Previous in Forum: Substation Generators Tripped -- Why?   Next in Forum: Ring Wave Test
Close
Close
Close
25 comments
Participant

Join Date: Nov 2009
Posts: 4

Gradual LED Failure

11/12/2009 8:31 PM

I'm hoping someone will have some insight on the most likely causes of gradual LED failure (noticeable dimming over a period of months).

The setup includes multiple strings of LEDs. Each string contains some 150 LEDs wired in parallel (the LEDs are set in built-in sockets along the strings); however, each LED also has its own 460Ω resistor wired in series with it. Each LED-resistor pair is supposed to consume 1/4 watt. The whole string is supplied by one 12VDC transformer (60W max). The transformers are powered from normal 110V, 60Hz AC. The supposed rated life of the LEDs is 40,000 hours. These lights are on approximately 10 hours per day.

Over a period of several months since installation, there have been a very few complete failures (which might be expected among thousands of LEDs). But, the real problem is an overall dramatic dimming of all the LEDs (to less than half their original light output, as measured with a light meter).

It seems this shouldn't be happening, and I'm trying to understand what the possible causes could be. The operating environment is fairly well-controlled, so I don't think heat or vibration would be an issue. Could occasional voltage spikes from the building power cause this sort of problem over time? That is, could a spike in the supply to the transformers also cause a problem in the transformers' output, and would this really lead to such gradual LED failure? The dimming seems fairly evenly spread across all the LEDs (apart from the few that are totally out). The transformers are definitely putting out 12VDC now, and at every time I've checked them, but perhaps there could be momentary problems? Or could these LEDs really be of such poor quality that they'd fail so far short of their rated lifespan? Thanks for any and all insights!

Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".
3
Guru

Join Date: Jun 2009
Location: South of Minot North Dakota
Posts: 8378
Good Answers: 774
#1

Re: gradual LED failure

11/12/2009 9:47 PM

I have experimented with LEDs as a back lighting system in my house for some years now and I have tested a number of different color LEDs by running them 24 hours a day non stop for many years now.

What I have found is the green, yellow, red, and most standard colors have very little perceived dimming effects on them even after nearly 7+ years non stop run time at their full rated input power levels. A reference LED from the same batch must be held up next to them to visually see any dimming that has occured in mowst cases.

The blue and white LEDs do however dim fairly fast in reference to their rated life expectancy when driven at any reasonable power level regardless of power supply quality and design or working environment conditions.

Every brand and type of white LED I have used reaches a perceived dimming of about 2/3 or less what a new one of its same batch shows after 10 -12 months of non stop run time on average. That seems rather disappointing considering they usually have 70 - 100K hour life ratings. At around three -four years they are so dim they cant be seen in day light and are useless in the dark. They are nothing more than pail yellow/white dots at that point.

I have several store bought under cabinet type White LED lights that are showing the same trend after about one year of continuous operation as well.

The phosphors aging is what I have been told causes the dimming problems.

Register to Reply Good Answer (Score 3)
Guru
Hobbies - Musician - Tube Amps Only Please!

Join Date: Apr 2009
Location: Los Angeles, California USA
Posts: 554
Good Answers: 1
#2

Re: gradual LED failure

11/13/2009 12:33 AM

This is a educational sight on LEDs and they sell the brightest LEDs I have ever used. https://www.superbrightleds.com/led_info.htm I had a professor in a analog class tell us that the half life of an LED is 40 years. I have a design that uses a IR LED and detector from Radio Shack that has been operating for 27 years almost every day and the design still works. Radio Shack has terrible LEDs. Did you know that LEDs are static sensitive? See the sight very useful info and great LEDs but they are expensive compared to your average LED. I have had the super brights running off and on for about a year and they blind you if you stare at them. Maybe reading this site will help you figure out the problem.

