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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.

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  • \$\begingroup\$ It doesnt. It limits the amount of light that reaches your eye. \$\endgroup\$ – PlasmaHH Apr 10 '15 at 9:35
  • \$\begingroup\$ That may be to reduce the current consumption by decreasing the duty cycle, especially if the lights are the type which doesn't require external resistive elements (e.g. pilot lights). \$\endgroup\$ – Mister Mystère Apr 10 '15 at 9:44
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    \$\begingroup\$ already answered: 49178/why-do-some-dc-powered-led-circuits-flicker \$\endgroup\$ – Chu Apr 10 '15 at 9:52
  • \$\begingroup\$ What is your question? Take note that LEDs are dimmable so the question in your title is invalid. \$\endgroup\$ – Andy aka Apr 10 '15 at 9:53
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    \$\begingroup\$ Software developers, that’s why. The 15-20 % of the population who suffers from the flickering from DLP projectors and new cars get to suffer in silence. \$\endgroup\$ – winny Feb 7 '18 at 15:00
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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.

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    \$\begingroup\$ This is not true for all LED materials, nor is it true for all current levels. \$\endgroup\$ – WhatRoughBeast Apr 10 '15 at 18:23
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    \$\begingroup\$ It was intended as a cautionary note. The increased efficacy vs current was applied 40 years ago to DPMs, but it is no longer much used. See, for instance lrc.rpi.edu/programs/solidstate/pdf/Gu-SPIE6337-17.pdf, "Therefor, to reach the same light level, PWM light dimming requires more power than CCR dimming". \$\endgroup\$ – WhatRoughBeast Apr 10 '15 at 18:40
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    \$\begingroup\$ I'd have thought it obvious. If you run an LED at 10x "nominal" current with a 10% duty cycle, you get the same apparent brightness as long as the luminous efficacy remains constant. Does it? For GAAsP, yes. For others, not so much or no. See, for instance, www.avagotech.com/docs/AV02-0342EN Note the efficacy curves are either straight or concave downwards. High-current low-duration pulses will give higher peak brightness, but this does not apply to situations such as brake lights, which are on for seconds at a time. \$\endgroup\$ – WhatRoughBeast Apr 10 '15 at 18:52
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    \$\begingroup\$ @WhatRoughBeast I think you may be confusing luminous intensity with perceived brightness. They are two very different things. Yes, on average, the amount of light isn't increased with pulsed current, but the instantaneous brightness at the time it is on is greater, and thanks to the fact that the human eye isn't a photodiode, but considerably more complex, with something called persistance of vision, those bright points add up to a greater perceived brightness. Like a capacitor that charges faster than it discharges - apply a pulsed current of less than 50% duty and the voltage rises. \$\endgroup\$ – Majenko Apr 10 '15 at 19:16
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    \$\begingroup\$ Sigh. OK. Real last comment. Can you provide backup? Not for generalized POV (wrt flicker frequency, etc), but "bright points add up to a greater perceived brightness" for PWM'd illumination. \$\endgroup\$ – WhatRoughBeast Apr 10 '15 at 20:32
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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.

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  • \$\begingroup\$ The second sentence of the question states, "But I've seen it on vehicles where brake lights and tail lights are separate luminaires, ...". \$\endgroup\$ – Transistor Feb 7 '18 at 13:22
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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.

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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.

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