Heatsink size

I am persuaded of the merits of mounting SSRs on the back of a heatsink that sticks out from the top of the control panel, as in Kal's build. Some of the heatsinks I have seen look excessively large though.

My panel will be a BrewTroller type thing with a 12V power supply inside, which lends itself well to forced convection by means of a little PC fan or two.

Presumably I could get away with a considerably smaller and cheaper heatsink this way. For example, the 'medium' heatsink from ebrewsupply (12" long, $38) corresponds approximately to 300mm of this heatsink from ABL:

http://www.abl-heatsinks.co.uk/index.php?page=extrudedproduct&product=184

Take a look at the graph on this website. Under natural convection, its thermal resistance is 0.5°C/W. Under forced air (i.e. with an appropriate fan), you could get the same cooling with a heatsink just 75mm (3") long.

Has anyone done any calculations of this sort?

I haven't done any calculations, but I'll mention that Heatsink USA (http://www.heatsinkusa.com/) seemed to have the best prices when I built my setup. It's not anodized but it seemed to be at least half the price of most other options out there.

alien said:

Has anyone done any calculations of this sort?

Click to expand...

Hundreds (I had 3 summer jobs working at a factory that built electroplating rectifiers when I was in college). The general idea is simple. There is a 'thermal impedance' between the die in the semiconductor (diode, SCR, triac, SSR....) and the ambient which is the sum of the thermal impedances of the individual components (heat sink, heat sink compound, die to device case etc). The temperature rise is the product of the heat being dissipated times the sum of the thermal impedances. Thus, if for example, the thermal impedance is 0.5°/W and the device is dissipating 10 W then the rise is 5 °. If putting a fan on/near the heatsink decreases the thermal impedance to 0.1°/W then the rise is 1 °.

You will have to go to device manufacturers' specifications to get thermal impedance numbers. You can calculate the power dissipated by multiplying the rms voltage drop across the device at load by the rms load current. For example, if a device is carrying 50 amps and the voltage drop across it is 1 volt then the power dissipated is 50 W.

I was having a bit of a hard time finding specs on heatsinks, especially from the surplus places where the stuff is cheap.

I ended up getting 3 big CPU heatsinks for $5 each. They are designed to have fans mounted on the top so there is that option, if it seems like they are getting too hot.