The schematic of an Automatic Amplifiers Cooler Fan is given here. The circuit automatically turn on the cooler fan whenever the temperature of the heat sink exceeds a preset level. This circuit can save plenty of energy becausef the cooler fan are going to be OFF when the amplifier is running on low volume. At low volume less heat are going to be dissipated and it'll not trigger the cooler fan ON.
The temperature is sensed using an NTC (negative temperature coefficient) thermistor R2. Junction of thermistor r2 and resistor R1 is connected to the inverting input (pin3) of IC1 that is wired as a comparator. The non-inverting input (pin2) is given with a reference voltage using the preset R3. As temperature will increase the resistance of NTC thermistor can drop and so do the voltage across it. When the voltage at the inverting input becomes but that of the reference voltage (set for a selected threshold temperature) the output of the comparator goes high and switches the transistor Q1 ON. this can activate the relay and the cooler fan are going to be switched ON. When the temperature decreases the reverse happens. LED D2 can glow when the fan is ON. Diode D1 could be a freewheeling diode.
