On Sep 11, 4:19=A0pm, John Larkin <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:> On Tue, 11 Sep 2012 00:32:46 -0700 (PDT), whit3rd <whit...@gmail.com> > wrote: > > >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. =A0Pay the extra nickel, use a REAL > >reference. =A0LM4140adj =A0is suitable, just add two > >resistors (the op amp is built in). > > I don't want a voltage reference, I want a low-capacitance current > source.You are being disingenuous. But you want a more or less stable current source, and the compensating drifts of the LED and the transistor gives the stable voltage that controls the current output. The transistor makes for a simpler, cheaper and more compact output than anything fast enough/low capacitance enough for you that you could wrap around an LM4140. If you needed something more accurate, you'd need a better reference voltage, but it seems that here you can get away with being a cheapskate. -- Bill Sloman, Nijmegen
LED reference current source
Started by ●September 9, 2012
Reply by ●September 11, 20122012-09-11
Reply by ●September 11, 20122012-09-11
On 09/11/2012 01:08 PM, John Larkin wrote:> On Tue, 11 Sep 2012 06:21:01 +0100, John Devereux > <john@devereux.me.uk> wrote: > >> 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! :) > > I think some science-project fun could be had with a windowed eprom. > > The first CMOS imagers were de-capped DRAM chips, where light affected > refresh times. I think some guys at JPL did that. > >Maybe Jim Janesick? Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●September 11, 20122012-09-11
On 09/11/2012 01:55 PM, Bill Sloman wrote:> On Sep 11, 4:19 pm, John Larkin > <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> On Tue, 11 Sep 2012 00:32:46 -0700 (PDT), whit3rd<whit...@gmail.com> >> wrote: >> >>> 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). >> >> I don't want a voltage reference, I want a low-capacitance current >> source. > > You are being disingenuous. > > But you want a more or less stable current source, and the > compensating drifts of the LED and the transistor gives the stable > voltage that controls the current output. The transistor makes for a > simpler, cheaper and more compact output than anything fast enough/low > capacitance enough for you that you could wrap around an LM4140. > > If you needed something more accurate, you'd need a better reference > voltage, but it seems that here you can get away with being a > cheapskate. >Congratulations! You've just won this months SED Snarkiest Way Of Agreeing With Somebody Award. This prestigious honour is hotly contested--it's almost as sought after as the Stupidest Flame War Award. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●September 11, 20122012-09-11
On Tue, 11 Sep 2012 10:55:24 -0700 (PDT), Bill Sloman <bill.sloman@ieee.org> wrote:>On Sep 11, 4:19�pm, John Larkin ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> On Tue, 11 Sep 2012 00:32:46 -0700 (PDT), whit3rd <whit...@gmail.com> >> wrote: >> >> >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). >> >> I don't want a voltage reference, I want a low-capacitance current >> source. > >You are being disingenuous. > >But you want a more or less stable current source, and the >compensating drifts of the LED and the transistor gives the stable >voltage that controls the current output. The transistor makes for a >simpler, cheaper and more compact output than anything fast enough/low >capacitance enough for you that you could wrap around an LM4140. > >If you needed something more accurate, you'd need a better reference >voltage, but it seems that here you can get away with being a >cheapskate.The board is 2" x 2" and it's brickwalled with parts. This current source is small, cute, glows a nice warm orange color, and is plenty good enough for my needs here. Is there some virtue to wasting more board area and more money, for no advantage? Actually, I've known some engineers who would usually select a more expensive part, because it apparently made them feel more important. I measured a TC of 90 PPM/degC, which is better than the spec on the metal-film emitter resistor on the breadboard. And as Phil points out, this circuit will have much lower noise than something based on a bandgap reference and an opamp. The LM4040s that we have in stock have TC specs of 150 PPM/degC. Cute circuit. I like it. -- 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 11, 20122012-09-11
On Tue, 11 Sep 2012 14:27:58 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 09/11/2012 01:55 PM, Bill Sloman wrote: >> On Sep 11, 4:19 pm, John Larkin >> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>> On Tue, 11 Sep 2012 00:32:46 -0700 (PDT), whit3rd<whit...@gmail.com> >>> wrote: >>> >>>> 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). >>> >>> I don't want a voltage reference, I want a low-capacitance current >>> source. >> >> You are being disingenuous. >> >> But you want a more or less stable current source, and the >> compensating drifts of the LED and the transistor gives the stable >> voltage that controls the current output. The transistor makes for a >> simpler, cheaper and more compact output than anything fast enough/low >> capacitance enough for you that you could wrap around an LM4140. >> >> If you needed something more accurate, you'd need a better reference >> voltage, but it seems that here you can get away with being a >> cheapskate. >> > >Congratulations! You've just won this months SED Snarkiest Way Of >Agreeing With Somebody Award. This prestigious honour is hotly >contested--it's almost as sought after as the Stupidest Flame War Award. > >Cheers > >Phil HobbsHey, it's only September 11. Give some other guys a chance. -- 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 11, 20122012-09-11
On Tuesday, September 11, 2012 7:19:15 AM UTC-7, John Larkin wrote:> On Tue, 11 Sep 2012 00:32:46 -0700 (PDT), whit3rd <whit3rd@gmail.com> > > wrote: > > > > >On Monday, September 10, 2012 7:53:37 PM UTC-7, John Larkin wrote: > > > > > >[about light sensitivity of a LED used as voltage reference]> > >... Pay the extra nickel, use a REAL > > >reference. LM4140adj is suitable, just add two > > >resistors (the op amp is built in). > > > > I don't want a voltage reference, I want a low-capacitance current > > source.[warning: bad ASCII art follows] (+)----+-----+ | | \-\ R1 ^ | / \----+ / \ | --+-- | | V +---| | \ R2 | | (current out) (GND) with added bypass capacitors in appropriate places... It has a temperature compensated voltage reference, gain, and drives the transistor irrespective of Vbe drop. The LM4041-adj is a kinda backward connected TL431, it regulates the cathode-to-sense voltage instead of the anode-to-sense voltage.
