"Michael Kennedy" wrote in message
...

"Arfa Daily" wrote in message
...

"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


All in all, I feel that pulse drive is terrible for tail lights. As
someone else stated they are irritating to the eyes. I can't stand driving
behind someone with LED taillights. They make my eyes do weird things and
it is not a plesant expirence. I already wear glasses and have some
trouble seeing at night, I don't need more trouble from tail lights that
are flashing at 30hz. No one else I know can seem to see that they are
flashing, but then again I can also tell if my monitor is set at 60 or
85hz just by looking at it. For me the flashing stops at about 85hz.

Mike
It was me who said this Mike, and I know what you mean. Nobody else in my
family knows what I am talking about when I tell them that I see a trail of
light 'dots' across my vision when I look at these things wrongly. There
used to be very strict constraints on car designers about where lights could
be positioned, and how far apart and such like. Now, there seems to be no
such restrictions, and anything goes. Some brake lights, for instance, are
blinding, and there's three of 'em ! Makes you wonder if any of this
contributes to accidents ...

Arfa