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cybersaga
join:2011-12-19
Selby, ON

cybersaga

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Fading LED Strip Light

About 2-3 years ago I bought 5 meters of 3528 LED strip lights on eBay and installed nearly all 5 meters under my cabinets in the kitchen. At first, it worked fantastic. I was actually wondering at first if it was too bright.

About a year later, they were much dimmer. Now it's to the point where I hardly notice a difference when I turn them on.

Now when I first hooked them up, I just used a 12V wall wart that I happened to have laying around. I noticed later that it was getting kind of hot. I figured it was pulling more current than it liked and made a mental note to replace it with something rated for more current.

It took me a long time to finally replace it. But when I finally did, I decided to measure the voltage on the wall wart: it was 16V. So it was probably pumping that extra voltage through the LEDs that whole time.

I think the answer is yes, but can running 16V through 12V LEDs make them dim very prematurely?

If that's all it was, I'll just buy another 5m and replace it.

tschmidt
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tschmidt

MVM

said by cybersaga:

I think the answer is yes, but can running 16V through 12V LEDs make them dim very prematurely?

LEDs do have a wear out function function but hard to tell if your problem is due to excessive voltage or crappy LEDs.

LEDs are current driven so ideally there should be a constant current source driving the string that takes the 12V (or what ever voltage) input and outputs a constant current. I'm guessing in your case the LED string is matched to the expected voltage source so the constant current driver was omitted. Without a driver even a little extra voltage results in significant excessive current.

At this point replace the string and use a switch mode wall-wort able to deliver the precise voltage and see what happens.

/tom

cybersaga
join:2011-12-19
Selby, ON

cybersaga

Member

I did end up replacing the wall wart with a new 5A 12V power adapter that kinda looks like a laptop charger (the wall wart was only rated for 1.5A). The voltage coming out of the new adapter is right.

Will that work ok? Or do I need something that's specifically designed to drive LEDs?

From the adapter, I split the wire into 4 wires (speaker wires that are really over sized for this application) that go down the inside of the 4 cabinets. Then under each cabinet, each wire splits into two to feed two strips of LEDs under each cabinet.

cowboyro
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join:2000-10-11
CT

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White LEDs *DO* fade in time, especially when driven at a current that is too high for what they were made.
Cheap devices drive them at a high current in order to achieve maximum brightness with the least cost. But to be fair, even driving them at low currents will cause them to fade in time, although not so fast.

SparkChaser
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join:2000-06-06
Downingtown, PA

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On IR, red and green diodes we saw a lot of die fractures until we got the supplier to fix it. We were buying close to a million a year so it was important to both of us.
quote:
The failure mechanisms of LEDs are divided into three categories: the semiconductors, interconnects, and the package. Semiconductor–related failure mechanisms include defect and dislocation generation and movement, die cracking, dopant diffusion, and electromigration. Interconnect-related failure mechanisms are electrical overstress–induced bond wire fracture and wire ball bond fatigue, electrical contact metallurgical interdiffusion, and electrostatic discharge. Package-related failure mechanisms include carbonization of the encapsulant, delamination, encapsulant yellowing, lens cracking, phosphor thermal quenching, and solder joint fatigue.
Microelectronics Reliability
journal homepage: www.elsevier.com/locate/microrel

EDIT: In other words over stressed cheap LEDS.

tschmidt
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said by cybersaga:

Will that work ok? Or do I need something that's specifically designed to drive LEDs?

What does the LED instructions say to use? If it says use a 12V supply then you should be fine.

/tom
tedmarshall
join:2000-12-02

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Actually, "white" LEDs have another failure mode: phosphor degradation. Almost all current white LEDs are actually violet or UV LEDs exciting phosphors in the package. These phosphors degrade with heat. Most white LEDs are rated for ~30,000 hours to 70% brightness at normal power (»apps1.eere.energy.gov/bu ··· leds.pdf). Dimming a white LED greatly extends its life and conversely, overpowering one, assuming you don't destroy the LED die itself, will greatly reduce the phosphor life.

I don't have a good idea, in the OP's case, if this is the only factor or if the LEDs were also of low quality.

printscreen
join:2003-11-01
Juana Diaz, PR

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said by cowboyro:

White LEDs *DO* fade in time,

At a prior job there was a machine used for filling bottles with pills (a pharmaceutical plant) that had an inspection system using cameras and white LEDs. The LEDs were mounted in a row and each row consisted of a number of individual modules that had 12 or so LEDs on it. One day, one of those modules needed replacement after several individual LEDs failed. The new replacement module had small but noticeable difference in color temperature from the older modules. And this was after a little over a year of use.

Jan Janowski
Premium Member
join:2000-06-18
Waynesville, NC

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I have encountered the same thing with my under counter lights... But the supply was 12.0V before and after, so it must be LED Degradation...

SparkChaser
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join:2000-06-06
Downingtown, PA

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said by tedmarshall:

Actually, "white" LEDs have another failure mode: phosphor degradation.

Are you referring to "phosphor thermal quenching" mentioned above?

