LED's most light for least current... - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: June 15, 2015, 12:32:52 am
content: One thing I notice with LED's at Digikey is that they have a "Millicandela Rating", but is there also a rating for how much light is produced per unit of current? Like the efficiency of an LED? How would you find ones that are the brightest for the least amount of current?

author: sleemanj
author_rank: Super Contributor
timestamp: June 15, 2015, 02:45:42 am
content: Technically, Lumens per WattBut human perception of light muddies the waters.https://en.wikipedia.org/wiki/Luminous_efficacy

author: alank2
author_rank: Super Contributor
timestamp: June 15, 2015, 01:17:11 pm
content: Even so, is there an efficiency rating or something? Something that could be used to find an LED that puts out the most light with the least current?

author: Zero999
author_rank: Super Contributor
timestamp: June 15, 2015, 01:35:26 pm
content: It depends on what colour you need. Red and blue LEDs are most efficient in terms of power out and the desired wavelength vs power in but the human eye is most sensitive to green light so green LEDs tend have a higher luminous efficacy.

author: Fungus
author_rank: Super Contributor
timestamp: June 15, 2015, 02:40:01 pm
content: Quote from: alank2 on June 15, 2015, 01:17:11 pmThere'sa massive can of worms, right there.First you have to define "most light" (good luck with that one, you'll need it...)

author: alank2
author_rank: Super Contributor
timestamp: June 15, 2015, 03:39:18 pm
content: Sorry guys. I'm looking for visible light for display or indicator purposes... I like LED displays, but they are so power hungry.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 15, 2015, 03:49:36 pm
content: White is easily the most visible light for the current.

author: jolshefsky
author_rank: Regular Contributor
timestamp: June 15, 2015, 03:56:32 pm
content: Quote from: alank2 on June 15, 2015, 03:39:18 pmPerhaps you should consider a more efficient technology.If you're going for indicators, then you need to also factor in optics and viewing angle. You can get fairly narrowly focused LEDs (which you'd need to view nearly straight-on) and run them at very small currents (<100µA) and provide a viewing shield (something to darken the surrounding area) for the most efficiency.Because of the huge number of variables, perhaps the best option would be to select several based on the advice here (e.g. green is more visible) and do a side-by-side test in your specific application. For ultra-low-current applications, stick to indicator LEDs (typically rated for 20mA, typically, not lighting LEDs rated for >100mA).

author: rdl
author_rank: Super Contributor
timestamp: June 15, 2015, 04:00:13 pm
content: LEDs have certainly improved greatly in recent times. I have some 20 and 30 year old red and green LEDs that are barely visible at 20 mA. I have new blue and white ones that are almost painful to look at with just a few mA.

author: Circlotron
author_rank: Super Contributor
timestamp: June 16, 2015, 03:53:46 am
content: ^^ Amen to that. Nowadays if I need an on-board indicator for an micro port pin I just run the pin through a10Kresistor to the red led. With a 5V supply to the micro a little over 300uA flows through the led but it is perfectly useable.

author: Fungus
author_rank: Super Contributor
timestamp: June 16, 2015, 09:07:01 am
content: Quote from: alank2 on June 15, 2015, 03:39:18 pmModern LEDs are still very visible at 0.5mA (or even less - I usually put a 10k resistor on my power indicator LEDs and I sometimes think they're too bright for indoor use).Probably the most efficient power-indicator LED would be a super-bright (ie. transparent body, not green plastic) green LED.20000mcd is no problem, eg.:http://www.ebay.com/itm/310621814375Sandpaper the lens a bit to make them more diffuse (fine sandpaper).Quote from: alank2 on June 15, 2015, 03:39:18 pmYou don'thaveto run them at 20mA each.

author: Fungus
author_rank: Super Contributor
timestamp: June 16, 2015, 09:12:19 am
content: Quote from: rdl on June 15, 2015, 04:00:13 pmI found some old LEDs the other day and I was like, "seriously??" They barely lit up at all.I thought it was just me or that the LEDs had degraded with time or something. I don't remember LEDs being that dim in the 1980s.

author: nfmax
author_rank: Super Contributor
timestamp: June 16, 2015, 11:26:54 am
content: Those of us who were around in the 80s (and even 70s) are realizing it isn't theLEDsthat have grown dim over the years...

author: sleemanj
author_rank: Super Contributor
timestamp: June 16, 2015, 12:08:21 pm
content: Quote from: Fungus on June 16, 2015, 09:07:01 amDo not for one second believe any mcd rating that a chinese vendor gives you. You have no way to verify it, and they very well know that you have no way to verify it, they certainly don't have any way to verify it, the manufacturer might have a way to verify it, but that's a long way back up the chain.

author: LukeW
author_rank: Frequent Contributor
timestamp: June 17, 2015, 09:01:39 am
content: It's tricky to predict how any given supplier of LEDs "will look" in your finished product under given conditions.Perhaps it's best to buy a few samples and experiment.I've had generic Wun Hung Lo LEDs before that are fantastic, and extremely bright with just a couple of mA. So you never know.

author: tautech
author_rank: Super Contributor
timestamp: June 17, 2015, 09:22:25 am
content: A long time ago I experimented with driving LED's at 50% duty cycle using a 555 just to comfirm power savings, the visible flicker being the determining factor as to how slow you could go.To the naked eye there was no difference in brightness @ 25Hz and 50% dc.For display purposes, the ~25 Hz and 50% duty cycle should result in savings.There is also the option to pulse them at higher than rated currents, providing you stay below their DC max current as an average.If a 1N4148 can withstand 20 Amp fast pulses, why can't a LED?

author: Zero999
author_rank: Super Contributor
timestamp: June 17, 2015, 10:14:13 am
content: Quote from: tautech on June 17, 2015, 09:22:25 amThat doesn't actually increase the efficiency because LEDs are less efficient at higher currents so pulsing the LED will actually reduce the efficiency, rather than increase it.http://spie.org/x108666.xmlhttp://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/weisbuch_droop_tampa2014.pdfThe eye acts as a low pass filter and is very good at compensating for changes in light intensity. Quite often the current can be halved with hardly any perceivable decrease in light intensity.Pulsing LEDs is normally used in data transmission because smaller LEDs can be used and photodiodes are very fast, compared to the naked eye so power savings are possible.

author: Fungus
author_rank: Super Contributor
timestamp: June 17, 2015, 02:36:32 pm
content: Quote from: sleemanj on June 16, 2015, 12:08:21 pmWell...OK, I'll give you that one.Chinese or not though, personal experience tells me they're seriously bright. Painful to look at directly at 20mA in a dark-ish room.

author: Kjelt
author_rank: Super Contributor
timestamp: June 17, 2015, 02:59:14 pm
content: Quote from: Fungus on June 16, 2015, 09:12:19 amThey sure had less lumens/watt then the ones even 10 years later.A friend of mine repairs audio equipment and was begging me for my old stock 80's red leds , because they were used in some sort of optocoupling device in some record players or something, and the new red leds were way to bright, he needed the old stuff to make it work again.

author: LaserSteve
author_rank: Super Contributor
timestamp: June 18, 2015, 08:01:52 pm
content: For Photopic Vision the Peak sensitivity is ~ 565 nm. (Yellow-Green)For Scotopic Vision the Peak sensitivity is ~ 520 nm (Green)Steve

author: poorchava
author_rank: Super Contributor
timestamp: June 18, 2015, 08:10:22 pm
content: Good white LEDs can be had at reasonable prices that have efficacies in the order of 140-150lm/W.Wys?ane z mojego HTC One M8s przy u?yciu Tapatalka