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HIDs need thousands of volts and have transformers (called ballasts) to get this, in turn meaning almost certainly an AC voltage being produced. Regarding strobing headlights, chances are they are HID lights not LED. #MAKE LED LOOK FLICKERY FULL#Brakes on, full power, running lights, dimmed. The flicker many people mention in slow motion footage of car LED taillights is almost certainly PWM dimming for combo brakes/running lights. For single diode lights (Christmas lights, dim indicators, or other decorative lighting) it is half the frequency and more noticeable. I have assumed above that we are talking about incandescent replacement globes which almost always have a full bridge rectifier. LED car headlights may fall into this category. Late E2+: Some battery powered things can use DC to DC transformers which can in turn cause strobing, so the above has caveats. Obviously with battery (DC) powered stuff, excluding dimming, there is no AC and so no strobing. #MAKE LED LOOK FLICKERY SOFTWARE#Incidentally, flourescents also strobe (though to a lesser degree) and most video cameras have special software to help hide this. #MAKE LED LOOK FLICKERY DRIVERS#In higher quality power supplies for LEDs, they use "smoothing capacitors" and/or purpose designed LED drivers to help the LED stay lit through the low/zero volt bits and this reduces the strobing effect. ![]() As this is happening at 100 or 120 Hz, most people wont notice it.Ĭheap or traditional triac based dimming can seriously exacerbate the issue with mains strobing. When the voltage to them starts to drop towards zero, the lights dim and turn off, coming back on again as it voltage goes back up. In LEDs, there is no fillament to heat and they react very quickly. This fillament takes time to cool down - much longer than the mains supply takes to go through zero - and so it can stay hot, keep putting out light, and there is (almost) no flicker. In traditional incandescent lights there is a fillament which is heated super hot to provide light. That means that 100 or 120 times a second the voltage is exactly zero. In your mains AC, the voltage alternates from positive to negative and back again 50 or 60 times per second. ![]() So why does this happen with LEDs but not other lights? Cheap camera phones also sometimes show it. #MAKE LED LOOK FLICKERY SERIES#You can often see the effect if you wave your hand back and forth while focusing on a stationary spot - instead of smooth motion blur you can see a series of hand images, like stills from a movie. Strobing is (historically) very common with LEDs driven from mains AC. My inbox kinda exploded and I've tried to answer repeated questions in the edits. This bit goes a little beyond ELI5 but hopefully still helps. You can also get strobing from HID headlights because they often use AC to get the thousands of volts they need to ignite. In theory PWM is too fast to be perceivable (when done right) but it seems a lot of people are actually sensitive to it! The LEDs are switched on and off really quickly - when they are on for half the time they look half as bright. Similar stuff happens when you dim LEDs (like LED car taillights when the brakes aren't on), though much, much faster through something called PWM. You can also see it when something is moving quickly across your vision. When blinking or looking away quickly your brain "preserves" what you saw in that instant and you can spot it. They look like they are strobing because (for most of them) on mains power they actually are! Much like when whatching TV, however, something called ”persistence of vision” smooths it all out for you. ![]()
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