Heatsink

[fadY]

Hello,
Do you know which heatsink would best fit the LM3875 (TA11B package) of this 80W amplifier?

 

[bogdanfirst]

well, basicly you can calculate like this: if you know the power disipated on the IC, lets say P=20W. then if you have a room temperature of about 20 dergees Celsius, and you allow the IC to heat up to 80 dergrees that means that you have 60 dergrees difference. that means that the temperature coeficient(i am not sure it is called like this) of the heatsink should be mminimum 60/20=3dereees/W
usually whae you buy a heatsink you can ask abuot that.
you might find sheets of aluminium with different thickness wich you can use to build your heatsinks an they say the heat conductance of it and you can calculate the surface of the heatsink.
now, back to your problem.....i dont think that you will have more than 20-30 W disipated on the heatsink so.....

 

[fadY]

I calculated it as you told me with 30W and 60°C of temperature difference, so I've found 2°C/W and I have choosen the RAWA 208- 14 which dissipates 2.2°C/W.

Do you think it's ok?

 

[kinjalgp]

Yes it will work. Bigger heatsink is always better.

 

[Chippie]

.......and you can always add fan cooling...


Thermal resistance is the term you were looking for 8)

 

[fadY]

Thank you all for your advices.

I hope I'm not gonna eat grilled components tonight :wink:

 

[bogdanfirst]

i dont think you can eat them, they smell awfull.... plus it has termal protection so you will not burn it.

 

[bogdanfirst]

i checked the datasheet and it says that the maximum disipation is 125W!!!!!!!!!!!!
here is the note for that: 'Note 3: For operating at case temperatures above 25°C, the device must be derated based on a 150°C maximum junction temperature and a thermal resistance of
qJC = 1.0°C/ W (junction to case). Refer to the Thermal Resistance figure in the Application Information section under Thermal Considerations.'
i dont really understand what it means(the language problems) but i think you shoud check the datasheet.

 

[fadY]

i checked the datasheet and it says that the maximum disipation is 125W!!!!!!!!!!!!

Where did you read that because on the datasheet of the LM3875 it is said that you have to apply this formula to calculate the maximum disipation: PDMAX = (VCC^2)/(2*(pi^2)*RL). I calculated it for VCC=50V and RL=4Ohm and I found PDMAX=30W...

 

[bogdanfirst]

yes, i misunderstood, it says that in any condition the disipation should not be more than 125W because it will destroy it....sorry for the confusion

 

[fadY]

I prefer that... Heat sinks are not so cheap!

 

[bogdanfirst]

well, at least you find them ...here(in my small town) they are almost impossible to find i get them from tv shops wich i buy along with a burned module, and i get some components too...
hm....hows your project working?
good quality?

 

[fadY]

Well, I tested the amplifier today and it works quite good I plugged my modulator as input and plugged the output on an inductor in series with a 2Ohm resistor and my ultrasonic emiter (see the topic :arrow: "Building a sound beam device").

I had good results . I had sufficient amount of audible sound which was amazingly directionnal. When you're in the beam you hear it but people around you don't hear anything. Another surprising effect is that you have more audible sound when you are something like 6m from the speaker than when you're closer to it.

I am now going to quantify these results by doing sound tests with a microphone.

I would like to thank all the people who helped me! :mrgreen:

 

[bogdanfirst]

how about shareing your directional amp, by adding some schematics of it and so on....
but how small is the "tube" of the sound...if you understand what i mean, wow much can you focus it?

 

[Mosfet]

Cheap Heatsinks

May I suggest one go to their local metal fabrication workshop. Ask nicely if you can look in their scrap bin. I use thick wall 2 inch square aluminum tubing. Often there are many small scraps less than a foot long that make terrific heatsinks. If you don't get them for free I will be surprised. Bigger heatsinks can be made by bolting pieces together with appropriate compound. If you have a heatsink that fits an IC use the scrap as a 'booster' to increase the cooling. "L" brackets are sometimes better for that.

I don't like fans on my audio equipment.

 

[fadY]

Quite clever :idea: 8)

Actually you're right. They're nothing more than a piece of aluminium...

 

[bogdanfirst]

thats correct Mosfet....
they are no more then a piece of a aluminium...i hot a sheet of Al about 30x40cm wich i cut from and make my heatsinks...but there is a bit of a problem....it is durealuinium, and its damn hard to cut it........

 

[kinjalgp]

Well, heatsinks are pieces of aluminium but specially deisgned so that maximum surface area comes in contact with air and thus heat transfer takes rapidly as compared to plain aluminium plate.

 

[bogdanfirst]

yep, they come in differend shapes and sizes....
some have copper too...i dont know why they dont use copper that much from what i know its therma conductance is as good as Al....

 

[kinjalgp]

Thats because copper corrodes very fast when comes in contact with air. If you use any protective coating on copper then its thermal conductivity will reduce.