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LED Traffic light flash

Discussion in 'Electronic Design' started by Chuck Olson, Nov 12, 2004.

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  1. Chuck Olson

    Chuck Olson Guest

    A few months ago I proposed a circuit to run LEDs on 120VAC 60Hz line using
    a capacitor for the current-limiting element, and putting two LEDs in
    parallel, oppositely polarized to conduct that current (maybe for
    night-light use). The forward drop of one would keep the reverse voltage low
    for the other. One of the comments made concerned the surge of current that
    would be conducted at the moment of turn-on due to the higher frequency
    content of the turn-on wavefront. Could this be the reason for the
    brightness flash of red LED traffic lights at turn-on? Certainly the LED
    array would have to plop right in where the incandescent lamp was, powered
    by 120 VAC. But it's hard to imagine all the rather large, high-voltage
    capacitors needed to do the current limiting on what I estimate may be 200
    or more LEDs. One comment I just read, concerning the flash, said it's due
    to poor regulation of the DC supply. But that implies the switching takes
    place between the DC supply and the LEDs, and that's hard to imagine, as
    well. Even worse, the use of capacitors on a large scale would wreak havoc
    with the utility's power factor, not to mention the arcing of switch
    contacts (but maybe the arcing is already taken care of since the old
    incandescent lamps also produced a surge of start-up current) . So do any of
    you know FOR SURE just how these LED traffic lights are powered, and why
    they flash brighter at turn-on?


  2. John Fields

    John Fields Guest

    Since it's imperative that LED arrays for traffic-light use exhibit
    extremely good reliability, I doubt whether the apparent increased
    brighteness flash during turn-on is really attributable to a current
    surge, unless it's within the spec's for the lamp. If I had to guess
    (which I'm doing) I'd think that it was due to the eye's AGC and its
    response to the sudden change in intensity.

    I don't know how traffic light LED arrays are designed, but I design
    largish LED arrays and the way I take care of turn-on surges is to
    make sure there _are_ none, by driving the array with the output of a
    full-wave bridge tied directly across the line. No series cap (with
    its attendant problems) in the line, and no filter cap on the output
    of the bridge, since you can't see a 100/120Hz ripple anyway. I get
    rid of transients with large series opposed Zeners across the mains
    side of the bridge, and size the series current limiting resistor for
    each string to limit the current through the string to the specified
    operating current at high line and take the brightness hit at low
  3. Bob Stephens

    Bob Stephens Guest

    That brings to mind a question that has interested me for a while. I know
    nothing about optics, so please bear with me. Do people who wear the
    polarized? eyeglasses which adapt to ambient light and become darker in
    bright sunlight weaken the eyes' 'AGC' response? It seems to me that if you
    always walked around in mild, filtered light, your eyes would become lazy
    and less able to react to a sudden intense busrt of light like a flashbulb.

    Anybody know anything about this?

    Probably should have OT'd this...

  4. John Smith

    John Smith Guest

    All states, AFAIK, require that LED traffic signals have a 0.9 power factor
    or better. This comes from the ITE (Institute of Transportation Engineers, specifications. Hence, all manufacturers, AFAIK, use
    PWM or PFC ICs to meet the specifications. The flash on turn-on is indeed an
    overshoot in the power supply usually due to improper compensation or
    topology. I speak from experience and the company for which I once worked
    will remain nameless.

    Usually, the 120 VAC is applied to a circuit containing a PFC (Power Factor
    Correction) IC with a transformer and, after rectification, puts out 30 to
    100 VDC with a high power factor input. The LEDs are usually in
    series/parallel strings of a sufficient quantity to meet the specifications
    for light intensity. The total LED current is sensed and regulated. Lenses
    may be used to focus the light, especially if the quantity of LEDs is small.
    You may have noticed the sharp reduction off-axis brightness of some of
    these signals.

    Controllers at intersections use solid-state switches. The ones with which I
    was familiar contained zero-crossing optical isolators to drive the triacs.

    There is a lot more to it than this, but this may already be more than you
    wanted to know.

  5. John Fields

    John Fields Guest

  6. John Smith

    John Smith Guest

    Well, let's see...

