Date: Tue, 18 Sep 2007 17:59:13 -0700
Reply-To: David Kao <dtkao0205@YAHOO.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Kao <dtkao0205@YAHOO.COM>
Subject: Re: 2 LEDs
In-Reply-To: <20070918232401.F234E1165C3@hamburg.alientech.net>
Content-Type: text/plain; charset=iso-8859-1
Mike, go buy a 31 LED flash light something like that. Open it
up and take the LED module out and verify it yourself that all
LEDs are in parallel. Not a single one in serial. I have quite
a few of such flash lights and all LEDs are in parallel.
Yes, these flash lights are probably from China. Very cheap sometimes.
But they do know how to design it correctly. Watch out the batteries
come with them. We know batteries from China are not that good.
Your example of LEDs in serial is regular signal LEDs not for lighting
applications. Regular LED runs at 10 - 50 ma small current. Luxeon
LEDs are working at 0.5 - 0.7 amp. Your serial resistor is 390 ohm.
The one for two Lexeon in serial with 12 volt supply will be less than
5 ohm. Try to short one out and see the smoke yourself.
There is no desire for me to convince you that you are wrong. Sometimes
I kept on bothering my wife to find my EE diploma with a Master seal on
it. I was afraid if I lose it I lose all my memory of what I learned.
I think I still remember how vacuum tube works. Memory sometimes
don't serve well. I am confused by Mark and Mike, obviously.
Am I talking about connecting in parallel or serial? Not in serious
for sure.
David
--- Mike S <mikes@FLATSURFACE.COM> wrote:
> 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.
>
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