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Misconception:"LEDs produce almost no heat". LEDs require MASSIVE HEATSINK

29370 Views 46 Replies 12 Participants Last post by  LightsRus
What goes in gotta come out. What a backward concept. :001_huh:


Many have heard that snow does not melt because LEDs are "so energy efficient". Along with that was a misconception that LEDs run cool and give off almost no heat.
http://www.engadget.com/2009/12/17/led-traffic-lights-dont-melt-snow-do-cause-accidents/

First, traffic lights is a different ball game from general lighting. LED is one of the most efficacious technology for producing highly saturated colors such as those sought after for traffic lights.

Incandescent traffic lights use colored glass filter to create the desired color and absorbs infrared as well, which warms up the lens.

In all electrical appliances, thermal dissipation into the room is equal to electrical input into the fixture, minus the radiant energy escaping out the window which is a drop in the bucket.

15W LED and 15W incandescent lamp takes in 15 watts of electrical power, and simultaneously rejects 15 watts in the form of conducted and radiant energy. Though LEDs produce more lumens, the overwhelming majority is rejected as heat (same goes for HIDs and fluorescent) Incandescent lamps reject heat through invisible infrared radiation. LEDs dissipate heat almost solely through conduction, therefore it gets hotter at the fixture.

LEDs are not much different from SCRs, transistors, CPUs,etc. They do not like heat and depend almost entirely on conduction by heat sink for cooling since they can't run at high enough temperatures to reject heat by infrared.

LED lighting for common applications in air conditioned spaces use MASSIVE heatsink(comparatively for their wattage). For special applications such as color changing stage spot lights, they're FAN COOLED, like computers.

You could trap a 20W halogen lamp in a beer bottle, pull a vacuum around it and it can still dissipate heat. The radiated infrared energy will induce heating on the brown bottle and whats not absorbed will escape.

Do the same with a 20W LED bulb and it will get hot until its broken or its glowing cherry red.

Even fluorescent lamps reject 37% of energy as infrared (IESNA Lighting Handbook)

Most LED lamps will not survive enclosed luminaires, commonly found for outdoor or other applications requiring good water resistance.

LED marketing boasts its performance in bone chilling weather, but lighting is used in applications where it gets hot too.

Externally heatsinked LED fixtures can still have trouble maintaining specification lumens-watt under actual application, such as 55C ambient in unconditioned high-bay mount warehouse lighting. No amount of cooling will bring the heat sink below 55C. Amalgam CFLs(induction also), HIDs and induction can handle such applications without an issue.

13W LED lamp after a few hours in an enclosed fixture

Oops, melt down....


It got hotter than 212F
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· Ambassador of Amps
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WHY do LEDs need such big ass heat sinks? You can power an LED from line voltage with a bridge rectifier, a resistor, and a capacitor. Even though the cap isnt really needed, but smoothes out the DC. Still.. where is all the damn heat coming from?

~Matt
 

· Light Emitting Decoration
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Discussion Starter · #3 ·
WHY do LEDs need such big ass heat sinks? You can power an LED from line voltage with a bridge rectifier, a resistor, and a capacitor. Even though the cap isnt really needed, but smoothes out the DC. Still.. where is all the damn heat coming from?

~Matt
At 64 lm/W,the efficacy is about the same as CFLs. Watts of energy radiated as light is a small proportion compared to 12.5 watts of input, so that heat must be rejected. Since it is not allowed to get too hot, all the heat must be dissipated through conduction.

The large heatsink is needed to provide enough surface area to provide adequate cooling by convection cooling. Thick heat sink is needed to provide sufficiently low thermal resistance from dies to heat sink surface.

This thing weighs 7 1/4 oz. It does not work with my swing arm desk lamp. It drops it down.
 

· Banned
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I still like my incandescents, thank you very much. :laughing:

Good info though. I was thinking about replacing the 35W HPS security light I have on my garage with an LED as an experiment, but it doesn't make any sense to do that economically.
 

· Light Emitting Decoration
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Discussion Starter · #9 · (Edited)
OH, It is interesting that people say LED is low power and runs cool... They run hotter than CFL, Much hotter Ive seen and felt.
All light sources we have is fairly inefficient if we compare the radiometric efficiency. (i.e. watts radiated in useful visible light range divided by input watt). As for the rest of the enregy, the less the lamp can reject that as infrared, the hotter the lamp or the fixture gets, given the same wattage.

A high power graphic card can make 150W of heat and requires substantial heat sink and fan cooling, because all the cooling must be made by conduction cooling. A 150W light bulb can just sit along and dissipate all the heat.

For the enclosed fixture test, it was placed in a lava lamp base, then covered up with a NSF rated plastic cup that can withstand thousands of harsh wash cycles in commercial dishwashers which operates at sanitizing temperature.

Meant to simulate outdoor enclosed and special application enclosed fixtures like:

Left is before: right is after



The cup still melted.
 