__________________
Regards, Maveric Manic - 'Knowledge is Power and Wisdom is knowing how to use it'
Register to Reply
Participant

Join Date: Nov 2009
Posts: 4
#3

Re: gradual LED failure

11/13/2009 1:20 PM

Thank you for the replies. tcmtech, these are indeed "white" LEDs (a fact I didn't think to mention in my original post), and your phosphor explanation sounds like it may be what's happening here.

Register to Reply
4
Power-User

Join Date: Jan 2009
Posts: 187
Good Answers: 9
#4

Re: gradual LED failure

11/13/2009 4:25 PM

One thing I've noticed is that LEDs cannot take much heat. Overheating by any means causes permanent degradation. Can you check the temperature right at an LED in its usual operating environment? If it even approaches the high limit of the specified operating range, this would be a contributing factor to your problem.

Is the resistor soldered to the LED lead, or is it on the other side of the socket? It may dissipate only 0.2 Watts (assuming a 3 V drop across a white LED), but close to the LED in a poorly ventilated environment, that could still be enough to cook it.

Register to Reply Good Answer (Score 4)
Participant

Join Date: Nov 2009
Posts: 4
#5
In reply to #4

Re: gradual LED failure

11/13/2009 8:39 PM

That's a good idea. I'll try to check the temperature. The resistor is soldered directly to the lead, so they are fairly close together, in sort of an epoxy casing that's made to fit into the socket.

Register to Reply
3
Guru
Hobbies - HAM Radio - New Member United Kingdom - Big Ben - New Member Fans of Old Computers - Altair 8800 - New Member Canada - Member - New Member

Join Date: Jun 2006
Location: Toronto
Posts: 3968
Good Answers: 119
#6
In reply to #5

Re: gradual LED failure

11/13/2009 10:32 PM

if you are using simple rectified AC your 12 volts peaks at 1.41 times that and if there is a capacitor, stays at that, minus the diode forward bias of .7 volts or 1.4 in a bridge.

That means your LEDS are drawing about 30-35 MA. This is on the high side for LEDS and will slowly 'cook' them. They are a doped thing and at higher temperature the dping material diffuses away. I suggest about 750 to 1000 ohms. In the dark they will about50% as bright(from new), but will have a long life. For use in darkness, walk lights etc, it should not be a problem compared to the dimming. In addition, more modren new higher efficiency LEDS might compensate for this. A little testing is needed.

__________________
Per Ardua Ad Astra
Register to Reply Good Answer (Score 3)
3
Member

Join Date: Mar 2009
Location: Bullcreek Western Australia
Posts: 5
Good Answers: 1
#7

Re: Gradual LED Failure

11/13/2009 11:01 PM

We found something similar in our instrumentation. We finished up running the LED's at about 80% of their nominal rated current by increasing the resistor values and don't appear to have this problem any more. The light output appears quite sufficient for our purpose (IR Leds).

Register to Reply Good Answer (Score 3)
Guru
Popular Science - Weaponology - New Member Safety - ESD - New Member Hobbies - Fishing - New Member

Join Date: Sep 2006
Location: Near Frankfurt am Main, Germany. 50.390866N, 8.884827E
Posts: 17989
Good Answers: 200
#9
In reply to #7

Re: Gradual LED Failure

11/14/2009 4:37 AM

Thats a good method.

__________________
"What others say about you reveals more about them, than it does you." Anon.
Register to Reply
Power-User

Join Date: Dec 2006
Posts: 409
Good Answers: 5
#8

Re: Gradual LED Failure

11/14/2009 1:06 AM

Good informative answers. Leds esp hi brite ones are critical in regards to power & heat- there must be cooling- even with the perfect set-up & the makers saying 100k life- reality is at 50k output is halved usually. The exception to the rules is by pulsing the leds- where life & brite can be maintained.

