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Why are LED pulsed?

Discussion in 'Electronic Basics' started by Mr. C, Jan 13, 2006.

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  1. Mr. C

    Mr. C Guest

    I am curious. I wonder why LED are pulsed on many of the consumer
    products I see. I have seen several devices pulse the LEDs
    and I can actually see the blinking, especially if my eyes scan across
    the display or a panel with discrete LEDs. Could it be that a super
    high intensity captures the eye and the eye retains the recognition
    for a while even when the LED is off? If so, then the LED would have
    to be pulsed often enough for the eye to think it is still on. But
    why is a shorter high current pulse better than a longer (or even
    continuous) duty cycle at a lower current? Maybe its related to a
    multiplexed drive arrangement where fewer lines are used if pulsed.
    Just curious.
  2. Rich Grise

    Rich Grise Guest



  3. Ralph Mowery

    Ralph Mowery Guest

    Not always so. They are pulsed because the total curent demand at one time
    is lower. If you have 10 leds drawing 10 ma each, that is 100 ma you have
    to design the power supply for. If you pulse them so only one is on at a
    time , you desing the supply for 10 ma.
  4. Impmon

    Impmon Guest

    That is most likely the correct answer. It is often cheaper to use
    multiplexing circuit to drive large number of LEDs (LCD and VFD
    display as well) than to control each single segment individually.

    Another factor is the current source. If it's just driving LED and
    nothing else, cheap design may not have AC to DC conversion and just
    feed AC straight into LED which results in 50hz or 60hz pulses.
  5. John Fields

    John Fields Guest

    Not true. If you have 10 LEDs in the array and each one is on for
    10% of the time and you want the apparent brightness of each LED to
    remain constant, then you'll need to drive each one at ten times its
    normal CW rating for 10% of the time and drive the array fast enough
    so that persistence of vision won't see the flicker.

    That means that if you have ten LEDs which are being driven at 100mA
    each, for a tenth of a second each, and you're driving ten LEDs that
    way, the average current out of the supply will be 100mA.
  6. Blake

    Blake Guest

    ----- Original Message -----
    From: "Mr. C" <>
    Newsgroups: sci.electronics.basics
    Sent: Friday, January 13, 2006 1:19 PM
    Subject: Why are LED pulsed?

    Many people believe that the eye does act as a peak detector, and therefore
    a pulsed LED will use less power than a continuously lighted one off the
    same apparent brightness.
  7. If we start listing the things that many people believe (in the
    absence of demonstrated facts), it is going to make a long list.
  8. Jasen Betts

    Jasen Betts Guest

    As with most stuff in consumer electronics it's to save money.
    something like that happens
    but the reason for the pulsing is that the way the LEDS are wired up
    they can only light a few of them at a time.
    Exactly. And those lines are often the most expensive part of the integrated
    circuits driving the LEDs.

  9. Because whoever designed it wants the LED to get attention, and they
    know that our "lizard brain" naturally focuses on things that change
    brightness about once a second.

    It's called the "Ooh, shiney!" effect. ;>)

    Mark L. Fergerson
  10. Brian

    Brian Guest

    Right on!

  11. Blake

    Blake Guest

    Not having any personal experience in the field, I'll stick to the "many
    people believe" line. But the idea does have some credence among the

    The following is an excerpt from On Semi ap note AND8067D

    "There are two main reasons why LEDs are brighter when pulsed. First, the
    human eye functions as both a peak detector and an integrator; therefore,
    the eye perceives a pulsed LED's brightness somewhere between the peak and
    the average brightness [4]. Thus, an LED driven by a high intensity low duty
    cycle light looks brighter in a pulsed circuit compared to a DC drive
    circuit that is equal to the average of the pulsed signal. The second factor
    controlling the improved brightness is shown in the relative efficiency
    versus peak current curves of an LED."
  12. And as an aside, pulsing or strobing reduces the power supply demands.
  13. How so? Show the math.
  14. John Fields

    John Fields Guest

    Not true.

