That LED Christmas light flicker

Discussion in 'Off Topic' started by reitz, Dec 10, 2010.

  1. reitz

    reitz Elite Member

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    Who else can see it?

    I first saw LED lights several weeks ago walking with my wife on the Southside (Pittsburgh neighborhood). One of the bars had white lights in the trees out front, and they stopped me dead in my tracks...it felt like I was looking at a glitch in the Matrix. Once the Christmas decorations came out in all the stores, I realized they were LED lights.

    Looking at them, even just seeing them out of the corner of my eye, is unpleasant; they cause a sensation that feels like a rapid buzzing deep in my brain. My two-year-olds love the Christmas trees at Target and Home Depot, so I've seen them a lot lately. It's gotten so I can easily spot them in someone's yard from my car while I'm driving.

    My wife can't see it at all. She also was never able to see the difference between 60 and 75 Hz on an old CRT monitor, though. During my long ago stint in Desktop Support, I would instantly notice when someone's monitor was still set to the 60 Hz default, and I always changed it first thing. Some users would break out into a big smile and thank me profusely, but most couldn't see a difference. I'd bet that some of the former might see the same thing I do with LED lights.

    I'm guessing it's due to the 60 Hz AC; I can see the flicker in fluorescent bulbs (and even more so in Europe where it's 50 Hz), but they don't induce the same sensation. Any thoughts on what causes it? Is it all bright AC LEDs or something unique to Christmas lights? I'm seeing these things for sale everywhere now, I'm thinking I'm going to have to stock up on the incandescent strings of lights while I still can...there's no way I could have those LEDs in my house without going nuts.

    Does anyone know what I'm talking about?
     
  2. ussfletcher

    ussfletcher Platinum Member

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    To the best of my knowledge what you describe is impossible because all practical and readily available LEDs require DC power.
     
  3. PieIsAwesome

    PieIsAwesome Diamond Member

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    Yes. I got some from Lowes for cheap after some deal. I was going to use them for ambient lighting on some of my projects, but the flicker killed that plan. Gave them away. I am guessing this is due to the crappy rectification of AC. Maybe they can be modified to run off a DC transformer.

    Edit: It seems that there are rectified and non rectified LED lights. AC is 120 hz, but AC goes both ways and LEDs only accept current in one direction, so they flicker 60 times a second. Rectified LEDs make use of the current in both directions by means of a rectifying circuit, so they flicker at 120 times a second. I'm no EE though.
     
    #3 PieIsAwesome, Dec 10, 2010
    Last edited: Dec 10, 2010
  4. rockyct

    rockyct Diamond Member

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    What he says and you say are both quite accurate and the reason why there is a flicker. AC flips the voltage between positive and negative 120 times a second (60 complete waves). However, the cheap LED strings have no rectifiers so they only light up on half of the AC wave. Half the time they are off, and half the time they are following the AC wave. A full wave rectifier with some caps would eliminate the flicker but that's too expensive for Christmas lights (maybe $1 or so in production costs).
     
  5. dighn

    dighn Lifer

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    could be remedied with caps for filtering but since the effect isn't noticeable to most, it isn't worth it to the manufacturers to put it in.
     
  6. rockyct

    rockyct Diamond Member

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    I found a site that explains it better and has a picture:
    http://www.foreverled.com/questions/how_does_full_wave_led_work.html

    [​IMG]
    The top is what AC voltage looks like going from +170 Volts to -170 Volts. The middle is basically the waveform each LED is getting. The bottom is what the good Christmas lights with no apparent flicker look like.
     
  7. ussfletcher

    ussfletcher Platinum Member

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    Ah, I had no idea they would put them on with no rectifier, that would definitely explain it. What do they do about the voltage for these strings? Resistors? I just figured they would have a pretty simple transformer/rectifier setup.
     
    #7 ussfletcher, Dec 10, 2010
    Last edited: Dec 10, 2010
  8. RelaxTheMind

    RelaxTheMind Platinum Member

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  9. rockyct

    rockyct Diamond Member

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    Caps alone would cause issues. First off, they'd have to keep the strings on for half the wave and that could be bigger caps and I think during the reverse cycle it would knock out the charge completely. Second, the LEDs are probably not designed to get the full peak AC voltage of 170 (120 volts AC is an average). Most Christmas lights aren't designed very well and typically overdrive the LEDs. A LED designed to get 3 volts could be getting 3 volts RMS now but would get 4.5 volts DC(ish) if you just put a big cap in.
     
  10. rockyct

    rockyct Diamond Member

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    Well, hopefully they put a resistor with each LED. I'm not sure if they do that though. If they don't, they are just using the voltage drop of enough LEDs in series to make it work. That's a huge problem though because the diodes are not uniform enough to have a consistent voltage drop for each LED and the whole string will fail quite quickly.

    Another issue if you wanted to fix the string is that there are typically two groups of LEDs. One half lights up on the positive AC wave, the other on the negative AC wave. You'd have to check the wiring on the string to see how they are put together and it may even be easier to just cut the string right there and rectify each half by itself.
     
  11. Leros

    Leros Lifer

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    I couldn't stand the flickering. It made me feel like the lights were moving around, which messed with my head. It was actually unpleasant to look at them.

    I built a simple full wave rectifier from some diodes and a capacitor. Cost me < $1 of components.

    Like this (without the transformer of course):
    [​IMG]
     
  12. ussfletcher

    ussfletcher Platinum Member

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    Not to mention the problems temperature would bring.
     
  13. thescreensavers

    thescreensavers Diamond Member

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    me too, so damn annoying, 60Hz gave me a headache.
     