Register to Reply
Guru
Popular Science - Weaponology - New Member Safety - ESD - New Member Hobbies - Fishing - New Member

Join Date: Sep 2006
Location: Near Frankfurt am Main, Germany. 50.390866N, 8.884827E
Posts: 17989
Good Answers: 200
#10

Re: Gradual LED Failure

11/14/2009 5:01 AM

Lots of good tips already, all should be read and understood fully.

I personally find that running such LEDs under the rated current (in your case dropping the voltage) is a good idea. You do not appear to be controlling the environment for the LEDs very well, so dimming is to be expected......sadly!

Also having a better quality system to monitor/supply the current is a must if the LEDs are to be on so much of the time.....there are many companies producing chips for this and easy to read documentation, TI for example (but there are many companies that can help) has a lot of info which is downloadable in .pdf format.....

Start here first:-

Texas Instrument LEDs

To my mind, you are treating the LEDs too simply. For a long life you need tight control of the temperature, current and voltage. Also do not drive them at the maximum "allowed" current, drop to around 80% or so.....less would be even better!!!

Buy LEDS that at 80% (or less) current supply the brightness that you are looking for.....or simply use more LEDs......at the lower current.

Also email (on CR4 email system) our good LED friend Campbell Lighting

He is a good CR4 LED lighting man for more and better help. Send him the weblink for this blog as well so he can read it all.......

Best of luck.

__________________
"What others say about you reveals more about them, than it does you." Anon.
Register to Reply
3
Power-User

Join Date: Aug 2008
Location: West Virginia
Posts: 185
Good Answers: 12
#11

Re: Gradual LED Failure

11/14/2009 9:17 AM

Something else to thinK about. If you are using 12 volts and resistors to drop this to 3 volts, you're wasting 3 quarters of your power. I'd look at a switching supply which would also allow you to regulate the voltage much closer. You could then probably tweak the voltage slightly lower and help control led heating.

Register to Reply Good Answer (Score 3)
Guru
United Kingdom - Member - Not a new member!

Join Date: Jun 2008
Location: USA/Europe
Posts: 4547
Good Answers: 68
#12

Re: Gradual LED Failure

11/14/2009 11:22 PM

hi cube,

If my Math is correct, at 10 hours a day the LEDs should last ~11 years.

I have used white LEDs and, the life although estimated to be as you say 40000 hours, a much more realistic figure would be around 3 years, at 10 hours a day.

I think it is the spikes caused by constant turning on and off which may be to blame. If they are in a position where they may get battered by things being moved or people walking by, or perhaps the branches from a tree, this would or could diminish the life.

To test the possible 'spike' theory, you could run some LEDs on a UPS (Un-interruptible Power Supply) or a Surge Protector.

You may like to think about using thyristors in the layout as well. All helping to supply a power supply which is gentle of the LEDs and the thyristors can take care and help control temperatures. (I think)!

From experience of using LEDs, I would say the life of LEDs is shortened by a very rough 70%, when they are used for just a few hours a day, and so are possibly liable to receive a spike each time they are switched on, compared to LEDs used for 24/7.

have you checked on-line for the particular LEDs you have and the test conditions they went through?

I think though cannot recall precisely, that using a variable copacitor, which can turn the LEDs on over a longer period (Comparitively speaking), of maybe 100th second, or anything up to a tenth of a second perhaps?

This is just a theory of mine, but automatically turning the power on over a short period may help prevent spikes and thermal shock, though I realise thermal shock may not be such a problem.

If you have so many LEDs it would be relatively easy to isolate a bunch and try this slowly switched power supply and compare it to other groups that have not been touched?

-----------------------------------------------------------------------------

I am not an electronic engineer so I suggest the above as a possible remedy to any spikes, but I only know they should be before the LED/s, and apart from a rectifier I will leave that as a suggestion for you.

