Let's say I found a random heatsink with no name or model or anything else from which I can find a datasheet and get its thermal resistance. Therefore, simply by knowing its sizes, how can I calculate its thermal resistance? I found a formula on this page: How do I calculate the thermal resistance of aluminum flat stock is it correct?
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2\$\begingroup\$ Compare it in size and shape to a commercial one and check the rating for that one? \$\endgroup\$– winnyCommented Dec 5, 2021 at 9:33
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5\$\begingroup\$ Or you could bolt a known wattage to it (e.g. a power resistor or power transistor) and measure the temperature rise... \$\endgroup\$– Ian BlandCommented Dec 5, 2021 at 9:43
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\$\begingroup\$ you can put it in hot water, then time how long it takes to cool down. once you weigh it you can calc how long it should take to cool that amount of whatever metal it is (the specific heat/time). \$\endgroup\$– dandavisCommented Dec 6, 2021 at 5:11
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A complete thermal analysis of a complex shape like an Aluminum finned heatsink, which I assume is what you have, is quite hard to do without thermal analysis software.
But in your case I doubt that you need that level of precision since you're just using a "random" heatsink rather than buying a new one from a supplier who can provide you with the parameters you are asking about.
My recommendation is to look through some of the commercially available heatsinks out there and find one that is similar to yours in material and geometry and use those parameters for your design.
As always in what I think is a DIY one-off circuit, leave plenty of margin since your heatsink and the one you use as model may not behave exactly the same.
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\$\begingroup\$ Thanks for your suggestion. \$\endgroup\$ Commented Dec 6, 2021 at 4:56