"James Sweet" wrote in message
news:rqqtj.374$we5.136@trnddc02...

"Dave Plowman (News)" wrote in message
...
In article ZSjtj.725$0%3.218@trnddc06,
James Sweet wrote:
They're pulse driven to give multiple brightness levels. Low as markers,
then they get bright when the brakes are activated. It has nothing to do
with increasing the max brightness.

Eh?



What?

The lights I'm talking about are used in lieu of twin filament
incandescent lamps for tail/brake lights. These have two brightness
levels, and to achieve that with LEDs they use PWM dimming to give two
levels.

I don't think that is an actual requirement though. A LED could just as
easily be dimmed by DC current reduction, although I agree, it would less
efficient in terms of wasted energy. Some time back, I did a lot of reading
on driving LEDs, and the manufacturers seem to advocate pulse drive for two
reasons. The first is that very high pulse currents can be used compared to
DC drive currents. As the light output is proportional to the current, this
means that very high level light pulses can be achieved, without hotting up
the die beyond what it can dissipate. The total result of this is a higher
average light output for a lower dissipation than would be achieved by DC
drive. The second reason is that pulse drive extends the useful 50% life of
the LEDs, by as much as 10 times. So I guess both you are right in that it
makes them easier to control, and so is Dave in that it makes them brighter
....

Arfa