· Light Emitting Decoration
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Discussion Starter · #10 ·
Is there an average percentage of energy wasted on just heat?
LED datasheet shows 37-53% with the LED bottom held at 25C.
datasheet shows output drops 10% at base temperature of 100C, which is realistic in open luminaire, but it doesn't say if electrical properties remain the same.

The driver has loss as well, so if let's say 5W of 12.5W becomes blue light.

The phosphor then takes that 5W and converts it to white light.


For a 60Hz fluorescent light, IES Lighting handbook says 53% of electrical energy is converted to ultraviolet light of which 26% is converted to visible light by phosphor coating.
 

· Registered
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This thread is dildos.

You throw out technical terms like someone selling PF correction for
residential. It appears the intent is what we used to call in the
computer industry "FUD", i.e. "Fear, Uncertainty and Doubt."

Bottom line is LEDs, fluorescents, incandesents may all have their
place depending on what is needed WRT installation cost, operating
costs, aesthetics, customer preference, code/energy requirements
etc. etc. etc.

While you're at it, why not throw in that the LED driver electronics
can cause harmonics? Oh, I forgot, you are pushing fluorescents,
and the same thing can happen there.

Another good one would be the change in efficacy/light quality
over time.

For simpletons like myself good LEDs have excellent Lumens/watt,
as well as excellent lifetimes. Negatives include high initial costs,
possibly declining output over time, etc. If people want good quality
light, and can afford them, Cree and others make some excellent
products.

To just throw out a lot of random technical comments on LEDs, and
stupid pictures of plastic cups melted, without relevant context just
seems useless to me.

If you want to market your product, I for one would appreciate
being spared the BS.

Just my opinion.
 

· Senior Moment
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A LED runs off .6 VDC, to use 120 AC they need a step down transformer and a rectifier, this is were the power is lost due to heat. LED itself produce very little heat.
 

· Resi lighting designer
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A LED runs off .6 VDC, to use 120 AC they need a step down transformer and a rectifier, this is were the power is lost due to heat. LED itself produce very little heat.
Sorry, but very little of that is correct.

Power LEDs use somewhere between 3.3 and 6 volts depending on manufacturer, model, and manufacturing variances.

LED lighting fixtures generally use constant current switched mode power supplies rather than linear power supplies. Heat generation by the power supply is non-trivial and must be sinked appropriately to avoid overheating the driver. The principles involved are not too far removed from the principles behind electronic ballasts for fluorescent lamps.

Heat generation within the LED, however, is extremely non-trivial and must be dealt with through fairly large heat sinks to avoid burning up the wires that connect the LED die to its case. Running a bare power LED without a heatsink will destroy the LED very quickly.



See the datasheets and design guides for Cree's XM-L bare LED:

http://www.cree.com/products/xlamp_xml.asp
 

· Senior Moment
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Sorry, but very little of that is correct.

Power LEDs use somewhere between 3.3 and 6 volts depending on manufacturer, model, and manufacturing variances.

LED lighting fixtures generally use constant current switched mode power supplies rather than linear power supplies. Heat generation by the power supply is non-trivial and must be sinked appropriately to avoid overheating the driver. The principles involved are not too far removed from the principles behind electronic ballasts for fluorescent lamps.

Heat generation within the LED, however, is extremely non-trivial and must be dealt with through fairly large heat sinks to avoid burning up the wires that connect the LED die to its case. Running a bare power LED without a heatsink will destroy the LED very quickly.



See the datasheets and design guides for Cree's XM-L bare LED:

http://www.cree.com/products/xlamp_xml.asp
I guess I'm behind the times, I am familiar with old school LEDs used in automotive and electronics. I haven't had the chance to tear apart the newer high output type.
 

· Light Emitting Decoration
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Discussion Starter · #19 · (Edited)
stupid pictures of plastic cups melted, without relevant context just
seems useless to me.
Just my opinion.
you don't have to read it if you don't want. Add me to ignore list if you will.

Just my opnion, but stupid picture of relatively heat resistant plastic that melted because of LEDs DOES raise a question about "LEDs producing almost no heat" for those that believes/believed such a thing.

I just picked up 10 LED PAR20's, the light output is awesome.
And you've had it for how long? The TRUE efficacy is?

They're guaranteed to last 15 years.
Or the life of company, whichever is less. Many LED sales company have existed for less than 1/5 the length of warranty they boast. Even if it's a lifetime guarantee, its not necessarily good. Most Harbor Freight Tools hand tools are lifetime guaranteed. When the junk breaks, they give you another junk, repeat as often as necessary. Time and fuel expense not covered. Would you pickup your entire set of tools of trade from Harbor Freight?
 

· LED Lighter
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re: Misconception:"LEDs produce almost no heat". LEDs require MASSIVE HEATSINK

LEDs used in illumination require heat exchangers.

There are three modes of heat mobility with any lighting technology to be understood by someone wishing to get to the core of that subject comparison.

You have known of this LED heat phenomenon all along; it's amusing to see that point raised by yourself.

This is called a Straw_man argument, and comes from your dislike of LEDs.http://en.wikipedia.org/wiki/Straw_man
 
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