On Mon, 10 Sep 2012 17:19:58 -0400, "krw@att.bizzzzzzzzzzzz" <krw@att.bizzzzzzzzzzzz> wrote:>On Mon, 10 Sep 2012 08:23:35 -0700, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >>On Mon, 10 Sep 2012 01:22:29 -0700, miso <miso@sushi.com> wrote: >> >>>On 9/9/2012 10:21 PM, Grant 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 >>>> >>>> I saw it decades ago for audio amp bias, claim was green LED >>>> (old-style now, not high modern intensity types) balanced Vbe >>>> tempco. >>>> >>>> Grant. >>>> >>>The high efficiency LEDs are double heterojuntion. When they first came >>>out, the existing generation LED drives had insufficient compliance and >>>produced a dimmer light than the standard LEDs. That is the high voltage >>>of the double heterojunction LED made the drive chips put out less current. >>> >>>It wasn't like the LED companies gave the chip companies a heads up on >>>the specs of the new generation LEDs so that the new driver chips would >>>be available. >> >>Here are some numbers for the Osram LB T67C series, beautiful parts. >> >>https://dl.dropbox.com/u/53724080/Parts/OsramCurrents.JPG > >The problem I had with Osram was that they melted in the RoHS process. >KingBright had a much better package.These don't. The outer white overmold is very tough. We once used some Panasonic parts that turned to mush at soldering temperatures. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
LED reference current source
Started by ●September 9, 2012
Reply by ●September 10, 20122012-09-10
Reply by ●September 10, 20122012-09-10
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.JPGOK, 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. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Reply by ●September 10, 20122012-09-10
On Mon, 10 Sep 2012 16:51:09 -0700, John Larkin <jlarkin@highlandtechnology.com> wrote:>On Mon, 10 Sep 2012 17:19:58 -0400, "krw@att.bizzzzzzzzzzzz" ><krw@att.bizzzzzzzzzzzz> wrote: > >>On Mon, 10 Sep 2012 08:23:35 -0700, John Larkin >><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >> >>>On Mon, 10 Sep 2012 01:22:29 -0700, miso <miso@sushi.com> wrote: >>> >>>>On 9/9/2012 10:21 PM, Grant 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 >>>>> >>>>> I saw it decades ago for audio amp bias, claim was green LED >>>>> (old-style now, not high modern intensity types) balanced Vbe >>>>> tempco. >>>>> >>>>> Grant. >>>>> >>>>The high efficiency LEDs are double heterojuntion. When they first came >>>>out, the existing generation LED drives had insufficient compliance and >>>>produced a dimmer light than the standard LEDs. That is the high voltage >>>>of the double heterojunction LED made the drive chips put out less current. >>>> >>>>It wasn't like the LED companies gave the chip companies a heads up on >>>>the specs of the new generation LEDs so that the new driver chips would >>>>be available. >>> >>>Here are some numbers for the Osram LB T67C series, beautiful parts. >>> >>>https://dl.dropbox.com/u/53724080/Parts/OsramCurrents.JPG >> >>The problem I had with Osram was that they melted in the RoHS process. >>KingBright had a much better package. > >These don't. The outer white overmold is very tough. We once used some >Panasonic parts that turned to mush at soldering temperatures.Osram wasn't the only manufacturer that made liquid LEDs but it must vary with the particular part numbers. The ones I was using were primarily 4-lead SMT and 0603/0805 types. They would claim RoHS but some could only take 235C. Others were maybe 250 for five seconds but I found that parts specified for>260C for ten seconds was the minimum needed.