Several things can effect the phosphor.
TheMG
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said by tschmidt:

I'm guessing in your case the LED string is matched to the expected voltage source so the constant current driver was omitted. Without a driver even a little extra voltage results in significant excessive current.

Pretty much all of those cheap LED strip lights that I've seen are in fact like that.

They simply use resistors for current limiting and they are designed to be powered from a regulated 12V supply (often an SMPS). The typical arrangement is 3 LEDs in series plus a resistor.

I'd say it is very likely that the excessive voltage from the unregulated power supply did overdrive the LEDs and cause them to begin to fail prematurely.

mackey
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join:2007-08-20

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said by TheMG:

They simply use resistors for current limiting and ... powered from a regulated 12V supply ... The typical arrangement is 3 LEDs in series plus a resistor.

This. I use these strips all the time, and it's just 3 LEDs and a resistor all in series.

We can calculate how much power you were forcing through them with your 16v supply: assuming they were targeting a LED current of 20mA at 12v and each LED has a Vf of 3.2v, they most likely used a 120 ohm resistor. Since you didn't change the resistor but fed the LEDs 16v, you actually had 53mA going through them, or 2.65 times the current they wanted.

Personally I always under drive my LEDs, targeting 10.5v-11v for a 12v strip. If it's too dim I simply add more. I really don't want to have to keep re-doing everything every few years.

/M

cybersaga
join:2011-12-19
Selby, ON

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said by TheMG:

I'd say it is very likely that the excessive voltage from the unregulated power supply did overdrive the LEDs and cause them to begin to fail prematurely.

That's good to know then. I ordered another 5m and I'll replace them. Hopefully they'll last longer this time.
said by mackey:

Personally I always under drive my LEDs, targeting 10.5v-11v for a 12v strip.

I just measured the output of the new power adapter and it's 12.04V. Based on what's been said here, I'd rather have it a little lower than a little higher, but still better than 16V.

Raphion
join:2000-10-14
Samsara

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LEDs "burn" really quickly when over-powered. As mentioned above, the cheap prefab assemblies have resistors in series with the LEDs that forms a very basic regulator in combination with the LED's characteristics. But it only makes them very slightly forgiving, just enough that 10% one way or the other won't devastate them. Without the resistor, you couldn't hope to avoid damaging these LEDs unless you used a constant-current power supply.

If I end up doing any LED lighting, I'll be using switching power supplies with adjustable output, because of the LED's sensitivity.
alphageek911
join:2007-08-10
Fresno, CA

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As tschmidt commented, the best power supplies for LED's are ones that supply a constant current, rather than a particular voltage. That type is uncommon in cheap consumer devices, though, usually relying instead on a series resistor to limit current, which is less desirable and also wastes power.

cybersaga
join:2011-12-19
Selby, ON

cybersaga

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How do you identify those types of power supplies? Is there something on the label?

tschmidt
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said by cybersaga:

How do you identify those types of power supplies? Is there something on the label?

They are typically small modules that fit inside a fixture.

This is a result of an eBay search: "LED constant current supply"
»www.ebay.com/sch/i.html? ··· _sacat=0

That is not going to work with your LED strips because they are randomly long. As long as you use a regulated power supply at or lower voltage then the strip is rated you will be fine.

/tom
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tedmarshall
join:2000-12-02

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Re: Fading LED Strip Light

said by SparkChaser:

Are you referring to "phosphor thermal quenching" mentioned above?

Several things can effect the phosphor.

I don't know the exact mechanism. My company makes equipment with small LCD control panels back-lit by white LEDs. We discovered that after a year or two of constant lighting of the LCDs, the brightness was noticeably less. The LCD manufacturer recommended setting the brightness lower and/or turning the backlight off when not in use. I implemented both timeouts to kill the backlight when not in use and user adjustable brightness.

Considering that the OP seems to have been overdriving his LEDs but the individual LEDs had not failed (no light), I have to assume the phosphors were badly damaged.
tomupnorth
join:2005-01-14
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Do folks here have recommendations for complete LED fixtures for this application (meaning both bright LEDs but integral power supplies)? When I look at Home Depot these days they sell striplights for anywhere from $16 bucks to this one from Lithonia for $120:

»www.homedepot.com/p/Lith ··· 03409289

I wanted to use some superbright LEDs perhaps yes for undercabinet lighting, but moreso in fact for OVERcabinet lighting in our lake cabin which is wood walls and wood cabinets and wood ceilings and thus the kitchen is overly dark.

I'm tempted to use the Philips high brightness T5 lamps (Lithonia again) that I've used successfully in my storage barn, but these fluorescents really don't like to be turned On and Off frequently i.e. are best if left On most of the time.

SparkChaser
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join:2000-06-06
Downingtown, PA

SparkChaser

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said by tomupnorth:

Do folks here have recommendations for complete LED fixtures for this application (meaning both bright LEDs but integral power supplies)? When I look at Home Depot these days they sell striplights for anywhere from $16 bucks to this one from Lithonia for $120

One would hope that the lamp in the fixture is designed with the right LED current. The problem is, as the OP found, just a strip of LEDS can be driven until they cry, depending on the supply you use.