Circuit Description

Automatic cooler fan for amplifiers is a circuit introduced for the purpose of saving power in amplifier circuits. This circuit switches ON and switches OFF the cooler fan of heat sink according to its temperature. The cooler fan will turn ON when heat sink temperature exceeds a certain value and will turn OFF temperature become lower than a certain value. Automatic cooler circuit does it always so that effective working of cooling fan occurs and that saves power.

In this circuit thermistors are used as temperature sensors. Here negative temperature coefficient thermistor (NTC) is used. NTC means as temperature increases the resistance of NTC thermistor decreases. Resistor R2 in circuit diagram is an NTC thermistor. Here LM311 IC is used as a comparator which is having an inverting (pin 3) and non-inverting (pin 2) input pins. The junction of R1 and R2 is connected to pin 3. At pin 2 a reference voltage is given using resistor R3. The comparator compares voltage at its inverting and non-inverting terminals. When voltage at inverting terminal becomes less than that at non inverting terminal, the output of comparator becomes high. This will turn ON transistor Q1 which triggers relay and cooler fan will turn ON. Fan will be turned off when voltage at inverting terminal becomes higher than non-inverting terminal.

12 V DC supply is used as power supply in this circuit. The relay K1 can be a 12V, 200 ohm, SPST relay. Calibration of this circuit can be done by adjusting R3.

Circuit Diagram

Automatic Cooler Fan Circuit for Amplifiers

Automatic Cooler Fan Circuit for Amplifiers


  • Use Vero board to assemble components.
  • Diode D1 is a freewheeling diode.
  • LED D2 acts as an indicator, it will glow when fan is ON.