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The scientific reason you don’t like LED bulbs – and the simple way to fix them

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Re: The scientific reason you don’t like LED bulbs – and the simple way to fix them

#51

Post by gargravarr »

torrent-of-ions wrote: post #17
abainbridge wrote: post #16

What gets me are side-lights on cars that flicker. Cars have DC electric systems in the first place. Where did the Audi engineers find 100 Hz to run the LEDs from? I guess there's a switch mode power supply to convert down from 12 V to something lower that the LEDs need. But AFAIK, switch mode power supplies don't switch at 100 Hz. I'm pretty sure you could show by experiment that flickering car side-lights are distr…

That is seriously annoying. Not only do they flicker, they are slightly blue as well. I have mistaken an Audi for an emergency vehicle in my rear view mirror on multiple occasions. I am sure that this is their plan. Such lights should be outlawed.

I've found the colour to be too close to emergency-vehicle blue as well, particularly if the car behind hits a bump and momentarily brings their headlights into my mirrors, it's almost exactly like the flash of emergency lights. I shouldn't have to be second-guessing whether there's an emergency vehicle behind me, I should be following my first instinct which is to ensure I'm not obstructing traffic. Not clearing out of the way for an Audi pulling a tonne to rocket past me.

Re: The scientific reason you don’t like LED bulbs – and the simple way to fix them

#52

Post by abainbridge »

KaiserPro wrote: post #19
abainbridge wrote: post #16

What gets me are side-lights on cars that flicker. Cars have DC electric systems in the first place. Where did the Audi engineers find 100 Hz to run the LEDs from? I guess there's a switch mode power supply to convert down from 12 V to something lower that the LEDs need. But AFAIK, switch mode power supplies don't switch at 100 Hz. I'm pretty sure you could show by experiment that flickering car side-lights are distr…

I suspect its not the PSU, its the PWM for the dimming. compared to actually lowering the voltage/current of a LED/light/other its far simpler and cheaper just to turn it off and on again at varying duty cycles/ frequency.

Good point. But there's no reason for the PWM to be in the 100 Hz range either. Even the crappest possible microcontroller using timer interrupts could handle 1000 Hz. Hardware PWM peripherals (that can switch at 500 KHz) have been trivially cheap additions to microcontrollers for decades.

Re: The scientific reason you don’t like LED bulbs – and the simple way to fix them

#53

Post by colanderman »

mollusk wrote: post #33
KaiserPro wrote: post #19

Earlier quoted context omitted.

I suspect its not the PSU, its the PWM for the dimming. compared to actually lowering the voltage/current of a LED/light/other its far simpler and cheaper just to turn it off and on again at varying duty cycles/ frequency.

Correct me if I'm wrong, but isn't it just the de facto way of dimming LEDs? Since as opposed to incandescent bulbs, they require certain voltage/current to be lit, if you pass them less, they simply won't emit light.

They require a certain voltage (to a first approximation). Their light output is (again, to a first approximation) directly proportional to the applied current.

PWM control is just a cheap way of reducing the current.

Re: The scientific reason you don’t like LED bulbs – and the simple way to fix them

#54

Post by colanderman »

KaiserPro wrote: post #19
abainbridge wrote: post #16

What gets me are side-lights on cars that flicker. Cars have DC electric systems in the first place. Where did the Audi engineers find 100 Hz to run the LEDs from? I guess there's a switch mode power supply to convert down from 12 V to something lower that the LEDs need. But AFAIK, switch mode power supplies don't switch at 100 Hz. I'm pretty sure you could show by experiment that flickering car side-lights are distr…

I suspect its not the PSU, its the PWM for the dimming. compared to actually lowering the voltage/current of a LED/light/other its far simpler and cheaper just to turn it off and on again at varying duty cycles/ frequency.

May not be for dimming. PWM control is used also to reduce the current (and therefore power, and therefore heat) required to achieve a particular apparent brightness. This utilizes the persistence of vision effect.

My guess is that, above a certain frequency, persistence of vision no longer applies, and the light is in fact seen simply as being dimmer (which is undesirable in this case). Hence ~100 Hz.

Re: The scientific reason you don’t like LED bulbs – and the simple way to fix them

#55

Post by colanderman »

abainbridge wrote: post #52
KaiserPro wrote: post #19

Earlier quoted context omitted.

I suspect its not the PSU, its the PWM for the dimming. compared to actually lowering the voltage/current of a LED/light/other its far simpler and cheaper just to turn it off and on again at varying duty cycles/ frequency.

Good point. But there's no reason for the PWM to be in the 100 Hz range either. Even the crappest possible microcontroller using timer interrupts could handle 1000 Hz. Hardware PWM peripherals (that can switch at 500 KHz) have been trivially cheap additions to microcontrollers for decades.

