How to cool (dissipate heat) LED in a sealed plastic housing

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YuvalS
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How to cool (dissipate heat) LED in a sealed plastic housing

I want to mod a headlamp with a sealed  plastic housing.
I will be using XML2 and want to have a L/M/H  250/500/1000 lm
Medium will be used for few minutes and high for only 30 sec a time.
I have some space for relatively big Aluminum "pill" to be used as a heat sink but in order to keep it waterproof LED and heat sink are sealed in plastic housing.
1. Is it enough to keep LED cool or I must allow the heatsink contact with the "outer world"?
2. Since it is sealed without air circulation does it matter what is the shape of the heatsink or only mass matter? 
3. How do I know what size of aluminum block I need?
TIA
 

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djozz
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You can calculate this.

1000 lumen equals about 3A. On a full battery 3A will run it at about 4V, so 12W. 30 seconds will be 30s×12W=360 Joule. You may like to heat up your sink from 20degC to 70degC (so that you are still just safe if the 30 seconds turns into 60) so that is 50 degrees warming up. Aluminium has a heat capacity of 0.9 J/g degC, times 50 that is 45 J/g for 50 degrees warming. So to store 360 Joule you need 8 gram alu heatsink. That is equivalent to a aluminium plate of 4 × 2 cm, 4mm thick. Or whatever fits, the shape is of no concern.
This is rough calculation of course, I have no idea if the common aluminium alloys have the same heat capacity as pure aluminium. But as a ballpark figure it should be correct.

For actually getting the heat out of the housing I would try to make as much phyisical contact between the aluminium and plastic as you can, maybe even heat paste in between. Having air between the heatsink and the housing unneccessarily much slows heat dissipation speed to the outside world. Plastic is a poor heat conductor but it is not zero, and it is where the heat must go in the end.

YuvalS
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Thanks Djozz,
So I guess there is no way to calculate the Cooling speed, ie how long do I have to wait before using High mode again  

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dthrckt
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I would probably make a through hole in plastic housing and put aluminum heating on the outside. Seal the hole that goes through the housing. Rtv might work well for that (flexible and won’t fail from the heat).

YuvalS
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dthrckt wrote:
I would probably make a through hole in plastic housing and put aluminum heating on the outside. Seal the hole that goes through the housing. Rtv might work well for that (flexible and won't fail from the heat).


That was my first plan but unfortunately the shape of the light make this impossible 

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pinkpanda3310
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djozz wrote:
.
This is rough calculation of course, I have no idea if the common aluminium alloys have the same heat capacity as pure aluminium. But as a ballpark figure it should be correct.

The different grades does affect the thermal properties some but it is a pretty minor variable if the heat is still contained within the plastic housing.

  

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Thermal properties of common metals -

https://budgetlightforum.com/node/69706

  

dthrckt
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YuvalS wrote:

dthrckt wrote:
I would probably make a through hole in plastic housing and put aluminum heating on the outside. Seal the hole that goes through the housing. Rtv might work well for that (flexible and won’t fail from the heat).


That was my first plan but unfortunately the shape of the light make this impossible 

Quite some time ago I did a mod w/ vaguely similar goals. I think mass would be the most important variable when you’re insulated from outside air. I added pennies and solder…it worked well enough for my purposes. The light was noticeably heavier, but high mode lasted much longer. Seems like a poor trade off for a headlamp.

Yokiamy
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I do like modding, since it is fun and gives a lot of satisfaction. But wouldnt it be easier to buy an aluminium headlamp?

djozz
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Plastic has superior shock absorbing properties over aluminium and does not deform (remember those ancient plastic Nokia’s?, you could drop them endlessly on concrete without failing, I know this as a fact because the pupils in my school did just that). Maybe for caving that is an interesting property?

YuvalS
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Yokiamy wrote:
I do like modding, since it is fun and gives a lot of satisfaction. But wouldnt it be easier to buy an aluminium headlamp?

I might be easier but less fun. Smile I like this headlamp and want to update it. This is an iconic caving lamp from the 80's

pinkpanda3310 wrote:
Thermal properties of common metals
  
Thanks that is some interesting info not sure how to use it but worth knowing   

dthrckt wrote:
Quite some time ago I did a mod w/ vaguely similar goals. I think mass would be the most important variable when you're insulated from outside air. I added pennies and solder...it worked well enough for my purposes. The light was noticeably heavier, but high mode lasted much longer. Seems like a poor trade off for a headlamp.

I don't mined adding some weight, I have a strong neck Smile

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chops728
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Are the cells in a separate compartment —like behind your head —- If so go for it —- As long as your driver has some kind of Step Down (timer/Thermal) it should be fine

YuvalS
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chops728 wrote:
Are the cells in a separate compartment --like behind your head --- If so go for it --- As long as your driver has some kind of Step Down (timer/Thermal) it should be fine

Yep, it has a rear battery box and I will be program the driver with a step down timer

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chops728
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YuvalS wrote:

chops728 wrote:
Are the cells in a separate compartment —like behind your head —- If so go for it —- As long as your driver has some kind of Step Down (timer/Thermal) it should be fine

Yep, it has a rear battery box and I will be program the driver with a step down timer

Then you should be able to customize perfectly —- I have used old Aluminum PC heat sinks with good results —From old video cards and the bridge chips