Blue LED at 3.3V
in [Design]
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From: John Fields on 31 May 2010 08:34 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. --- > Suppose the average current during the pulse is 50 mA. Doing that for 4 >every 10,000 nanoseconds would make the average current 20 microamps. > > Since a usual blue LED is likely to be more efficient at 50 mA than at >20 uA, I would expect it to be brighter with this than with 20 uA steady >DC. (Ratio of photometric output to current usually peaks around 1.5-4 >mA with these LEDs.) So, I would expect brightness typical of 30-40 >microamps steady DC. That sounds to me on the dim side for an indicator >LED, but I do expect this to be visible. --- OK. --- > If this blue LED is switched to 3.3V through a MOSFET, such as in a CMOS >IC, then the full 3.3V is available. --- No, there will be some drop across the MOSFET. --- >A fair number of blue LEDs nowadays >on average need no more than 3.3V to push 20 mA through them, and that >would make them so bright at 20 mA that some of these come with warnings >to not stare into them. So, it sounds easy to me to get half a milliamp >or a milliamp through them from 3.3V without a boosting circuit. And at >this much current, many blue LEDs get plenty bright. --- I don't have any data for Vcc = 3.3V, but for HC at 2V with a 1mA load I see a drop of about 400mV in the gate's output, so you're probably right.
From: John Larkin on 31 May 2010 13:17 On Mon, 31 May 2010 01:17:08 +0000 (UTC), don(a)manx.misty.com (Don Klipstein) wrote: > If this blue LED is switched to 3.3V through a MOSFET, such as in a CMOS >IC, then the full 3.3V is available. A fair number of blue LEDs nowadays >on average need no more than 3.3V to push 20 mA through them, and that >would make them so bright at 20 mA that some of these come with warnings >to not stare into them. So, it sounds easy to me to get half a milliamp >or a milliamp through them from 3.3V without a boosting circuit. And at >this much current, many blue LEDs get plenty bright. We use blue LEDs on our VME modules to indicate bus access. I started with Cree SiC parts, driven by two paralleled 74F38 sections, 5 volts, 27 ohms. That was just barely OK. As the parts kept getting better, my customers started complaining about the blinding blue LEDs. So we keep ratcheting up the resistor. We currently use a 2N7002 or BCX70 and 1K to +5. This can't go on forever, since the LEDs have to eventually stop at 100% efficiency. John
From: John Larkin on 1 Jun 2010 18:24 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? John
From: John Fields on 2 Jun 2010 20:06 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...
From: Don Klipstein on 2 Jun 2010 21:32
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. -- - Don Klipstein (don(a)misty.com) |