The plots do show the yellow is the stiffest.
LED reference current source
Started by ●September 9, 2012
Reply by ●September 11, 20122012-09-11
Reply by ●September 11, 20122012-09-11
On Sun, 09 Sep 2012 18:50:25 -0700, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:> >Has anybody done this? > >https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPGCurious idea, why don't i see any indication of a light tight box around it? ?-)
Reply by ●September 11, 20122012-09-11
On Mon, 10 Sep 2012 20:50:07 -0700, josephkk <joseph_barrett@sbcglobal.net> wrote:>On Sun, 09 Sep 2012 18:50:25 -0700, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >> >>Has anybody done this? >> >>https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG > >Curious idea, why don't i see any indication of a light tight box around >it? > >?-)Because there isn't one? The LED is a voltage reference and a pilot light. What's the schematic symbol for a light-tight box? -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●September 11, 20122012-09-11
On Mon, 10 Sep 2012 17:05:40 -0700, John Larkin <jlarkin@highlandtechnology.com> wrote:>On Sun, 09 Sep 2012 18:50:25 -0700, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >> >>Has anybody done this? >> >>https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG > > >OK, here's some numbers. > >https://dl.dropbox.com/u/53724080/Circuits/Current_Sources/LED_Isrc_bb.JPG > >https://dl.dropbox.com/u/53724080/Circuits/Current_Sources/LED_Isrc_data.JPG > >The orange looks good for my application. I don't understand what's >going on with the green. > >The supply regulation isn't as good as a bandgap or something, but is >OK for what I'm doing. > >There's zero evidence of light sensitivity. I can blast my >mini-maglite, or the Mantis illuminator, into the LED and nothing >happens.Sunlight on green LEDs is used for solar tracker applications, turns up on various DIY sites. Grant.
Reply by ●September 11, 20122012-09-11
On 9/10/2012 10:01 PM, Bill Sloman wrote:> On Sep 11, 2:38 am, bitrex <bit...@de.lete.earthlink.net> wrote: >> On 9/10/2012 11:14 AM, Jim Thompson wrote: >> >> >> >> >> >> >> >> >> >>> On Mon, 10 Sep 2012 09:48:23 -0400, Phil Hobbs >>> <pcdhSpamMeSensel...@electrooptical.net> wrote: >> >>>> "langw...@fonz.dk" wrote: >> >>>>> On Sep 10, 3:50 am, John Larkin >>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>>>>> Has anybody done this? >> >>>>>> https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG >> >>>>> seen it done since forever for audio amps, I believe some claim they >>>>> are >>>>> also less noisy than regular diodes, just remember to shield the LED >>>>> from light >> >>>>> http://www.ka-electronics.com/Images/pdf/Junction_Temperature_LED_Tem... >> >>>>> looks like somewhere between red and UV will get close to zero tempco >> >>>>> -Lasse >> >>>> Good paper, thanks for the reference. >> >>>> Cheers >> >>>> Phil Hobbs >> >>> But an LED forward is not "stiff", so where's the benefit? Or is this >>> just one of those "fun things to do"? >> >>> ...Jim Thompson >> >> An interesting aside: About a year ago I built a simple "LED curve >> tracer" that used an i2c DAC, an Arduino and some PC scripting to >> automatically generate pretty plots of the I-V characteristics of LEDs, >> and ran tests on just about every LED I could get my hands on. >> >> Medium intensity diffuse yellow 3mm LEDs, like this one: >> >> http://www.taydaelectronics.com/media/catalog/product/cache/1/image/2... >> >> seemed to have a remarkably "stiff" forward voltage across a wide range >> of currents. On a log-linear plot of current v. voltage, with current >> on the x axis, the slope of the line was very slight over operating >> currents from microamps to about 35 mA. >> >> Here's the plot of the yellow LED:http://cdn.instructables.com/FD3/IS7A/GOW48BVH/FD3IS7AGOW48BVH.LARGE.jpg >> >> For comparison, here's a diffuse red:http://cdn.instructables.com/FRA/EIKZ/GOW3YQLI/FRAEIKZGOW3YQLI.LARGE.jpg >> >> and a high intensity green:http://cdn.instructables.com/F1L/MDGE/GOW48BUA/F1LMDGEGOW48BUA.LARGE.jpg > > Strange data, and an implausible fitting function. You'd expect the > LED voltage to be a logarithmic function of current at low currents, > then start going up faster at high currents as the ohmic resistances > became significant. > > You "exponential" fit is crappy at low currents, okay at medium > currents and crappy again at high currents. You need to fit a more > physically realistic function to your data. > > -- > Bill Sloman, Nijmegen >Yes, I'm aware of the shortcomings of the fit. As you say, at high currents ohmic resistance causes a deviation from log/exponential behavior. At low current, there's deviation due to base current errors from the current source feeding the LED, which was just a jellybean op amp and a couple of transistors. There are also errors due to Early effect, and the fact that in this setup the current is not sensed directly. In this case I wanted the measurement circuit to be as straightforward as possible for Arduino users/hobbyists who might not be too well versed in electronics. With a little work it should be straightforward to develop a much better current source for LED characteristic measurement, and the test firmware and script could accommodate it with very little modification.
