LED Bulb Efficiency vs. Operating Life (2023 Update)

This can be offset by operating the leds vastly below capacity. E.g. I have a flash light costing 3.5 euro with 37 leds. I estimate that the leds have an effectively infinite life span. Unlike light bulbs you can diminish the current and have approximately proportional light.

Groetjes Albert

The catch is that if they are all in series with a current source or worse rectified mains then the first one to fail takes the entire chain out so MTBF is about 1/N th of the N components in the chain. I have seen some where there were ~60 white leds in series.

Traffic lights seem to have about 4 chains that are independent so you don't get nice even illumination if one fails but they don't just stop completely one day like the old filament bulbs did.

I guess one way to build a highly-reliable LED bulb might be to use LEDs manufactured with intrinsically high forward voltages, and then boost instead of buck the AC -> DC to maybe 400 volts, so you could eliminate the electrolytic cap also.

Another thing I've seen is an IC with a switch matrix that monitors the line voltage and cuts series/parallel strings in and out in proportion to the instantaneous line voltage to keep illumination mostly constant

Depending on the construction of the lamp, there are a few things that cause LED output to degrade:

1. Darkening of the package material with time, heat, and optical dose. This is what's usually responsible for old optocouplers gradually failing.
2. Loss of fluorescent efficiency. This is slow for white LEDs, since the fluor is mostly inorganic Ce:YAG.
3. Propagation of dislocations into the active region, leading to lots of nonradiative recombination.

Cheers

Phil Hobbs

Old traffic lights had special hi-rel, often dual-filament bulbs.

Especially when they start blinking

A regular-size red or amber common cathode 7 segment display, in series with a TL431 configured as a zener, and the segments driven from 5 volts looks just about perfect brightness to me! Seems stable with temperature, too.

I can't stand the fast blinking LED tail lights on cars. Everytime I move my line of view, they create a dozen spots of light. When there are more than one it gets insane looking. Seems like most people don't even see this. The first time it happened to me I was trying to merge where there was no merge lane and the ramp was coming from an angle, rather than merging while driving parallel, so the rear view mirror didn't show anything useful. A quick look over my shoulder showed one car passing me, just as I needed to either go or stop. As I turned my head back, the tail lights (those tall Cadillac tail lights) suddenly blossomed into a dozen pairs of lights and I thought it was a bunch of cars! I had to hit my brakes to avoid an accident, only to find there was only the one car. Insane that they would create this sort of hazard, even if everyone doesn't see it. It's the sort of thing that would be changed on an airliner, after the first accident it causes.

I've never found out how rapid the blinking is.

I usually see those flashing brake light mods on street racer-type cars around Providence RI, kids put them on their old Eclipse etc. to make them look cool. I don't know that they're much improvement over a regular high-level lamp at avoiding being rear-ended, though.

There's some failure mode of LED street lamps and outdoor floodlights, etc. where the whole unit blinks at about 2 Hz.

We haven't used a 7-seg in ages. This is the last one, displaying a unit number from 0 to 7.

LCDs are much better.

We aren't talking about the same thing. I'm talking about tail/brake lights where the brightness is adjusted by PWM. It saves a few cents by leaving off the inductor too smooth the waveform into a level.

The problem is that they PWM pulse them at a frequency that some people can see in their peripheral vision (which is much more flicker sensitive). I guess the engineers who designed it didn't think about their choice of frequency too hard. Anything above about 300HZ frequency would look pretty much continuous but there seem to be several car makers (and street furniture makers that use ~100Hz at a guess).

People who see this as a problem are the same ones who don't get on with walls full of TVs or large screen monitors at 60Hz refresh rates.

I think we are talking about different things. I don't "see" the flickering. I see the resulting pattern of images when my view changes, like the sign sticks you wave in the air to display text. I find it disturbing because when I'm driving, my spidey sense is always watching the lights of other cars. When I move my eyes and the field of tail lights explodes from two or three cars, to a dozen, it is very disturbing. This has nothing to do with peripheral vision "seeing" the flicker.

They need to speed up the rate of flashing, so that this is not visible. It's interesting that not all cars do this. I first saw the flashing in a Cadillac some 20 years ago, or more. So, even today, either not all cars have LED taillights, or some car makers design them to not blink, or blink very fast so it is not observable. The difference in circuitry is very slight. Essentially it requires an inductor and capacitor and maybe a diode, to smooth the current rather than just blinking the LEDs. Or with two strings of LED, it requires two control circuits to set the brightness of each string.

It has everything to do with peripheral vision seeing the flicker. That is why the LED light motif breaks up into several strobe flash images.

If the frequency was about 3 or 4x higher then they would overlap. FWIW I do see some of them flicker in my peripheral vision and with those slower ones the flash images are further apart. It is the eye cadence movements that make them so prominent after dark.

I agree - increase the frequency and it would smear them out and look much more like a continuous light source. They can be very distracting.

It seems unlikely to me that they had leds bright enough for automotive indicators 20 years ago. The breakthrough in high efficiency LEDs was later than that. The first ones I can recall seeing used in anger were on motorway service vehicles for the "please don't hit me" yellow warning lights. They had sophisticated optics in front of the emitter which made them very strongly collimated beams that were bright from a great distance but never dazzling when you got close to them.

It would be good enough to just up the frequency by a factor of 5x - it isn't like they are even close to losing efficiency by switching losses at what are essentially audio frequencies. I guess they don't see it as a problem since plenty of automakers have exactly the same fault.

What Ricky's talking about is when you sweep your eyes eg left to right and see a line of red dots, rather than a smudged single line as you would with an incandescent lamp. Particularly noticeable in the dark of course.

Many years ago I diagnosed a faulty instrument without looking inside because I noticed the power-on LED was showing this effect and it was supposed to run from a DC rail supplied from the mains via transformer, rectifier and reservoir capacitor. The capacitor had become disconnected. The service engineer was mightily impressed - "Clive can see 100Hz!"

On a sunny day (Fri, 20 Jan 2023 10:25:32 +0000) it happened Clive Arthur snipped-for-privacy@nowaytoday.co.uk> wrote in <tqdq6s$20t98$ snipped-for-privacy@dont-email.me:

I have used the effect many times to its advantage:

My cheap satellite receiver 7 segment channel indicator has the same problem if I move my view fast.

Indeed one day something will go wrong and you get legislation to fix this.. Long time ago I proposed here something like the first link on the side of your car for advertising...

Sometimes I think you literally can't understand the written word. I am the guy experiencing the issue, yet you are telling me what I saw. It was looking straight at the object and moving my eyes a small amount. Any amount of movement creates the separate images of the flickering light. This is not a matter of peripheral vision.

Talk to Cadillac. You are aware that a tail light is only red, and you can use a large number of LEDs, no?

If you say so.

I made one of those, also on Veroboard, though mine had 16 red LEDs. I had the idea of mounting it on a rear windscreen wiper to send polite and friendly messages to the car behind. Then I got a proper job.

I assume that high efficiency means more light for less watts.