No frills here—just inexpensive functionality. This circuit activates a cooling fan when the temperature of a target high-power-dissipation device, such as a processor chip, exceeds a predetermined limit.
The key element in the design is the Epcos PTC Temperature Limit Sensor (see the datasheet at www.epcos.com/inf/55/db/ptc_03/01900191.pdf), which cost about $0.50. Sensors are available with limits of 75°C, 85°C, 95°C, 105°C, 115°C, 125°C, and 135°C. At room temperatures, the sensor has a resistance of about 470 O. The resistance increases to about 100 kO at temperatures above the specified limit (Fig. 1).
The sensor comes in an SMT 0603 package that you thermally couple to the targeted power device. One effective arrangement is to locate the power device on the top of the board and locate the sensor on the bottom side directly under the power device.
The sensor controls a 1-W fan driver made from a 555 timer IC (Fig. 2). Tying both inputs (pins 2 and 6) together turns the timer IC into a Schmitt trigger. Tying both outputs (pins 3 and 7) together doubles the IC’s sink output capacity so that it can easily drive the 1-W fan load. One recommended fan candidate is the popular Sunon 40-mm tubeaxial fan, which draws 82 mA from a 12-V source. The 555 also runs well on 5 V, though, so the circuit will work equally well with a 5-V fan.
If you need more than 1 W, you can add a transistor driver to the output (Fig. 3). Because the transistor driver inverts the output signal logic, you will need to invert the input signal by reversing the positions of the sensor and its companion resistor.