On Thu, 07 Jan 2010 14:35:21 -0500, Spehro Pefhany
wrote:

On Thu, 07 Jan 2010 12:30:23 -0600, Don Foreman
wrote:

On Thu, 07 Jan 2010 10:28:57 -0500, Spehro Pefhany
wrote:

On Wed, 06 Jan 2010 22:22:56 -0600, Don Foreman
wrote:

http://members.goldengate.net/dforeman/yellowlight/

Velly nice!

What do you use for a power supply?

That's my next project. G It needs to be a 700 mA current source
with load voltage circa 3 volts or so. Previous version was 350 mA so
an LM317 and sense resistor epoxied on the top of a wallwart sufficed.
Might work here too if I can find a 4.5 or 6 volt 700mA wallwart.
Else, maybe a little switcher.

It's cheating a bit, but have you seen these?

http://www.leddynamics.com/LuxDrive/...1-BuckPlus.pdf

$15.99 in one-off for the 700mA version and another $1 for the one
with dimming.

After thinking about that some today, I'm leaning toward a dirt-simple
hysteretic switching current regulator comprised of a 220 uH inductor
(35 cents at AxMan), MCP6022 opamp, MOSFET, Schottky diode, sense
resistor and voltage reference. Maybe 2 more resistors to get a 0.5
volt ref from a 1.23V bandgap device. All bench stock. I sometimes
build such simple circuits "in air", no circuit board. Just solder
the various parts together with perhaps a bit of hookup wire and then
pot the lot in epoxy on top of the wallwart xfmr that supplies the
low-voltage line-isolated DC.

I need to go to a switcher because of my available xfmr choices. I
have several that have the necessary 2.4VA ratings but they're all
rated for more voltage and less current than I need. A switcher
solves that nicely.

Current ripple will probably be about 200 mA P-P, which doesn't matter
at all in this app. The biggest inefficiency is in the current sense
resistor where maybe a quarter watt becomes heat, oh dear! Uh no, by
far the dominant inefficiency and heat generator will be the xfmr.