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Author Topic: LED lights  (Read 4151 times)
Jeremy
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« Reply #15 on: December 24, 2009, 05:19:23 AM »

I've a question for Tim Strommen or anyone else with a technical knowledge of LEDs - can they be damaged by too little voltage? I have a string of dirt-cheap LED lights in the loft of my house - the kind that are designed to be used as 'under counter'-type lights, powered by three 'AA' size batteries. They only have three LEDs in each fixture, but with a decent reflector behind and they do a fair job of lighting my loft. I have nine of them (total cost less than £10/$15), wired in parallel and powered by an old cell 'phone charger. The cell 'phone charger produces 3.7v, whereas the LEDs are expecting 4.5v (3x AA batteries) - but I figured there would be no problem as they were previously powered for a long time on rechargeable AA batteries which have a nominal voltage of 1.2v rather than 1.5v. The cell 'phone charger produces 350mA, but I assume this is irrelevant as the lights will presumably have internal resistors.

Five minutes ago I noticed one of the LEDs had apparently failed. I haven't investigated yet so it might just be a wiring problem, but either way I am interested to know how tolerant LEDs are to incorrect voltages.

Jeremy
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Tim Strommen
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« Reply #16 on: December 25, 2009, 12:45:03 AM »

Hi Jeremy,

The question about LEDs being undervolted - LEDs require a certain voltage before they begin to conduct.  Once they start to conduct, the manufacturer has a current that will output light at the specified rating, which is matched to how much waste-heat the LED pachage can get rid of. if you increase the voltage to the part it will overcome the resistance of the led die a little more allowing more current to run through the diode (this generates more heat, and a little more energy is converted to light - but you may notice a color shift as the heat generated starts to build up more and more).  Once the voltage gets high enough, the current starts to run-away (the resistance across the junction drops and more and more heat is generated until the LED burns either a gold wire or the LED die delaminates from the base/holder - or even more spectacularly, it superheats and melts/vaporizes... usually with a loud "pop").

Undervolting an LED will not hurt it - but that is only to a point.  If you go below zero too far (reverse biasing an LED), you can do some real damage.  If you have a 10x-magnification loupe or microscope, you can take a look at the LED to see if you have a bad gold bonding-wire.  If not it's probable that you have a bad connection somewhere.  One thing to note, the charger if it is really old, may have a current limiting function built in that raises the voltage until the load draws a specified current - in most cases now, the phone or laptop has the final charge regulation circuits, so the "wall-wart" may not be as tightly regulated...

Lastly, you mentioned "dirt cheap" and "battery-powered" - these two things suggest to me that you will want to open one of the dead fixtures to see how they acually wired the LEDs up.  I suspect they might not have a resistor for current regulation - but instead rely on the output characteristics of the AA batteries for voltage/current regulation...  In this case, by hooking the fixture up to a high-power supply (relative to the AAs), you may have exceeded the current capability of the LEDs (350mA is far higher than most 20mA LEDs will take).

Best of luck in doing the failure analysis...

-Tim
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« Reply #17 on: December 25, 2009, 12:22:51 PM »

Can't I put a "cream colored" shade over my led's to make the temp warmer?  Best case.....50% of the light? 

Thanks,

John
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Just Dallas
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« Reply #18 on: December 25, 2009, 01:04:21 PM »

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« Last Edit: July 17, 2010, 02:01:01 PM by Now Just Dallas » Logged

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« Reply #19 on: December 25, 2009, 03:41:24 PM »

OK....OK,

I did say "shade".  And lamp shade does come to mind.  At least now it does. Tongue

What I meant was that my "stock" RV fixtures have a plastic "cover" that makes the stark light from the tungsten bulbs very nice or diffused.  Couldn't I treat those LEDs the same way and even in the same fixture?  I know a diffusing cover/shade saps candle power to do its job and I don't know if having a completely different temp would work as well.  That 3 to 6 dollar apeace cost for the "warm temp" LED leaves me cold.

And Big D:  No I am not going to glue a lot of'em together and take them to the beach and use'em to block the sun. SSSSSHHHHEEEEEEEHHHH Ya gotta be sooo kerfull with some folks....LOL "D"....LOL

Thanks for catching that,

John
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"An uneducated vote is a treasonous act more damaging than any treachery of the battlefield.
The price of apathy towards public affairs is to be ruled by evil men." Plato
“We can easily forgive a child who is afraid of the dark; the real tragedy of life is when men are afraid of the light.”
—Pla
Tim Strommen
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« Reply #20 on: December 27, 2009, 06:10:51 PM »

...Can't I put a "cream colored" shade over my led's to make the temp warmer?  Best case.....50% of the light?...

Yes, you can use a filter over the LED, but...

While light is generally additive when applied to an environment, a filter or shade is a color subtractor.  Generally if you want to get a certain color of light - you need to have enough light in the color spectrum that you want generated by the lamp you are using that when you filter out the parts that you do not want, you end up with the desired light spectrum.  LEDs are not really broad spectrum emitters like say halogen filaments.  There isn't that much light generated that isn't in the target spectrum (there are peaks in the blue/red wavelengths and a "hump" from the green through the yellow spectrum - but it isn't as close to a slope as Halogen or sun-light.  This means that if you filter out the blue and red (retaining a yellowing cast to the light) to lower the color temperature, you may be reducing the light output by as much as 70%...

That leaves you with about 30% of the original output to use, and doesn't take into account the losses from the lenses and reflectors (they are never "perfect", and always have at least 3% loss for each element...).

