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LED Cooling

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tom_pay

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Hi

I have decided to make a LED torch. I am going to run it at 450mA, (max. is 500mA) off a 555 timer (30KHz, 50% Duty Cycle).

The only problem is cooling it. I have attached a small heat sink. A big one is not possible, the housing is too small and I cant have a bigger one.

Is there an electronic way of ensuring the LED stays cool without loosing too much light intensity?


Thanks Heaps

Tom
 
Hi Tom.
LEDs generally do not get hot. The only problem could be if you drive them above their max current, and in that case they will just burn out internally. As long as you keep it below 500mA they should stay pretty cool.
Good luck!
Der Strom
 
Hi

This is one of those high intensity star LED's.

After about 5 min on the 555 circuit the heat sink on the LED gets so hot I can barely touch it.

Tom
 
Oh, I see. How large is the heat sink you are using? Do you have any photos of your setup?
Der Strom
 
Hi

I have attached a heat sink, it is the biggest that I can have. And it is still running too hot!!!

Here is a picture of my LED and heat sink.


Tom
 

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  • LED.jpg
    LED.jpg
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You mention you are making an LED torch - presumably the body of the torch is made from aluminium. If you connect the LED's heatsink to the body of the torch, that will help with the dissipation, as the whole of the torch will heat up.
 
Hi

I had thought of that. There is only one problem. The body is made of bright yellow plastic. :-(

Oh well onto the next idea

Thanks

Tom
 
Last edited:
Hi

I got some thermal paste put it on the LED back and then used roof sealant to mechanically attach it.

I used roof sealant because the tube said it was good for 175 degrees C. Well above the maximum junction temperature of the LED.

Tom
 
Thats no good ,friend.

You dont want the LED to get near those temperatures. Even though the roof sealant will survive, it has no thermal conductivity. The heat HAS to get from the LED to the heatsink.

The roof sealant is acting as a blanket, holding the heat in.

No good.

When you find a larger heat sink, drill 2 holes and use screws to hold it down. Self-tapping screws work great for this, and can be found everywhere.

Your heat sink is MUCH TOO small.

If that were a solid chunk of aluminum, it would work better, OR add a fan.
 
The "star" that the LED is mounted to, is shaped like that so you can use screws or rivets to attach them to the heat sink.

You only want a "SUPER THIN" layer of thermal paste. This makes up for flaws and slight scratches in the metal. A PERFECT heat-sink join would have no grease, and a PERFECTLY flat and polished surface. The bottom of the LED would have a PERFECTLY flat and polished surface too, this way there is no air (even microscopically) between the two.

Since this is pretty much impossible to achieve, we use thermal grease. It fills the microscopic air holes/scratches and transfers the heat into the heat sink.

SO, once the thermal grease is there, you want to bond the star to the heat-sink as firmly as possible. You want it to feel like they are ONE PIECE.

You have far too large of a gap between the star and the heat sink.
 
Have you thought of attaching some sort of fan to blow cool air across the heat sink? This, I imagine, would cool it down significantly.
Der Strom
 
In a plastic torch, with limitted space, and no external fan drawing in cold air from outside and venting hot air, your only option is to reduce the power going to the LED.

Or use a higher power LED, which are usually more efficient (eg run a 1A LED at 500mA).

If you want to use that LED at 500mA, you will really need a metal torch, or a much bigger heatsink.

Andrew
 
Ok,

I think that I will get a disc of aluminum about 15mm thick and a bigger diameter as possible.

Another problem is that I cannot use screws. I have got a small reflector / lens and it clips onto the LED leaving no room for any screws. Got any other ideas?

Last thing. Is it possible to use steel instead of aluminum?

Thanks

Tom
 
Do not use steel, Use Copper or Aluminium.

Convectional heat transfer is better than radiated heat transfer. Therefore use a fan. If u are not using a fan, then anodise the aluminum black to give an extra 5% thermal performance.

Worst case cooling: Seal the LED fixture/heatsink with silicone oil filled in. That will provide thermal inertia/ convectional cooling and transfer some heat to the case.
 
Hi

I have just had a look at the lens if I cut the base out of it I might be able to use small nuts and bolts. Then I would have to glue on the lens, it would not clip on anymore.

Would paint do instead of anodizing the chunk? Or will the paint act as an insulator?

Tom
 
Paint is not good for these low temps. It will insulate before radiate.

see this for DIY anodising:

**broken link removed**
 
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