Just reading of LED needing to be kept cool and I was wondering why. Is it
because standard components are used which will fail, or is there some
electrical principle that will always cause failure at a particular
temperature? Could some re-design make them happier at higher temperatures?
200C storage. Genereally 180C operating junction temps.
But lifetime is a lot less at that sort of temp.
Dopant actually migrates through the semiconducting barriers as they
age, and heat speeds it up.
“But what a weak barrier is truth when it stands in the way of an
I have some kitchen downlights (Halers) where the driver assembly is
separate from the lamp unit. They have a 7 year warranty - I guess this is
the way to go. Imagine the electronics in a "ceiling rose) and the lamp on
a pendant. I guess that would work. However, makers have felt pressure to
make everything fit a conventional lamp envelope.
Yes, we're in a transition phase at the moment. The problems are really
with LED lamps which have to retrofit an existing, often unsuitable,
luminaire. The purpose made LED lights should be much better in this
respect, and with the greatly improved lifetime, replacing a whole unit
rather than just a lamp shouldn't cost more in the long run.
True. And the same applied to CFL. Making any energy saving type which
needs a power supply of some sort with the PS built in is invariably a
compromise. I'm rather surprised new standard lighting wiring for LEDs -
perhaps with a central power supply or two - hasn't been devised.
*I dropped out of communism class because of lousy Marx.*
Dave Plowman firstname.lastname@example.org London SW
But this does not solve the problem. Bare LEDs (the diodes themselves)
are not happy at a fixed voltage, they need circuitry to control the
current through them. So a 12V 'LED bulb" is little different from a
240v one in the complexity of circuit required.
And even if it was possible to run LEDs from a remote power supply,
there would still be the problem of a keeping the actual diode, itself a
semiconductor, adequately cooled.
The only good engineering solution is a light fitting containing the
circuitry to work from a fixed voltags supply (which might just as well
be 240v AC as anything else) and designed to allow appropriate cooling
both for the LED and the associated electronics.
A DC supply would be better as you wouldn't need relatively large
electrolytics in the internal LED switch mode PSU which have a limited
life or a high cost. If you're using DC, 48V would be a good choice as
it's high enough not to need too thick wires (given that LEDs use less
current than incandescents) and low enough not to be a shock hazard.
No point replacing existing wiring, but maybe one day for new buildings.
It probably wouldn't be done that way. You can design highly efficient
voltage to constant current converters which provide the drive that LEDs
actually require. They are current based devices with a terminal voltage
that varies somewhat with applied current and temperature.
Crude resistor based current limiting is used in some torches but chips
to do this job cheaply and efficiently are quite common now.
Yes, but you don’t need to when you have a central power supply
or two and have that isolated from the heat that the LEDs produce.
That’s how the LED strips work, the power supply is physically
separate from the LEDs.
For a constant current drive then yes, but one advantage of a lower
starting voltage is that some multiple of LED voltage drop plus a simple
passive resistor can be made to work without any failure prone
electrolytic capacitors. Plenty of torches use this method although an
increasing number now use a cheap small flyback step up current source
and just a single cell battery.
The problem invariably boils down to thermal management of the waste
heat generated in the LED chip itself otherwise the phosphor gets
degraded and they are less efficient when run hot. It isn't for nothing
that manufacturers maximum output claims are based on 25C and a nearly
infinite heatsink compared to what a consumer lamp will have.
It is invariably the capacitors that go first when used in existing
luminaires designed for hot incandescent bulbs.
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