Reply by ●September 11, 20122012-09-11
John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> writes:> On Mon, 10 Sep 2012 20:50:07 -0700, josephkk > <joseph_barrett@sbcglobal.net> wrote: > >>On Sun, 09 Sep 2012 18:50:25 -0700, John Larkin >><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >> >>> >>>Has anybody done this? >>> >>>https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG >> >>Curious idea, why don't i see any indication of a light tight box around >>it? >> >>?-) > > Because there isn't one? The LED is a voltage reference and a pilot > light.Hey, I used to be able to crash a quartz-windowed 8749 microcontroller by taking a picture of the machine it was in with a flash camera! :) -- John Devereux
Reply by ●September 11, 20122012-09-11
On Sun, 09 Sep 2012 19:01:02 -0700, Jon Kirwan <jonk@infinitefactors.org> wrote:>On Sun, 09 Sep 2012 18:50:25 -0700, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >>Has anybody done this? >> >>https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG > >Must have been, since I just saw some discussion about it on >LTspice group less than a month ago. > >JonJust doing a few searches found these: http://www.ciphersbyritter.com/RADELECT/MEASNOIS/NOISMEA1.HTM Also, AN-211 from Analog Devices discussing the use of a pair of back to back 2N3904 BE junctions zenered to supply a 15V reference with very little noise (bypassed with 10uF tants.) See Figure 3 on it, Q14 through Q17. And check out DIYAudio, from 2002: http://www.diyaudio.com/forums/parts/35821-some-noise-measurements-leds-z= ener-diodes.html Jon
Reply by ●September 11, 20122012-09-11
On Sunday, September 9, 2012 7:03:48 PM UTC-7, Jim Thompson wrote:> On Sun, 09 Sep 2012 18:50:25 -0700, John Larkin > > <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:> >Has anybody done this?> >https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG > > > > What is it supposed to buy you? Does the LED forward TC match a > > Silicon Vbe TC?Both the LED and the transistor have the same 0.3% per Celsius temperature sensitivity, and nothing cancels. The current through the resistor would only remain constant if the resistor had a compensatory thermal characteristic, and was heatsinked to the other devices. There's also the issue of photons getting at the sensitive part of the LED junction. Photosensitive references: just say NO. A way to make a temperature-dependent diode drop into a non temperature dependent reference is well known: it's the bandgap reference.
Reply by ●September 11, 20122012-09-11
On 10/09/2012 21:15, Jon Kirwan wrote:> On Mon, 10 Sep 2012 09:11:10 +0100, Martin Brown > <|||newspam|||@nezumi.demon.co.uk> wrote: > >> On 10/09/2012 08:26, Tim Williams wrote:>>> I once had a chemistry lab about bandgap. We took a battery, LED and >>> styrofoam cup of LN2, and proceeded to observe the color change upon >>> putting them together. Theory from lecture said, cold --> crystal >>> shrinks --> wavelength shrinks. >> >> That LN2 demo was always much more fun with the early generation of low >> efficiency red LEDs. The quantum efficiency rockets up when the lattice >> is cooled and thermal vibrations are decreased much more rigid lattice. >> Unfortunately they die after a few hundred cycles or less. >> >> I don't recall seeing the wavelength shift perceptably though the >> brightness changed by a very significant amount. Admitedly on a red led >> this could be both quantum efficiency and it producing shorter >> wavelength red light which is perceived as bright. >> <snip> > > This may be due to human color perception sensitivities to > wavelength shift in the red region, which is quite poor per > unit wavelength shift. Discrimination between nearby > wavelengths in the red region is not nearly as good as > elsewhere. Did you use a spectrophotometer?No. It was done as a public demonstration of early light emitting semiconductors and was entirely qualitative. I will try it with a spectrograph next time I am near some LN2 (and have spare time). -- Regards, Martin Brown
Reply by ●September 11, 20122012-09-11
On Monday, September 10, 2012 7:53:37 PM UTC-7, John Larkin wrote: [about light sensitivity of a LED used as voltage reference]> Well, I measured it this afternoon. There is no discernable > > photoelectric effect, even with intense illumination. I didn't expect > > any, and Phil didn't expect any.When I put a light chopper on a laser and detect a weak response signal with a lock-in amplifier, the sensitivity will be much more clear. Pay the extra nickel, use a REAL reference. LM4140adj is suitable, just add two resistors (the op amp is built in).