The details below are just a short part of a whole lot of detail on different types of LEDs and way to power them. They do seem to say thermal shock over time is a known reason for white LEDs losing brightness. I found it to be a great piece and you can follow it up if you think the same as the site has several different ways to stabilise and supply power to various types of LED. Check it out below.

http://www.arrowne.com/innovation/archive/print_10_2006.pdf

Small lighting LEDs such as those mentioned for camera applications
can be readily designed into a new product. On the other hand,
for sustained operation at light output levels close to those of
traditional incandescent lights, additional factors must be borne
in mind. For example, since the light output from an LED falls as
the internal temperature of the chip increases, large increases in
temperature will lead to visible variations in light output. These may
impair the visual effect of lighting products or systems using high
brightness LEDs. Excessive internal temperatures will also reduce
the lifetime of the LED, leading to early failure.
High brightness LED components tend to exhibit more thermally
aware design than traditional indicator LEDs. However, care must
be taken to ensure low thermal resistance paths away from the
device. This may comprise a thermal substrate such as IMS, as well
as efficient bonding to purpose-designed heatsinks or other large
metallic surfaces.
Driving a high brightness LED also demands a different approach,
compared to design for ordinary LEDs. For example, the standard
approach is to use a series resistor to limit the forward current
and thereby regulate light output. In a high brightness application
drawing several hundreds of milliamperes, the series resistor may be
required to dissipate several Watts. Power resistors are expensive,
and also bring additional overheads such as the need for hand
soldering and attachment to an additional heatsink. On the other
hand, adding extra LEDs to a given string, to reduce the current in
the resistor, makes the LED array more susceptible to variations in
supply voltage, causing larger current variations in the LEDs.
To allow a high brightness LED to be operated close to its maximum
capacity, despite variations in temperature, supply voltage, and also
manufacturing variations from device to device, a constant current
drive may be desirable. This requires extra design work to configure
a linear or switch-mode current source. This approach is naturally
more complex than a series resistor arrangement. However, a
current source can be built efficiently using SMT components and
allows lower LED forward voltage conditions, higher supply voltage
and reduced power dissipation. This typically allows the LED count
to be reduced, leading to lower costs, easier assembly and smaller
overall dimensions.

---------------------------------------------------------------------------------

Good luck and please let me how you get on, OK?

__________________
Take it easy, bb. >"HEAR & you FORGET<>SEE & you REMEMBER<>DO & you UNDERSTAND"<=$=|O|=$=>"Common Sense is Genius dressed in its Working Clothes"<>[Ralph Waldo Emerson]
Register to Reply
Power-User
United States - Member - Donald here, Campbell Lighting Co. Engineering Fields - Retired Engineers / Mentors - New Member

Join Date: Sep 2006
Location: central arkansas
Posts: 337
#13

Re: Gradual LED Failure

11/15/2009 3:26 AM

Thanks Andy for that good word.

When LED's for lighting purposes first started to circulate, most were overpowered, thereby causing some of these premature failure problems.

A basic understanding of LED operation will help in choosing the proper LED.

Red and yellow/orange 3-5 millimeter low power type run on 2 volts each, while white, green, and Blue run on 3 volts each.

Care must be taken to use the proper MA transformer, as both undersizing, and oversizing will shorten life.

As was mentioned, heat is another factor in short life, as well as misuse of capacitors.

We have a 2 watt/60pc white Led them has a IC chip that is holding up well in Chicken house environment, and it is fully dimmable.

I have just received a request from quite a large customer to design a LED type soft neon application that must work in a country that has extremely high summer heat.

My first inclination is to use UV stabilized PC tubing, with a built-in air moving design to maintain tolerable temperature levels.

Anyone else have any ideas?, like you Andy??

This customer claims the enclosed regular soft neon runs too hot, and doesn't hold up well.

Donald

__________________
Check out our home page for specs on "Soft Neon"
Register to Reply
Guru
Popular Science - Weaponology - New Member Safety - ESD - New Member Hobbies - Fishing - New Member

Join Date: Sep 2006
Location: Near Frankfurt am Main, Germany. 50.390866N, 8.884827E
Posts: 17989
Good Answers: 200
#14
In reply to #13

Re: Gradual LED Failure

11/15/2009 6:34 AM

Hi Campbell Lighting we have missed having you around, still very busy?

