John Fields wrote:
On Sun, 8 Apr 2007 19:52:26 -0500, "Anthony Fremont"
wrote:

John Fields wrote:

I seriously doubt whether the LED manufacturers are the weak link in
the chain. If it exists at all, I suggest that it's 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.

Exactly, not the LED manufacturer, but the light assembly
manufacturer is the one that I blame. Normally, these lights are
not used continuously, but are for looking cool while the bike is
parked outside the bar for a couple hours. These are targeted
towards a market segment where they would likely see much less than
100 hours. I suspect that on the display board they were actually
seeing a bit less voltage than it normally would installed, and
under allot better conditions. It's just that 12 hours per day
operation in a static display is an outrageously heavier than normal
duty cycle.

---
I missed those last two sentences earlier.

If they were seeing a bit less voltage than normal then there's no
way their forward current could increase to the point where the
lifetime of the lamp would decrease. Other than reverse bias
failures, what causes LEDs to die is high junction temperatures, and
that comes about by forcing more current into them than they're
designed for. And that's caused by increasing the voltage across
them, not decreasing it.

I agree, and to put it simply that's all I've been saying. The maker of
these lamp assemblies is allowing them to be over driven for whatever reason
(brighter, planned obsolescence, chinese manufacturer subbed parts, who
knows). ;-)

Also, in other than high-power pulsed service, LEDs are designed to
be operated continuously. The example you cited in another post is
for high power operation.

I know that, but that was to refute this ridiculous claim:

"Ther are no LEDs made that have a spec that declares that they should not
be on all the time." (sic)

Consider: If that weren't true, then the POWER ON LED on a device
designed to be ON 24/7 would have to be turned off every once in a
while, and that's just silly.

To echo your earlier statement, well duh. ;-) I will say that I have seen
commercial products that had power LEDs that dimmed over time. This seems
to indicate that it was likely being driven a tad harder than it liked (and
most certainly harder than was necessary).

On a slightly different tangent: Maybe it's just me, but the little hairs
stand up on my neck every time someone suggests driving LEDs with 20 or 25mA
to a noobie. Almost all LEDs that anyone is likely to encounter today will
illuminate quite well with 5mA and will most certainly last allot longer.