[ATM] LED's and Diode Electronics

["Richard Schwartz" <richas@earthlink.net>]

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
_______________________________________________
ATM mailing list http://www.atmlist.net/