September 25, 1997

Fan controller adapts to system temperature

Kerry Lacanette, National Semiconductor, Tucson, AZ

Any piece of electronic equipment--whether it be a lab instrument, an audio amplifier, or a PC--that relies on a cooling fan to control internal temperature is a potential source of noise pollution or even hearing damage. Most of the time, the fan's noise level is probably much higher than necessary, because it has enough capacity to move enough air to handle worst-case conditions. In many systems, the fan can either be off or running slowly most of the time with no damage to the system. Keeping the fan off or running slowly benefits you by reducing ambient noise and possible fatigue, irritated nerves, or even hearing damage. The circuit in Figure 1 adjusts a fan's speed according to temperature.

IC₁ is a temperature sensor with an internal voltage reference and two comparators. Three external resistors set input thresholds for comparison with the sensor's output voltage. When IC₁'s temperature rises above a comparator's input-threshold level, that comparator's open-collector output goes low. At low temperatures, output 1 is off, so the common-base npn transistor is also off. The gate of the p-channel MOSFET is high, keeping the cooling fan off. As temperature increases to above the first setpoint (60ºC in this example), output 1 pulls down on Q₁'s emitter, turning on Q₂ and supplying power to the fan. The fan, which has a speed-control input, rotates at low speed (1900 rpm in this example) when the speed-control input sees 10-kiloohm impedance to ground.

If temperature continues to increase to beyond the second setpoint (80ºC with the values shown), IC₁'s comparator output 2 pulls the speed-control pin low, which causes the fan's speed to increase to its 3800-rpm maximum value. Set the temperature thresholds based on the sensor's output voltage of V_TEMP=395 mV+6.2 mV/ºC and the 1.25V reference. R₁+R₂+R₃ should be approximately 27 kiloohms. The circuit in Figure 1 uses ±1% resistors; you can substitute ±5% resistors if you can tolerate setpoint errors on the order of 5ºC. Figure 2 shows fan speed vs temperature. Note the 5ºC hysteresis around each setpoint. This hysteresis is inherent in the LM56 to prevent the fan from "hunting" between two speeds when the temperature is near one of the setpoints. (DI #2086)

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