Why do LED rear lights flicker?

Jonny69

Jonny69

Jonny69

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I don't understand why LED rear lights flicker like a fast strobe. They are being fed 12V DC aren't they? So they have no reason to strobe. Educate me OcUK, why do they do it?
 
Only thing I can think is they maybe use the same set of LEDs for rear and brake lights, strobing them on a reduced duty cycle to give reduced brightness as lights then giving them 100% duty cycle as brake lights. Must admit I've only noticed the strobing when on film rather than in real life.
 
I can see it quite clearly in real life, it's quite high frequency, I'd say over 30hz but I can see it. Both on brake lights and the sidelights. I've also noticed it on the new LED cats eyes in the road.
 
It's probably your eyes. Light technically does flicker, but we don't see it that way (Or so I'm led to believe, but I am thick n that department). Maybe you're an alien?
 
If you have sensitive eyes, then you can see the flicker - most LEDs (especially when low-power usage, such as cat's eyes) are driven by PWM - Pulse Width Modulation - it "pulses" power to and from the LED, and the faster it's done, the brighter it will go. With most LEDs, you can drive them at ~50% modulation, and they'll appear to most people as static, and on fully.

I'm guessing you're quite sensitive to CRT monitors, too? Also I bet you can see the "rainbow effect" on DLP projectors?
 
I've noticed the flicker on the rear lights of the BMW 7 series, quite clearly. Other cars' lights have seemed more solid, though.
 
ConfusedTA said:
I'm guessing you're quite sensitive to CRT monitors, too? Also I bet you can see the "rainbow effect" on DLP projectors?
Click to expand...

That sounds like me when i had my old crt monitor i got splitten headaches using it under 100hz.

Now i have tft its all good.
 
I must have sensitive eyes like Jonny as I can see a flicker myself. Then again I found CRT monitors on anything 75Hz and below uncomfortable.
 
Surprised the flicker can be seen, I would have thought they would be driven at pretty high frequencys.

<goes out to point a cam corder at a car with LED lights>
 
I need to borrow some of you fellow freaks to prove to my wife & family I'm not nuts.

Someone needs to come look at my TV and confirm that the picture flickers on and off very quickly. I'm constantly complaining about it but no one else can see it so I just get funny looks/default specsavers comments.

TV's and Monitors flicker at me. LCD's dont. Not sure about LED lights - I'll have to check.
 
Ehhhm.. im sure they will,, not all LEDs get constant DC currents, giving them pulses is the only effective way of lowering the intensity of leds. maybe thats what they are trying to do...
 
ConfusedTA said:
If you have sensitive eyes, then you can see the flicker - most LEDs (especially when low-power usage, such as cat's eyes) are driven by PWM - Pulse Width Modulation - it "pulses" power to and from the LED, and the faster it's done, the brighter it will go. With most LEDs, you can drive them at ~50% modulation, and they'll appear to most people as static, and on fully.

I'm guessing you're quite sensitive to CRT monitors, too? Also I bet you can see the "rainbow effect" on DLP projectors?
Click to expand...
Yes, I can see both.

Doesn't make sense to me though, why PWM a LED when it's a DC device? Surely use a lower power item in the first place or is it just me that thinks powering up a load of control circuitry is kind of defeating the object of using LEDs in the first place? Or are we talking light output regulation here because the voltage is not very stable in a car? Ie it fluctuates between 11V and 14V so runs a lower duty cycle when the voltage is higher?
 
I suspect, it's the same reason we PWM drive our system display LED's.. longevity..

The LED junction temperature is quite high on 'bright' LED's, driven with DC, this actually leads to a relatively short 'half life' of brightness... so PWM is a simple way of maintaining the brightness whilst prolonging their useful life..
 
As Demon said, it's partly to increase the lifespan of the LEDs, and it's partly as you said to ensure the light output is constant.

The input voltage will be regulated, and then the output will be adjusted via PWM - it's a simple-ish way also of getting multiple brightnesses - for brake light you just increase the cycle duty to the desired level. You have one regulator/control circuit, reducing costs further.

Also the high-brightness LEDs that are used are very power hungry, and do actually generate a lot of heat - so if you can run them for only part of the time, it reduces the power consumption and heat generated.
 
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