Sounds to me like you already have a good idea on that problem you mention with the cooling air, my only extra that I can think of is a) filter the air first, you don't want dust build up inside the tube, b) have a temperature sensor where the air exits and drive the fan so as to maintain that temperature or less, which means that the first ones to be cooled will be cooler, but that is a problem that you can live with I am sure!

It just struck me that maybe the air should not exit, but just circulate and be cooled at some point - think about it!!

Keep in touch.

__________________
"What others say about you reveals more about them, than it does you." Anon.
Register to Reply
Power-User
United States - Member - Donald here, Campbell Lighting Co. Engineering Fields - Retired Engineers / Mentors - New Member

Join Date: Sep 2006
Location: central arkansas
Posts: 337
#15
In reply to #14

Re: Gradual LED Failure

11/15/2009 1:22 PM

Hey Andy..

Yes, lately I am Very busy, as my son that has been running my company has left to run his Bow Shop. So, they caught the old man and put him to work. Quite frankly, these last weeks have been a little hard on me. I am used to staying up late, reading and posting on CR-4, and spending time with Mama.. haha

However, we have a very good business that has taken many years to cultivate, and it must be taken care of. I guess my new business ventures will have to "fit in" to the normal daily business of taking care of the worlds Chicken House's

I like your idea of "being Cooled at some point".

Now I will "rack" my brain, (or is it wrack my brain)?, as to the very best source of cooling in an area where the outside temps go to 135 F..

Is it the air conditioning system, or how about a dedicated small freezer with coils inside to cool the air?.. What about condensation?, we certainly don't want moisture/electronics to mix. :o( Hey, this sounds like a New topic forming.. I know this one guy in Taiwan that is cooling and LED Bulb with liquid, I forget what kind..

Hey, I actually built an LED Bottle... I took my little Polycarbonate Tubes that we build the little bright inspection lights for the RCMP, and sealed them with epoxy, and obviously sealed in the heat as well, then I sealed this up in a small juice bottle with plain water.. Worked well, and water never boiled..

Hey, maybe that would work, a tube inside a tube, where the outer tube circulated water to a heat exchanger. The heat exchanger could be a simple old style swamp cooler, (evaporative cooling). This would not only be less expensive than refrigeration, but even the water used, (less evaporation of course), could be recovered for washing their limos or feeding their swimming pools.. haha

Hey, what about this, use UV resistant tubing, (tube in side tube), with non potable water plumbed into the outer tube, and the heat of the LED making the water warm, and I guess use your imagination as to what to do with that warm water.. :o)

Nice to hear from you Andy..

Don

__________________
Check out our home page for specs on "Soft Neon"
Register to Reply Off Topic (Score 5)
Power-User

Join Date: Jan 2009
Posts: 187
Good Answers: 9
#16
In reply to #15

Re: Gradual LED Failure

11/15/2009 1:50 PM

You might want to put spacer rings (like plastic washers with holes or slots in them to let the water flow through) around the inner tube to make sure it doesn't touch the outer tube and cause a thermal short circuit that doesn't get cooled.

Register to Reply Off Topic (Score 5)
Guru
United Kingdom - Member - Old New Member

Join Date: Jul 2005
Location: South east U.K.
Posts: 3471
Good Answers: 87
#18
In reply to #15

Re: Gradual LED Failure

11/16/2009 5:42 AM

We use a home built lamp as part of a photocathode production process. The lamp has a blue LED & a UV LED & is cooled using a peltier system. The lamp has to sit on top of a vacuum chamber that is at about 120°C. The blue LED is fairly stable but the UV LED degrades quicker. We run these at about 80% max.

