Re: [ATM] LED's and Diode Electronics

[Dominic-Luc Webb <dlwebb@canit.se>]

On Sat, 18 Jun 2005, Richard Schwartz wrote:

> 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?


One can try. Caps still have resistance. Maybe you could find some
cute trick to give shorter duration pulses, but generally speaking,
these circuits (discrete transistors or timer ICs) have generated
additional heat and waste. Maybe selection of batteries with most
appropriate ratings would be the best solution. If the battery is
exactly matched to the LED, you can bypass all these manipulations.
Some of these circuits are very sensitive to the charge status of
the battery. Getting more out of your batteries might be possible
by building simple circuits that can use the complete power available.
My Canon A85 CCD camera, for instance, stops working when even one of
the battery's voltage (without load) drops to 1.3 volts. That same
battery can continuously run in my Sony CD player for well over 4
hours. Clearly, some circuits do not take advantage of the full power
available in batteries. Indeed, in the case of my Canon A85 camera, it
now seems clear that I am only exhausting maximum 20% of the available
energy before needing to change or recharge batteries (otherwise, I
like the camera very much and have recommended it to friends).


If you want the flashlight to last a long time with high reliability,
one could build a solar powered charger circuit with diode to block
discharge from re-chargable type batteries. This should continuously
force the battery in the charged state.

Dominic-Luc Webb



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/
>

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