Automatic Temperature Controlled Fan Circuit using Thermistor

This is a simple automatic temperature controlled fan circuit using NTC thermistor. The main components of this temp control fan circuit is a negative temperature coefficient thermistor (NTC) which detects the temperature, a comparator which is used to compare the preset temperature value and a relay circuit to drive the fan. Power supply for this circuit is given by a step down transformer and a bridge rectifier. A filtering capacitor is used to get perfect DC. We will be posting a circuit on how to control fan speed as speed control of fan is not discussed in this thermistor applications circuit.

Circuit diagram of Automatic Temperature control Fan

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Components required

IC LM324
Transistor BC 548
Diode 1N4007 (4 Nos)
Resistors (1K 2Nos,4.7K)
Variable resistor(10K 2Nos)
Thermistor NTC
Capacitor (1000uF/16V)
Relay 12V
Transformer 230V/0-12,500mA
FAN 230V

Working of Automatic Temperature Controlled Fan

Step down transformer reduces 230V AC into 12V AC. It is then converted into DC by a bridge rectifier circuit. A capacitor (1000uF) is used to avoid the unwanted AC ripples.

IC LM324 has four comparators each having an inverting and non inverting terminal. Here we use the first comparator, inverting terminal of which is connected to a reference voltage made by two voltage divider resistors R1 ,R2 and a variable resistor RV2.

Non-inverting terminal is connected to thermistor and resistor RV1. The temperature at which fan becomes ON is set by the variable resistor RV1.

When there is a temperature increase, thermistor resistance will decrease as it is a negative temperature coefficient NTC thermistor. Then the voltage drop across RV1 is increased and if it becomes greater than the voltage on the inverting terminal of the comparator, then it output will go high(+Vsat)

The high comparator output will turn ON the transistor and relay will switch ON the fan as phase of AC supply is connected to the NO (Normally Open) connection of the relay.

When transistor is ON, the resistors R1 and RV2 become parallel decreasing the voltage drop(reference voltage). Hence fan will not be OFF though fan ON time temperature is reached. This difference of temperature can be varied by using the variable resistor RV2.