Blue LED at 3.3V
in [Design]
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From: John Larkin on 2 Jun 2010 21:43 On Wed, 02 Jun 2010 19:06:27 -0500, John Fields <jfields(a)austininstruments.com> wrote: >On Tue, 01 Jun 2010 15:24:36 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Mon, 31 May 2010 07:34:44 -0500, John Fields >><jfields(a)austininstruments.com> wrote: >> >>>On Mon, 31 May 2010 01:17:08 +0000 (UTC), don(a)manx.misty.com (Don >>>Klipstein) wrote: >>> >>>>In article <jno5061i4ghd8r6uo2hvvjrtgqg46jb280(a)4ax.com>, John Fields wrote >>>>in part: >>>>>On Fri, 28 May 2010 09:13:40 -0700, John Larkin >>>>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> >>>>>>>>>With that in mind it's clear that even with a perfect driver, your >>>>>>>>>circuit, which pushes 150mA spikes which decay to essentially 0mA in >>>>>>>>>4ns every 10 microseconds or so through the LED, won't work. >>>>>>>> >>>>>>>>It actually does work with a perfect driver, as your sim shows. >>>>>>> >>>>>>>--- >>>>>>>If the criterion determining "works" is whether the LED will emit >>>>>>>light which can be detected by the human eye in a casual manner, then >>>>>>>I submit that 4ns wide pulses occurring at a rep rate of 100kHz won't >>>>>>>quite fill the bill. >>>>>> >>>>>>Average current makes light. Eyeballs respond to average light. Are >>>>>>you suggesting that X microwatts of light are visible if DC, but >>>>>>invisible if the same average amount of photons arrive in bunches? >>>>> >>>>>--- >>>>>Not at all. >>>>> >>>>>What I'm suggesting is that if the duty cycle is low enough, even if >>>>>the LED is 100% efficient in turning current into light, it'll be >>>>>invisible. >>>> >>>> 150 mA decaying to near zero over a 4 nanosecond pulse repeated every 10 >>>>microseconds: (Did I get that right?) >>> >>>--- >>>Yes. >>>--- >> >>In the first sim you posted, the really fast one, the LED had an >>average forward current of close to 5 mA. Measure it. >> >>The later sims had nearly the same average LED currents. Do you know >>why? > >--- >Hmmm... > >Since you weren't up front enough to explain why, or if, that's true, >and maybe take some lumps if you were found to be wrong, it sounds to >me like you're trying to set a trap so that, no matter what my >response might be, you'd impugn it with garbage which would then have >to be shown to be garbage and refuted. > >Generally a huge PITA since the rate of new garbage in from a >detractor seems to increase as the old garbage in is disposed of. > >As usual, it's much easier for a scoundrel to make up charges than it >is for his target to refute them, so unless you reveal what you're >holding in abeyance, I'm outta here... > OK, you don't know why all of your sims pumped about 5 mA into the LED. Of course it's because a charge pump outputs an average current I = C*V*F where V is how much the voltage across the cap changes every cycle. Assuming a typical diode and blue LED, when the schmitt is low, there's about 2.7 volts across the cap. When it's high, there's about 0.3 in the other direction. So delta-V is around 3. At 150 KHz and 10 nF, that works out to 4.5 mA, close enough. It's that simple. As long as the schmitt can get close to the rails by the end of each transition, the internal impedance of the schmitt, and its rise/fall times, don't matter. What's weird is that you used sims to prove that "my" circuit wouldn't work, and all of your sims showed that it *does* work, but you wouldn't believe it. John
From: John Larkin on 2 Jun 2010 22:43 On Thu, 3 Jun 2010 01:32:35 +0000 (UTC), don(a)manx.misty.com (Don Klipstein) wrote: >In article <0v1b06d3jmrjo2me91oqn8g9hi01e8jt7r(a)4ax.com>, John Larkin wrote: >>On Mon, 31 May 2010 07:34:44 -0500, John Fields >><jfields(a)austininstruments.com> wrote: >> >>>On Mon, 31 May 2010 01:17:08 +0000 (UTC), don(a)manx.misty.com (Don >>>Klipstein) wrote: >>> >>>>In article <jno5061i4ghd8r6uo2hvvjrtgqg46jb280(a)4ax.com>, John Fields wrote >>>>in part: >>>>>On Fri, 28 May 2010 09:13:40 -0700, John Larkin >>>>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> >>>>>>>>>With that in mind it's clear that even with a perfect driver, your >>>>>>>>>circuit, which pushes 150mA spikes which decay to essentially 0mA in >>>>>>>>>4ns every 10 microseconds or so through the LED, won't work. >>>>>>>> >>>>>>>>It actually does work with a perfect driver, as your sim shows. >>>>>>> >>>>>>>--- >>>>>>>If the criterion determining "works" is whether the LED will emit >>>>>>>light which can be detected by the human eye in a casual manner, then >>>>>>>I submit that 4ns wide pulses occurring at a rep rate of 100kHz won't >>>>>>>quite fill the bill. >>>>>> >>>>>>Average current makes light. Eyeballs respond to average light. Are >>>>>>you