It's likely not for dimming but for reducing power usage without reducing apparent brightness. See my comment sibling to yours.

Re: The scientific reason you don’t like LED bulbs – and the simple way to fix them

#56

Post by sevensor »

cjsuk wrote: post #8

I've had this problem and decided to test the problem away. Used a Vishay visible spectrum phototransistor and my scope. Think it was a BPV11 transistor. Philips bulbs are 100% clean. Poundland ones are not. Cheap no brand ones from Amazon are not. The latter two have a 100Hz output. I suspect the hatred for LED bulbs stems from the cheap ones which use the lowest cost solution to get mains voltage into a constant cu…

When I bought a pack of cheap LED bulbs, the harsh flicker really surprised me. I thought cheap would mean that the diodes would be uneven in brightness and have a very narrow spectrum. I was OK with that. It had not crossed my mind that they'd decide to slice a nickel off the cost of each bulb by leaving out capacitors. Whoever designed those bulbs needs to spend a week living under their horrible 60Hz flicker as punishment.

Re: The scientific reason you don’t like LED bulbs – and the simple way to fix them

#57

Post by gustavam »

You can see the flicker of your light sources for yourself if you have a slow speed camera. iPhone 6s/SE/7 can record at 240FPS. Point that at your LEDs, fluorescents and incandescents. Try your computer screen as well. I did this half a year ago and can confirm. Cheap LEDs are really bad, some more expensive ones don't flicker at all. Cheap computer screens flicker as well. Whenever I find a light source that I don't like somehow but can't quite say why, I record it slowmo and usually it turns out it flickers.

Re: The scientific reason you don’t like LED bulbs – and the simple way to fix them

#58

Post by seanp2k2 »

Johnny_Brahms wrote: post #49
lucaspiller wrote: post #21

Earlier quoted context omitted.

It's not just cheap LEDs unfortunately. I bought some expensive recessed lights with Osram drivers (~€80/light), and even those flicker a little.

I have a different experience. Osrams dont flicker (measured using the cheapo way: deferent types of cameras), whereas Philips LEDs do. But otoh the osram ones have all failed or started blinking due to what I think is over heating.

Yeah, adequate cooling is actually a big deal with LEDs, and the problem is compounded in cheap bulbs with minimal cooling and lower-quality components all around. These things are supposed to last decades. http://www.ledsmagazine.com/articles/print/volume-11/issue-7... talks about this with a specific focus on the fixture, since it affects thermals so much. The LED automotive headlights I purchased recently have substantial aluminum fins and cooling fans ( https://www.amazon.com/dp/B00WVJQO58 ) plus external drivers also with aluminum cases. There are tons of cheaper options on Amazon with less thermal consideration. Given the 80w rating (for the pair) compared with the kind of heat sinking you need for a CPU with a ~80w TDP to keep it happy and below 90*C, these looked to be at about the lower end of what would do the job. The fans definitely spin fast and move a bit of air; you can hear them quite well with the ignition on and the motor off.

Re: The scientific reason you don’t like LED bulbs – and the simple way to fix them

#59

Post by rhizome »

cjsuk wrote: post #15
rhizome wrote: post #12

Earlier quoted context omitted.

the cheaper ones seem to have a blue/white tone whereas the expensive ones have a yellow/white tone and are warmer. This is just normal color temperature and both come in all price points. I have several warm bulbs I got at Target on clearance for $2/2. I think it's just that the warm cheap LEDs are usually sold out, so all that's left are the prison-colored ones.

It's not quite that. The cheap ones use white LEDs which are basically phosphor doped blue LEDs. They have a distinct blue peak which is far brighter than the overall output. The more expensive ones use blue LEDs with separate phosphor outside the LED chip. Philips do that. Seems to result in far better filtering of the blue part of the spectrum.

Thanks, I never looked into the technical reasons behind this.

Re: The scientific reason you don’t like LED bulbs – and the simple way to fix them

#60

Post by TMSZ »

garaetjjte wrote: post #37
kabes wrote: post #20

But then wouldn't our 60Hz screens have a much bigger impact on office workers than the lights above them?

On CRT screens yes, but backlight in LCD screens is LED/fluorescent driven by much higher frequencies. Though some LCD screens still flicker, but it is visible only on special patterns: http://www.lagom.nl/lcd-test/inversion.php

Not necessary much higher. I've measured screen flickering in Lenovo T500 (probably one of the oldest laptops with LED backlight) with small solar cell and oscilloscope and it's 220Hz, with lowest brightness setting only 100Hz (although this brighness level is not too useful): http://commonemitter.blogspot.com/2017/06/lenovo-t500-measur...
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