In article ,
Jim K writes:
On Nov 27, 7:13 pm, (Andrew Gabriel)
wrote:

If you are able to manfacture your own, the component LED parts are
not expensive (at least, nowhere near as expensive as commercial
efficient LED lighting).

what's involved?

Two parts: designing/building a luminare (light fitting),
and designing/building a power supply.

The luminare needs to look aesthetically pleasing (depending on
location), direct the light where it needs to go, and handle the
heat dissipation from the LEDs. I mostly use the 3W LED elements
which are around a few quid each. Above 3W, they get proportionally
much more expensive and it's usually possible to use multiple 3W
ones to achieve what you're after. These are pretty much point
sources of light which would be very unpleasent to view directly
due to high illuminance, so you will need some optics to handle
the light. If you want a beam, there are a set of minature high
efficiency lenses made for these which create accurate beam angles
from 5° to 25° and also an asymmetric 5°x20° version. If you want
more diffuse light, you will need to use a translucent diffuser,
or bounce the light off a high efficiency diffusing surface.
The LEDs dissipate heat - assume 3W for a 3W LED (although that's
not strictly true). This doesn't sound like much but the big
problem LEDs have is that the junction can't run very hot, and the
LED efficiency rapidly drops as the junction temperature increases.
Light output is always quoted at 25C junction temp, which is not
achieveable in practice. I usually aim for no more than 40C at
the mounting surface, which probably equates to a 55C junction,
at which point 10% of the efficiency at 25C is lost. The LED
elements I use come either on a star of aluminium for easier
mounting (although not that easy), or bare to design your own mount.
Either way, you need to attach to a larger piece of metal which
acts as a heat sink, removing the heat from the LED and dissipating
it. You are most likely going to want white LEDs, which come in
warm white (which is not that warm, at typically 3300K), or plain
white which is very cold (verging on blue) at over 5000K and not
very useful in my view. Unfortunately, the warm white are the
least efficient (another serious problem the LED industry needs
to fix), but that's what I use. They are also available in various
colours which are mostly more efficient than white.

The LED elements need to be fed from a constant current power
supply, so you can't connect them directly to a voltage supply.
You can buy LED drivers which do this, but they are stupid prices,
like everything in the commercial LED industry. You could use a
dropper resistor as is done for indicator LEDs, but the power
dissipated in it is wasted, and that defeats the object of building
an efficient light. I guess I'm lucky in that I can design/build my
own switched mode power supplies, and that's what I do for low
voltage supplies. For mains supplies, I buy a small efficient SMPSU
from CPC which are well under a tenner, and modify it to add constant
current limiting to the feedback circuit. One power supply can
drive many LEDs in series. Allow voltage headroom of about 4.5V
per LED plus another few volts in total for the current sensing and
stablising overhead, which means you could drive 10 from a 50V
supply. In practice, you would have to consider various failure
modes and max dissipation a failed one might generate (half the
total power output by Thévenin's theorem), and that might
restrict total number per chain to fewer. I drive a series string
of 5 of them down the path by my house from a 24V SMPSU modified
to include current limiting. In this case, I'm driving the 3W ones
at about 2W each (500mA), because that gave plenty enough light
and will result in longer life.

--
Andrew Gabriel
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