Sizing Heatsinks for a Given Application - Page 1
Forum section: electronics / beginners

author: BobbyK
author_rank: Regular Contributor
timestamp: March 16, 2013, 01:55:57 am
content: Hi,If you buy a heatsink from a reputable source, you will get data on the thermal resistance/conductivity, which greatly helps when trying to pick the right sink for a given semiconductor junction temperature and max heatsink temperature. These heatsinks also cost a lot sometimes, a small TO220 5W heatsink can cost up to 5 bucks, and large aluminium extrusion are atrociously expensive when purchased from electronics suppliers. On the other hand, broken equipment or surplus sales, or even eBay can deliver the goods at a tiny fraction of that cost. The only problem is, how can we calculate, or guesstimate the thermal resistance/conductivity of a given aluminium (or sometimes copper) heatsink, based on it's weight, dimensions and geometry? Are there any simple rules of thumb based on weight and/or surface area? material? etc...?Would love to hear if any of you guys know how to make that estimate short of FEM modeling of the thing

author: IanB
author_rank: Super Contributor
timestamp: March 16, 2013, 02:41:38 am
content: One way would be to compare with other heatsinks of known properties. Two heatsinks of similar material, surface area and fin pattern are going to have similar heat dissipation abilities.Another way would be to test it and see. Run the heat sink with the desired duty and see how hot it gets. If it gets too hot it is likely too small.

author: BobbyK
author_rank: Regular Contributor
timestamp: March 18, 2013, 11:07:53 pm
content: Hmmm... tried the comparison route. I also checked out a whole bunch of "calculators" and they invariably give higher resistances than when I put in the data fromhttp://www.aavid.com/product-group/extrusions/search. So I basically took all the dimension data from Aavid, and plugged it into different calculators. From Aavid I invariably get a lower thermal resistance than from generic simple calculators. Do you know how heatsink manufacturers test their sinks, I mean do they do it in a fridge, or at extremely temperature gradients,...or maybe dip it in water or something?I guess what I will be doing is taking the manufacturer data for a similar heatsink as the most optimistic estimate, and taking the worst calculated value from the many online calculators as the pessimistic design, and see how close I can afford (literally in dollars) to get close to the pessimistic estimate

author: IanB
author_rank: Super Contributor
timestamp: March 19, 2013, 02:27:54 am
content: I think you may find that online calculators give you a conservative, or "safe", estimate, whereas manufacturers are trying to sell their products so they are more likely to give you an "optimistic" estimate.Remember also that no thermal performance measure of a heatsink is exact. It depends tremendously on the air flow and the surroundings of the installation. (This is just the same as with electrical power cables, where the allowable current rating depends on whether the cable is freely suspended, in a wall, in a conduit, etc. There is no single number.)When you size a heatsink you should allow a safety margin. The cooler your device runs the longer it will last and the better it will perform. Conservative sizing just gives you a better design.How much difference are you seeing between the "calculated" values and manufacturer data? If the numbers are within, say, 30% they are effectively the same.

author: BobbyK
author_rank: Regular Contributor
timestamp: March 30, 2013, 02:08:35 am
content: differences varied from 50% to 100%. I ended up picking up a heasink that was looked like it was rated approximately to my dissipation, but slapped on two fans. Doesn't go over 60 degs celcius, and I am happyI guess a little bit of experimenting is hard to avoid when dealing with thermal design, at least until you are good enough to have a feel for these things :|