  14. Rubycon

    Rubycon Madame President

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    ALL lights running on AC (even incandescent) flicker. The longer decay time due to cooling wolfram acts as a buffer to make it gentler but it's there. Hook a small solar cell up to the mic input of your PC sound card and point it at a table lamp with a standard light bulb and listen to that hum. If your power is 50Hz it's definitely noticeable.

    With LED lighting the flicker is much more pronounced because their rise and decay times are nearly instantaneous. This also makes them good for sending/receiving data. ;)

    If you really wanted to use flicker free LED lighting you should build a switching power supply to deliver 120 volts DC with little ripple. The lighting will truly be CW and exhibit no flicker at all regardless how fast you turn your head.

    LED tail lights on automobiles have this annoying flicker as well because their driver circuits are switching too low or so noisy this is passed into the output - visibly!

    With CRT displays flicker was highly dependent on not just the refresh rate but also the persistence of the phosphor used. Problem is too long a persistence would cause "trails" on fast moving bright objects on a black background. Too short a persistence meant a higher refresh rate (typically 85 or 100Hz) was required and at higher resolutions this required a faster RAMDAC in the display as well as more video bandwidth on the monitor. The combination of the two made things very tricky from cabling (13W90 or RGBHV worked better than plain HD15) to the display itself. Generally the result was blurry text and shadows.
     
  15. bobdole369

    bobdole369 Diamond Member

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    Has anyone actually o'scoped an LED light that is bothersome to the eyes?
     
  16. Rubycon

    Rubycon Madame President

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    Yes actually I have. :$
     
  17. Homerboy

    Homerboy Lifer

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    I refuse to buy the (cheap) LED lights. They hardly have that "warm glow" that Christmas lights should have. They simply aren't "comforting". Very harsh.

    There are nicer, soft-glow LEDs that I have seen for sale at Target. I've given those consideration, but at $13 a line.... forget it. $1.99 throw-away specials for me!
     
  18. geno

    geno Lifer

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    I've definitely noticed it, same for LED tail lights on some newer vehicles. You can see it from a mile away with a blink or a movement of the eyes. It's not necessarily annoying, but definitely noticeable.
     
  19. IndyColtsFan

    IndyColtsFan Lifer

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    Why am I not surprised? :D
     
  20. Analog

    Analog Lifer

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    They blink at 120Hz actually. I notice it if I move my head rapidly. No biggie.
     
  21. Mark R

    Mark R Diamond Member

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    You're right. It's due to AC supply. Normally, these strings are powered off a small transformer, with a full wave rectifier. The problem is that LEDs have a threshold voltage, so they only come on at the crest of the wave, and are off when the voltage is lower. As a result, they pulse on-and-off at 100 or 120 Hz. Usually, in order to save cost, the LEDs are put together in longish series strings, with small ballast resistors. There may even be a current limiting capacitor before the rectifier. This is quite common when LEDs need to be powered directly from a 120 or 240 V AC line without a transformer. Resistors would waste too much energy and get too hot - so, instead a ballast capacitor is used, followed by a full-wave rectifier. The capacitor doesn't waste (a relevant amount of) energy, so the heat problem and energy efficiency problem of the resistor are avoided.

    Traditional fluorescent lamps show the same effect - but they still produce some light at low voltage, and the phosphors smooth things out a bit, so the effect is nowhere near so pronounced.

    Of course, it would be easy to avoid the flicker of LEDs in this configuration by adding a capacitor to store energy. However, this adds cost - so it isn't done for cheap lighting. An alternative, better but more expensive technique is to use a SMPS to power the LEDs. These can be configured in constant-current mode, so don't need any additional ballasting, and avoid visible flicker by operating at very high frequencies (20-50 kHz). High-end LED lighting uses this type of power supply.

    The flicker seen in car LED lamps is present for a different reason. LEDs change properties with current - in particular, their color shifts if operated away from their recommended supply current. Further, limiting current in a DC system to dim LEDs is undesirable due to the need for wasteful and hot ballast resistors.

    The recommendation way of dimming LEDs is therefore to operate them at full power, but switch them on-and-off rapidly, in order to achieve the required brightness. So, variable brightness LEDs - e.g. car tail/stop lamps operate are designed to be rapidly switched on/off in tail mode, but full-on in stop mode. Depending on the speed of the switching circuit, this could produce visible flicker (particularly if your eyes, or the vehicle is moving).
     
  22. Rubycon

    Rubycon Madame President

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    Tint control is very much achievable without perceivable flicker though. They are being cheap about this! So much stuff made in China that's just cheap utter crap these days! Intolerable!
     
  23. rockyct

    rockyct Diamond Member

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    I think the main reason for the flicker though is that there isn't a full wave rectifier in the cheap LED strings. One half of the string gets lit up when the voltage goes positive, the other half gets lit up on the negative voltage. Incandescent bulbs can light up on both the positive and negative wave but LEDs can't because they are of course diodes (current flows one way only).

    Again, if anyone tries this, take note on how the string is wired. Typically, just giving it 120 V DC will only light up half of the string.
     
  24. Rubycon

    Rubycon Madame President

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    Have you tried this before?

    13 nine volt batteries in series should be enough. ;)
     
  25. bobdole369

    bobdole369 Diamond Member

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    LED's change colors. Specifically the more exotic stuff like blues and violets, so I wonder if its a simple way to stave off the effect for a bit longer. Full on DC voltage would cause the effect as quickly as possible, while rectified AC would take a bit longer.