The schematic of an automatic cooler fan for audio amplifiers is given here. The circuit automatically switch ON the cooler fan whenever the temperature of the heat sink exceeds a preset level. This circuit will save a lot of energy because the cooler fan will be OFF when the amplifier is running on low volume. At low volume less heat will be dissipated and it will 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 which is wired as a comparator. The non-inverting input (pin2) is given with a reference voltage using the preset R3. As temperature increases the resistance of NTC thermistor will drop and so do the voltage across it. When the voltage at the inverting input becomes less than that of the reference voltage (set for a particular threshold temperature) the output of the comparator goes high and switches the transistor Q1 ON. This will activate the relay and the cooler fan will be switched ON. When the temperature decreases the reverse happens. LED D2 will glow when the fan is ON. Diode D1 is a freewheeling diode.

Circuit diagram.



  • The circuit can be assembled on a Vero board.
  • Use 12V DC for powering the circuit.
  • The circuit can be calibrated by adjusting the preset R3.
  • K1 can be a 12V, 200 ohm, SPST relay.
  • LM311 must be mounted on a holder.


    • seetharaman

      to increase the drive current of the IC such that the relay picks up

  1. akrishna602

    Please help me with connection of Relay,.

    i tried a lot but either fan doesnt operates at all, or simply keeps operating w/o any effect of temperature,..

    Also, does range of NTC be specific?? of any NTC will work??

    • This schematic is nonfunctional. Q1 can’t be turned ON because there is not positive bias.
      Pin 7 is the open collector of the output transistor(in LM311). Its collector should be tied to +V(12V) via a resistor (2.2kΩ) in order to bias Q1 into saturation. Base of Q1 should be tied to pin 7(no resistor needed!!).

      And employ some positive feedback(hysteresis), otherwise the relay will chatter around trip point. Tie a resistor(100k…470kΩ) from collector of Q1 to pin 3(inverting input!!!).

  2. what is the use of NTC thermistor? and how can we connect relay which has 5 terminals? plz give me the answer?

  3. Your automatic-cooler-fan-for-amplifiers circuit will not work as it is. The LM311 sinks the output when turned on. So change the value of R4 to 2.7k and add a 10k from the base of the transistor to +12V. Then it works good.

  4. what is K1 and what will it do?
    how many ohm the ntc thermistor?

  5. how to connect relay?? there are 5 pins? plz help.

    • Der r 2 terminals of coil conect it as shown in fig. & 1 is common wer it has to conected in series with NO/NC(normaly open/normaly close). Here initialy relay has NO. leave NC no connected. 🙂

  6. does the NTC thermistor connected to amplifier heatsink or something for sensing the heat, or just put it in on a pcb board?

  7. Seetharaman

    This is a closed loop as the temperature is the feedback information for the air flow.

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  9. Seetharaman

    Hi Gabriel you are correct negative should be connected common earth.(Common line of IC1 Pin1, Q1 emitter, 12volt fan motor negative etc)

  10. gabriel gomez

    Sincerely, I’m a fan of electronics and your site. I’d like clarification on the this circuit about the negative supply voltage feed that is connected to earth (GDR). Thank you in advance
    Waiting for your prompt response

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    • Nhelie Casing

      Hi. What value of Thermistor if you have no NTC thermistor? Thanks!:)

      • seetharaman

        Hi It can be any available NTC from 1k to 4.7K

          • Nhelie Casing

            Another thing, how can you use spdt relay instead of spst? Thanks!