Change the tag to [EE].

On 2/23/06, William Chops Westfield <westfw@mac.com> wrote:
>
> 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
>

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