Why is PWM used to control LEDs even when they're only used On or Off?

Question

I've noticed on cars with LED tail lights that they have a flicker to them - I presume because they're PWM controlled.
But I've seen it on vehicles where brake lights and tail lights are separate luminaires, therefore are just on or off and do not have varying brightness. I understand that LEDs need to have current through them limited, but I don't see how PWM would achieve this?

Edit: bad phrasing - sorry. Better phrasing: why would the LEDs be flickering even though there is no apparent feature to allow changing of the brightness? Perhaps the brightness is factory set, or maybe there is a feedback circuit to keep brightness constant. But could there be any other reason?

Thoughts: because a vehicle's battery has a significantly different voltage across it when the engine is off or on, (12v vs 14.5v) perhaps there is a controlling PWM circuit that keeps brightness constant over a range of voltages. Saying that, I'm convinced that smart phone screen back-lights have some flicker to them even on full brightness, yet their Li-ion batteries' voltages are near constant.

Answer 1

Contrary to initial intuition, it's actually to increase the brightness.

LEDs can be driven at a constant current, or they can be driven with a pulsed current.

With constant current you have to limit the current to a relatively low value - for instance many common small LEDs are limited to a constant current of say 20mA. That gives good brightness for indication purposes, but it's not that great.

LEDs, when driven with pulsed current, can be driven with a considerably higher current - maybe 5 to 10 times as much, or even more. That could be say 100mA for what would normally be a 20mA LED. However, there are restrictions on what the pulses can be - typically with limits on the frequency and duty cycle - maybe as little as 1% duty.

The end result is that the higher current increases the perceived brightness of the LEDs, since more photons are being emitted when they are on, but at the cost of some flicker, which is only really noticed when the LEDs are in motion.

A research group at Ehime University developed a pulse drive control method to make LEDs look twice as bright by leveraging the properties of how people perceive brightness.

The group was led by Masafumi Jinno, an associate professor of Dept of Electrical and Electronic Engineering at Graduate School of Science and Engineering of Ehime University.

When a short-cycle pulse voltage with a frequency of approximately 60Hz is applied to an LED at a duty ratio of about 5%, the LED looks about twice brighter [sic] to human eyes than that driven by a direct voltage, the research group said.

- Nikkei Technology - Human Perception Studied to Double LED Brightness

So you get more perceived brightness from smaller and cheaper LEDs without using more current (often less current) on average than if they were on constant.

The report above goes on to explain the effect in more detail:

There are two principles, the Broca-Sulzer effect and the Talbot-Plateau effect, involved in how human eyes perceive brightness. The Broca-Sulzer effect refers to a phenomenon in which light looks several times brighter to the eye than it actually is when exposed to a spark of light, such as a camera flash.

In addition, the Talbot-Plateau effect is a principle where human eyes repeatedly see flashes and sense the average brightness of the repeated lights. Thus far, "it has been believed that, due to the Talbot-Plateau effect, the brightness perceived by human eyes would not change even if an LED is pulse driven," Jinno said.

"The Talbot-Plateau effect is a principle found in the days when fluorescent mercury lamps and other light sources driven by a power supply with a longer voltage cycle of about several hundred milliseconds were used," Jinno said.

Thus, the group decided to drive the LEDs using a power supply with a shorter voltage cycle of about several hundred microseconds. As a result, the group discovered that, when a pulse voltage with a frequency of approximately 60Hz is applied at a duty ratio of about 5%, the impact by the Broca-Sulzer effect becomes greater than that of the Talbot-Plateau effect so that the light emitted from the LED looks brighter to human eyes.

Answer 2

Not saying this IS how it's done, but this is certainly how it COULD BE done:

A car has red lights at the rear. Red is used both for tail lights and for brake lights, so why duplicate them?

They are duplicated with incandescent bulbs because their intensity can only be controlled by controlling the resistance of the filaments. That's why dual-filament bulbs are/were commonly used as combined tail/brake bulbs.

However, with LEDs you can easily control intensity with a PWM circuit. Same red LEDs; different duty cycle and therefore different intensity.

Look again at those flickering tail lights in front of you: when the brakes applied, the flickering stops.

Answer 3

As I worked in this field, the answer is that it is due to factory adjustment in the software when doing lumen measurement in a test environment, to calibrate light precisely according to regulatory standards or requirements with a simple firmware upgrade.

This is a NEW EDIT on my previous answer, which is going to be deleted, since it will completely discard my initial thoughts.

Answer 4

It is a very good question! +1 I've asked to myself the same thing. Even on signal LED lights of other devices (hard disk, tachimeter of the car, whatever) which are only on or off. Why use PWM instead of a cheap resistor.

I've thought that could be related to a better LED efficiency, driving it in overcurrent (current which is unsustainable if continuous) with a given duty cycle. But this is just an assumption made by reading power led/drivers datasheets. As mentioned (but I think it is for a different reason) here: Why do some DC-powered LED circuits flicker?

Still dark the answer of doing it even on NON-power led. These LED still have a resistor inside stuff that I disassembled, so it could not be an economical reason. Any answer?

A possible answer is due to the better perception of flickering with a side view. I tend to spot better these lights instead of non-PWM ones. This could explain why in my car (in Europe, don't know if standard colors are different in USA) the dipped headlights are signalled to be active with a green, slow PWM dimmed (few milliwatts) LED near the tachimeter. And, due to better sensitivity to blue of the rods of human eye, the full headlights are signalled with a blue light (of still few mW) which is not PWM modulated but visible when looking at the street without moving the eye on it.

Edit: If negative votes, please tell why. I think that there is a misunderstanding in the question. We all know that leds are dimmable, but they must have a very precise reason if digitally dimmed even when power is not a concern (signal LEDs) even on old systems.

Answer 5

The flicker you notice is not unique to led bulbs. First, led bulbs, atleast signal ones not headlight ones, tend to be simple ballast resistor circuits. They are not stable with voltage change, just like incandescent bulbs.

Second, cars are noisy. The voltage fluctuates depending on the engine and alternator state. Next time you're in your car at night, pay close attention to your dashboard at idle, then Rev the car up and notice the light difference.

In my car, the electric noise, the ripple is enough that the bulbs, led and incandescent, noticeably flicker. That is what you are seeing.