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One night, raining, crossing a street, I noticed strange thing about one of the stopped cars: In front of that car's headlamps, it rained dashed lines. I assumed that the car used LED for lights, running on AC, all with the same phase. (I couldn't recognize the model of the car.)

But what benefit do the AC headlights give? Most car lights run on battery so you can use it even when the engine is off. What powers AC car lamps when the engine is off?

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    $\begingroup$ Are you positive these are LED lights and not HID lights? LED's are after all, diodes and current only flows one direction. $\endgroup$ – DLS3141 Aug 17 '16 at 19:22
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The headlight LEDs are using DC, but the voltage regulation on all modern power supplies is PWM or Pulse Width Modulation. The electronics switch to full on quickly and full off quickly so there is very little loss in the switching components. This is used in lots of electronics, but in the application you saw it is called a DC DC converter, and permits the car to go from a variable battery voltage to a constant led voltage (or constant current for higher quality LED power supplies).

Usually a high enough frequency is chosen that it is not visible or displeasing to the eye. If you can estimate the velocity of the rain drop and length of the illuminated path you can calculate the frequency they were using. Velocity in m/s divided by the length in meters for example.

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LEDs, especially high output ones are often run off pulsed DC, this is a convenient and efficient way to control current and thus brightness, especially in a vehicle electrical system where the voltage may vary somewhat.

This isn't identical to AC which implies a switching the polarity of the current but rather just a series of on/off pulses.

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    $\begingroup$ At least in the US, I usually hear this called "Pulse Width Modulation." $\endgroup$ – Ethan48 Aug 17 '16 at 17:15
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    $\begingroup$ PWM is typically done at frequencies in the kHz range, well above the observable flicker threshold $\endgroup$ – DLS3141 Aug 17 '16 at 17:40
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I would say the the lights were not LED's, but HID lights. The flickering you observed is not due to the difference between using AC vs DC for the headlamps, but due to voltage fluctuations in the output of the ballast. Some of the cheap ballasts don't provide a stable output voltage and there's noticeable flicker. Think of a florescent light in a building where the ballast is going bad. This is pretty common with cheap or aging HID ballasts.

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  • $\begingroup$ It's very unlikely that this is unintended in an LED driver, it is likely intentional dimming to control heat, brightness, and/or life of the led. $\endgroup$ – Ethan48 Aug 17 '16 at 17:16
  • $\begingroup$ The OP only assumes that the lights were LED, this fluctuation happens in cheap/older HID ballasts. $\endgroup$ – DLS3141 Aug 17 '16 at 17:38

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