Blue LED at 3.3V
in [Design]
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From: Rich Webb on 26 May 2010 07:58 On Wed, 26 May 2010 12:26:16 +0200, Uwe Hercksen <hercksen(a)mew.uni-erlangen.de> wrote: > > >Tim Wescott schrieb: > >> AFAIK a white LED is just a blue LED chip with some fluorescent material >> in the package that makes enough "whatever else" to make it look white. >> >> They look exceedingly blue to me -- I don't know if that's because they >> are, or because I'm color deficient in green and don't see them the same >> as other people do. > >Hello, > >they mix the blue light with yellow light from the fluorescent material, >but it is a very narrow blue band and a very broad yellow band. If you >look at a spectrum diagram of the resulting light it looks very >different to white light from the sun or from a ligth bulb with a >glowing tungsten wire. Some may indeed by bi-chromic but the ones that I've examined recently with a handheld spectroscope have a remarkably even spectrum from about 420 to 680 nm. It's not a quantitative instrument but there were no obvious emission or absorption lines. Surprised me, in a good way. -- Rich Webb Norfolk, VA
From: David Eather on 26 May 2010 08:03 On 26/05/2010 8:10 PM, Jan Panteltje wrote: > On a sunny day (Tue, 25 May 2010 19:40:25 -0700) it happened John Larkin > <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in > <vd2pv55vqabddc91bahg7rpva3o204d1tb(a)4ax.com>: > >> On Tue, 25 May 2010 12:16:10 -0700, Tim Wescott<tim(a)seemywebsite.now> >> wrote: >> >>> On 05/25/2010 11:27 AM, Tim Wescott wrote: >>>> On 05/25/2010 11:00 AM, John Larkin wrote: >>>>> On Tue, 25 May 2010 10:18:11 -0700 (PDT), rich >>>>> <rsoennichsen(a)gmail.com> wrote: >>>>> >>>>>> I need to drive a blue led from 3.3V. Most of the SMD blue leds I >>>>>> find have a Vf equal to or greater than 3.3V. >>>>>> >>>>>> I am curious how others are dealing with this. >>>>>> >>>>>> Thanks >>>>>> >>>>>> Rich >>>>> >>>>> I have some nice Osram blues that are OK at 1 mA and 2.65 volts, >>>>> bright at 10 mA, 3.1 volts. So you could just get by with a resistor >>>>> or current limiter from 3.3. You could use one of my famous beta >>>>> limiter circuits. >>>>> >>>>> I sometimes make my "3.3" volt supplies actually 3.5 or 3.6. Most >>>>> other parts don't mind. >>>> >>>> How is it at cold, though? >>>> >>> A quick look at a blue LED data sheet shows a 3.3V nominal forward >>> voltage at 25C, with a 20% increase at -20C and a 40% increase at -40C. >>> >>> "Fading blue"? >> >> Since LEDs get more efficient when they're cold, there is a thevenin >> drive impedance that results in nearly constant brightness over >> temperature. >> >> John > > I wanted to say: > Just use resistor to heat up the LED. > Perhaps a NTC could be used, not sure if it would be easy to fidn teh right one, > and mount it next to the lED in parallel with the supply... > > Now somebody could invent a LED with all that build in, photo sensor too, > to keep light output at a progammable (I2C perhaps) level. > Little switcher inside? Current limiter... This is the age of integration, > logic level I2C input. > Nah, A one wire interface with parasitic power.
