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


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

In: 2

A simple way to do “voltage conversion” in DC power to reduce the voltage is to not actually do the conversion. Instead, power to the LED is pulsed on and off at high frequency. Rather than pushing half the power to LED, we’ll push 12 volts on a 50/50 on/off cycle at high speed. To the human eye this looks good enough.

To a camera with a shutter time though, you can actually observe the LED in the “off” state if the cycle isn’t quite fast enough.

This is called Pulse Width Modulation (PWM) if you want to read more about it. I hate it, but this is what the industry does.

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.

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

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.

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.