I have confirmed that the correct Vf bin code ranges in 3.51 - 3.75[V] for my white Luxeon III Emitter power led. I have to confess that my mind a little bit confused at this stage. After taking account the proper Vf value i decided to use max Vf value in my calculations (or do i have to choose the typical 3.6V value?) and based on the "DS45 Luxeon III Emitter" datasheets' page 12 "Forward Current Characteristics" figure; "i power the luxeon 3 white emitter with 0.7[A] i can get max normalized luminous flux". So i chose 0.7[A] as a second parameter of simple equation. That makes R = (24 - 6x3.6[V]) / 0.7[A] = 3.4[Ohm] (i will chose proper approximate R value for that) The dissipated power will be ~2.5[W] in this configuration. I think i have to lower the R value a-bit for maximum capacity. I will try my simple experiment tomorrow morning. Do you think the result predictable, in other words do we say that we get stable 0.7[A] current or at the start we get 0.5[A] and increasing to 0.7[A] slowly? Another question for red color Lux3 power leds; Whats the wiser solution to get the best current / normalized luminous flux trade-off? ( If i want to get maximum flux i have to drive the led at 1400[mA] but that will cause overload on dissipated power and if i drive the circuitry with avarage current i will only get half of available radiation. Whats wrong with that? I've lost a bit in this lighter area. Hope you clarify the dark areas... 2006/2/23, William Chops Westfield : > > > 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