I recently installed some decorative LED shelf lighting (specifically these if it makes a difference). The manufacturer says they will last 50,000 hours, where "last" means maintaining 70% of lumen output. (I realize this might be inaccurate.)

I would like to keep these lights on most of the time. Question: If I keep them dimmed, will they last longer?

More specifically: If I keep them at half-brightness, will they last twice as long? Or somewhat less than that? Or no longer at all?

  • Even with them being on "most of the time" (let's say 75%), they're expected to last nearly a decade. Not quite good enough? – isherwood Jul 14 '16 at 21:04
  • @isherwood: I have a long horizon :) Really --- the installation is such that they'd be very difficult to replace,and yes, I do care how they look a decade from now. So --- given that they'll always be dimmed to about 50% (because that's how they look best) I'm wondering, if I keep them on 75% of the time, should I expect one decade or two? (And yes, I realize nobody knows the answer to that, but I'm looking for best guesses). – WillO Jul 14 '16 at 22:21

Here's what doesn't apply to you, but does apply to most consumer LEDs (sold as primary lighting devices, i.e. screw in bulbs, fluorescent replacements etc):

They are significantly overdriven from spec, for instance an LED will be sold as a 10 watt emitter, however the datasheet will spec it for baseline performance at 1050ma at around 3.3 volts - now hold on, that's only 3.6 watts, not 10! As you read on, you discover they achieve the "10 watt" nameplate by driving it at 2800ma at 3.6 volts, which is allowed, but with lower efficiency and higher junction temperatures requiring bigger heatsinks. Of course that overdrive figures into the LED's life, as the higher junction temperature is one factor in lamp aging.

The other factor is the driver circuit which converts 120/230VAC into the low-voltage, constant-current the LED emitter requires. For a packaged consumer product, this driver circuit is the weak link in the chain, and is the likely cause of death, especially since there are so many ways for bottom shelf brands (FE, UT, LoA) to cut corners here.

In that light, here's what does apply to your LED strip accent lighting.

Those strips usually run on 12VDC or sometimes 24VDC, but they don't include that complex driver circuit. They use a simple, bulletproof resistor for current limiting. Those strips don't have cooling fins, so they don't overdrive the LEDs much. The resistor is also sized for worst-case 14V (or 28V), which is what a car's alternator is putting out to charge the battery. At actual 12V they derate considerably - I've seen a 16 foot "24 watt" strip drop to 15 watts actual. These factors combine to make them dimmer than they could be, but conversely, more long-lasting.

Generally, DC LED strip installations have an AC power supply that is a separate item. It is a failure point, but you can replace it separately.

The other thing is, people really have no idea how long LEDs will actually last. The numbers they throw down are guesses. Realistically they will probably last a lot longer than claims made, but the claims are tempered to avoid appearing unrealistic. My rule of thumb is your installation won't live long enough to see the LED fail - you'll modernize, sell the home or die!

The upshot is, you're already in pretty good shape, and so dimming will help, but not a lot.

I would say the bigger reason to dim it is that lights bright enough to be useful workspace lighting would be annoyingly bright as ambience lights.

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  • You write: "the bigger reason to dim it is that lights bright enough to be useful workspace lighting would be annoyingly bright as ambience lights". Yes, I definitely want them dimmed for that reason. But the choice is between leaving them on, dimmed, pretty much all the time, or frequently turning them off. And a key input to this is "How long will they last dimmed?". Recognizing that nobody knows the answer, I wondered if the best guess is different from the best-guess answer to "How long would they last undimmed?". Thanks for this detailed reply. – WillO Jul 14 '16 at 20:43

The answer depends more on the (triac) dimmer. LEDs have a sweet spot for voltage and will operate for a long time in the sweet spot. LED/triac dimmers actually just turn on and off rapidly. LEDs can handle the on/off better than other kinds of light, but the "flickering" would be the factor in break-down. At 0V obviously the LED would last for a very long time so when the LED is off for an extended amount of time (dimmer on low- like 10%) the light should last longer. But at half dim/brightness the dimmer is switching between 0V and (say) 3V and the damage due to flickering is probably somewhat significant. Theoretically, the LED would last twice as long, because it is off half the time, but turning off and turning on means the voltage will be increasing and decreasing; even though it is a rapid transition, time will be spent driving the LED at below specification (say 0.1 - 1.9 volts). This may or may not cause the LED to fail prematurely; remember, at 0V the LED will last even longer than it would at 3V.

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The only way to know would be to test the lights at various "dimmings"... with different dimmers. The more rapidly the circuit/dimmer can go from 0V to 3V to 0V, the less time the LED will be diven at below specifification. A good quality LED dimmer might allow the LED to last longer, a cheap LED dimmer might cause premature failure... depending on the breadth of the LED's voltage specifications.

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  • I read "the more rapidly the circuit can go..." as fast switching (ideally a rectangular curve) being more favorable. But isn't the opposite the case? At least with "normal" (i.e. non-semiconductor) things it sure is because the inrush current gets huge spikes. Which is why on/off cycles are almost always the lifetime-limiting factor for a product. Some machines try to mitigate the inrush current by explicitly starting slow. Now, pulse modulation dimming is, by definition, billions of on/off cycles. – Damon Dec 21 '17 at 10:23

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