__________________
I didn't have a really important life, but at least it's been funny (Lemmy Kilminster 1945-2015)
Register to Reply Off Topic (Score 5)
Commentator

Join Date: Oct 2006
Location: Pacific Northwest, USA 45.952N -123.976W
Posts: 73
Good Answers: 3
#17
In reply to #14

Re: Gradual LED Failure

11/15/2009 4:46 PM

Hello All,

Andy just reminded me of a test set-up we used for an old sensor project. The salient points are that we used 'tube-in-a-tube' cooling and as the breadboard was used both indoor and out, UV Stabilized PVC was the choice for the external jacket of the cooling media. Second, there was the issue of "the first ones to be cooled will be cooler". Long, linear strings of small diameter tubing create a lot of back pressure. Consider that the effective cross-section of the 'jacket' is reduced by the inner tube diameter. What we ended up with was a manifold arrangement that was similar to an automotive type radiator. This was constructed in staggered 'halves' to allow assembly. By using the 'radiator' concept, we were able to significantly reduce input to output delta t.

Also, the manifold arrangement reduced the back pressure from our original linear string. As (my enfeebled mind) recollection serves, we used the (12Vdc) water pump from one of my colleague's motor homes (his sink has never worked the same, since) and started with plain water as the coolant. This got us into the ballpark. The next generation used automotive coolant (ethylene glycol) and was the most thermally effective. The final iteration used a glycol derivative that was not as toxic, slimey or hard to manage when spilled. As the pump in question was a 'demand' type, I don't think the system ever ran at over 6 PSI.

As for the spacers required to keep our tubes concentric, we ended up with the center splines from a lot of Lord shaft to shaft couplings. NOT particularly elegant, but then when you are in the Mojave Desert, you make use of what is in ready supply (and is open to be 'dog-robbed').

I'm tossing these 'notes' out in the hope that one or more might bear the slightest resemblance to the application at hand. Maybe, Cube Wave, you could post a few photos??

Please keep in touch on this one and let us know of developments/ultimate solution.

Warmest Regards,

GLB

__________________
“Any sufficiently advanced technology is indistinguishable from magic.” Sir Arthur Charles Clarke
Register to Reply
Participant

Join Date: Nov 2009
Posts: 4
#19

Re: Gradual LED Failure

11/16/2009 8:59 PM

Wow, this thread really took off over the weekend! Lots of helpful information here. I can elaborate on the setup, and I'll see if I can come up with some photos.

The application is an exhibit display case. The inside is not easily accessed, and it's full of delicate objects, hence the need for low-maintenance lighting. The LED/resistor assembly comes already together from the manufacturer, as do the sockets and wiring (and they provide the power supplies). (I am talking to the manufacturer about this dimming issue, too. I imagine they are not the original manufacturer of the LEDs themselves, and they have so far resisted giving me the full specifications of the actual LEDs, so perhaps something is up.)

I did manage to get a temperature probe inside the epoxy casing during operation and found a highest reading of 52 degrees Celsius very close to the resistor (versus an ambient temperature of 21 degrees Celsius). The casing is in two parts: one fully encloses the LED (except for the leads) and then nests in another which contains the resistor. Then this whole assembly goes in the socket.

Unfortunately, these LEDs are intended not as indicators, but as illuminators of other objects, so brightness is critical. Voltage spikes would be the easy thing to fix, but it seems as though things may be thornier than this if heat is the big issue. You've given me quite a few good suggestions and directions to go in. I'll update with more developments as I have them. (I'd prefer fiberoptics for an application like this, actually, but switching at this point would be impractical at best.)

Thanks!

Register to Reply
2
Power-User

Join Date: Jan 2009
Posts: 187
Good Answers: 9
#21
In reply to #19

Re: Gradual LED Failure

11/17/2009 1:41 AM

52 C is too warm at 20 mA according to the data sheets for a T 1 3/4 LED I checked. It's even worse if it's bottled up in a little plastic case. At that temperature, the absolute maximum current is 18 mA, so using the 80% rule-of-thumb, the current should be about 15 mA. Either somebody didn't pay much attention to thermal design, or assumed the string would only be run in a fully ventilated ambient, perhaps outdoors in the wintertime.

