At 10:17 PM 9/18/2007, David Kao wrote...
>In fact I fell into the same mistake a few years back in connecting
>some
>LEDs in serial.
>You do not know how exactly manufacturers design their LED modules.

I do know how the major LED manufacturers suggest using their products.
I don't particularly care how the poor design of some cheap Chinese
flashlight is done. I've already pointed out, and provided a link to,
how Luxeon sells a regulator which supports series connection of their
high power LEDs.

Hewlett-Packard/Agilent, who at the time this tech note was written was
a major LED manufacturer, advised a series connection: "The LEDs in a
device could be electrically connected in parallel, with each LED
having its own current limiting resistor. ...A more effective approach
is to connect the LEDs within a light bar in a series/parallel circuit
combination..." - www.avagotech.com/assets/downloadDocument.do?id=273
(Avagotech is the now spun-off HP/Agilent Semiconductor Products Group,
and claim to be "#2 worldwide in LEDs for signs and signals")

Here's a semiconductor manufacturer who makes a white LED driver chip,
"specifically designed to drive up to 12 white LEDs (6 white LEDs in
series per converter)... Series connection of the LEDs provides
identical LED currents resulting in uniform brightness and eliminating
the need for ballast resistors and expensive factory calibration." -
http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1094,C1766,P34513

So, you've said I'm wrong and challenged me to do an experiment - I
did, and it worked fine. Then you said that you meant with high power
LEDs, to which I pointed to a manufacturer's site and their explicit
support of connecting LEDs in a way you say causes problems. Then you
return to saying it won't work right with standard LEDs. HP says
otherwise. Not once have you provided any authoritative reference at
all for any your claims. I've provided links to manufacturers of both
regular and high power LEDs, and to a semiconductor manufacturer, all
of whom discuss the advantages of series connection of LEDs.

The 30-LED flashlights don't run the LEDs in series because they run on
~4.5V (3 alkaline cells, typically), and white LEDs have a voltage drop
of around 3.5v, so they can't run two in series. If you were an EE, you
would know that. But, the discussion is in regard to using automotive
12V power, were up to 3 such LEDs can easily be connected in series,
with a 2/3 power savings over connecting them individually, which is
what you advise.

Please go argue with the manufacturers, who have very much more
experience and knowledge about the matter than you.