    The current for the red LEDs have to just about double at high temperature
    because of the drop in efficiency and Vf of the LEDs. But the green LEDs
    required only about a 10% increase, as I recall. Yellow LEDs act about like
    red ones and blue ones act about like green ones.

    An early design I inherited was a boost PWM circuit with a string of LEDs
    adding to about 300 V. There was a FET switch in series with the string of
    LEDs which opened when no AC was applied. Why? Well, the filter cap was
    large enough to cause the LEDs to stay on for awhile after AC was removed.
    You can imagine what that does to traffic if it is a green signal. Worse
    yet, the series FET switch failed a couple of times. It turns out that green
    LEDs are so efficient that the signal may glow for minutes and be very
    visible at night under a shorted switch condition. Very bad to have an
    intersection with both directions of traffic seeing green. I learned
    something very important from this. Always build a switching supply with a
    transformer. That way, if your switch fails, output goes to zero. This is
    true as well for getting 5V from a 12V battery for a computer. If you use a
    series switch and it fails, good by computer. Yes, I know one can provide a
    crowbar and fuse, but I don't like crowbars for reasons I don't want to get
    into. My preference is for the power supply output to go dead for any power
    supply fault I can think of.

    Traffic signals have a CMC (Conflict Monitor Circuit) built in (this might
    be specified by the ITE). Its function and its name have a distant
    connection. Traffic controllers, as I said, use solid state switches
    consisting of triacs and optos. If a triac fails shorted (or leaky), you've
    got a problem with a signal not being able to turn off (a conflict). So, the
    controller has a monitor which measures the voltage going out to the signal
    and, if the SS switch is supposed to be off but the voltage is actually
    above a threshold, it stops all normal signal sequencing and puts the
    intersection in flash (flashes red in all directions simultaneously). In the
    early days, the snubbers around the triacs allowed relatively hefty leakage
    currents to flow. No problem for a 135 Watt incandescent to suck it up, and
    it would keep the voltage low when the triac is off. Not so with an LED
    signal. The nature of an LED signal is such that it will usually allow
    enough voltage to build up at it terminals for the controller's monitor to
    throw the intersection into flash mode. So, a circuit is added can draw the
    snubber's current and hold the voltage to just a few volts unless the full
    line voltage is applied.

    And then there is the use of the CMC to force the controller into flash mode
    if a certain percentage of LEDs fail. We used a cheap uC to monitor several
    strings of LED currents and then disabled the CMC or, better, blow the line
    fuse which accomplishes the same thing. I think the specs say that the
    signal must appear as something like 500k Ohms or more for this fault.

    We also had an autodimming option. We actually put a phototransistor in the
    signal looking out the front. It caused a reduction in LED current at night
    for two purposes. First, green signals are very bright to the eye at night.
    They are blinding. This improved on that a lot. Second, it saves about 40 or
    50% on power consumption. The spec said that all the signals in an
    intersection must be coordinated, that is, dimmed at the same rate and time.
    I didn't do that because there was no communications system. But, I learned
    that nobody could tell the difference by eye. We always stated that we did
    not meet the letter of the specifications. I don't think the end user cared.
    But, I don't think many municipalities ordered it.

    Our pedestrian Walk/Don't Walk Countdown signal had an 8405 in it. The micro
    was used to learn the walk and don't walk cycles and then provide a
    numerical display to show how many seconds remained to get out of the
    crosswalk. It had to compensate for aperiodic sequences. Did you know that
    policemen may be given a pushbutton to plug into an intersection controller
    to manually control the walk/don't walk cycles? Usually near a stadium or
    auditorium. I never saw this, but I was told that it was a possibility.

    I've run down.

  7. John Fields

    John Fields Guest

  8. Ok John Fields, this is for you. You remember a while ago when I stated that
    LEDs run in voltage mode have some advantages over constant current mode and
    you called me every name in the book. Well this is why some traffic lights
    As stated the LEDs must be Power Factor Corrected (PFC) and act like a
    incandescent lamps which has a power factor of one. The PFC unit runs in
    voltage mode and if the LEDs are to be driven by a constant current, another
    driver and control loop must be added. The LEDs must turn on like
    incandescent lamps and that is quite fast. At 60 cycles it is difficult to
    get the current loop up and settled in 75mS with no overshoot, hence the
    light flash. I wrestled with this problem for some time until I relised if I
    threw out the current loop section and ran them in voltage mode, it would
    follow the AC line at turn on with no overshoot and 1/2 the parts. Also the
    temperature / intensity problem is greatly reduced.
    Oh yes the LEDs must be matched in the array but when you buy them in large
    batches, they come in different bins (voltage grades).
    Come to think of it, I think only Tony Williams did not call me a SOB.
    Thanks Tony.

