For maximum reliability, cooling-fan speed should be based on current rather than temperature. Commonly available thermal-sensing fan-control ICs require elevated temperatures to start fans, and even higher temperatures to turn them on harder. In contrast, sensing current supplies immediate information of actual system thermal load. Consequently, current sensing enables fan control. This keeps systems as cool as possible—rather than hovering at whatever elevated temperature is required by thermally based control methods.
In the figure, the circuit uses a current-output current-shunt monitor IC to control a pulse-width modulator connected to a fan. The shunt resistor could be in series with the CPU in a computer, output stage of an audio amplifier, or power supply. The current-shunt monitor (IC1) has a maximum output of 20 µA when 100 mV is developed across the shunt at its input. IC2 is a modified comparator oscillator where IC1's output current is implemented to offset the comparator threshold, with increasing current resulting in an increased positive duty cycle.
C1 and R1 set the nominal frequency. In this case, the selected frequency is 10 Hz at a 50% duty cycle and varies from 7 Hz at a 10% duty cycle to 15 Hz at a 90% duty cycle.
R5 and Q1 do more than just drive the fan. They also clamp the output level of IC2 to keep input excursions (which result from the combined effects of feedback via R3 and R4, and the current from IC1) within the common-mode limits of IC2.
The circuit is designed for a 50% duty cycle at the half-scale output of IC1. At the full 20-µA output of IC1, it's designed for 100% duty cycle.
Acknowledgments: Thanks to Cesar Quijada for his help in breadboarding and testing this circuit.