[sdiy] Re:

[Theo]

Agreed,
but pulsed leds are not seen by the eye as brighter,
the eye sees the average brightness.

Also the pulse frequency has influence on the efficiency/brightness of the
LED.
According to a HP application bulletin LEDs need to be pulsed at 1khz or
more for the brightness to be equal to the brightness of a unpulsed LED with
equivalent current.

In other words, a LED pulsed with 100mA and 10% dutycycle at 10hz
is less bright than a unpulsed LED at 10mA.
While a LED pulsed with 100mA and 10% dutycycle at 1khz
has the same brightness as the unpulsed LED at 10mA.
The effect has to do with the relative junction temperature or something.

Theo


From: harry <harrybissell at prodigy.net>

> The Voltage Follower is running in linear mode... the transistor is not
> saturated. In the PWM example the transistor is saturated, so the power
> dissipation is very low.
>
> LEDs running at low currents will operate in an area where brightness
> differences
> are very obvious. By pulsing them at a higher current, the eye is fooled
into
> thinking
> that the LEDs are brighter, and the physical limits of the LED are
overcome at
> the
> same time. The eye sees the flashing LED as brighter than the average...
it is
> more of a peak detector in that regard.
>
> If you want to "light up" a synth we are looking at several hundred
millamps
> to
> possibly a couple of amps. Every advantage helps.
>
> H^) harry
>
> matti wrote:
>
> > How is this better and more efficient than a voltage follower and a
> > current-limiting resistor?
> >
> > enjoy
> >
> > On Sun, 3 Feb 2002, harry wrote:
> >
> > > Ahhh... you want a voltage to pulse width converter. Its easy. You
need
> > > a
> > > triangle or sawtooth wave (only one per synth) at a fixed (you choose
> > > it) frequency... and a comparator for each LED channel. You make it
work
> > > so that increasing voltage equals a longer pulse... then drive all the
> > > LED from a transistor on the
> > > comparator output.
> > >
> > > Caveat: This is a "switching" power supply. Unless you are careful,
all
> > > these modulated LEDs are going to wreak havoc with your power supply
and
> > > or
> > > radiate into the audio. It is the best and most efficient way to
> > > modulate an
> > > LED intensity.
> > >
> > > H^) harry
> >
> > *snip*
> >
> > -
> > you stupid vulgar greedy american death-sucker
> > some trillions of years ago a sloppy dirty giant flicked grease
> > from his fingers
> > one of those gobs of grease is our universe on the way to the floor
> > *splat*
>
> --
> Jihad Terrorism Conspiracy New World Order
> Revolution Black Helicopters Freedom of
> Speech First Amendment Rights: Carnivore Bait
> go ahead and READ my e-mail I have nothing to
> hide... how about YOU ???
>
>
>