Re: LED Traffic light flash

From: John Smith (kd5yikes_at_mindspring.com)
Date: 11/13/04 

Date: Sat, 13 Nov 2004 17:31:46 GMT

"Jeff Stout" <nobody@nowhere.com> wrote in message
news:cn5crb$mu7@library1.airnews.net...
> In our system, it is the controller which dims the intersection. The
> controller
> would pulse the field relays (solid state relays) at the zero crossing to
> change
> the intensity of the lights. The controller keeps track of the time of
> day
> and
> 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.
> Questions
> like that drove me nuts when I designed our first Conflict Monitor.

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.

> 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
> should
> be, but what they actually are) and looks for conflicts. An example of a
> conflict
> 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
> time.
>
> Jeff Stout
>
>
> "John Smith" <kd5yikes@mindspring.com> wrote in message
> news:3scld.2100$G36.950@newsread3.news.pas.earthlink.net...
>>
>>
>
> [snip]
>
>>
>>
>> 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.
>>
>
> They probably didn't order it because dimming is usually done in the
> controller.
>
> [snip]
>

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.

John

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