It's often better to get the LED you want rather than fix it on the back-end if possible.

-Tim
« Last Edit: December 27, 2009, 10:37:29 PM by Tim Strommen » Logged

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« Reply #21 on: December 27, 2009, 06:22:34 PM »

Tim,

Excellent!  Thank you.  I now remember, thanks to you, a post that advocated mixing in yellow and red LED's to "adjust the temp of the light.  Does that seem like it would work?  I also read, here recently I think, that the color adjusted LED's that mimicked sunlight were VERY expensive.  I would welcome your comments.

Thanks,

John
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"An uneducated vote is a treasonous act more damaging than any treachery of the battlefield.
The price of apathy towards public affairs is to be ruled by evil men." Plato
“We can easily forgive a child who is afraid of the dark; the real tragedy of life is when men are afraid of the light.”
—Pla
Tim Strommen
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« Reply #22 on: December 27, 2009, 11:30:52 PM »

...I now remember a post that advocated mixing in yellow and red LED's to "adjust the temp of the light.  Does that seem like it would work?  I also read, here recently I think, that the color adjusted LED's that mimicked sunlight were VERY expensive...


Hi John,

Adding a few Yellow or Amber LEDs will add color in the areas that are a bit weak with the "cooler" LEDs.

This is a grab from a Philips-Lumileds Rebel White Data sheet (which can be found here).

This image shows the spectrums for three general color-temperatures of white, 6500K which is very blue/cool, 4000K which is typically called "neutral", and 3000K which is generally pretty red-ish/warm.

If you take a look at Wikepedia, you'll see this is the spread for light wavelengths...

Violet = 380-450nM
Blue = 450-495nM
Green = 495-590nM
Yellow = 590-620nM
Red = 620-750nM

I think it's usefull to see the difference in the contents of the light that is emited in order to know what you want to change. So, here are the same three spectrums overlayed on eachother:


You will note that generally the blue content is pretty similar - this is due to the use of a Blue LED which also emits a little UV that excites the Phosphor-Slurry that Lumileds applies to their die (this phosphor is a tuned mix of different phosphors that look like white when viewed over a general area).  You'll note that for the 6500K LED, the most output of light is in the Blue and Green spaces, while for the 3000K LED they have increased the relative presence of Green, Yellow, and Red-ish Orange.

I know it's a bit long winded to go into this description, but I figure we're about learning here and people should know "why" it works...

So to put your answer simply, yes, you can add yellow LEDs to an already 6500K LED lamp and get a warmer-white color - you shouldn't need to add more green as our eyes are already picking up enough green  Wink.  If you want the 6500K LED to appear even warmer, you can use Amber or a mix of White, Yellow, Amber, and Red-ish Orange to move it even warmer (down to 2700K, about the warmth of a candle's color which is actually around 1800K).

If you want to adjust the color temperature dyynamically, you can put the different LED colors on separate "channels" of PWM dimming and adjust the brightness of each channel to get the target.  This is how they are doing sunset effects for ceiling washes these days - start with a cool 6500K white to simulate noon, then slowly add yellow, then amber, then red-ish orange, then dim out the White, then yellow, then amber, then dim out the red-ish orange to simulate night-time ("dark" implies night... Right? Wink ).

Hope this helps...

-Tim


Non-Compensation Statement: Philips-Lumileds does not compensate me to mention them nor am I an employee, they are simply my LED manufacturer of choice (and have been for some ten years or so).  Link to the datasheet was obtained through their public website and the data provided in the datasheet is compiled by Philips-Lumileds, so if you have spotted something wrong with their data... it's their problem Grin. -T
« Last Edit: December 28, 2009, 10:29:22 AM by Tim Strommen » Logged

Fremont, CA
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DD 6V92TA (MUI, 275HP) - Allison HT740
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« Reply #23 on: December 28, 2009, 10:20:44 AM »

Tim,

That is a superb response. Shocked  You answered questions I wasn't knowledgeable enuf to assemble and that certainly expedites the learning process...for me.  I "think" I have a concept for what would be the be-all end-all lighting system for me.  And it is a brand new thought.

The LED seems to be the most efficient light source possible and the prices for the individual lamps seems to be dropping dramatically....exactly what I need.  From an efficiency standpoint there seems to not be any down side in the least.  Seems instead to be strong motive to convert all lights to this technology.  I have reading lights, baby spots, that are intense but being Tungsten are more yellowish/orange.  They are very comfortable but still I prefer my flo tubes that are cooler for reading a lot.  For work I use overhead lights and that light is intense and very cool.  It is used for cleaning and such and it is never left on for long periods.  I have other low intensity lights that I use for ambient or background lighting.  So that makes three or four types of lighting that I use.  They overlap very little due to their relative intensity and color.  If I could adjust the  general intensity of each and the color or each, as well, I could overlap the applications of each fixture and undoubtedly the comfort of my environment.  Lasts forever and uses no power AND creates no heat for my AC to deal with.  Where is the down side?

It has been mentioned on the board, Boogie the Cat I think, that there are new LEDs that are super bright but they are very expensive.  I can gang enuf lights for intensity, I think, to achieve my goals.  Sounds like a great project to me.  I hope we can get together and explore this further.

Thank you,

John
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"An uneducated vote is a treasonous act more damaging than any treachery of the battlefield.
The price of apathy towards public affairs is to be ruled by evil men." Plato
“We can easily forgive a child who is afraid of the dark; the real tragedy of life is when men are afraid of the light.”
—Pla
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