Reply by ●September 11, 20122012-09-11
On Tuesday, September 11, 2012 12:29:34 PM UTC-7, whit3rd wrote:> > >... Pay the extra nickel, use a REAL > > >reference. LM4140adj is suitable, just add two > > >resistors (the op amp is built in).> ...The LM4041-adj > > is a kinda backward connected TL431, it regulates the cathode-to-sense > > voltage instead of the anode-to-sense voltage.My apologies for the wrong part number in the first posting... Typical tempco (not worst-case) for the LM4140 is better than the measured performance of the LED circuit; if the LED were a perfect planar diode, and matched the diode equation, both the transistor Vbe and LED Vak would track at 1/T (at room temperature, circa 3400 ppm/C), and so would the transistor emitter current. Collector current will have a bit of correction for base current, of course.
Reply by ●September 11, 20122012-09-11
On Tuesday, September 11, 2012 12:29:34 PM UTC-7, whit3rd wrote:> > >... Pay the extra nickel, use a REAL > > >reference. LM4140adj is suitable, just add two > > >resistors (the op amp is built in).> ...The LM4041-adj > > is a kinda backward connected TL431, it regulates the cathode-to-sense > > voltage instead of the anode-to-sense voltage.My apologies for the wrong part number in the first posting... Typical tempco (not worst-case) for the LM4041-adj is better than the measured performance of the LED circuit; if the LED were a perfect planar diode, and matched the diode equation, both the transistor Vbe and LED Vak would track at 1/T (at room temperature, circa 3400 ppm/C), and so would the transistor emitter current. Collector current will differ a bit because of base current, of course.
Reply by ●September 11, 20122012-09-11
On Tue, 11 Sep 2012 12:55:56 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:>On Tuesday, September 11, 2012 12:29:34 PM UTC-7, whit3rd wrote: > >> > >... Pay the extra nickel, use a REAL >> > >reference. LM4140adj is suitable, just add two >> > >resistors (the op amp is built in). > >> ...The LM4041-adj >> >> is a kinda backward connected TL431, it regulates the cathode-to-sense >> >> voltage instead of the anode-to-sense voltage. > >My apologies for the wrong part number in the first posting... > >Typical tempco (not worst-case) for the LM4041-adj is better than the >measured performance of the LED circuit; if the LED were a perfect >planar diode, and matched the diode equation, both the >transistor Vbe and LED Vak would track at 1/T (at room >temperature, circa 3400 ppm/C), and so would the >transistor emitter current. Collector current will differ >a bit because of base current, of course.That would work, but I'd want to bypass the 4041, and add a base resistor, to better control the impedance that the base of the PNP sees. But it wouldn't glow in the dark! -- 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 11, 20122012-09-11
On Sep 11, 8:30=A0pm, John Larkin <jlar...@highlandtechnology.com> wrote:> On Tue, 11 Sep 2012 10:55:24 -0700 (PDT),BillSloman > > <bill.slo...@ieee.org> wrote: > >On Sep 11, 4:19=A0pm, John Larkin > ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > >> On Tue, 11 Sep 2012 00:32:46 -0700 (PDT), whit3rd <whit...@gmail.com> > >> wrote: > > >> >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 expe=ct> > >> >> 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. =A0Pay the extra nickel, use a REAL > >> >reference. =A0LM4140adj =A0is suitable, just add two > >> >resistors (the op amp is built in). > > >> I don't want a voltage reference, I want a low-capacitance current > >> source. > > >You are being disingenuous. > > >But you want a more or less stable current source, and the > >compensating drifts of the LED and the transistor gives the stable > >voltage that controls the current output. The transistor makes for a > >simpler, cheaper and more compact output than anything fast enough/low > >capacitance enough for you that you could wrap around an LM4140. > > >If you needed something more accurate, you'd need a better reference > >voltage, but it seems that here you can get away with being a > >cheapskate. > > The board is 2" x 2" and it's brickwalled with parts. This current > source is small, cute, glows a nice warm orange color, and is plenty > good enough for my needs here. > > Is there some virtue to wasting more board area and more money, for no > advantage?Absolutely none. One definition of an engineer is somebody who can do for $1 what any fool can do for $2. Cheapskate is a term of approbation in this context, though Phil Hobbs - as a physicist - doesn't seem to realise this.> Actually, I've known some engineers who would usually > select a more expensive part, because it apparently made them feel > more important.The world is full of lunatics, but lunacy tends to take different forms in different people.> I measured a TC of 90 PPM/degC, which is better than the spec on the > metal-film emitter resistor on the breadboard.I tended to use 15ppm/C metal film resistors where it mattered - they aren't that expensive and they are easy to get hold of.> And as Phil points out, > this circuit will have much lower noise than something based on a > bandgap reference and an op amp.Perhaps not. You've probably got a lot more bandwidth than you'd end up with if you were engineering something around a bandgap reference. Phil's being a physicist rather than thinking about the circuit.> The LM4040s that we have in stock have TC specs of 150 PPM/degC. > > Cute circuit. I like it.You invented it - you mightn't have been the first to invent it, but you still love it in the same way that we all love our brain-children. -- Bill Sloman, Nijmegen
