Blue LED at 3.3V
in [Design]
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From: Don Klipstein on 26 May 2010 01:04 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. 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. 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. 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). Also I would want the oscillation frequency to be not-too-close to the common ones for TV/VCR/DVD common consumer devices. But if the LED emits at a shorter wavelength, as in/near blue, that is less-likely a problem. If that problem comes up anyway, I would primarily put a longpass filter in front of the consumer device sensor - as in Wratten preferably 92, secondarily 29, tertiarily 25 or equivalents including Schott glass longpass optical filters (regardless ofwho supplies longpass optical filters based on Schott glass products), having part numbers starting with RG and including afterwards 630 to 670. -- - Don Klipstein (don(a)misty.com)
From: Don Klipstein on 26 May 2010 01:11 In article <_5udndgDGoqUv2HWnZ2dnUVZ_sSdnZ2d(a)web-ster.com>, Tim Wescott 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"? How are these data as function of temperature faring when current is 25 or 20 or 15 % of "characterizing current", when efficiency is improved by current into the range of 15-25% of "characterizing current", in addition to lower temperature that favors increase of ratio of light output to LED current, more-so when the LEDs in question are underpowered? -- - Don Klipstein (don(a)misty.com)
From: Jan Panteltje on 26 May 2010 06:10 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.
From: Uwe Hercksen on 26 May 2010 06:26 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. Bye
From: David Eather on 26 May 2010 08:00
On 26/05/2010 4:10 AM, John Larkin wrote: > On Tue, 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. >>> >>> 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 on www.filedropper.com or www.filefactory.com or something similar - I just can connect. |