On 2012-12-21, Andrew Gabriel wrote:

In article ,
Adam Funk writes:
On 2012-12-21, Andrew Gabriel wrote:

These are made by companies which previously made CFLs.
CFLs have no exposed metalic parts, except the lamp cap.
Now they're making LEDs where the most difficult part is
the thermal design, and this generally requires a significant
proportion of exposed aluminium to dissipate the heat.
They don't seem to have the experience required to do this
safely, i.e. keeping the mains potential a safe distance
from the exposed aluminium whilst still having a good thermal
contact between it and the LED junction (often at mains
potential). I was a bit surprised at Philips falling into
this trap though.

(I'll take the risk of asking a stupid question here.)

It's not a stupid question at all - it's the essence of the
problem.

:-)

Isn't that
problem --- keeping the mains potential a safe distance from the
exposed [metal] whilst still having a good thermal contact --- rather
like designing kettle & immersion heater elements?

Yes and no.

It's like trying to make a kettle or immersion heater which
requires no earth, i.e. it's double insulated, as there's no
earth connection to a light bulb. This is not the case with
immersion heaters and kettles, which are normally earthed.

Also, there's no requirement for good thermal contact between
the resistance wire and the element casing. The wire gives off
3kW, and it will simply get hot enough to pass that 3kW across
the electrical insulator, regardless of how thermally insulating
it is (as long as the resistance wire remains below its melting
point). It might well be running at 800C+ above the casing at 100C.
That won't work with an LED where the challenge is to keep the
junction at as low a temperature as possible, and the thermal
conductivity must therefore be as good as possible, or both the
efficiency and the life of the LED drops rapidly.

Interesting, thanks. Out of curiosity, what is the stuff they use
between the resistance wire & the casing of a water-heating element?