On Fri, 08 Jan 2010 23:45:09 -0600, the renowned Don Foreman
wrote:

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.


That should work nicely. The ripple will give you more heating than
otherwise, of course. Usual LED mfr recommendations are 5-20% maximum
current ripple.

Here's another "canned" solution:
http://www.sure-electronics.net/DC,I...s/LE-LL209.pdf

At the other end of the scale, do you have any of those 5V switchers
laying around? Last I looked, a surplus place near me has a big stack
of brand new compact 5V wall plug switchers for about $2-3. Inside is
a flyback converter using a UC3843 (clone), with the usual TL431 2.5V
reference/optoisolator feedback, so changing the resistor ratio on the
TL431 would allow the voltage to be lowered to something like 3.5V. (I
gently crack the ultrasonic case welding open with a vise, then they
can be glued shut again with epoxy- usually you can't tell they were
ever opened without looking very closely.).


Best regards,
Spehro Pefhany
--
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