Lowest current LED color - Green mystery LEDs - Page 1
Forum section: electronics / projects

author: antenna
author_rank: Frequent Contributor
timestamp: November 25, 2023, 12:37:49 am
content: I just watched a video by IMSAI guy about capacitance leakage testers and one example he had was a simple series arrangement of resistor, capacitor and LED. He states that blue has the lowest turn-on current (which I have heard elsewhere), but when testing this myself, I found that I have a few green ones that take far less current to light up.I am using old LEDs from a battery powered Christmas string that quit working (salvaged the LEDs). The blue is noticeably lit at 25uA, whereas the green ones are noticeably lit down to 300-400nA (about where my meter stops measuring). I do have a couple newer green ones that don't do this, but now I am curious how these old green ones draw such little current to light up when manufacturing has supposedly gotten better. Is that normal for green and maybe my blue ones draw too much? Maybe the manufacturing process changed?? Are these old green ones a relic from the past that I should be saving?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 25, 2023, 12:51:32 am
content: Green is close to the eye's peak sensitivity, so needs less light to be visible

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 25, 2023, 01:43:46 am
content: Quote from: antenna on November 25, 2023, 12:37:49 amKeep in mind, that part of the curve will not be one the vendors focus on !There are also photons/electrons going in both directions, as LEDs also work as photo-diodes.Some will give uA currents under external light, so if you want to generate light at uA currents, you likely need a LED that is poor at photodiode operation, or only use it in the dark.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 25, 2023, 07:43:03 am
content: I suspect you have a variety of things. Classic GaP greens are distinguished by the 2-2.2V Vf, and are rather inefficient, particularly at low currents (recombination is strong). Modern high-efficiency greens are based on GaInN, as blue, white and UV types are, hence the 3-3.6V Vf, but despite the higher voltage, they have higher overall efficiency because the quantum efficiency is just so high. Not to mention the efficacy because green visual sensitivity.Green is obtained by... I forget what exactly, some combination of bandgap tuning (obviously not much here, if Vf is what it is), quantum dot structures or phosphors, but they get pretty narrow spectrum in any case. Oh uh, optical response time, not sure, it can't be too fast if it's narrow phosphors, but the other kinds could still be fast. I know blue (plain GaInN) and red ("heavier" GaAlAsP alloy, also good to high quantum efficiency) are fast, haven't heard anything about greens or measured it myself.Not sure of what date ranges apply here; 2010s at least, not sure if GaInN greens were available much before then?There's a color difference; it's subtle, with GaP being slightly yellower and GaInN being slightly cyan-er. Easier to see side-by-side, tricky to see alone.Anyway, there shouldn't be anything GaInN green in things "old", but, I mean, everything is in the past, "old" is relative. Any ideas on actual age?Tim

author: SeanB
author_rank: Super Contributor
timestamp: November 25, 2023, 10:53:24 am
content: I had some of the early, 1990's era, hyperbright 5mm green LED units, and they were bright at 20mA, enough to be able to throw a nicely focussed die image across the room at night. I tried a few at low current, and one or two were quite capable of glowing with a visible glow, using a CR2032 cell and a series resistor, and with a 4M7 resistor they still would emit light.To see just how good they are as photodiodes I assume that you have 5mm flat top blue ones, so take 2 and use some tubing to hold them face to face. Then connect the one to drive it at 20mA, and the other you connect direct to a high sensitivity red LED, and you will see it light up, though you might need to use a dark room, and cover the other 2 to exclude the spill of blue light from them. Also works with the 2 coupled ones being white, though with lower efficiency. Best with 2 near UV ones though, which can actually just about light a regular blue LED, or a white one that uses a blue LED as light source.

author: Zero999
author_rank: Super Contributor
timestamp: November 25, 2023, 05:06:24 pm
content: Under low light conditions, the response of the eye is shifted towards the blue, rather than green, with the peak being around 500nm, so I would expect a cyan LED to be the most efficient at low currents, when it's dark.

author: antenna
author_rank: Frequent Contributor
timestamp: November 25, 2023, 07:00:55 pm
content: Aiming one at the other taped together gave me about 200mV into my DMM with 4mA into the other LED, so it does generate a little voltage from light hitting it. But not much. At 4mA, these things are very bright. This is with a 4.7MΩ resistor in series with it and my power supply set to about 5v (current through LED measured at 0.6uA). It is easily visible in a lit room, and in the dark I can see it lit down to about .3uA.

author: Someone
author_rank: Super Contributor
timestamp: November 25, 2023, 10:25:22 pm
content: Quote from: antenna on November 25, 2023, 12:37:49 amSave for what?LEDs are rarely tested/specified/optimised for low current so it's a bit of a lottery. To replicate your experiment I pulled out two dozen or so LEDs across a range of ages/materials/colours/packages, at 1uA only a single AlGaInP red model would produce visible indication in a lit room. That model is one of the few LEDs designed for sub mA drive.Efficiency drops off at both high and low currents:https://www.eevblog.com/forum/blog/eevblog-869-counting-led-photons!/msg918646/#msg918646If you had enough volume/demand I'm sure the manufacturers would be able to produce parts designed for single digit uA drive.

author: antenna
author_rank: Frequent Contributor
timestamp: November 26, 2023, 01:15:14 am
content: Quote from: Someone on November 25, 2023, 10:25:22 pmNo lottery here, every single green one from that string behaves the same, fairly bright at 1uA. To answer the "for what" question, I already found a (rather silly) use for one. I made a little voltage probe. Just the LED, a series resistor (4.7MΩ) and a probe tip stuck into a section of old telescopic antenna tube. My body acts as the ground. It lights up touching the probe tip to all sorts of things. It's not all that useful, but its really fun to play with. It's probably the simplest voltage tester I have now. I think I will take it apart and put a few resistors in series just to make it safer for higher voltages as the resistor presently inside it is physically small.

author: Buriedcode
author_rank: Super Contributor
timestamp: November 26, 2023, 08:54:21 pm
content: I noticed that the backlight on the newer "cheap" green 1602 LCD modules from china work at ridiculously low currents - <1mA and it still lights pretty bright. They are also painfully bright at 5mA. They've also removed the transflective backing so there's less the backlight has to shine through, and tinted the display green rather than let the backlight determine the colour. It's a yellow -green but definately not the 570nm of the older types (which were never that efficient - this new one a single die at 5mA is as bright as a 420mA 4.2V array of the older 570's).The Vf is >3.1V and they do fluoresce amberish under UV light so I'm guessing a blue die with phosphor. Whilst desoldering the backlight off I left the iron on a tad too long and one of the legs came out the lightguide (its molded directly in the lightguide - not a 3mm LED inserted/epoxied in there) with a single bond wire, so its not two dies, just one.I'm assuming the market is now flooded with extremely efficient phosphor LED's, as they're brighter ( = "better" for marketting), and if they all have the same Vf much easier to design circuits that drive multiple strings but setill having different colours. That and the number of colours available is much larger for phosophor LEDs.