Thermal Characteristics of an aluminum Heat Sink

Now that pretty much all my friends have left to their respective universities, I've had a lot of time to do the very boring yet nonetheless interesting experiments I've not yet had the time to do.

One of these is calculating the emissivity of an aluminum heat sink I have on an amplifier. The heat sink dissipates the heat from two large bipolar junction transistors that are used as the second stage of a two stage class A stereo amplifier.

The amplifier is only about 20% efficient, consuming about 50 watts and sending a max of only 5 watts to each of the speakers. As a result, the transistors get quite hot.

The heat sink is a square piece of aluminum measuring 12.7 cm x 12.7 cm x .635 cm.

Over the course of 1 hour I tracked the temperature given by a digital temperature probe every 30 seconds. The probe was resting between the heat sink and the side of the amplifier. Although this disrupts the ability of the heat sink to dissipate heat ideally, it allows for relatively smooth temperature changes to take place.

As an additional test, I also took the temperature every 30 seconds for an hour as the amplifier heated up from room temperature. The results are illustrated in the graph below.




After plugging in the Stefan-Boltzmann relationship I calculated an emissivity equal to .884. I understand that black body radiators have emissivity equal to 1 so this reduced efficiency of radiation is likely due to the shape of the heat sink and the placement of it right next to the wall of the amplifier. 



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