On Fri, 15 Jan 2010 21:02:27 -0500, Charlie Smith wrote:

Group,

Many thanks for all the innovative and informative posts on the LED
lighting scheme. Many of you have been quite generous with your time
and knowledge and I am most appreciative.

Certainly there are many different ways to put this together and each
has a particular strength. I would prefer to stay with PWM techniques
for dimming versus voltage changes. With 20 (and perhaps more) LEDs in
the circuit, some are certain to begin to shut off before others when Vf
starts falling too low. I have taken as many of your suggestions as I
could understand
(Sorry JosephKK, but for me, yours was grad school.
Beta sounds like a coefficient of some sort. Perhaps you could post a
primer for me.)

No problem. Simply stated beta is the ratio between base current and collector
current. So 10 mA base current yields about 500 mA through the collector (with
something like 2 V collector to emitter. With R3 = 1K your base current will be
about 10 to 12 mA. A bit stronger drive by reducing R3 to 500 ohms will give you
better consistency for the LED drive because the 3055 will be fully switching.
Do make sure that your 555 can source at least 50 mA for this to be reliable.

and put them into a modified design. I have de-rated
the LEDs somewhat and switched to a low noise regulator that uses a trim
pot to adjust Vout. This way, should I need to go to two or even three
LEDs in series, I can accommodate this in the design.

Here I have calculated the total power consumption of the circuit at
4.56W with the LEDs consuming 1.23W (27% efficient). At 14V, it should
require 0.33A to run. The most critical part is the junction temp of
the regulator. It seemed to me to run too hot as is so I put in a
series resistor (12 ohms, 7W) to absorb some (3.075W) of the power. I
now estimate Tj to be 31.5°C hotter than ambient with an allowable Tmax
of 125°C on the chip. Safe even in the desert.

I also fixed a couple errors on the original schematic. As always, I
would appreciate some experienced eyes looking at this and letting me
know if I have seriously screwed something up somewhere. I'm not at all
familiar with linear low-noise regulators.

Thanks,
Charlie