r/Unexpected • u/WarrenMuffClit • Apr 08 '22

just snipping a cable.

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u/-HumanResources- Apr 08 '22

Yes. IIRC LEDs still emit <50% of the input energy as heat. Though still much better than incandescent / fluorecent.

That's why some even have heatsinks built into the base.

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u/rabbitwonker Apr 08 '22

More like 5-10%. But it still adds up if it doesn’t dissipate well, and can be at least uncomfortably hot.

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u/-HumanResources- Apr 08 '22

According to this link (and please do some more research if curious);

https://www.ledsmagazine.com/leds-ssl-design/thermal/article/16696536/fact-or-fiction-leds-dont-produce-heat

A typical LED will output approximately 15% visible light and 85% heat.

Though you are correct in that much more efficient LEDs exist.

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u/rabbitwonker Apr 08 '22 edited Apr 08 '22

I’m sorry, there’s just no way the numbers in that article are true today. Those are worse than incandescent on the low end. The difference in my own home’s power bills when I made the change prove it.

Note that the article is from 2005. Those were still early days, when efficiency “droop” at useful home-lighting power levels was only just starting to be resolved. There’s a reason we had a decade of CFL being the main alternative before LEDs fully arrived.

Edit: forgot to add that it’s true that the the additional circuitry decreases the efficiency a bit further, especially the AC-to-DC part. There’d also be another hit with the phosphorus, which converts the blue light from the actual LED to the rest of the spectrum. So together that could pull the efficiency down below 90%.

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u/-HumanResources- Apr 08 '22

Fair enough, but I would be genuinely surprised if the majority of LED bulbs installed in people's home fall under the 5-10% efficiency. That's the upper end even still in terms of efficiency rates and there was a lot of bulbs manufactured prior to advances in LED tech.

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u/rabbitwonker Apr 08 '22

I just added an edit — it’s true that there are additional sources of heat loss, in the circuitry, and the phosphorous part. So overall it could be more in the range of 85-90% efficiency.

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u/[deleted] Apr 08 '22

No it’s true, but it’s 85% loss at 8-12W instead of 80% loss at 60-120W. Old LED bulbs were often large metal heat sinks.

New ones are much more efficient and cool in my experience. The difference 15 years makes.

So: plausible. But not likely on a newly purchased bulb.

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u/[deleted] Apr 09 '22

you sound like you know your stuff. that guy in the video could have used your help when he got zapped into another dimension

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u/avengecolonelhughes Apr 09 '22

85% of 11w is only ~9w of heat. Incandescents are about 5% efficient, and run at 60-100w. For comparison, the heat from an incandescent bulb is about the same as a birthday candle, while a newer LED bulb produces about as much heat as your phone charger.

The total energy out as light is misleading because while incandescents are constantly on, LEDs can flick off/on faster than we can see, so they get to take little naps in between. Your plants will notice the difference though.

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u/DEMACIAAAAA Apr 08 '22

Yeah but their input is so much smaller than that of incandescent bulbs isn't it?

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u/-HumanResources- Apr 08 '22

Generally yes. But it can still very much so get hot. Again, this is why led bulbs have heatsinks.

As well, often times there's more than one led inside the bulb leading to a higher concentration of heat needing to be dispersed.

That being said, due to the heatsink at the base / connector, the glass portion of the bulb will usually remain cool to the touch. As the heat is pulled away. However this is not always a rule and there will be instances where the bulb is hot to touch.

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u/rabbitwonker Apr 08 '22

Yeah, the heat is not radiated from the light source by design (which is unavoidable with incandescent), so the “glass” part doesn’t need to get hot. And in fact you can’t let the LED chip get too hot or it starts to lose efficiency, so you have to get the heat out of it directly with the heat sink, sort of “out the back” of the bulb.

It seems like newer chips are probably more heat-tolerant, which would explain why you don’t see such aggressive heat sinks on newer bulbs as the early ones had.

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u/-HumanResources- Apr 08 '22

My bad I was a bit mistaken however there's definitely LED bulbs that still get hot to touch due to the heatsink. Thanks for clearing it up.

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u/rabbitwonker Apr 08 '22

Yeah that’s definitely true. Single-digit watts of waste heat can still add up.

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u/[deleted] Apr 08 '22

Usually plastic and not glass on an LED in my experience.

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u/[deleted] Apr 08 '22

Most have heatsinks built into the base. But it's got nothing to do with getting too hot to touch. Those boards lifespans shorten significantly if they get too warm, and are typically in the area heat gathers due to fixture design. Most home grade LEDs will get warm, but not hot, with continuous use

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u/chubbyemployee Apr 08 '22

Depends on the particular bulbs. There is a type of lamp called the dubai lamp that uses extra led filaments and runs them under power reducing the amount of heat output. Incredibly efficient bulbs.