    Now JF, spank me with your keyboard.
  9. John Smith

    John Smith Guest

    I made a mistake. It was not an 8405 but an 8055 in that pedestrian signal
    of the last paragraph. Some other products (LED message signs) used up to 50
    or so Cypress 8-bit micros.

    Best compliment I've had in a long time and the only compliment ever from a
    respected authority. Thank you.

  10. Jeff Stout

    Jeff Stout Guest

    In our system, it is the controller which dims the intersection. The
    would pulse the field relays (solid state relays) at the zero crossing to
    the intensity of the lights. The controller keeps track of the time of day
    knows when it is night and day. Our controller even kept track of daylight
    savings time and leap year and leap day and would change its behavior
    depending on time of day or the day of week.

    But getting back to the light dimming issue. Dropping half sin waves as
    a method of dimming the intersection really does a number on the NEMA
    specification. NEMA specifies that a phase (a signal light) is on when the
    when the voltage is more than such and such RMS and on for more than
    T milliseconds. What does that specification mean when every third half
    sin wave is missing? The RMS voltage at night is now lower than the
    specification, but all the lights can be seen on the intersection.
    like that drove me nuts when I designed our first Conflict Monitor.

    For those who don't know, a Conflict Monitor monitors the actual voltages
    on every signal light at the intersection (not what the controller says they
    be, but what they actually are) and looks for conflicts. An example of a
    is if all green lights in all four directions are green. A second example
    is if all
    the lights (red, yellow and green) in a single direction are on at the same

    Jeff Stout

    They probably didn't order it because dimming is usually done in the

  11. John Smith

    John Smith Guest

    Hi, Jeff -

    I am aware of this method of dimming. We sent an engineer to investigate
    problems at an intersection after the signals had been changed from
    incandescent to LED types. I don't remember the exact complaint, but I think
    there was flickering of the lights, perhaps some lights refused to even
    light up. LED signals regulate the current through the LEDs so that they can
    meet specifications at low or high line voltage as well as at temperature
    extremes. By trying to regulate the light level with line voltage, you get a
    confused regulator in the traffic signal. It doesn't like anything but a
    constant line voltage and, regardless of what the line voltage is doing, it
    will try to hold a constant LED current even with line fluctuations.

    The problem was fixed by disabling the controller's dimming capability. They
    later installed the signals with built-in dimming and everything was fine.

    Just because ours had a problem, doesn't mean other manufacturer's LED
    signals won't work with controller dimming. I would urge careful evaluation
    with LED type signals if the controller is allowed to do the dimming. At the
    very least, contact the LED signal manufacturer and ask about this function.
    A lack of communication may cost someone a lot of money for air fare, auto
    rental, and motel.

  12. John Fields

    John Fields Guest

  13. Jeff Stout

    Jeff Stout Guest

    I can understand how the design of an LED signal would more difficult if the
    controller is dropping half sin waves periodically. But then I realized
    that the controller can be programmed to not dim the intersection at night.
    But then, the dimming problem remains.

    Jeff Stout
  14. John Smith

    John Smith Guest

    Not just difficult. I would think that designing a current-regulated power
    factor controlled power supply would be next to impossible where the input
    voltage has missing half cycles arbitrarily. But then there are a number of
    brilliant design engineers in this group who could probably do it with their
    eyes closed.

    The problem disappeared when the controller was programmed to not dim and
    the signals with the dimming option were installed. Everybody was happy.

  15. David Lesher

    David Lesher Guest

    Why the concern/spec over PF? Can you even notice that load on the
    grid when you have 100's of gas-discharge street lamps running?
  16. Rich Grise

    Rich Grise Guest

    The I^2*R losses in the wires are very real, indeed. And more than
    they would need to be with the power factor corrected.

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