How to find similar LEDS (what makes them special)? - Page 1
Forum section: electronics / projects

author: zapta
author_rank: Super Contributor
timestamp: February 11, 2014, 03:37:05 am
content: I got these leds from Sparkfunhttp://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/LED/Q150R4.pdf// redhttp://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/LED/S150ANG4.pdf// greenThese LEDs are very sensitive, I can drive with with 5V and 3.3K resistor and they are still very visible. Much brighter than similar LEDS i have from another source.How do find similar parts on let's say DigiKey? What parameter in the data sheet indicates that the LED is very visible at let's say 1ma?

author: Psi
author_rank: Super Contributor
timestamp: February 11, 2014, 04:45:46 am
content: If its max light output at low current your looking for, red and yellow AlGaAs or AlGaInP are usually the most efficient.You can use the mcd rating and the viewing angle to calculate the total light output in lumens using this online calchttp://www.ohmslawcalculator.com/mcd_to_lumens.phpBut it doesnt work to well for leds with vastly different viewing angles.Once you have the lumens you can divide that answer by the mA value of the test current for that mcd rating (it will be in the datasheet near the mcd rating somewhere).That will give you the light output per mA of input current which is a figure you can use to compare various leds to each other.Note: You should really be compared mW instead of mA but for simplicity sake its ok since most red/yellow ALGaInP leds have the same forward voltageThese 3mm digikey leds are really good, i use them quite oftenhttp://www.digikey.co.nz/product-search/en?vendor=0&keywords=754-1588-NDThey are nice and bright indoors at 1mA and still perfectly visible in a well lit room at 0.1mATo find them i bought around 30 leds from digikey that looked good and soldered them all to some veroboard to test before making my big order.

author: mariush
author_rank: Super Contributor
timestamp: February 11, 2014, 05:01:29 am
content: Well, first I'd filter the leds by view angle.. because there are leds that are basically like a laser pointer (just 15 degrees around the center or even less) or there's leds that behave like regular bulbs (45 degrees, 90 degrees, 100+ degrees).Then, I would decide on the maximum forward voltage I'm willing to work with but this depends a lot on the color of the led. If color doesn't matter, then for example, if your circuit/microcontroller runs at 3.3v, I may choose a 3v led instead of a 1.9v led because the 3v may be brighter at lower current compared to a 1.9v led, so I'm not going to waste energy in the current limiting resistor, dropping (3.3v - 1.9v) volts.Then I would probably sort by the millicandela rating.. the higher the rating, the brighter the led can be. But this is somewhat tricky ... you don't know exactly at what current this millicandela rating is calculated, without checking the datasheet. The majority will test at 20mA, but there are leds that can handle 30mA , or 50-60mA without any problems.For example have a look at this one:http://www.digikey.com/product-detail/en/C503B-GAS-CB0F0791/C503B-GAS-CB0F0791-ND/1922939In the table it says 53650mcd but if you go in the datasheet, it says on page 3 at the top right:Bin Code Min.(mcd) Max.(mcd)B0 16800 23500and this is rated for 20mA but the led can work at up to 30mA with no problems... and further down on page 5, you see the first one showing 16800-90500 milliCandela rating ... the maximum is probably for very small pulses at the maximum current of 100mAI think the millicandela rating is proportional with the current, but I may be wrong, I have to admit I didn't study led technology as much as I'd like to.

author: ConKbot
author_rank: Super Contributor
timestamp: February 11, 2014, 12:05:10 pm
content: Youre looking for high efficiency LEDs. You used to be able to see right on digikey "High Efficiency Red" or the like but it doesnt appear to be that way anymore. As PSI said, AlGaAs/AlGaInP LEDs are high efficiency type, with a low Vf compared to others. Their maximum brightness will be less than other LEDs which concentrate on more output rather than efficiency.