[ELI5] Why do LEDs in cars flicker? Aren’t they powered by DC?

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I understand why we see them flicker in videos, but why do LEDs flicker at all when they’re powered with DC?

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12 Answers

Anonymous 0 Comments

Light up all lights and it use “lot” of power which usually end up creating heat somewhere and that can be hard to manage at some point. (Hard and/or costly)

Our eyes (or brain?) is kinda slow at reacting to light. So we found out that we can cycle trough leds fast enough to make them look like all of them are light up.

On a parallel subject, I don’t know if this is our eyes/brain or leds themselves, but if you start playing with the on/off ratio time you see them like they are dimmed. This is called PWM (Pulse Width Modulation)

Anonymous 0 Comments

Light up all lights and it use “lot” of power which usually end up creating heat somewhere and that can be hard to manage at some point. (Hard and/or costly)

Our eyes (or brain?) is kinda slow at reacting to light. So we found out that we can cycle trough leds fast enough to make them look like all of them are light up.

On a parallel subject, I don’t know if this is our eyes/brain or leds themselves, but if you start playing with the on/off ratio time you see them like they are dimmed. This is called PWM (Pulse Width Modulation)

Anonymous 0 Comments

They don’t need to be powered by DC. You can throw them in an AC circuit, it will just only let the current flow one way. LED stands for light emitting diode, a diode is just a component that only let’s current flow one way, also known as a half wave bridge rectifier. Picture a sine wave, and just remove all the bits That are negative. Now you just have the light turn off and back on at the same frequency of the sine wave.

Anonymous 0 Comments

They don’t need to be powered by DC. You can throw them in an AC circuit, it will just only let the current flow one way. LED stands for light emitting diode, a diode is just a component that only let’s current flow one way, also known as a half wave bridge rectifier. Picture a sine wave, and just remove all the bits That are negative. Now you just have the light turn off and back on at the same frequency of the sine wave.

Anonymous 0 Comments

LEDs don’t dim evenly – you get colour changes and the dimming is not consistent between LEDs, even if their full brightness is consistent.

In cars, it is desirable to have LEDs all look evenly bright, even when the lights are dimmed – for example, tail lights might be dim when just the headlights are on, and the brightened when the brake pedal is pressed or the fog lights are on. Similarly, different countries have different regulations about daylight running lights – so some cars may have to have a higher brightness set. The DRL may also need to dim, if the turn signal is on.

In order to get reliable dimming with LEDs, it is better to run them at full power but pulse them on and off. If they are on half the time, then they are exactly half as bright, and all the LEDs dim consistently.

Anonymous 0 Comments

LEDs don’t dim evenly – you get colour changes and the dimming is not consistent between LEDs, even if their full brightness is consistent.

In cars, it is desirable to have LEDs all look evenly bright, even when the lights are dimmed – for example, tail lights might be dim when just the headlights are on, and the brightened when the brake pedal is pressed or the fog lights are on. Similarly, different countries have different regulations about daylight running lights – so some cars may have to have a higher brightness set. The DRL may also need to dim, if the turn signal is on.

In order to get reliable dimming with LEDs, it is better to run them at full power but pulse them on and off. If they are on half the time, then they are exactly half as bright, and all the LEDs dim consistently.

Anonymous 0 Comments

Follow-up: why is this problem so much worse with motorcycles?

Anonymous 0 Comments

Follow-up: why is this problem so much worse with motorcycles?

Anonymous 0 Comments

Your car uses an alternator to generate AC power, because an alternator is lighter and cheaper than an equivalent DC generator. This AC power is then converted into DC using a rectifier. The rectification circuitry used in most cars is not very high quality, so the DC power fluctuates a lot around the desired (~14v) level. Powering LEDs off of this poor quality DC power will make them flicker, at least a little bit. Depending on the quality of the rectification circuit, and the LED driver circuit, the flicker may be more or less noticeable.

Anonymous 0 Comments

Your car uses an alternator to generate AC power, because an alternator is lighter and cheaper than an equivalent DC generator. This AC power is then converted into DC using a rectifier. The rectification circuitry used in most cars is not very high quality, so the DC power fluctuates a lot around the desired (~14v) level. Powering LEDs off of this poor quality DC power will make them flicker, at least a little bit. Depending on the quality of the rectification circuit, and the LED driver circuit, the flicker may be more or less noticeable.