Blue LED at 3.3V
in [Design]
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From: Jim Thompson on 27 May 2010 19:02 On Thu, 27 May 2010 11:13:56 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Wed, 26 May 2010 22:01:00 -0400, Spehro Pefhany ><speffSNIP(a)interlogDOTyou.knowwhat> wrote: > >>On Wed, 26 May 2010 21:36:02 -0400, the renowned Phil Hobbs >><pcdhSpamMeSenseless(a)electrooptical.net> wrote: >> >>> >>>Might need a cap at the input, but otherwise OK, I think. If you make >>>the resistor big enough, the input capacitance of the inverter should >>>make it work fine. Nice low parts count. >>> >>>Cheers >>> >>>Phil Hobbs >> >>A typical blue LED won't actually be off with a Si diode in series and >>3.3V, even without considering temperature and tolerance on the 3.3V. >> >>Can be fixed with two Si diodes in series the way I drew it (but even >>that might be a bit marginal) or use a red LED for the diode. >> > >This is nice: > > +3.3 > | > | > led > | > | > c > +3.3-------R-------b BCX70K > e > | > | > | > gnd > > >R is 330K maybe. Current will be around 5 mA down to Vcc-Vled of maybe >0.2 volts. The BCXs are beta-binned, plenty tight enough for an LED. > >John > Surf on "LM3909 replacement with low power consumption" ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: Bitrex on 28 May 2010 19:56 > Bitrex's circuit likes an asymmetric duty cycle, which is maybe why he > has the thresholds where they are. The asymmetry struck me > immediately, because an HC gate oscillator would be nearly symmetric. > I'm still new to LTSpice, I was a little confused about how the model sets the thresholds and hysteresis. In datasheets the hysteresis is defined as the difference between the high transition point and the low transition point, in the model Vh seems to be the difference between the high and low transition points and the midpoint, which is defined by the Vt parameter. I think I've rectified most of my errors now...:)
From: John Larkin on 29 May 2010 01:06 On Fri, 28 May 2010 19:56:36 -0400, Bitrex <bitrex(a)de.lete.earthlink.net> wrote: > >> Bitrex's circuit likes an asymmetric duty cycle, which is maybe why he >> has the thresholds where they are. The asymmetry struck me >> immediately, because an HC gate oscillator would be nearly symmetric. >> > >I'm still new to LTSpice, I was a little confused about how the model >sets the thresholds and hysteresis. In datasheets the hysteresis is >defined as the difference between the high transition point and the low >transition point, in the model Vh seems to be the difference between the >high and low transition points and the midpoint, which is defined by the >Vt parameter. I think I've rectified most of my errors now...:) If the Spice definitions aren't clear, you can just look at the gate input triangle to see where it actually switches. Most cmos schmitts will have their switch limits straddling Vcc/2, maybe a bit less. A simple RC (or R with no obvious C!) should make a pretty symmetric square wave. I wonder how big R can be. 1 Gohm easily. Maybe 1 Tohm if everything is clean. An open cmos gate input will hang high or low, whichever way you leave it, for seconds at least. I had one floating cmos level last week that took about 15 seconds after powerup to change state. That might roughly correspond to a pA of leakage. John
From: John Fields on 30 May 2010 19:49 On Fri, 28 May 2010 09:13:40 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Fri, 28 May 2010 09:54:50 -0500, John Fields ><jfields(a)austininstruments.com> wrote: >> >I should hope it works. It's a simple charge pump. Your latest example >puts about 5.5 mA average forward current into the LED. You have >proved that my charge pump works, even with the unrealistic schmitt >model. --- Well, I must admit that I dropped the ball on this one, but I think you have too, in that you haven't considered how much current an HC14 (which is what I think the OP said he wanted to use) can source or sink using a 3.3V supply. I'd supply the numbers _I_ found, but since you seem to think it's everyone's job but yours to flesh out the skeletons you dangle in front of them, I'll leave that up to you, if you choose to pursue it. --- >>>>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. --- >Those big roadside billboards use multiplexed LEDs, each LED operating >at a low duty cycle. These are invisible to you? --- Straw man. --- >>>It works better with a real-world CMOS part and properly chosen values. >> >>--- >>Neither of which you addressed earlier, with your cockamamie circuit, >>while now you're pontificating as if you did. >>--- >> >>>You don't believe in charge pumps? >> >>--- >>Idiot. >> >>What's that supposed to do, put me on the defensive? > > >If you claim charge pumps can't work, you've done that to yourself. --- But, since I've never claimed that charge pumps, per se, can't work, your statement is nonsensical. --- >If you can't come up with values to make this charge pump work nicely, >you've compounded the damage. --- Surely, since you've taken ownership of the circuit away from Speff, it's incumbent on you, not me, to do the grunt work required to get a satisfactory solution for the OP if he can't work it out for himself. --- >You so badly want it not to work that >you've stopped thinking like an engineer. --- Au contraire! I noticed, initially, that in your circuit you had drawn in a Schmitt buffer instead of an inverter and commented that your circuit couldn't work as drawn which was, and still is, true. That's engineering thinking, I think, and I was just pointing out something anyone in your organization should comment on, and which you'd welcome, during a design review You, of course, since we seem to be at odds with each other and you want to be the king of sci.electronics.design, took umbrage at my comment and, instead of a simple "Thanks, good catch," (which you posted re. your earlier data sheet error on which I commented), decided to play the "everyone knew what I meant" game instead of owning up to having fucked up. No big deal back then... --- >That's a really bad sign, >letting your emotions suspend your ability to design. --- Why would you think that emotions suspend one's ability to design? have you never heard Mozart? --- >If you wanted it to work, you'd let it work. --- It's not me that's holding it back, if _you_ want to prove it works, flesh it out. Let's see what you've got. --- >In all the cases you've simulated, it does work. So many people do >things like this that it's hardly "my" circuit. It was done with tubes >before I was born. --- Then it should be simple for you to come up with something which really works instead of trying to slough the work off on someone -anyone- else. JF
From: John Fields on 30 May 2010 20:14
On Fri, 28 May 2010 22:06:53 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Fri, 28 May 2010 19:56:36 -0400, Bitrex ><bitrex(a)de.lete.earthlink.net> wrote: > >> >>> Bitrex's circuit likes an asymmetric duty cycle, which is maybe why he >>> has the thresholds where they are. The asymmetry struck me >>> immediately, because an HC gate oscillator would be nearly symmetric. >>> >> >>I'm still new to LTSpice, I was a little confused about how the model >>sets the thresholds and hysteresis. In datasheets the hysteresis is >>defined as the difference between the high transition point and the low >>transition point, in the model Vh seems to be the difference between the >>high and low transition points and the midpoint, which is defined by the >>Vt parameter. I think I've rectified most of my errors now...:) > >If the Spice definitions aren't clear, you can just look at the gate >input triangle to see where it actually switches. > >Most cmos schmitts will have their switch limits straddling Vcc/2, --- None that I've seen... --- >maybe a bit less. --- How much is "a bit less"? Take a look at Philips' HC14. it looks more like ECL than CMOS. --- >Take a look at A simple RC (or R with no obvious C!) should make a >pretty symmetric square wave. --- Now there's the John Larkin I love... Vague, himself, while demanding quantitative data from his critics... --- >I wonder how big R can be. 1 Gohm easily. Maybe 1 Tohm if everything >is clean. An open cmos gate input will hang high or low, whichever way >you leave it, for seconds at least. I had one floating cmos level last >week that took about 15 seconds after powerup to change state. That >might roughly correspond to a pA of leakage. --- See what I mean? |