On Feb 23, 2006, at 12:45 AM, G=F6khan SEVER wrote:

> I have to design a led strip with 6 whitepower led.
   :
> The strip will be sourced with 24V DC Meanwell smps.
   :
> So i calculated required resistor value for driving the strip at 1A =

> which is 24-6Vf =3D R (The datasheet of Lux III Emtter notes that it has =

> 3.90 forward voltage at 1A)
   :
> i have chosen the Resistor value 0,75R / 5W. But when i conntected
> the serie to the power supply i only get 500mA

I think you have fallen victim to two pieces of misleading data.

First of all, the Vf spec of an LED is somewhere on the "knee"
of the voltage applied vs current conducted curve.  If you apply
a current source whose max voltage is exactly Vf, you'll be on
that knee instead of in the "current limitting" range where you
want to be.  Your supply of 24V only allows 0.1V "extra" for
each of your LEDs, which may not be enough to get them "ON" all
the way.  (I'm not saying this QUITE right; we might get into
another "voltage across vs current through" debate.)

More seriously, you're interpreting a Vf spec for TYPICAL
performance as what you'll actually see in practice.  If you
look at the Luxeon document "Luxeon Product Binning and Labeling"
(<http://www.lumileds.com/pdfs/AB12.PDF>), and in particular the
section of Vf binning, you'll see that a white Lux III CAN have
a Vf as high as 4.47V (or as low as 3.03V, which is quite a range.
(the datasheet has the full range even for 700mA))  Even if
you have the "middle" bin ("L") you could have Vf as high as 3.99V.
If you have a full reel, you can check which bin you have.  I don't
think individual emitters are marked, though.  If you have even a
couple of emitters in a higher Vf bin, you're going to run over
the 24V of total Vf you can get away with (or at least be even
further in that "knee" that I was talking about.)  Also, you
junction temperature isn't going to be 25C at that sort of drive
(although this appears to be helping you; thus the warmup to
higher current.)

Easiest solution: can you tweak the power supply output up via
some adjustment or remote sense capability?  (OTOH, I think
1A current is pushing a Lux III quite a bit.  That's close to
4W even at typical Vf...)  Note that each instance of your
light bar will need separate tweaking.

The rather broad range of luxeon characteristics seems to be a
bit of a sore point among the lighting crowd.  Apparently you
have to be really BIG customer before you can ask for particular
bins instead of taking your chances...

BillW

-- =

http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist