Author Topic: Is it possible to dissipate most of the LED heat by radiation only?  (Read 436 times)
merc
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Is it possible to dissipate most of the LED heat by radiation only? « on: November 16, 2018, 02:36:08 PM » Author: merc
This recently bought LED module (see the story below the picture) is 30W and should withstand long time operation in a relatively small enclosed luminaire with a plastic diffuser even if room temperature is 30C (86F).
Particular dies are placed in small reflectors that allow to radiate the heat through the diffuser outside the fixture (can be felt with a hand laid on the diffuser). There is 2 times more reflectors than active dies (see the lit fixture in the bottom of the picture) and those may possibly also help with dissipating the heat in a way. (They're not empty a what's inside them is a question.)
But I'm still not sure if 30W (-light output) of heat can be dissipated so well to increase the inside fixture ambient temperature +10 C only (from 30 to Ta=40C).
Tc (temperature reading of the case of the semiconductor device) is 75C. Tj isn't specified.

Manufacturer's product information Czech - complete / English - partial.
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Medved
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Re: Is it possible to dissipate most of the LED heat by radiation only? « Reply #1 on: November 17, 2018, 02:40:32 AM » Author: Medved
Practically no. You may do the math via known thermal radiation equations the heat is here radiated mainly from the PCB and so you will get the temperature needed to radiate the 30W.
This is normally cooled by either convection or diffusion to the backing material and the wall material itis attached to. And even the small draft of air behind the fixture counts.

With this design I see the weakest spot not that much the LEDs alone (these can operate nearly at 100degC), but the used PCB material, which can not go over 60..70degC on its hottest spot if it is supposed to last some time. When hotter, it will bulge and (because the hottest spots are there) damage the LEDs by the resulting mechanical stress as the consequence.

Normally this construction is good, if the air can flow arround the module (so if the cover glass is not sealed, if it is eithr sitting on a metal body with air flow underneath or if the air gap onderneath is open to the surroundings).
The heat is generated in already evenly distributed manner, so don't need any extra heat conductor. But it still needs to go somewhere...
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merc
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Re: Is it possible to dissipate most of the LED heat by radiation only? « Reply #2 on: November 17, 2018, 01:03:43 PM » Author: merc
Thanks, Medved.
And even the small draft of air behind the fixture counts.
That's what I though too and rather used ~4mm spacer pads to create extra space above the fixture to open all mounting holes.
(The original fluorescent electronic ballast didn't need such a care and could last there with a 40W lamp (+ its own heat) for almost 30,000 hours. The unevenly drilled holes (by me) used to be above the plastic lamp base that had used to crack by the heat. Last 2 lamps with this precaution EOLed without the cracked base.)
Normally this construction is good, if the air can flow arround the module (so if the cover glass is not sealed, if it is eithr sitting on a metal body with air flow underneath or if the air gap onderneath is open to the surroundings).
The module is attached by those 4 magnets (~25mm long). That's the space above it. The plastic diffuser is avg. ~4cm underneath. Drilling some holes into it would definitely help the air to flow better but holes aren't the thing I wish to look at all the time. Smiley The module wasn't so expensive (compared to native LED panels with the same lumen output) and it's super easy to replace it with another one so if it lasts only a half of the rated life, I'm happy.
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Medved
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Re: Is it possible to dissipate most of the LED heat by radiation only? « Reply #3 on: November 17, 2018, 02:09:00 PM » Author: Medved
By the way out of curiosity I did a quick calculation about the radiated cooling of 30W via a 20cm diameter black body disc (that is about the panel size, the emisivity is nearly 1 in the IR for most organic materials). With 40degC ambient (that is the heat received from ambient radiation) the disc will warm up to 125degC when radiating just via single side (one side on a heat insulating supplrt), and to about 100degC when radiating via both sides (near a cold absorbing surface or completely in a free space). Clearly not up to the job for a 30W panel, but roughly 10W for that disc size should be fine (it then ends up at about 75degC)
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funkybulb
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Re: Is it possible to dissipate most of the LED heat by radiation only? « Reply #4 on: November 17, 2018, 03:30:38 PM » Author: funkybulb
this lamp prolly comtainated with bit of air
best thing to do is get Electronic F96 slimeline
ballast and tie both outputs together.  run this lamp
until until it strikes and leave it runnig for a good day
to clean up the gas inside the lamp.  then transfer it
to VHO.   containated tubes will run hot.  that means lap
have impurtity in them.
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