White box model

06.05.2012 20:50

In a month or so, we will be deploying around 50 VESNAs to a wireless sensor network in one of Slovenian smaller cities. VESNA boards, sensors and other electronics will be mounted in white plastic boxes and fixed on top of light poles alongside public streets. It will look somewhat similar to this temporary mock-up we set up in the Jožef Stefan Institute courtyard the other day:

VESNA sensor node mounted on a light pole

While these boxes are certified to be resistant to the elements and provide adequate shielding for sensitive electronics within, one question remains open. In the summer the air temperature can reach 40°C and with the boxes exposed to direct sunlight the inside could get uncomfortably warm. The ARM microprocessor on the VESNA core module only dissipates around 100 mW at worst, however my UHF spectrum sensing receiver, which is also going to be inside the box, has to shed close to 1.5 W of heat while not heating up over 120°C. If the air inside the box is at 80°C from the sun alone, this requirement might require some extra measures to meet.

Unfortunately, there are no exact specifications on how the boxes behave in this respect. To get a rough estimate of the situation I defined the following thermal resistance model of the box and performed some experiments in an attempt to determine some limiting values on the environment my electronics will have to endure.

White box thermal resistance model

Here, Ta is the environment air temperature, Tb is the temperature of the box itself and Tc is the temperature of the air inside the box.

First experiment was to determine the contribution of the sun (Psun). I put a VESNA node running on batteries inside the box which was mounted in a sunny position on a south-facing balcony. VESNA was running a temperature logging application which recorded the ambient temperature every few minutes.

Temperature inside the box, exposed to direct sunlight

In the ten days the experiment was running we had 8 sunny days. If you look at the highest peak, the temperature inside the box was somewhere in the vicinity of 55°C, while the air temperature outside the box (not drawn on the graph) was around 25°C. So, in early May the sun already heated up this white object to 30 K above the ambient.

The second experiment was to determine the contribution of the UHF scanner's power dissipation on internal air temperature. I placed a ceramic resistor inside the box and set up a lab power supply so that 2 W were dissipated on the heater (Pc). This time I used two battery-powered VESNA nodes to log the air temperature inside and outside the box.

Temperature inside and outside the box, with 2 W heater inside.

While this experiment was performed inside and in the shade, the daylight still somewhat affected the ambient temperatures by heating up the room, so the thermometer outside the box recorded a slight increase during the day. During the day I also turned on the heater (I didn't want to leave it running over the night), which resulted in around 5 K increase in the internal air temperature.

If you look at the model I pictured above, these measurements are unfortunately still inadequate to determine both thermal resistances. From the second experiment you can calculate that Rab + Rbc is around 2.5 K/W, but to get individual resistances you would need to know the thermal input from the sun, which depends on the unknown albedo of the box.

However, at this point this detail is irrelevant. From the first experiment in know that the sun will contribute around 30 K and from the second experiment I know that internal power dissipation with add another 5 K above the ambient. If I take the worst case air temperatures outside to be 40°C, this adds up to 75°C inside the box. From this I can now calculate the maximum allowed thermal resistance between the tuner IC's silicon and the inside ambient and from that choose the appropriate cooling solution.

Posted by Tomaž | Categories: Life

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