    For the same apparent brightness when the LED isn't driven CW, the
    power supply will be called upon to deliver high peak currents which
    it may not be capable of delivering. That won't reduce the power
    supply demands, it'll aggravate them and require the power supply to
    be designed to supply those peak loads.
  15. Impmon

    Impmon Guest

    Assume a typical 3v 20mA LED:

    continuously on = 0.06 watts per hour

    50% duty cycle (such as direct off AC source) = 0.03 watts per hour

    10% duty cycle = 0.006 watts per hour

    Simple math. Unlike incascedant buld, LED doesn't suck extra power on
    initial illumination.
  16. It would have been more helpful to the discussion if you provided the
    I would have to read the referenced document, "4. Smith, George,
    “Multiplexing LED Displays: Appnote 3,” Siemens Semiconductor." before
    I was willing to give this any credence. I think they are overusing
    the reference.

    Here is the closest I could find to this reference:

    (an excerpt)
    "The luminous intensity, or the luminance of GaAsP LEDs, is
    essentially proportional to forward current over a wide range, but
    certain phenomena modify this condition. At low currents, the presence
    of nonradiative recombination processes results in less light output
    than the linear relationship would predict. This effect is noticeable
    just below 5 mA per segment (for 1/ 4 inch characters). The result is
    that noticeable difference in luminance from segment to segment can
    occur at low currents. At high currents, the power dissipation in the
    chip causes substantial temperature rise, and this reduces the
    dissipation efficiency Figure 4.

    As a result, the light output versus forward current curve falls below
    the straight line, at high currents (Figure 5). It should be
    emphasized that this latter effect is entirely due to self heating. If
    the power dissipation is limited, by running short pulses at low duty
    cycle, the output follows the straight line up to very high current
    densities. Whereas 100 A/cm2 may be used in DC operation, as much as
    104 A/cm2 can be used under pulsed conditions, with a proportionate
    increase in peak intensity. (If this did not occur, GaAsP lasers could
    not be built.) Gallium Phosphide, however, has an inherent saturation
    mechanism that causes a drastic reduction in efficiency at high
    current densities even if the junction temperature remains constant.
    This effect is due to competing non-radiative recombination mechanisms
    at high current density. As a first approximation the brightness of a
    pulsed LED will be similar to being operated at a DC forward current
    equal to the average pulsed current."

    I think such peak response effect of the eye is weak to the point of
    nonexistence for pulse frequencies that produce no visible flicker.
    Sight is a photochemical process that produces output proportional to
    the rate of photon arrival.
    The curve shown is incorrect, if efficiency refers to light produced
    per watt of electrical power. The curves show light output versus LED
    current, not efficiency. Since all curves roll off to the right,
    actual efficiency goes down above some current not much above the
    normal 20 mA for all examples. This is science by salesman.

    I suggest you set up the experiment. Design a circuit that alternates
    a second at a fixed current with a second at a pulsed drive, with
    the period, duty cycle and peak current settable. For many
    combinations of period and duty cycle, visually find the pulse drive
    current that produces no visible brightness changes on alternate
    seconds. The pulse current setting would be unknown till you conclude
    the no flicker condition, to reduce experimenter bias. I would like
    very much to see your experimental results.

    Then we can refer such discussions to your paper.
  17. Sjouke Burry

    Sjouke Burry Guest

    At 50 percent you have to doublee the current to
    get the same brightness,so where is the advantage??
    Or do you accept halve the lightlevel??
    Because thats wat you get .
  18. 0.06 watts, not watts per hour. Watts are equivalent to joules per

    And apparently, half as bright as the DC case.

    You could, instead, lower the current to 10 ma, and get the same light
    output, but the LED voltage would go down a little below 3 volts, so
    the power would go a little lower than 0.03 watts, beating your 50%
    duty cycle case.
  19. Blake

    Blake Guest

    Actually, I have done this experiment in a casual way. Just a bit of fun at
    lunch hour, and I won't make any claims about the validity of my results.
    The result was that I was unable to verify any improvement in efficiency
    with the pulsed driver.

    But do I misunderstand the OP's question? I think he asked why some
    designers pulse LEDs. My answer is that some designers pulse LEDs because
    they believe it will improve efficiency. Is this not a correct answer to the
  20. I guess it is possible that some designers do believe that pulsing
    LEDs makes raises the luminous efficiency and some apnote writers are,
    apparently, trying to convince designers that it is the case.

    I think they are mistaken.
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