From: Spehro Pefhany on 26 May 2010 08:07 On Wed, 26 May 2010 10:10:52 GMT, the renowned Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >I wanted to say: >Just use resistor to heat up the LED. >Perhaps a NTC could be used, not sure if it would be easy to fidn teh right one, >and mount it next to the lED in parallel with the supply... > >Now somebody could invent a LED with all that build in, photo sensor too, >to keep light output at a progammable (I2C perhaps) level. >Little switcher inside? Current limiter... This is the age of integration, >logic level I2C input. Maybe a TEC and controller to control the wavelength? Do you really want LEDs with 80 page manuals (and 5 pages of errata)? ;-) Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff(a)interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
From: John Larkin on 26 May 2010 10:24 On Wed, 26 May 2010 05:04:00 +0000 (UTC), don(a)manx.misty.com (Don Klipstein) wrote: >In article <ti4ov551ahdt2po32gjcie9km7gg4a1h8g(a)4ax.com>, John Larkin wrote: >>On 25 May 2010 10:22:59 -0700, Tim Wescott <tim(a)seemywebsite.now> wrote: >> >>>On 05/25/2010 10:18 AM, rich wrote: >>>> I need to drive a blue led from 3.3V. Most of the SMD blue leds I >>>> find have a Vf equal to or greater than 3.3V. >>> >>>That pretty much demands a voltage boost of some sort. Depending on how >>>many lights you have, how much power you're willing to waste, how much >>>design time you want to spend and how expensive you want the final >>>product to be, your choices sort of boil down to a switcher with >>>inductors and diodes and all that, or a current pump. >>> >>>Most of us would solve this problem by looking for a suitable IC. >>>_Some_ of us would do it with two transistors, an inductor, and a cap, >>>then brag about only needing one $.001 resistor instead of three. >> >>One resistor: >> >>ftp://jjlarkin.lmi.net/LED_boost.JPG > > Dear John Larkin, > > I like that one! It's notably simple, as in being a DC-to-DC >converter having only 4 components (excluding the often-advised >capacitor across the power supply leads of the IC - .1 uF 25V cheapie >ceramic capacitor is an often-advised occaisionally-necessary item >that gets the component count up to 5, 6 if you also count the LED. I usually do multilayer boards with power and ground planes. A few bypass caps will do for the whole board, so I don't put one per chip like in olden days. > > Component count may get to 6 excluding the LED, 7 including it, if >this boost converter gets good enough to overpower the LED or deliver >more output power than desired, so as to necessitate adding a resistor in >series with the LED. The average LED current is set by the R and C values, so you don't need another resistor. > > I would like to add that efficiency is likely to improve if the non-LED >diode (a reaistor is offered as a workable alternative) is a Schottky one. >I would look into Schottky diodes with breakdown voltage 30V at most, >maybe 20V, and rated to handle 1 amp or less, maybe much less. Come to >think of it, much less to get improvement towards shorter switching times. The danger is that the drop of the diode+LED had better be less than the max supply voltage. So silicon might be better in some cases. > > It does appear to me that the shown capacitor and resistor are "left to >the student". I would like to make that capacitor .01 uF merely from >knowing that one is a common cheap part. I could gain desire to make it >smaller in consideration of likely oscillation frequency considering a >desired value for the shown resistor... > > I would want to make that resistor 100K max to "make this cleaner", and >I have a liking to get oscillation frequency into the 50 to >mildly-above-100 KHz ballpark, in order to make the oscillation frequency >ultrasonic to humans and most pets (even though dB acoustic pressure is >impressively low to negligible likely less than 30 1 meter away). R and C have to be selected to get the desired LED current. There's a second invisible capacitor, Cin of the gate, that's involved too. Hmmm, neither C may matter. It might all cancel out. Somebody should do the math. John
From: George Herold on 26 May 2010 10:31
On May 26, 8:00 am, David Eather <eat...(a)tpg.com.au> wrote: > On 26/05/2010 4:10 AM, John Larkin wrote: > > > > > > > On Tue, 25 May 2010 10:22:59 -0700, Tim Wescott<t...(a)seemywebsite.now> > > wrote: > > >> On 05/25/2010 10:18 AM, rich wrote: > >>> I need to drive a blue led from 3.3V. Most of the SMD blue leds I > >>> find have a Vf equal to or greater than 3.3V. > > >>> I am curious how others are dealing with this. > > >> That pretty much demands a voltage boost of some sort. Depending on how > >> many lights you have, how much power you're willing to waste, how much > >> design time you want to spend and how expensive you want the final > >> product to be, your choices sort of boil down to a switcher with > >> inductors and diodes and all that, or a current pump. > > >> Most of us would solve this problem by looking for a suitable IC. > >> _Some_ of us would do it with two transistors, an inductor, and a cap, > >> then brag about only needing one $.001 resistor instead of three. > > > One resistor: > > >ftp://jjlarkin.lmi.net/LED_boost.JPG > > > John > > Hi, > > Could you post his circuit onwww.filedropper.comorwww.filefactory.com > or something similar - I just can connect.- Hide quoted text - > > - Show quoted text - +3.3V----+-----------+ | | |\| V This is diode or R | \ - +--+| >--+-CC---+ | | / | | | |/| | V light comes out here +------RR--+ - | GND The IC is a schmitt trigger. (And connected to ground also... connection not shown) George H. |