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
-- 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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Good paper, thanks for the reference.
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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
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
--
| 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 |
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I love to cook with wine. Sometimes I even put it in the food.
We used to run 50 mA through the Cree blue SiC led's as a VME bus-activity indicator, a pair of F38s and a 33 ohm resistor or something like that. As time went on and led's got better, we started blinding our customers. We use about 1 mA now, into the Osrams.
I got dark adapted and tried to find the minimum current that a green Avago led needs to produce detectable light. It was around 800 pA.
-- 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
Here are some numbers for the Osram LB T67C series, beautiful parts.
-- 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
Wow, that's impressive, except that I need PNP in this case. I figured I'd use an MMBTH81, min beta 60, kinda high capacitance but tolerable in this application, since I'm charging a capacitor anyhow. I'm doing a one-shot sort of thing that's not too critical, pulse width programmable from 2 ns to 200 ps.
PNPs suck. NE97833 has the speeed and capacitance of the NESG4030, but typical beta is 30. Maybe I could live with that, since beta changes, wot, 1% per degree or something.
-- 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
Great data; thanks for that one.
My orange Osram may compensate the Vbe pretty well. My board is only
2" square, and it's brickwalled with parts already, and it needs a power indicator anyhow, so I figured I'd use the LED twice.-- 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
The dynamic impedance is pretty low, so in the voltage-reference mode, you can wrap an op amp around it to provide its own bias. The benefit is mainly getting low noise at low impedance and low voltage without needing BFCs. You can probably do a series-shunt feedback approach to combine the V_BE drop and constant excitation current, but I haven't looked at it in any detail.
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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
...
o s=A0 =A0 ...Jim Thompson
yeh, from Johns data it looks like they about par with Zeners in dynamic impedance
-Lasse
In my circuit, I think you can add a resistor from the PNP emitter to ground, to cancel the non-zero dynamic impedance of the LED. Too bad I'm out of room on this board. My supply is pretty stable anyhow, and I don't need 10-bit accuracy on the timing here.
-- 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
Red led? I've seen this many times in Elektor.
-- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------
It's a power-on indicator, too!
-- 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
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?
Jon
I've
;-) I used the above circuit with green (SiC) LEDs directly off a LiIon battery to keep the intensity reasonably constant. A 20% variation over the
3.3-4.2V range was close enough (IIRC, it did a little better than that).
The problem I had with Osram was that they melted in the RoHS process. KingBright had a much better package.
on? I've
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.
-- Bill Sloman, Nijmegen
tion? I've
D t eI've seen learning remotes that used the IR led as a receiver when learning codes from other remotes, but I wonder how much external light will affect a LED that is already on.
might wanna check if that current source gets a 50/60Hz ac on it when exposed to light
-Lasse
I've
Yes, it has. What is your contribution?
Numbers matter. Nanoamps over milliamps equals parts per million.
It's easy enough to try. I'm soldering up a breadboard now.
-- 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
I've
LEDs are really crappy photodiodes, though, because they're direct-bandgap devices with only a few nanoseconds' minority carrier lifetime. High-level injection conditions (e.g. a small diode with a few milliamps of forward bias) make them a lot crappier.
It would be very difficult to get more than a few hundred nanoamps out of a LED used as a photodiode in indoor ambient conditions, even with the box taken apart. So assuming it has a few milliamps' forward bias, that's maybe 1 part in 10**4 if you're unlucky, i.e. about 2.6 uV out of a volt or so. Since nobody is proposing to use LEDs for references for
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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
I've seen learning remotes that used the IR led as a receiver when learning codes from other remotes, but I wonder how much external light will affect a LED that is already on.
might wanna check if that current source gets a 50/60Hz ac on it when exposed to light
++++++++++++++++++++++++++++++++++++++++++++Also check under CFL light. They put out a lot of trash.
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