EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: eevdude on September 25, 2020, 09:54:36 am
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Probably a very stupid question, but I'm trying to figure out the best surface mount leds for a project, i've search here and across the internet and i keep reading that people suggest using high efficiency leds at a low current (1-2ma) but i can't see anyone suggesting actual led part numbers.
When i search for them (digikey/mouser/lcsc), there aren't many 1 or 2ma current leds, and they are rated at a low brightness (mcd rating) and have a max current of 20-30ma, but specify anywhere from 5 to 40mcd @2ma. When i look at 20ma leds, they can often be rated from around 300 to over 1000 (I assume this is subjective by the manufacturer), but have a very similar current / brightness curve chart in their datasheets to the "low current" ones.
Are these basically the same - for instance taking a 20ma led and just using it at the minimum current it will light, such as 2ma? or is there something i'm overlooking - for example, is the material more important than the current rating?
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High efficiency LEDs are highly efficient at lower currents than the rated test current, as well. The fact you can run them at 20mA and that they were tested at such high current, does not mean you have to. These are very bright.
20mA seems to be the most usual test current for standard indicator LEDs. Just sort for intensity as measured at 20mA. Pick the highest values. Use them at 1-2mA for indication purposes, especially if you can directly see the LED emitter.
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Originally, "high efficiency" LEDs were simply anything newer and higher efficiency than the original, standard, GaAs-era red, green and amber LEDs which are quite dim by today's standards.
Now there are a bazillion different types and chemistries, including ones with phosphors so you have a very, very wide assortment to choose from to match the properties you need to a given application.
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the terms of "high efficency LED" and "Low curren LED" are terms invented in the '80s, when most LEDs were barely visible at 20mA.
Then the world moved on.
In this modern world the average LED is blindingly bright at 20mA and running a simple indication LED at 1mA or so is more sensible.
Led's have a maximum current rating, but all LED's I've seen work well down to very low currents.
White LED's, especially the high power LED's designed for lighting tend to loose CRI (Color Rendering Index) at (very) low currents.
But LED's are cheap. If you have any doubts, just order a handful of different parts. These are always handy for experiments, breadboards and such.
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LEDs have changed much over the years and aren't battery hogs they used to be.
Even 1-2 mA is too much with some new ultra efficient LEDs. More like 100-200 µA. Best is to order them a few of different colors and evaluate them by eye, as it is impossible to tell how they'll work out by numbers alone.
For example, I used a 3 mm green diffused LED (HuiYuan Opto 3034G3D-EHE-A) in a project and recently had to change its 10K resistor to 15K because it was damn too blinding bright @ 8V.
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Awesome responses thanks, really clears it up, I guess like everything tech moves on and that can leave some advice out of date.
Will definitely get a few and try some different resistors to see what looks best!
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Early LEDs required high current densities for good efficiency making them unsuitable for low drive currents and low power applications with the exception of GaP "high efficiency red" which could be used below 2 milliamps with reasonable brightness.
High brightness LEDs were much brighter at higher currents but not at low currents.
Better efficiency can be achieved by pulsing a high brightness LED at high currents making them especially suitable for multiplexed displays. The old GaP LEDs are unsuitable for this because of their poor efficiency at high currents.