Since the convective cooling is not sufficient, I decided to improve the airflow by installing a small fan. None of the popular PC fans from my junk box was small enough to fit on the board, I needed a smaller blower. The heat sink is a 28.5mm by 28.5mm, so either 30mm or 25mm fan should work. There is a power connector on the board for a fan, but it provides only 5 volts, so 12V fans will not work well. Having the requirements set, I started to shop for a suitable device.
The best place to look for such parts is Ebay. There are many Asian sellers who have them, but I didn't want to wait several weeks for delivery, so I decided to buy form a seller in California. 4 days later I found in the mailbox an envelope with two of such beauties:
The fans came with a 0.05" plug, so I had to cut it off and crimp a standard 2-pin 0.1" connector. The on-board power connector for the fan is located between the micro-SD socket and the host USB connector. Polarity is marked on the silk screen. Black wire should be connected to negative terminal, red to positive. Twisting the wires helps to keep them tidy.
- Board cooled down to room temperature is powered on with fan removed.
- After the temperature stabilizes, Spectrum Analyzer application is started.
- When the die temperature reaches steady state, fan is installed and powered on.
- For the final measurement the application is switched off and the board is allowed to cool down.
The plot below shows the results. The red line shows the die temperature, blue - CPU load during the experiment. Strong, thick lines show data smoothed using FIR filter. Raw measurements are shown below them, using faint colors. The oscillations visible prior to sudden changes in value are artifacts produced by the filter.
With convection cooling the idle board reaches almost 68°C. Running the application causes the temperature to rise by 4.5°C. After installation of the fan the temperature drops by over 20°C. The difference between loaded and idle system is almost negligible - only about 1 degree. This shows that despite being small and quiet, the cooling fan works extremely well.
While effective, my solution is not perfect. The most obvious problem is that the fan will interfere with expansion boards. I can see two solutions for this issue:
- Use raised headers for the expansion boards,
- Install the fan on a side and channel the airflow to the heatsink.
Updated on 2014-04-02: corrected number of ADC bits.