Re: LED's

[Brett Dikeman <brett@pdikeman.ne.mediaone.net>]

>c a l i b a n wrote:
>>
>>         interesting.  also, a plus for led's is that they
>>         light up much closer to totally instantly than regular
>>         incandescent.  this is a ctually a major reason that
>>         they are becoming standard.
>
>I don't know if that's the reason or not, but it is an added bonus. The
>rise time of an LED is usually under 20 ns, while you can watch the
>incandescent stop lights and tail lights gradually turn on or off. HP
>has cited studies showing that drivers are more likely to notice brake
>lights and stop lights that change so quickly, like LED's do.
>

Ford used neon 3rd lights in the Explorers for this.  Same reason. 
Caddilac was an early adopter of the LED's; one STS had a row of led's from 
one side of the car to another.  Looked even dumber when some of them 
stopped working, like missing teeth.

Other advantages:
-high reliability(when driven properly)
-pure light output(no filter needed.)


>> also their low voltage
>>         usage has made them popular in warning signs, often
>>         able to be solar powered and totally off that pesky
>>         grid.  traffic signal red lights in many cities have
>>         been switched over - and they have this weird effect
>>         of "flashing" on, which is probably a good safety
>>         feature.  i dunno if it is intentional or not.
>
>I've seen a few of the ones you describe, which seem to have a spike in
>brightness right when they come in. Whether it's intentional or not,
>it's not a characteristic of the LED itself; it could be an
>unintentional characteristic of the drive circuit,

It could be a poor design.  If the LED is being overdriven(unlikely), the 
junction will heat up very quickly and the brightness will go down.

More likely, this is the pulse driver circuit coming up and it just appears 
that the light "comes up flashing"; LED's are many times more efficient 
when pulse-driven at higher voltages, because you can take advantage of a 
lower junction temperature but higher voltage and current.  No burnout, and 
much brighter light; the pulses are so fast that the human eye can't see 
anything but continuous light(however, you do get an eerie feeling that 
they're blinking, say  when you look side to side.  Cool effect.)


About the brightest thing I've seen generally around right now is hovering 
in the 2-4 C range.  I'm considering buying a set of 4 large 4C red LED's, 
and seeing how it looks in the 3rd light on my 200.  I looked into it about 
2-3 years ago.  Some of you may remember my making queries into how light 
intensity was measured, and how bright a Candella was, so on etc...


I work right near the main US post office in Boston, and they're converting 
most of the fleet to using LED clusters that replace the lens+everything on 
those round signal lights.  They have an extremely definitive on-off like 
action that really catches your attention.  New tractor trailers, built 
within the past year or two, use them as well.

Brett
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Brett Dikeman
brett@pdikeman.ne.mediaone.net
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