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Typically it's not the emitters -- the LED's themselves -- that fail. If driven correctly, those have lifetimes of tens of thousands of hours. That's what the manufacturer is advertising on the box. Yes, an individual LED when driven correctly will probably last 20,000 hours. (Usually more, depending on how pedantic you want to get. The 20,000 hour figure often quoted is the point where the emitter drops to 80% of its original light output.)
LED "bulbs," the type that replace filament bulbs in consumer fixtures, typically fail in their driver hardware. LED's run off of low voltage DC and in the base of all of those LED conversion bulbs is a power conversion assembly that steps down and rectifies 120v/240c AC to whatever DC voltage the LED array in there expects. These are inevitably made out of whatever the cheapest passives and semiconductor components the manufacturer thinks they can get away with. These don't last 20,000 hours, especially not in where they're usually installed.
The main killer for all semiconductor electronics, which includes both LED's themselves and their driver circuitry, is heat. This is often exacerbated by the fact that LED replacement modules are usually stuck in enclosed light fixtures designed for filament bulbs that have insufficient ventilation to get rid of the waste heat from the components in an LED module. The insides of those enclosed ceiling light fixtures, the ubiquitous "boob light," gets hot, even with only LED modules installed. Filament bulbs don't care because they don't have any electronics in them and how they work is literally by getting so hot the glow. But LED modules in that kind of environment will invariably suffer an early failure.
The best way you can get your LED modules to last longer is to install them in a fixture where they'll have a lot of air circulation available or at the very least which is not fully enclosed.
Hah, I have one three bulb boob light that’s my “trash” fixture because it gets whatever mismatched leftover I can find. At one point I had an LED, and incandescent and a CFL in it. It doesn’t have the extra ventilation that modern ones do so I’m sure it gets hot, but even that doesn’t seem to shorten the lives much
Excellent.
Also, the D stands for diode, which is a one-way passage for electricity, some rectifiers may use diodes in their circuitry. So another way to cut costs is to not rectify completely or well.
Case in point, cheap LED Christmas lights are often not rectified at all so they flicker at 50/60 Hz depending on your country's electrical supply...
I freaking hate that flicker so much. I can always tell when someone has done a cheap aftermarket headlight conversion because I'll see the flicker out of the corner of my eye and my rearview mirrors and it's incredibly distracting.
Slightly different. The alternator on a car has a very variable frequency due to change in engine speed and it's fed into the car's regulator/rectifier, so the typical power supplied is stable. However, many cars will use the high beams at half-power to function as daytime running lights (DRLs). This is usually done by pulse width modulation (PWM) meaning it's chopping up the power supplied at a nearly imperceptible speed. An incandescent bulb will have so much fade time that the choppy power will go unnoticed. From there, theses two possibilities that cause the bulb to flicker. Very cheap bulbs will show their choppy power supply directly by flickering, making them noticeable as the move across your vision. Some mid-range bulbs will have cheap smoothing circuits (since vehicle power is "dirty") so there will be charge time and discharge time as the capacitors charge up and down, allowing and disallowing the emitter to light, creating a slower flash pattern. Higher end bulbs (ignoring the part where their beam pattern is still usually trash) should be able to accommodate the chopped power and run dimmer.
You may also notice a similar flicker on LED tail lights where the brake light is a brighter tail light instead of a dedicated element. Such cars will use PWM for dimming and may flicker as they move across your view as well. Some of my car's dash uses LEDs for backlighting and dimming the dash is done via PWM. If I glance across the steering wheel from side to side, it looks like "cruise" gets stamped across the view