On Sun, 8 Apr 2007 07:00:08 -0500, "Anthony Fremont"
wrote:
John Fields wrote:
On Sat, 7 Apr 2007 08:28:07 -0500, "Anthony Fremont"
wrote:
MassiveProng wrote:
On Fri, 6 Apr 2007 08:10:25 -0500, "Anthony Fremont"
Gave us:
We will have to see on that. It sure looks like they're burning
out in traffic signals and car tail lights fast enough.
That's bull****. Bus Lines, as well as traffic controllers
switched to them SPECIFICALLY due to the FACT that they have a
longer life span, and lower consumption rate.
And you don't see failures? Take a good look at traffic lights, you
can usually find dead pixels in them. They might last longer, but
they aren't eternal.
---
Good quality LEDs have lifetimes in excess of 50000 hours with, I
believe, the criterion for failure being a halving of the light
output.
---
I don't disagree with that, but that's under proper operating conditions.
Too much current and the brightness rolls off real fast.
---
Well, duh...
---
On a slightly different note, I'm disturbed at the number of
vehicles I see that don't have working tail lights. Not old
vehicles, but newer ones (3 years old) that shouldn't have these
kinds of failures. It looks to me like over-all reliability is
going the other way while expense and complexity are going up. I
know those failures are for other reasons, but they are failures
none the less.
---
I suspect that in cars, early LED failure is due to the nasty
electrical environment, and primarily to high-voltage reverse bias
spikes PIVing the diodes.
---
Sounds good to me. I was referring to the number of vehicles with total
failures due to high side drivers or something along those lines. Probably
switch failures more than anything. I really didn't like LED tail lights at
first. Because they illuminate so much faster than incandescant bulbs, they
would trigger some kind of sense of urgency in my mind. As if my
subconcious was interpreting it as though the guy in front was slamming on
his brakes. I seem to have adapted for the most part now. They're plenty
bright enough.
The light source will be
expensive and it will have a limited life (man-made at least).
Bulbs yes. LEDs, no. That stats already exist.
Like always, they will be operated at the maximum power dissipation
that gives a life expectancy just longer than the warranty period.
Why do you think it would be any different? Just look at regular
light bulbs, a 10% reduction in voltage greatly increases life
expectancy. Given that, it stands to reason that manufacturers
could just make the filament a little sturdier to accomplish the
same thing. Why aren't any manufacturers doing that?
---
Make the filament any thicker (sturdier, but lower resistance) and
you'd need a lot more power to get the same brightness as well as
making the lamp cost more. Make it any thinner (less sturdy and
higher resistance) and you'd have to make it shorter and it would
burn out more quickly.
Sure, but there are heavy duty bulbs that are less resistant to blowing from
a shock. Since different wattage bulbs are simply different resistance, it
seems entirely possible to build an almost indestructable filament of any
desired brightness for only a little more money.
---
While different resistances are what's required to dissipate the
specified power, the thickness and length of the filament is going
to determine how much power is going to be radiated as visible
light.
See: Voltage, light output, and lifetime at:
http://en.wikipedia.org/wiki/Incandescent_light_bulb
The failure mode for the lamp (other than gross mechanical abuse) is
evaporation of the tungsten from the filament to the point where the
filament can no longer support its own weight and it breaks.
There is no such failure mode in an LED. See "Failure Modes" in:
http://en.wikipedia.org/wiki/Light-emitting_diode
Well ok, but the end result is the same.....darkness. ;-) I saw a display
showing some of these neato LED assemblys for autos/motorcycles. After only
a few weeks, brightness was noticeably reduced and some LEDs had failed in
the strings. Too much current for no sensible reason. By lowering the
current by 25%, brightness would have probably only been slightly reduced
but lamp life would have been years instead of weeks. Obviously these
weren't meant to be on static display 24 hours/day, but I think it makes my
point that the manufacturer intends for it to fail.
---
I seriously doubt whether the LED manufacturers are the weak link in
the chain. If it exists at all, I suggest that its somewhere down
the chain where the unscrupulous are trying to trim the
dollars-per-lumen bottom line by overdriving fewer than the number
of LEDs required to generate the desired light level reliably.
Also, it's possible the display could have been rigged to run the
lamps very brightly as "attention grabbers".
--
JF