Automotive external signaling LEDs - Why so much flicker?

[Jeff7]

More and more vehicles are being equipped with LEDs for things like brake lights and turn signals. A lot of them flicker very noticeably, and I was curious as to why this is.
Does the alternator put out an AC waveform, pulsed DC, or steady DC? Are these lights being PWM'd at a very low frequency to control brightness/power?

I'm just wondering what the origin of this flicker is. Some cars have LED signals and exhibit no visible flickering, so it definitely can be controlled.

 

[bmaverick]

Some have a flicker that is noticeable while others I have not seen it happening.

Maybe EMI/RFI is getting into the lines that power them.

 

[PottedMeat]

if it varies with frequency, probably the alternator. the alternator outputs multiphase AC dumped into a rectifier pack, further smoothed by the battery.

i don't see any reason for the pwm controller to change the driving frequency.

 

[Sukhoi]

(Garage or OT...I pick OT because this is more on the electronics end of things, in my opinion.)

More and more vehicles are being equipped with LEDs for things like brake lights and turn signals. A lot of them flicker very noticeably, and I was curious as to why this is.
Does the alternator put out an AC waveform, pulsed DC, or steady DC? Are these lights being PWM'd at a very low frequency to control brightness/power?

I'm just wondering what the origin of this flicker is. Some cars have LED signals and exhibit no visible flickering, so it definitely can be controlled.

You complain about 60 Hz CRTs don't you?

 

[thescreensavers]

^ I do its horrible lol

But with leds I really dont notice it

 

[Jeff7]

You complain about 60 Hz CRTs don't you?

Briefly - then I change the monitor's rate to something tolerable.
I ran my 21" Trinitron at 120Hz.

I prefer PWM frequencies somewhere in the whole-integer kilohertz range.
The low-frequency flicker of many LED lights gets quite distracting in traffic if there are several such vehicles visible - it starts to look like flashing emergency lights all over, and it can get disorienting/dizzying too.

^ I do its horrible lol

But with leds I really dont notice it

Really? Huh...I'm pretty sure LEDs have a turn-off time that's many times faster than the decay rate of the phosphor coating in a CRT. (That's of course why incandescent bulbs don't appear to flicker, despite running on plain AC - the filament doesn't have time to cool off enough that you can see any change in brightness.)

 

[Howard]

Ripple in the power supply, I suppose. Unless the brightness is being modulated (which I think is not the case), there wouldn't be any PWM.

 

[Jeff7]

Ripple in the power supply, I suppose. Unless the brightness is being modulated (which I think is not the case), there wouldn't be any PWM.

Brake lights would need to have two brightness levels - running mode, when the headlights are on, and fully-on when the brakes are engaged.
But when fully-on, I wouldn't expect there to be any flicker. I guess that does point to a pulsed or rectified DC supply then.

 

[Sukhoi]

I've never noticed it on car LED lights. But I do get as irritated about 60 Hz CRTs as you do.

 

[Zap]

I'm pretty sure LEDs have a turn-off time that's many times faster than the decay rate of the phosphor coating in a CRT. (That's of course why incandescent bulbs don't appear to flicker, despite running on plain AC - the filament doesn't have time to cool off enough that you can see any change in brightness.)

This is correct. I've read forum threads about modding LED Christmas lights with resistors or something to reduce flickering caused by alternating current, basically converting AC into DC. (This also makes the lights brighter)

My guess is that the cars with flickering LED lights get that way from the alternator. Same reason why some aftermarket car stereos have a whining noise.

 

[MaxFusion16]

LEDs are driven by pulse width modulation, so technically they are flickering really really fast, usually unnoticeable. I guess you are just really sensitive.

Also, car alternators output DC via rectifier diodes, no AC anywhere in your car.

 

[Jeff7]

Kind of forgot about this thread...oops.

LEDs are driven by pulse width modulation, so technically they are flickering really really fast, usually unnoticeable. I guess you are just really sensitive.

They can be driven that way. It's just a popular way of dimming them, because it doesn't affect the voltage, and thus it doesn't affect their color - if you change the voltage to change brightness, the color will shift slightly.

Good driving of an LED at full-brightness is done with a proper constant-current source. (Some of these LED drivers also accept a PWM input, which they pass along to their power output line.)

My take is that PWM should be done well into the multi-kilohertz range, where no one could possibly notice it.

Also, car alternators output DC via rectifier diodes, no AC anywhere in your car.

But if they internally produce AC, which is then passed through a bridge rectifier, then they are outputting pulsed DC, which will result in flicker. LED Christmas lights can be driven by this sort of waveform, which results in flickering, which I perceive to be very noticeable.

This is correct. I've read forum threads about modding LED Christmas lights with resistors or something to reduce flickering caused by alternating current, basically converting AC into DC. (This also makes the lights brighter)

My guess is that the cars with flickering LED lights get that way from the alternator. Same reason why some aftermarket car stereos have a whining noise.

They probably stick a capacitor onto the rectifier's output to help fill in the gaps. It could also risk overdriving the LEDs, depending on how they're being driven in the first place.

 

[clumsyalex]

I've noticed this flickering alot on range rovers in the morning as they come into my view. they flicker for about 1 or 2 seconds. it's quite noticeable on them at the least. even my passenger notices it all the time.

 

[guus]

Hi, I'm happy to finally find a discussion about this subject. I noticed this flickering of car LED's from the very beginning. Indeed, the car leds do only flicker when they are at the low light level, not when they are at the high level, so it must be a matter of switching them off and on as a simple means for reducing the light level. I must be a little sensitive for this because I see it always, with car led lights. The frequency is certainly not high, I guess about 50 Hz. I don't see it when I directly observe the light. The way to easily observe this is the following: quickly look from left to right (with your eyes!) when a car passes you. Observe the "light-trail" left by the light inside the "phosphor" of your eyes. If it is a LED light, you'll notice that this trail is broken-up in light and dark parts. Otherwise you'll notice a continuous trail. With some practice, this becomes very easy indeed. (Keep your eyes also on the road, though!) Downside of this method is, that once you master this, you'll always notice the flickering with LED lights of cars.

I wonder if all these flickering lights in modern traffic are not someway distracting or tiring, even when you are not fully aware of them. When you quickly skip your eyes from left to right, to observe the traffic, it must have some influence, I guess.

 

[jlee]

Welcome to AT!

 

[sdifox]

You complain about 60 Hz CRTs don't you?

Can't stand 60Hz crt...minimum 72Hz for me.

 

[us3rnotfound]

In terms of flickering, here are the brake lights of doom imo:
http://images.dealerrevs.com/gallery/photo.php?id=31136812

 

[us3rnotfound]

Can't stand 60Hz crt...minimum 72Hz for me.

It was a balancing act. The lower refresh rates offered better text clarity. I kept it 80-100Hz at 1280x960 on my Sony E540. If I put it at the 120 Hz, text would no longer look exact; it'd be fuzzy, however any flicker that was once there for refresh rates <=80Hz would not exist.

If I put it at the more sensible 1600x1200 resolution, text clarity went to absolute shit. Therefore I'd have to lower the refresh rate until I found a compromise. But at that resolution, I'd take frequent brakes from the computer.

 

[Howard]

My take is that PWM should be done well into the multi-kilohertz range, where no one could possibly notice it.

It's probably the cost of the engineering more than anything else. I suppose this issue would easily be fixed if people complained about it more.