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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

#61

Post by cardiffspaceman »

colanderman wrote: post #54
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.

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.

Another technical consideration would be to have grid-based scanning of the LEDs. That is you have some sort of row output, one row at a time, and the columns are multiple parallel outputs. This inherently pulses the LEDs as the rows aren't all lit simultaneously. The persistence-of-vision effect is a great side effect and can be capitalized on. The grid is best for limiting the number of wires to access a substantial number of LEDs (like 16 or so) in a group. I don't think a grid for the turn signals of a car would be a great idea, at least not at the level of the individual turn signal diodes. Perhaps within a single corner's turn signal assembly.

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

#62

Post by Osmium »

inferiorhuman wrote: post #29
Osmium wrote: post #7

Earlier quoted context omitted.

Yeah, I think my Ikea ones are phosphor, the Philips Hue are RGB though and still fine.

I went to LEDs in my apartment about two years ago. 3 Hue bulbs + hub, some 48" T8 LEDs, and some E26 Ikea bulbs. The Ikea bulbs are great, power on instantly and slowly fade off. The T8s take about 3/4 second to turn on, and are nice and bright (and a far cry from the power consumption of the T12 florescent bulbs they replaced). The Hue bulbs... well, one's developed a noisy transformer. Replacements are, what, $40?…

Not trying to sell Hue here, but their bulbs have gone through several iterations by now. They were one of the first big names to push consumer LED bulbs, I wouldn't be surprised if their earlier versions had flaws just by being new. But maybe not, I don't know.

(Also their color bulbs are $40, but they have much cheaper white ones, and also an in-between model they call ambiance which are white but allow you to adjust color temperature)

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

#63

Post by cjsuk »

sevensor wrote: post #56
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 pu…

I don't think they leave out capacitors. The typical Mylar units they use are cheap. They only put enough capacity in to get them out of the door.

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

#64

Post by cjsuk »

seanp2k2 wrote: post #58
Johnny_Brahms wrote: post #49

Earlier quoted context omitted.

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 sub…

All the cheap ones I've had that blew up were either an SMD resistor blowing out or one of the LEDs blowing out. When the LEDs blow out if you peel the board off the heatsink you will always find a missing bit of goop under the LED that blew.

Interestingly I stuck one on my bench supply (30v 2a) and I couldn't get one to blow out at all with that!

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

#65

Post by Osmium »

seanp2k2 wrote: post #22
Osmium wrote: post #7

Earlier quoted context omitted.

Yeah, I think my Ikea ones are phosphor, the Philips Hue are RGB though and still fine.

Anyone know what the LiFX bulbs use? We have a dozen of them and the adjustable color temp is great. I also have a box of smaller LED bulbs from Home Depot. Neither hurt my eyes and I don't (didn't, will see now that I'm paying attention) notice a difference, but I'm sure there is one. I also wonder about e.g. The Anker desk lamps, where the bulb is built in. I have one behind my monitor with the head turned round (i…

LiFX let you set the color right? So by definition they're going to be RGB (have three colored LEDs inside). Generally, LEDs that are single color are more likely to be phosphor (blue LED inside with a phosphor that converts to yellow).

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

#66

Post by joezydeco »

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…

Cadillacs are even worse. I could swear their brake lights run at 20Hz.

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

#67

Post by oasisbob »

w458cmau wrote: post #38
oasisbob wrote: post #14

Earlier quoted context omitted.

Agreed! It took several tries to get LEDs which worked well for vanity (i.e. Mirror) lighting. I thought it was a simple matter of color temperature - so wrong! High CRI strip LED arrays are surprisingly hard to find. The end result was worth it though - skin looks natural, and the nearest commercial version is upwards of $800 - and I'm not entirely sure they got the CRI right.

Which solution did you end up with?

I ended up with a strip from colordiode, they're available on ebay. Was described as CRI>95 3000~3200K SMD2835, 90/m. Came with a pretty decent datasheet, seemed well-characterized. Cost was ~ $10/meter. They were responsive enough to include a few samples of other color temperatures with the order when I asked nicely.

The housing used was extruded aluminum u-channel by Torchstar (U05, semicircular), very nice clean finish. Available on Amazon.

Like I mentioned, this was for vanity lighting - there's a full length mirror in the closet that's wedged on the wall between a closet door and a window. Not much room for anything else. Hung two vertical strips, ~ 4ft long, on each side of the mirror.

Light ended up soft, diffuse, and flattering - but still directional enough for good modeling. Significant other couldn't have been happier.

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

#68

Post by Paianni »

Zak wrote: post #13
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…

Another issue is that many LEDs have peaky spectra. This makes cool white harsh and warm white dingy. This isn't limited to cheap products, as producing a fuller spectrum with LEDs is less efficient. I'll take the efficiency hit for better light; the most common measure used, printed on the packaging of some bulbs is CRI (color rendering index). It's based on blackbody radiation, so incandescent bulbs and sunlight bo…

Low pressure sodium lamps (SOX) are negative, -44. High pressure sodium lamps (SON) are positive, +20.

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

#69

Post by arantius »

gargravarr wrote: post #50

I find this a problem with my LED-backlit LG monitor. It's ridiculously (by which I mean, blindingly) bright at full brightness, but anything below and the imperceptible flicker gives me migraines. So I have the choice of impossible brightness with no flicker, or acceptable brightness with headaches. I don't get why LED bulbs need to flicker though - the bulbs don't dim, and if they're providing steady light, they sh…

> I don't get why LED bulbs need to flicker though

It's _much_ cheaper/easier to blink an LED on/off for 75% of the time than to drive a constant but lower current to produce 75% of the brightness. Most products are cost optimized.

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

#70

Post by lucaspiller »

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.

I have multiple Osram drivers (different models) and some flicker, some don't - so I don't think as a general rule you can say "X brand doesn't flicker". I'd assume the choice of LED, and how many you have makes a difference as well, as it might be based on power draw.
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