suggesting that X microwatts of light are visible if DC, but >>>>>>invisible if the same average amount of photons arrive in bunches? >>>>> >>>>>--- >>>>>Not at all. >>>>> >>>>>What I'm suggesting is that if the duty cycle is low enough, even if >>>>>the LED is 100% efficient in turning current into light, it'll be >>>>>invisible. >>>> >>>> 150 mA decaying to near zero over a 4 nanosecond pulse repeated every 10 >>>>microseconds: (Did I get that right?) >>> >>>--- >>>Yes. >>>--- >> >>In the first sim you posted, the really fast one, the LED had an >>average forward current of close to 5 mA. Measure it. >> >>The later sims had nearly the same average LED currents. Do you know >>why? > > In that case, the blue LED will really shine. Even if the current flows >through the LED during high current pulses that the LED handles half as >efficiently as it handles 5 mA steady DC, the LED will still be plenty >bright. Consider how bright most of these LEDs appear with 2-2.5 mA of >steady DC. That is usually 12.5-16% of the brightness that they achieve >with 20 mA of steady DC. Many of these are characterized at 20 mA and are >rated 30 mA maximum average and a fair subset of those come with warnings >that staring at them could cauase eye damage. > > They are usually "good-and-bright" at 1 mA and they usually >achieve typical "indicator LED brightness" at ~~ .5 mA. The nice Infineon right-angle surface-mount blues look good as panel indicators, backlighting a window in a sticker, at 2.5 mA. Our next gadget will use light pipes to go from a top-firing surfmount LED to just below a sliding cover. Under ideal conditions, a good Agilent green LED up against my eyeball, dark adapted, I could just detect light at about 700 pA. John
From: JosephKK on 3 Jun 2010 07:36 On Tue, 25 May 2010 18:16:27 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >On a sunny day (Tue, 25 May 2010 11:00:42 -0700) it happened John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in ><b93ov5hd2c0t71ma2adoaejiop1o81615j(a)4ax.com>: > >>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. >> >>Or use a booster. >> >>John > >Yup, my blue one drops 2.66 V >Extremely bright at 3.5 mA. > ftp://panteltje.com/pub/low_current_LEDs_img_1964.jpg >in that picture it is at 10% PWM with 180 Ohm in series fro ma 3.3V PIC output. > Yikes. What are the greens running, 0.5 mA?
From: Don Klipstein on 3 Jun 2010 18:57 In article <1s4f065mqg26l3vj71ct3hk9akb9eaa84p(a)4ax.com>, JosephKK wrote: >On Tue, 25 May 2010 18:16:27 GMT, Jan Panteltje ><pNaonStpealmtje(a)yahoo.com> wrote: > >>On a sunny day (Tue, 25 May 2010 11:00:42 -0700) it happened John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >><b93ov5hd2c0t71ma2adoaejiop1o81615j(a)4ax.com>: >> >>>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. >>> >>>Or use a booster. >>> >>>John >> >>Yup, my blue one drops 2.66 V >>Extremely bright at 3.5 mA. >> ftp://panteltje.com/pub/low_current_LEDs_img_1964.jpg >>in that picture it is at 10% PWM with 180 Ohm in series fro ma 3.3V PIC >>output. >> >Yikes. What are the greens running, 0.5 mA? I sem to figure the greens may be dropping 2.6 volts. So, I figure about 4 mA, 10% of the time to get an average current of .4 mA. Since these LEDs are probably a little more efficient at 4 mA than at .4 mA, I would expect brightness about typical for .5 mA. And I have some Cree and Nichia green LEDs that are plenty bright at .5 mA. -- - Don Klipstein (don(a)misty.com)
From: John Larkin on 3 Jun 2010 19:08
On Thu, 3 Jun 2010 22:57:45 +0000 (UTC), don(a)manx.misty.com (Don Klipstein) wrote: >In article <1s4f065mqg26l3vj71ct3hk9akb9eaa84p(a)4ax.com>, JosephKK wrote: > >>On Tue, 25 May 2010 18:16:27 GMT, Jan Panteltje >><pNaonStpealmtje(a)yahoo.com> wrote: >> >>>On a sunny day (Tue, 25 May 2010 11:00:42 -0700) it happened John Larkin >>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>><b93ov5hd2c0t71ma2adoaejiop1o81615j(a)4ax.com>: >>> >>>>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. >>>> >>>>Or use a booster. >>>> >>>>John >>> >>>Yup, my blue one drops 2.66 V >>>Extremely bright at 3.5 mA. >>> ftp://panteltje.com/pub/low_current_LEDs_img_1964.jpg > >>>in that picture it is at 10% PWM with 180 Ohm in series fro ma 3.3V PIC >>>output. >>> >>Yikes. What are the greens running, 0.5 mA? > > I sem to figure the greens may be dropping 2.6 volts. So, I figure >about 4 mA, 10% of the time to get an average current of .4 mA. Since >these LEDs are probably a little more efficient at 4 mA than at .4 mA, >I would expect brightness about typical for .5 mA. And I have some Cree >and Nichia green LEDs that are plenty bright at .5 mA. My Osram greens, beautiful parts, are "dim" at 0.5 mA, 2.62 volts, and "bright" at 5 mA, 2.9 volts. John |