Reply by ●September 10, 20122012-09-10
On 9/10/2012 11:14 AM, Jim Thompson wrote:> On Mon, 10 Sep 2012 09:48:23 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> "langwadt@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_Tempco.pdf >>> >>> 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/220x/9df78eab33525d08d6e5fb8d27136e95/y/e/yellow_led_3mm_3.jpg 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
Reply by ●September 10, 20122012-09-10
On Sep 11, 12:51=A0am, John Larkin <jlar...@highlandtechnology.com> wrote:> On Mon, 10 Sep 2012 14:22:03 -0700 (PDT),BillSloman > > > > > > > > > > <bill.slo...@ieee.org> wrote: > >On Sep 10, 9:34=A0pm, John Larkin <jlar...@highlandtechnology.com> > >wrote: > >> On Mon, 10 Sep 2012 01:04:36 -0400, "k...@att.bizzzzzzzzzzzz" > > >> <k...@att.bizzzzzzzzzzzz> wrote: > >> >On Sun, 09 Sep 2012 21:28:55 -0700, John Larkin > >> ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > > >> >>On Sun, 09 Sep 2012 23:59:32 -0400, "k...@att.bizzzzzzzzzzzz" > >> >><k...@att.bizzzzzzzzzzzz> wrote: > > >> >>>On Sun, 09 Sep 2012 18:50:25 -0700, John Larkin > >> >>><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > > >> >>>>Has anybody done this? > > >> >>>>https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG > > >> >>>Is there any significant matching between the LED and the Si BE jun=ction? I've> >> >>>used this (or the opposite sex) with6 really good results: > > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 Vcc > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 +--------+ > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0| > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0V -> > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0- > >> >>> =A0 =A0 =A0 =A0 =A0 =A0.-. =A0 =A0 =A0 | > >> >>> =A0 =A0 =A0 =A0 =A0 =A0| | =A0 =A0 =A0 | > >> >>> =A0 =A0 =A0 =A0 =A0 =A0| | =A0 =A0||-+ > >> >>> =A0 =A0 =A0 =A0 =A0 =A0'-' =A0 =A0||<- > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 +-----||-+ > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0| > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 =A0\| =A0 =A0 =A0| > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 |------+ > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 =A0<| =A0 =A0 =A0| > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 .-. > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 | | > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 | | > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 '-' > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0| > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 +--------+ > >> >>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 Gnd > > >> >>>(created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de) > > >> >>That will have a tempco of around -0.4% per degree C. In theory an L=ED> >> >>tc cancels the Vbe tc some. I just don't know how much. Google is > >> >>ambiguous on the subject. > > >> >>I guess I'll have to build it and try it. We have some Osram LEDs th=at> >> >>are a gorgeous orange color, about 1.8 volts drop at 1 mA, so they a=re> >> >>a good candidate. > > >> >If it's just an LED, who cares? > > >> It's a power-on indicator, too! > > >LEDS are - incidentally - photodiodes, so if you can see it, it can > >see enough of the external illumination to compromise its value as a > >voltage reference. This has already been pointed out elsewhere in this > >thread. > > Yes, it has. What is your contribution?Minimal. I'm just coupling your proposition that the LED can also be a power-on indicator with the earlier proposition that it's a photodiode. There's obviously a potential problem - it could be negligible but without numbers, I can't say. Phil Hobbs thinks he's got numbers, and can be more explicit. You seem to be willing to imagine a few numbers, which isn't quite as helpful. "Numbers matter. Nanoamps over milliamps equals parts per million." Real numbers matter. Imagined numbers are just cheap rhetoric. -- Bill Sloman, Nijmegen
Reply by ●September 10, 20122012-09-10
On Sep 11, 2:38=A0am, 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? =A0Or is thi=s> > just one of those "fun things to do"? > > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0 =A0...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. =A0On 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/G=OW48BVH/FD3IS7AGOW48BVH.LARGE.jpg> > For comparison, here's a diffuse red:http://cdn.instructables.com/FRA/EIK=Z/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
Reply by ●September 10, 20122012-09-10
Jon Kirwan wrote:> On Sun, 9 Sep 2012 22:04:13 -0400, "tm" > <No_one_home@white-house.gov> wrote: > >> >> "John Larkin"<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message >> news:iphq48dvaejq1p99fnjp2jj159pod18823@4ax.com... >>> >>> Has anybody done this? >>> >>> https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG >>> >>> -- >> >> Do you need or want the base resistor? > > He spec'd an RF transistor and JL sticks base resistors on > BJT bases like others put bypass caps on ICs. Everywhere. > > Jon..and for an _excellent_ reason that always works.
Reply by ●September 10, 20122012-09-10
