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Aluminium for a sink

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ebuka_2

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Hi guys,
I have a project at hand now, it uses 2N3055-a power transistor. As we all know it needs a heat sink. But as it is now, I can't find a heat sink. So, my question is: If I use an aluminium sheet as a heat sink, will it work fine?
 
I don't see why not. Heatsinks are little more then sheets of aluminum in the first place. Just make sure if its just a sheet with no ribs that its big enought to dissapte the heat.
 
That depends on the size of the piece of aluminium and the power dissipated by the transistor.
 
Hero's right, how thick is the aluminum you have? More physical mass will allow better heat transfer to the heatsink itself but without a large surface area and airflow the actual power dissapation from the heatsink won't be very high. Careful consideration in placements of fans and determining the airflow in your case can usually solve that problem though.
How about some pics of your case and circuit board when installed showing the fans and airflow (if any)
 
In at least one of my voltage regulator handbooks (National, Fairchild or someone like that) I found an excellent couple of chapters on heat sink design. One chapter provided the methods assuming you had factory data on a heat sink. The other provided methods for estimating the performance of home-made heat sinks such as you describe. Search semiconductor mfr's website for applications papers.

Doing those calcs is a lot of work. You might examine projects that use 2N3055s and have a look at the heat sinks as a way to begin a cut and try method.
 
Generally you can pretty well guess what size heatsink might be needed, from what the circuit is doing. Then you use one of that guessed size, and see how hot it gets - changing it as required.
 
I would also get some heat transfer grease, that silicon stuff to put between the transistor and the heatsink. Its expensive, but worth it. If it still gets really really hot and blows the transistor (unlikely but possible), put a small computer fan on the back (with it blowing air away from the transistor) to pull more heat off it. I did that for my H-Bridges.
 
If possible you should always use a thin layer of thermal grease, even the tightest direct physical connection will still have some air stuck inbetween, best case scenario is it helps insulate heat, worst case is the air pocket super heats and starts melting the packaging.
 
Thanks

Thank you, now I am convinced of what I am about to do. But, is there any substitute for the thermal greese?
 
One alternative to the thermal compound is no compound. No intent to be funny here.

The thermal compound improves the heat transfer from device to sink and I'd expect that it makes the rate of transfer more uniform and predictable.

The power handling capabilities of the device and heat sink size are related to that transfer rate. Reducing the power handled (add transistors in parallel) or increasing the heat sink size might be thought of as alternatives to the compound.

The do make a stick of material that melts at specific temperatures. Tempilstik is one brand. You might simply try something and test the device case temperature - possibly at 167 deg F, 212 deg F. You might find candle waxes or crayons where you might estimate the melting temp as alternatives to the Tempilstik.

I do not know if other greases or fluids would make the problem worse than no grease. I am thinking that some of these might act as insulators.
 
ebuka_2 said:
Thank you, now I am convinced of what I am about to do. But, is there any substitute for the thermal greese?

Silicon grease is often a major ingredient, and if you can source silicon grease easier?, then it's a slightly inferior alternative.
 
Silicone rubber sheets? That stuff is a good insulator usually not a very good conductor of heat. I have oven mits made out of the stuff.
 
Sceadwian said:
Silicone rubber sheets? That stuff is a good insulator usually not a very good conductor of heat. I have oven mits made out of the stuff.

You're thinking of something else, pretty well all modern semiconductor insulating kits are now silicone rubber, as it saves needing grease.
 
Didn't know that. Then again polymers are very flexible, only slightly differening the mix can give it completly different mechanical/thermal properties. Apparently some compounds even use gallium (liquid at even slightly elevated temperatures)
 
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