Re: ATM DIY LED Flashlight Conversion

[Richard Schwartz <richas@idt.net>]

-- [ From: Richard Schwartz * EMC.Ver #2.5.02 ] --

> A red LED will have a voltage drop of around 1.6-1.7 v   which changes
very
> little with current (I believe I masured about 0.09 v   for a factor of 10
), that
> is why you need a limiting resistor. Common types are   rated at about 30
mA
> maximum, and to drop 1.3-1.5 V you need about 47 ohms   or a bit more. 
(making a
> constant current generator takes 2 transistors and 2   resistors).

I sure don't want to use a resistor or current limiter: I would end up
wasting a lot of power.  What happens if you charge a small capacitor and
then connect it (through a fet) to the LED?  Perhaps you do no damage if the
capacitor is small enough.  And the brightness is proportional to the
frequency.

Another method might be to build up a current through an inductor, then
switch the LED into the current path.  

Both of the above methods would eliminate energy lost in current limiting.

> Green or yellow LEDs have greater voltage drops, a bit   over 2 volts.

I will be working this problem soon; I want to incorporate a really bright
LED into a foucult tester.