On 8/10/2015 8:23 AM, philo wrote:
On 08/10/2015 09:00 AM, sms wrote:


An LED requires about 2 volts . Though the heat an LED produces is
negligible, the problem is in the self-contained power supply.

Wrong.

The source of the heat is the semiconductor junction of the LED. No heat
to speak of on the surface of the plastic part where the light emits
from, but significant heat from the junction. Look at the heat sink on
an LED bulb. Look at the heat sink on high-power LED flashlights.
Vehicles with LED headlights often use fans for cooling the lamp, or
they need an elaborate passive heatsink.

The reason so many LED lamps use multiple LEDs is that it's so difficult
to bleed off all the heat from a very high power LED. The junction is
very small and you need active cooling for very high power LEDs.

Good article about this he
http://www.ledsmagazine.com/articles/print/volume-8/issue-6/features/active-cooling-can-boost-lumen-output-in-led-lighting-magazine.html.

Read the section "Thermal issues in LED lighting."




My experience with LED's goes as far as pilot lights which produce no
heat...thanks for the correction on those used for lighting.

Yes, that's where the misconception comes from. Indicator lamp LEDs are
totally different.

I recall in the bicycling Usenet group someone was extolling the
benefits of LED lights and he was thrilled that there was no "white hot
filament." True enough, but that was replaced by a very hot
semiconductor junction which is even harder to cool than a white hot
filament in a hot bulb. Obviously he didn't understand anything about
semiconductor physics and junction current and temperatures (Tj). Sadly,
he claimed to be a physics professor at a university!

There's no free lunch here. There is heat, a lot of heat, to deal with
in both LEDs and incandescent sources. I was just looking at LED car
headlamps afer the last incandescent bulb I put it lasted only six
months. The problem with them is that the rating for hours that they
last is pretty low. Some models have little fans for cooling, some use
metal braiding out the back and explain that you have to be sure that
there is clearance for the heat sinking.