On Sun, 26 Sep 2004 10:49:36 +0100, "R.Lewis" <h.lewis-not this
If you bother to learn something about the subject you would not need to ask
such silly questions.
The statement above leads me to think that I did not explain myself
clearly. Here goes again:
Intent: We are trying to achieve maximum perceived brightness using a
plulsed LED, but using the least power consumption.
1) If we leave the LED on solid, without pulsing it, then we will use
up too much energy and we will not achieve a high brightness because
the LED can not handle hundreds of milliamps for a long time.
2) If we pulse the LED using a 1ms pulse, we will have to drive so
much current into the LED in order to achieve any brightness, that it
will be destroyed.
3) If we give the LED a strong 20ms pulse then we will achieve a
blinding flash without destroying the LED. Note however, that dumping
the spike from a fully charged inductor into an LED directly, puts
tremendous thermal stress on the die; so we should strive to shape the
waveform such that thermal cycling can take place more gently. Some
kind of pulse forming LC network would be good. I thought a radar
modulator type "simulated transmission line" energy storage network
might be interesting to try because it would avoid the spike, but
since we are not talking about huge power levels here, perhaps slowly
switching a transistor on and off might do the trick. Here is a simple
PWM soft start circuit for doing that:
In---1M--+--100K--1uF--+
| |
| |
+--|>o--------+--Out to driver stage
| 4584
|
1nF
|
GND
4) When I did a little personal experiment in my home lab and
delivered a 2 millijoule pulse of "energy" to a red LED, I quickly
found that a 10ms pulse appears much brighter than a 1ms pulse or a 1
second pulse. Furthermore, I found that if I operate the LED just
below the threshold of destruction then the 15ms pulse gave me the
most percieved brightness for the least ammount of energy.
hotometry of Flashing Lights from here