If the cost of a different setup is substantial, I'd look into rebuilding the sockets with 3 white LEDs in series with a 60-120 Ω resistor (the exact value depends on the average voltage drop of the LEDs). The cheapest high-brightness LED I found at Mouser Electronics is the Optek OVLEW1CB9, which is $239 for a bag of 500 (you may not need that many). This part has only a 15° viewing angle, but it is literally blindingly bright at 16,800 mcd minimum intensity with a 20 mA drive current. (The data sheet warns that looking directly into the light within the viewing angle can cause retinal damage.) Mount these on small circuit boards with two LEDs angled away from the central LED to spread the light around, and reflect the light off of light-colored stained wood to correct the excess blue component and prevent direct viewing of the light. The LED circuit boards can be made to fit the original LED cases with the top removed.

Since more power goes into light production and less into heat, and the LEDs are now directly exposed to air, the heat problem should be remedied. You may have more light than you need. If that is the case, there are two options available. You can omit lamps from the string and cover the remaining LEDs with small Fresnel lens caps to spread the light beams out more evenly. One such part ($0.19 in 500 quantity) is the VCC CLB300CTP

Note: the part numbers I provided are only suggestions. Another LED model that trades off center brightness for a wider viewing angle is probably more suitable, unless you would like to have highlight spots.

You can also put a 0-3 Amp adjustable pulse width modulation current source in the circuit, using a 2-wire cable tied to the leads of one of the LED resistors to provide current feedback (the other LED assemblies, up to 149, will run at the same voltage as the one used for feedback; this will give them about the same current). I suggest this scheme because you will get more consistent color if you keep the current the same, but reduce the fraction of time it is on to dim the light. I assume you don't want the spatial variation in light you would get by omitting lamp units, otherwise this would be a really low-cost solution. Reducing the current will cause color shifting since less of the phosphor is activated (Have you noticed that a white LED flashlight gets more bluish as the battery fades out?).

There are several vendors that produce single-chip PWM current source modulators specifically for driving LEDs. You would probably have to use one capable of driving an external transistor to get 3 Amps. LT Semiconductor (AKA Linear Technology), National Semiconductor and Texas Instruments are probably the best-known in the USA.

I would not be surprised if Campbell Lighting already has a suitable current source that can be connected between your transformer and the lighting string.

Register to Reply Good Answer (Score 2)
Guru
United Kingdom - Member - Not a new member!

Join Date: Jun 2008
Location: USA/Europe
Posts: 4547
Good Answers: 68
#20

Re: Gradual LED Failure

11/16/2009 9:53 PM

Hi to all,

If you can find time to answer a small query I would be most obliged.

I wrote in post #12, "I think though cannot recall precisely, that using a variable capacitor, which can turn the LEDs on over a longer period (Comparatively speaking), of maybe 100th second, or anything up to a tenth of a second perhaps?"

I have not actually used a capacitor, (and this time I also spell checked!) but I would be interested if this would work? Or have I got it completely up-side-down?

This just me trying to understand whether it perhaps should be a capacitor or resistor just before the LEDs, to try and prevent the spiked current which may be shortening the LEDs life?

Sorry to be a PITA.

__________________
Take it easy, bb. >"HEAR & you FORGET<>SEE & you REMEMBER<>DO & you UNDERSTAND"<=$=|O|=$=>"Common Sense is Genius dressed in its Working Clothes"<>[Ralph Waldo Emerson]
Register to Reply Off Topic (Score 5)
Power-User

Join Date: Jan 2009
Posts: 187
Good Answers: 9
#22
In reply to #20

Re: Gradual LED Failure

11/17/2009 2:15 AM

Well, umm... a variable capacitor large enough to generate a significant rise-time would probably be about the physical size of a 300 Watt incandescent floodlamp and cost a big bundle of money. If you were thinking about putting it in parallel with the LED, you'd need about 20 µFd for a 1 msec. RC time constant with the 460 Ω resistor. This would have to be an electrolytic capacitor, which is not that good at eliminating really fast spikes, and it would be about as large as the LED itself. The resistor is essential to limit the current. Spikes are not a significant problem with a properly regulated supply, given that ESD sources (people) are not supposed to be inside the display case.

