A simple proportional fan controller can be built using the MIC502 from Micrel (www.micrel.com). With this IC the speed of the fan runs slowly at low temperatures, reducing noise and wear. Any fan can be controlled using the pulse width modulated output signal via a driver transistor. Using PWM control has the advantage that the fan can be run much slower than using variable DC control. Up to two NTC thermistors can be connected. The second control voltage can alternatively be derived from a DAC output from a processor system, for example.
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