LED Traffic lights

Over the last couple of years I have seen quite a few LED traffic lights being installed as replacements for the incandescent traffic lights.
This is in West Auckland, New Zealand
Of course the long lifespan is very favourable ± 100,000 hrs and lower maintenance and powerconsumption.
The very bright and even colour are certainly noticable , so is the instant colour change over.
I have noticed though that some faillures have occured, especially in the orange lights that 4 or 5 LED's have failed.
It could be due to a faulty batch, but it's interesting that the orange LED clusters are mostly affected, although today I noticed a green LED lamp with 3 LED's out.
Question How are these lamps assembled and what size are the dropper resistors? I think that there are circuit design faults in some of those LED lamps. Anyone can post a photo on this forum?
I have used LED's on 240 Volts for years with 2*1 Watt 33 or 39 k Ohm resistors and 1N4007 diode in series and 1N914 antiparrallel with LED in harsh conditions with spikes and NO faillures. (Inom LED is ± 3 to 5 mA).

see if these images post

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LED Traffic light at Mc Leod rd, Te Atatu, Auckland, NZ ( 4 LED's out)

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LED Traffic light cnr Swanson rd, Henderson, Auckland, NZ ( 2 lots of 4 LED's out)

Most of the LED arrays for this application simply place the LED's in a series configuration with single dropping resistors per line. The arrays are usually designed to allow for the smallest dropping resistor possible. The failures seen are almost always from mechanical stresses on the assembly, heat vibration, etc, cracking the PCB or solder joints. In the free-hanging traffic lights the wind twists the entire housing, and the middle assembly ( amber ) takes the worst of the twist. If each end of the housing is mounted, expansion and contraction could still flex the centre of the housing that is unmounted and allowed to flex. Shock ring mounting helps, but some mechanical stress always remains.

I have seen these failures also when heavy vehicles and buses are retrofit to LED without appropriate mounting.

Some of the more durable lamps like those from Gelcore are complete modules, having built-in switching supplies that operate from a range of input voltage. Their units are very heavily potted and overbuilt. These are mostly used on rail systems where electrical and environmental conditions are often worse than average. Unfortunately the Gelcore lamps are pretty expensive.

Thanks zevon8 for your reply, I think the flexing of the housing isn't the problem. In New Zealand all traffic lights are mounted on poles and not on overhead catenary wires like in the USA.

The arrays are usually designed to allow for the smallest dropping resistor possible.

Click to expand...

The heat built up in the series resistor is what I think as the culprit, it may either burn out or create a crack in the circuitboard.
Of course costs in the design dictate for smaller components to be used, but early faillures are not a good sign.
I will try to get hold of a faulty unit from the council to do some research on it.
Also I'm curious what the actual running current of the LED arrays is.

The question is when an LED fails what normally happens to it?

Does it go open circuit or close circuit with a slight reisistance?

If the former is true then they can't be connected in series because one LED failing will take them all out, but if the latter is true then it's probably better to connect them in series.

I suspect that they don't use dropper resistors, I suspect they use a switching power supply to save energy.

I didn't post very clearly, what I meant was that they are in parallel groups of LED's that are in series, bringing each of the strings close to the supply voltage, then adding a very small dropping resistor to set the current suitable to the supply voltage. The resistor also acts as a sacrificial part, incase the string shorts, leaving the rest of the strings working. Some of the modules I have used contain a switching PWM supply, some only have a plain LM78xx regulator, depends on the supply voltage available, and the features required. If the supply is well controlled or has the PWM circuitry, less complex modules are used.

Modules designed for the highest efficiency are usually driven by a PWM supply, carefully designed to drive the LED's as hard as possible to get maximum brightness, yet lowest average power consumption, and longest life. They usually last about 5 years. One of the most common ones I use draws about 40mA average, for an array of 39 LED's, and is equivalent in brightness to a standard 69 Watt high output traffic lamp.