Hello there everybody. I've been on this forum for quite some time but this is my first thread i think. 
Okay! I've got these two big heat sinks, 290*200*70 millimeters, with a base plate thickness of 15 millimeters. The distance between the fins are about 8 millimeters. The heat sinks are salvaged from two broken frequency converters.
I'm thinking of using them in my PASS F5 amp witch I am building, but will they work? I mean I've heard rumors that these kind of heat sinks doesn't dissipate heat at all without a fan pushing air through them, but is that true?
I can understand that they need some fans helping them cool the frequency converters when they are doing their thing at full throttle, but I'd like to think that they can handle a F5 without fans, but I'm not sure so that's why I'm creating this thread and hoping that someone's got the answer. 🙂
P.S I can post some pictures of them if it's necessary.
Okay! I've got these two big heat sinks, 290*200*70 millimeters, with a base plate thickness of 15 millimeters. The distance between the fins are about 8 millimeters. The heat sinks are salvaged from two broken frequency converters.
I'm thinking of using them in my PASS F5 amp witch I am building, but will they work? I mean I've heard rumors that these kind of heat sinks doesn't dissipate heat at all without a fan pushing air through them, but is that true?
I can understand that they need some fans helping them cool the frequency converters when they are doing their thing at full throttle, but I'd like to think that they can handle a F5 without fans, but I'm not sure so that's why I'm creating this thread and hoping that someone's got the answer. 🙂
P.S I can post some pictures of them if it's necessary.
Since the heatsink increases the thermal conductivity from heat generation (die) ans surrounding air, ALL heatsinks work. The question is how well do they do its job. Ones do better, others worse, but all do it.
Well, here I beg to differ!
Running with a to small heatsink will cause components to overhead and therefore fail within a very short timespan.
So while they might dissipate a bit of heat, they will not work for all applications.
But thats just nitpicking, isn't it ;-)?
Running with a to small heatsink will cause components to overhead and therefore fail within a very short timespan.
So while they might dissipate a bit of heat, they will not work for all applications.
But thats just nitpicking, isn't it ;-)?
You need to do a bit of maths to ascertain the surface area of the alluminium in the heatsinks.
It is the surface that gets rid of the heat.
The Thermal Conductivity Degrees C/Watt is approximately equal to 50/ Sqr Root (Area in cm^2). Remember that each fin has two sides and the base itself has surface area.
This formula relies upon the heat being transferred to all the fins equally. i.e. a massive thin alluminium sheet will not be as good as a properly designed heatsink. Your sinks will have been commercially designed which is borne out by the thickness of the base of them.
It is the surface that gets rid of the heat.
The Thermal Conductivity Degrees C/Watt is approximately equal to 50/ Sqr Root (Area in cm^2). Remember that each fin has two sides and the base itself has surface area.
This formula relies upon the heat being transferred to all the fins equally. i.e. a massive thin alluminium sheet will not be as good as a properly designed heatsink. Your sinks will have been commercially designed which is borne out by the thickness of the base of them.
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Hmm. Thanks for the answers. I don't know if the calculations will help me because the heat sinks are much bigger than the most F5 builders use so in that way they are hopefully good to go. The thing that I am hung up on is the distance between the fins, it's only 8 mm and the fins are also ribbed so the heat sinks are almost solid.
Well I think I'll go on with the build and if it later shows that the heat sinks doesn't dissipate the heat I just have to build a new chassis. 😱
Well I think I'll go on with the build and if it later shows that the heat sinks doesn't dissipate the heat I just have to build a new chassis. 😱
Doing a bit of guess work, the fins are probably along the longest length so there will be 25 of them.
The fins are therefore approximately 29.0 x 7.0 x 2 cm^2 each = 405 cm^2
There are 25 of them so 25 x 405 = 10150 cm^2
Each heatsink is therefore approx = 0.5 Degrees C/Watt
With properly mounted MOS-FETs this will be sufficient for an F5.
You MUST mount the heatsinks so that the fins run up and down from the floor to the ceiling. If you mount them horizontally you will SERIOUSLY degrade the heatrsink performance.
My only concern is the 8mm spacing of the fins, that is quite tight and may have been designed to have assisted (fan) cooling.
It should be OK, try it, you can always add fans if required - you wont need turbo spinning fans, just a gentle breeze. Running a 12V fan at say 5V is almost silent.
The fins are therefore approximately 29.0 x 7.0 x 2 cm^2 each = 405 cm^2
There are 25 of them so 25 x 405 = 10150 cm^2
Each heatsink is therefore approx = 0.5 Degrees C/Watt
With properly mounted MOS-FETs this will be sufficient for an F5.
You MUST mount the heatsinks so that the fins run up and down from the floor to the ceiling. If you mount them horizontally you will SERIOUSLY degrade the heatrsink performance.
My only concern is the 8mm spacing of the fins, that is quite tight and may have been designed to have assisted (fan) cooling.
It should be OK, try it, you can always add fans if required - you wont need turbo spinning fans, just a gentle breeze. Running a 12V fan at say 5V is almost silent.
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You are good at guessing I must say. There are 29 fins, but I was wrong, there is only 6 mm spacing between the fins and that makes everything a little bit worse.
Yes the fins will run vertically and I'm going to polish the mounting surface so the heat transfer will be at it's best. The amp is going to be of a "tower" model just because of the design of the heat sinks.
Now when I'm at it I'll post some pics of the heat sinks.
Wow them are big. Odd design though. The fin thickness is the same at the root as it is at the free end. If you got a couple hundred CFM through one of those, the ends would fall to ambient even if 500 watts were dumped into it. They look cool as hell, but thermodynamically not optimum (from a sink volume per max dissipation perspective). They may be optimized for a quite moderate forced flow or chimney. Plain convection would be OK, more reasonable if they were cut in half height-wise, since at very low flow the upper half of the sink (at 290mm) is going to be bathed in already heated air.
Anyway, they will be just fine for your project.
Anyway, they will be just fine for your project.
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Just an example
The nature of the heatsink is that it will not be very good at dissipating point loads due to the thin fins and the even thickness of the base.
You will need to help it along by spreading the heat sources across the heatsink.
If you calculate the actual area of each fin with all the fins it will be better than 0.5 Deg C/Watt. However, as the fins are very thin at their bases the heat will not travel up the fins very well.
The ideal Heatsink should have thick roots and thinner tips.
The nature of the heatsink is that it will not be very good at dissipating point loads due to the thin fins and the even thickness of the base.
You will need to help it along by spreading the heat sources across the heatsink.
If you calculate the actual area of each fin with all the fins it will be better than 0.5 Deg C/Watt. However, as the fins are very thin at their bases the heat will not travel up the fins very well.
The ideal Heatsink should have thick roots and thinner tips.
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Yes I noticed that directly when I got them that the thickness of the fins are even and I thought that doesn't matter, but now when you are saying it makes perfect sense that they are not optional.
Well I think I'll go on and give them a try. If they overheat then I know not to use sinks like these. Who lives shall see.
Well I think I'll go on and give them a try. If they overheat then I know not to use sinks like these. Who lives shall see.
Ah, they look like they where designed for forced airflow.
so mount them close together and but a fan below and be happy!
Michael
so mount them close together and but a fan below and be happy!
Michael
Yeah I think so too. Hmm, I don't think fans would be a option for me because the heat sinks will be visible from the outside of the chassis. Would be kinda odd looking. 😛
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