On Sep 11, 2:05=A0am, John Larkin <jlar...@highlandtechnology.com> wrote:> On Sun, 09 Sep 2012 18:50:25 -0700, John Larkin > > <jjlar...@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_b... > > https://dl.dropbox.com/u/53724080/Circuits/Current_Sources/LED_Isrc_d... > > The orange looks good for my application. I don't understand what's > going on with the green.Whatever it is, it's also going on with the green LEDs measured in the paper that Lasse dug up http://www.ka-electronics.com/Images/pdf/Junction_Temperature_LED_Tem... Their green LEDs come out at about -5mV/C, about twice what you get on a transistor Vbe.> The supply regulation isn't as good as a bandgap or something, but is > OK for what I'm doing.Most cobbled-together voltage reference circuits need a bit of tuning - usually by fiddling with the current through the "reference diode" - to give respectable performance. Even the 1N823-29 reference diode series were only reference diodes at exactly 7.5mA, and to get the stability you were paying for with the 1N829 you had to make that 7.50mA pretty exact. -- Bill Sloman, Nijmegen
Reply by ●September 10, 20122012-09-10
On Mon, 10 Sep 2012 18:52:12 -0700 (PDT), Bill Sloman <bill.sloman@ieee.org> wrote:>On Sep 11, 12:51�am, John Larkin <jlar...@highlandtechnology.com> >wrote: >> On Mon, 10 Sep 2012 14:22:03 -0700 (PDT),BillSloman >> >> >> >> >> >> >> >> >> >> <bill.slo...@ieee.org> wrote: >> >On Sep 10, 9:34�pm, John Larkin <jlar...@highlandtechnology.com> >> >wrote: >> >> On Mon, 10 Sep 2012 01:04:36 -0400, "k...@att.bizzzzzzzzzzzz" >> >> >> <k...@att.bizzzzzzzzzzzz> wrote: >> >> >On Sun, 09 Sep 2012 21:28:55 -0700, John Larkin >> >> ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> >> >> >>On Sun, 09 Sep 2012 23:59:32 -0400, "k...@att.bizzzzzzzzzzzz" >> >> >><k...@att.bizzzzzzzzzzzz> wrote: >> >> >> >>>On Sun, 09 Sep 2012 18:50:25 -0700, John Larkin >> >> >>><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> >> >> >>>>Has anybody done this? >> >> >> >>>>https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG >> >> >> >>>Is there any significant matching between the LED and the Si BE junction? I've >> >> >>>used this (or the opposite sex) with6 really good results: >> >> >> >>> � � � � � � � � Vcc >> >> >>> � � � � � � +--------+ >> >> >>> � � � � � � | � � � �| >> >> >>> � � � � � � | � � � �V -> >> >> >>> � � � � � � | � � � �- >> >> >>> � � � � � �.-. � � � | >> >> >>> � � � � � �| | � � � | >> >> >>> � � � � � �| | � �||-+ >> >> >>> � � � � � �'-' � �||<- >> >> >>> � � � � � � +-----||-+ >> >> >>> � � � � � � | � � � �| >> >> >>> � � � � � � �\| � � �| >> >> >>> � � � � � � � |------+ >> >> >>> � � � � � � �<| � � �| >> >> >>> � � � � � � | � � � .-. >> >> >>> � � � � � � | � � � | | >> >> >>> � � � � � � | � � � | | >> >> >>> � � � � � � | � � � '-' >> >> >>> � � � � � � | � � � �| >> >> >>> � � � � � � +--------+ >> >> >>> � � � � � � � � Gnd >> >> >> >>>(created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de) >> >> >> >>That will have a tempco of around -0.4% per degree C. In theory an LED >> >> >>tc cancels the Vbe tc some. I just don't know how much. Google is >> >> >>ambiguous on the subject. >> >> >> >>I guess I'll have to build it and try it. We have some Osram LEDs that >> >> >>are a gorgeous orange color, about 1.8 volts drop at 1 mA, so they are >> >> >>a good candidate. >> >> >> >If it's just an LED, who cares? >> >> >> It's a power-on indicator, too! >> >> >LEDS are - incidentally - photodiodes, so if you can see it, it can >> >see enough of the external illumination to compromise its value as a >> >voltage reference. This has already been pointed out elsewhere in this >> >thread. >> >> Yes, it has. What is your contribution? > >Minimal. I'm just coupling your proposition that the LED can also be a >power-on indicator with the earlier proposition that it's a >photodiode. > >There's obviously a potential problem - it could be negligible but >without numbers, I can't say. Phil Hobbs thinks he's got numbers, and >can be more explicit. > >You seem to be willing to imagine a few numbers, which isn't quite as >helpful. > >"Numbers matter. Nanoamps over milliamps equals parts per million." > >Real numbers matter. Imagined numbers are just cheap rhetoric.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. -- 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 9/10/2012 6:25 AM, Phil Hobbs wrote:> Mr Stonebeach wrote: >> >> On Sep 10, 11:17 am, miso <m...@sushi.com> wrote: >>> On 9/9/2012 6:50 PM, John Larkin wrote: >>>> Has anybody done this? >>>> https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG >>> >>> Analog Device SSM-2220 data sheet uses that scheme. Nelson Pass amps >>> have used it in production. LED biasing was a wave or sorts in high end >>> audio. >> >> Also in SQUID readout amplifiers, like Pasquarelli & Rossi in the >> ISEC'97 conference. >> >>> The RF transistor I presume is for low capacitance. The low beta could >>> be an issue. >> >> What about NESG4030, hFE=400 typ, Cre=0.12pF typ? I know, costs way >> too much... >> >> Regards, >> Mikko > > The LED trick is about 20 dB quieter than a bandgap, and reasonably > competitive with buried zeners for noise, but of course not for > stability. I haven't done this recently enough to have tempco data on > modern LEDs. (I've only used the LED + emitter follower voltage > reference form.) > > Cheers > > Phil Hobbs >Can you elaborate on the noise aspect? After all, most bandgaps are filtered. Or are you referring to an op amp circuit using the bandgap to create the current reference? It seems questionable to make such a blanket statement about the LED reference being 20dB quieter without describing the competing circuit.