In general practice, it is simpler and just as effective to put a bypass capacitor across the entire load. This may be an electrolytic capacitor in parallel with a small high-frequency ceramic or film capacitor. (If a current source is used to drive the LEDs, the bypass capacitor should be at the supply terminals of the current source circuit rather than the output, not counting any capacitor that may be part of the current source circuitry. Putting capacitors across LEDs in a current-controlled circuit could cause a lot of trouble with response times, perhaps causing destructive oscillations.)

Unfortunately, cube wave is in a bit of a corner with his existing circuitry. It almost looks like he has a string originally designed for 12 V incandescent bulbs that was retrofitted with LEDs. He measured the temperature at an LED, and it is too high to run the LED at 20 mA (above a certain temperature, which is dependent on the LED model, the LED current must be reduced to prevent damage). In another post, I suggested a solution, but it isn't cheap or easy.

Register to Reply Off Topic (Score 5)
Guru
United Kingdom - Member - Not a new member!

Join Date: Jun 2008
Location: USA/Europe
Posts: 4547
Good Answers: 68
#23
In reply to #22

Re: Gradual LED Failure

11/17/2009 1:42 PM

Hi MNIce,

Really appreciate your reply post, thank you.

I should explained myself better, I had a thought of how cube or anyone else could possibly get out of trouble. What I was thinking about was as you said, a capacitor just after the power supply but before the LEDs. But that would be restrained and dependent of how many LEDs the capacitor can handle.

It does sound like cube either started with a system already fitted, or tried to convert a system not knowing all the likely pit-falls.

I thank you once again for your help with my ideas. I have no plans to work on this type of circuit.

Cheers.

__________________
Take it easy, bb. >"HEAR & you FORGET<>SEE & you REMEMBER<>DO & you UNDERSTAND"<=$=|O|=$=>"Common Sense is Genius dressed in its Working Clothes"<>[Ralph Waldo Emerson]
Register to Reply Off Topic (Score 5)
Guru

Join Date: Aug 2005
Location: Hemel Hempstead, UK
Posts: 5155
Good Answers: 279
#24
In reply to #22

Re: Gradual LED Failure

11/18/2009 1:56 AM

I agree with almost everything you say except that capacitor technology has come on quite fast. This is a 2220 surface mount 22µf multilayer ceramic for under a dollar:-

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=445-3495-2-ND

In fact you can now get 22µf in 1206.

This is a 1206 resistor (in the picture) a capacitor would be a bit "fatter"

__________________
We are alone in the universe, or, we are not. Either way it's incredible... Adapted from R. Buckminster Fuller/Arthur C. Clarke
Register to Reply Off Topic (Score 5)
Power-User

Join Date: Jan 2009
Posts: 187
Good Answers: 9
#25
In reply to #24

Re: Gradual LED Failure

11/18/2009 3:07 AM

I'm glad to know that. Thanks!

Register to Reply Off Topic (Score 5)
Register to Reply 25 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".
Copy to Clipboard

Users who posted comments:

Andy Germany (3); aurizon (1); babybear (3); Campbell Lighting (2); cube wave (3); GLB (1); maveric_manic (1); MNIce (5); Neil Kwyrer (1); Nigh (1); Randall (1); Rebuilt (1); SafOcean (1); tcmtech (1)

Previous in Forum: Substation Generators Tripped -- Why?   Next in Forum: Ring Wave Test

Advertisement