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LED car lights flicker - no need!

Discussion in 'Beginner Electronics' started by Major Scott, Apr 22, 2013.

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  1. Major Scott

    Major Scott Guest

    Surely they can design LED lights on cars to have a higher frequency PWM? Even £100K cars flicker dramatically, especially when filmed. It makes them look really cheap. All it would take is a higher frequency PWM, or a smoothing capacitor?
     
  2. Guest

    They flicker for a reason. If they smoothed the current they might just as
    well use DC direct from the battery. I don't know the technical reasons why but
    apparently using the equivalent DC voltage required to get the same brightness
    as you can get by strobing them would burn them out. I'm sure some electronics
    guru on here can explain more. But it does lead to interesting effects on
    video as you say :eek:)

    NJR
     
  3. Dave Plowman

    Dave Plowman Guest

    Pulsing an LED is a way of getting a higher light output from it without
    overheating. Overheating an LED kills it in short order. Seeing a flicker
    from them on a video is the same effect as wagon wheels appearing to turn
    backwards on old cowboy and indian films - stroboscopic effect.
     
  4. Guest

    Its odd though isn't it. The way they're constructed must mean the amount of
    heat generated for a given voltage or current must slowly tail off so although
    they'd heat up too much at constant voltage X you can pulse them at for
    arguments sake X*2 producing the same or even more total light but without a
    doubling of the heat generated so allowing for cooling down to safe levels
    during the OFF periods of the pulse. Or something like that.

    NJR
     
  5. DavidR

    DavidR Guest

    The effect relies on the persistance of the eyes to make it appear that the
    average brightness is higher. Smoothing at source would be less energy
    efficient.

    Agreed the effect is not pleasant. It would help if they could introduce
    softer start for indicators and brake light dimming when conditions suggest
    a slow moving queue.
     
  6. Major Scott

    Major Scott Guest

    Take for example the brake/tail lights. These are often pulsed for tail and on for brake. So what you said doesn't make sense. Anything less than full voltage on (as for brake) will be lower heat.
    It's way worse than that - the duty cycle is quite a lot less than 50%, so you see them off, with the occasional on.
     
  7. Major Scott

    Major Scott Guest

    Easy enough to double the frequency of the flicker, then you wouldn't notice it. Remember 50Hz CRT monitors?
    I don't believe you. Switched mode power supplies are very cheap nowadays, especially compared with the cost of a car, especially a £100K carwhich has the same problem.

    You can get a very smooth DC voltage of any level out of one - just lookat your PC power supply then think of a smaller version of it. There are in fact smaller versions of it on your motherboard changing 12 volts to the CPU voltage (which is in fact variable).
    I prefer them to go on and off suddenly. The only problem I have is flickery tail lights.
    I don't agree with different brightnesses of brakes. We already have two brightnesses of red - tail and brake. Adding more would just lead to confusion, you would wonder if it was a tail or a brake.
     
  8. Major Scott

    Major Scott Guest

    No, provide them with a lower DC voltage to make them dimmer. Say 12V for brake and 9V for tail.
    So you're saying that on brake they are also strobed? I have never noticed a brake strobing. It's the tails that do it.
     
  9. Guest

    So you want tail lights made dimmer? Oh, ok.

    Next...

    NJR
     
  10. Dave Plowman

    Dave Plowman Guest

    LEDs are current, not voltage, driven.
     
  11. Dave Plowman

    Dave Plowman Guest

    I'd suggest you look up the Ladybird book of electronics to get a clue
    about how LEDs work.
     
  12. DavidR

    DavidR Guest

    What is generated
    ------ ------
    | | | |
    | |______| |_____

    What the eye perceives
    -------- --------
    | \ | \
    | \___| \____

    By averaging the power in the top waveform, the peak intensity is reduced
    and would not use eye's ability to fill in the gaps. Therefore a
    smooth waveform requires more power at source.

    Picking a Cree led at random, the data sheet shows that the increase in
    luminous output falls relative to the increase in current (ie, doubling the
    current produces less than a doubling of output), so at first sight it would
    seem that pulsing is counterproductive. Which means that there are other
    factors that make it advantageous.
    If you can't maintain a gap in a slow moving queue without the help of brake
    lights, are you sure you're competent?
     
  13. Dave Plowman

    Dave Plowman Guest

    Why would you have a 'series resistor' if they are pulse driven?

    But in any case it is irrelevant. It's the current they are driven with
    that matters - not the voltage.
     
  14. Guest

    No, the series resistor merely determines the maximum current that could
    flow through the LED.

    Daniel
     
  15. Ian Jackson

    Ian Jackson Guest

    Nothing is really only 'current driven'. It's more correct to say that
    LEDs need to be driven from a power source which provides a fairly
    well-defined current. Even when you drive them with pulsed current, the
    amplitude of the pulses will be determined by the voltage producing the
    pulses. At any instant, the power dissipated in the LED is simply the
    product of the voltage across it (typically 2V, depending on the colour)
    and the current flowing through it. When pulsed, the average power is
    also determined by the mark-space ratio of the pulses.
     
  16. Dave Plowman

    Dave Plowman Guest

    No idea. They are crap and I wouldn't have one in the house.
     
  17. Dave Plowman

    Dave Plowman Guest

    Very well defined if you're driving them hard. The source voltage is
    irrelevant. Provided it is more than the forward voltage drop of the LED
    or LED chain.

    As I said, it's the current flow through the LED that you design for.
     
  18. DavidR

    DavidR Guest

    Because a large number of morons in and around the motor industry have
    become attached to the idea that if some lighting is good, brighter lights
    must be better.

    Thy don't seem to recognise that the the current standard of lighting is
    making the roads more dangerous.
     
  19. Steve Firth

    Steve Firth Guest

    The LED bulbs I have used have been anything but "crap". They use 1/10th
    the electricity of equivalent halogen bulbs and can be bought as flood or
    spotlight versions.
     
  20. Ian Jackson

    Ian Jackson Guest

    In message
    <2082196069388490010.841939%steve%-september.or
    I was under the impression that they did use a pulsed supply in domestic
    lighting.
     
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