Blue LED at 3.3V
in [Design]
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From: Spehro Pefhany on 25 May 2010 14:38 On Tue, 25 May 2010 11:29:20 -0700, Tim Wescott <tim(a)seemywebsite.now> wrote: >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. Usually a blue chip with yellow phosphor. There maybe some that are made differently. >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. They make them with different color temperatures these days, especially those designed for illumination.
From: rich on 25 May 2010 14:58 On May 25, 11:10 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> 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 FTP link does not work...
From: Jan Panteltje on 25 May 2010 15:02 On a sunny day (Tue, 25 May 2010 11:58:36 -0700 (PDT)) it happened rich <rsoennichsen(a)gmail.com> wrote in <730ce605-fd72-4d2c-8954-6cd81feb5096(a)23g2000pre.googlegroups.com>: >> One resistor: >> >> ftp://jjlarkin.lmi.net/LED_boost.JPG >> >> John > >FTP link does not work... Works OK here.
From: Dave Platt on 25 May 2010 14:21 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. Google "Joule Thief". This is the way it's usually done in small LED flashlights - the sorts powered by one or two AA or button cells. -- Dave Platt <dplatt(a)radagast.org> AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads!
From: Tim Wescott on 25 May 2010 15:16
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"? -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com |