT TEST COATED MIRRORS WITH IT.
Where has everybody been? TRY THE INCANDESCENT FILAMENT.
Dave
"If it ain't broke, don't fix it"
How can you improve on the wheel? Better bearings? Been around a hundred
years. How can you improve on a tungsten filament? Not possible. LEDs?-
Trash can 'em. LEDs get sand kicked in their faces at the beach. They are
"90 lb weaklings". They know it and won't go to the beach with a girl.
Dave
----- Original Message -----
From: "Richard Schwartz" <richas@earthlink.net>
To: <atm@atmlist.net>
Sent: Saturday, June 18, 2005 10:55 AM
Subject: [ATM] LED's and Diode Electronics
Last night's PPC/CHHU meeting was mostly about LED's. Richard Nelson
brought some samples that he picked up at ALL Electronics, and we looked at
the I-V curve of one such device. The equation for current through a
diode is I = Io*(exp(mE/nkt)-1), but I am not certain if that is the
appropriate equation for a LED. LED's seen to operate at higher voltages
than ordinary forward-biased silicon junctions. I guess the first thing to
do is curve fit the data we have...
Particularly, there was interest in bright LED flashlights, and the question
came up of how to supply power to a LED. The usual method is through a
current limiting resistor, and that is wonderful for you if your name is
Eveready or Duracell. But why not a capacitor? The initial current would
be very high, but it would be for only an instant. The idea is to put all
of the power into the LED and not put some of it into a resistor.
Along with this question, there is another one: How does the photon output
of the LED vary with current? You need to know this to calculate the flux
during a capacitor discharge through the LED. You want to choose the
voltage, capacitance, and firing frequency to deliver the most photons per
joule of energy while not overstressing the diode. All of this is highly
dependent on temperature.
Finally, note that white LED's use blue light from the semiconductor to pump
a phosphor that emits white light. There is a noticable time delay in the
phosphor that renders a white LED unsuitable for applications that require
high frequency modulation.
Hopefully somebody will get interested in this and we will have brighter,
more battery-efficient testers.
. . . Richard
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