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What current is best, longevity wise, to run an LED at?

For example in the below data sheet, an RGB LED from sparkfun, the absolute current limit is listed as 20ma. However running an LED at 20ma or close to it is likely to degrade that LED an an accelerated rate, correct?

If I want to achieve decent brightness but maintain a long life for the LED what current should I provide to an LED? And if I do provide less than optimum current in order to increase life time, what sort of returns are expected?

Is it negligible or worth it?

https://cdn.sparkfun.com/datasheets/Components/LED/YSL-R596AR3G4B5C-C10.pdf

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It is the luminous intensity of LEDs that decreases over the life of a LED. Higher the current you supply to LED, higher will be its power dissipation, and its temperature will increase. This induces thermal stress and electrical stress on LED and speeds up the rate of decrease of luminous intensity. What you are asking for is the derating of forward current for improving reliability of LED.

Answering your first question, if you supply 20mA forward current then the luminous intensity will degrade by the percentage shown in below graph.

You should determine if this percentage of degradation is acceptable to you. If not, you have to reduce forward current.

You also need to consider effect of temperature on maximum forward current. 20mA maximum current is at 25 degree C ambient temperature (mentioned in your datasheet). But you also need to derate (reduce) this with increase in temperature.

Please see this datasheet form Kingbright

PS: I have used it only as example. The ratings are different from your LED.

Derating curve

This curve shows the derating you need to consider as effect of ambient temperature. So consider the maximum ambient temperature in which your LED will operate and then determine the maximum forward current at that temperature.

And in doing all this, the result you get is decreased luminous intensity, which is shown in graphs as below.

enter image description here

To conclude, in your case the power dissipation is too less (~150mW). So just limiting the current to 15mA will be fine if you are satisfied with the intensity.

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  • \$\begingroup\$ Seems a bit unclear to me whether the current limits are for all 3 LEDs 'ON' at once or for one at a time. \$\endgroup\$ – Spehro Pefhany Jun 15 '18 at 20:19
  • \$\begingroup\$ @SpehroPefhany The current limits are for single colour/LED. \$\endgroup\$ – chiragp Jun 16 '18 at 4:12
  • \$\begingroup\$ @Misunderstood The source of that table is the datasheet OP himself has shared. The graph temp vs current is for derating current when ambient temperature is higher that 25 degree C. I have added this graph so that OP knows he has to first derate current based on his ambient temperature. \$\endgroup\$ – chiragp Jun 16 '18 at 4:18
  • \$\begingroup\$ @chiragp Per LED, of course, but there is mutual heating so the maximum current for green should be less if red and blue are on, but that interaction is not quantified. \$\endgroup\$ – Spehro Pefhany Jun 16 '18 at 4:29
  • \$\begingroup\$ Yes it's my mistake. The table seems wrong. I am sorry I didn't bother to verify it as it was in datasheet. \$\endgroup\$ – chiragp Jun 18 '18 at 8:57
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Here is a typical degradation curve. You have nothing to worry about!

enter image description here
Source: Lumileds

How long do LEDs last?

LED luminaire useful life is often described by the number of operating hours until the LED luminaire is emitting 70 percent of its initial light output. Good-quality white LED lighting products are expected to have a useful life of 30,000 to 50,000 hours or even longer.

Source: US Dept of Energy

NOTE: This is applicable to LED light bulbs. The lighting surface of a light bulb LED gets HOT, too hot to touch. When an LED is run cool (under 50°C, it will last A LOT longer.

My LED light bulbs are guaranteed to last for 23 years. At 20 mA your LED is going to last a lot longer than 23 years.


Is it negligible or worth it?

Negligible. At 20 mA the heat is insignificant. It's heat that accelerates the mortality curve of any semiconductor, LEDs included. If kept under 50°C you are not going to have any heat issues.

Blue  0.02 amp x 3.2V = 64 mW
Green 0.02 amp x 3.2V = 64 mW
Red   0.02 amp x 2V   = 40 mW
Total                 =168 mW

At least 20% of the wattage is radiant watts (light) so
168 mW - 20% = 134 mW heat., probably under 100 mW.

Any degradation will not be perceptible in your lifetime.

Select the intensity by measuring the temperature.
Run all colors at 20 mA and if you can hold your finger on the LED with no discomfort everything is fine.
If you burn your finger or you have to remove your finger after a few seconds then it is too hot and decrease the current.
IF too hot, Rinse and Repeat.

I have been designing LED lighting for years now. The LEDs you are using will not get hot.

Below is a 22" strip sitting on top my monitor. Vf = 46V x 0.250 amp = 11.5 watts / 48 LEDs = 240 mW/LED, about twice the max of your LED. The temperature of the LED is 41°C, slightly warm to the touch.

I guarantee you I can run this strip 24/7 for 5 years (43,000 hrs) and you will NOT perceive any difference in intensity.

enter image description here

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  • \$\begingroup\$ Frequency of faulty LEDs on new vehicles in Toronto I see daily suggests MFG defects are not included by above results which are a supplier variable not published that are defined as infant mortality rates often with many causes; wire bonds sheared by thermal stress, metallic shorts in substrate, and power supply failures \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 27 '18 at 5:24
  • \$\begingroup\$ Where the car density is concerned and slowest freeways in North America (fact) and my level 3 automotive , and LED factory experience is concerned , you are blind. I have picked from the best of over 100 factories and know some of MTBF info that you don’t. Even north Chicago Interstate has 5% of its LED lamps out of service. And if you’re saying you’ve never seen a bad LED in a traffic light , you live a sheltered life. I can show you even Cree’s failure reports \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 27 '18 at 15:07
  • \$\begingroup\$ I was Test Engineering manager of a CE who were qualified as Tier 3 to Chrysler with 100k PCA’s / yr so I know about supply chain MTBF reporting and root cause forensics. And supplied over 1 Million LEDs to 1 client all defect free until customer overstressed them from lack of awareness in a dozen areas from plastic injection molding E-field stress to thermal gradient stress in soldering process etc. So you are less aware than I on this subject yet superior to me in horticulture application of LEDs \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 27 '18 at 15:44
  • \$\begingroup\$ @TonyStewartolderthandirt I'd like to see the Cree failure report. \$\endgroup\$ – Misunderstood Jun 27 '18 at 16:31
  • \$\begingroup\$ Sure but it will take a while to dig up. In the mean time you ought to be aware of en.m.wikipedia.org/wiki/List_of_LED_failure_modes. And every company has escapes . \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 27 '18 at 17:11

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