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" (), 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