Date: Tue, 18 Sep 2007 19:09:36 -0400
Reply-To: Mike S <mikes@FLATSURFACE.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Mike S <mikes@FLATSURFACE.COM>
Subject: Re: 2 LEDs
In-Reply-To: <350351.60037.qm@web82709.mail.mud.yahoo.com>
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At 06:32 PM 9/18/2007, David Kao wrote...
>I never said it won't work.
The record speaks for exactly what you've said.
>Try two 3 watt LEDs with a resistor of single digit value.
All of the examples I gave, which you said were wrong, used standard
LEDs and resistors. I've also said that Luxeon type high power LEDs are
properly driven with constant current sources, and the manufacturer
agrees: "LEDs should be driven at a constant current" - Luxeon. A
properly designed regulator would adapt if one of the LEDs shorted out.
Here's a simple current source based on a cheap LM317:
http://led.linear1.org/a-cheap-current-regulated-luxeon-star-driver-design/
I suspect that all but the cheapest Chinese high power LED flashlights
use a similar circuit. If someone wants to try running them in series,
it would be easy to see what happens in the very unlikely event one of
the LEDs shorts out - try running a single one from 12V. Luxeon
certainly recommends constant current sources for driving their LEDs.
Driving LEDs in series is highly desirable for battery operated
applications, since otherwise a lot of efficiency is wasted as heat
when using low cost linear buck regulators. With a constant current
source driving 3 LEDs in series vs. driving 3 LEDs in parallel, one
uses only 1/3 of the power.
They even sell a current regulator which can drive multiple LEDs in
series (
http://www.luxeonstar.com/item.php?id=1747&link_str=240::241::1396&partno=2008B-700
). As they say "The 2008 accepts a wide input voltage range and can
power up to (6) High-Power LEDs in series." (but you can only do 3 in
series from a 12V source, because the Luxeon modules have voltage drops
of around 3.5V) I'd just build a cheap LM317 one, though.
All of your arguments about voltage drop differences causing different
output, and one LED failing causing others to fail is just plain
incorrect.
But, if you want to persist with your argument, I suggest you contact
the manufacturer and tell them they're wrong to sell a product which
allows using their products in a manner which you don't approve. Please
forward their response.
