This is a simple automatic temperature-controlled fan circuit using an NTC thermistor. The main components of this temp control fan circuit are 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. The power supply for this circuit is given by a step-down transformer and a bridge rectifier. A filtering capacitor is used to get the perfect DC. We will be posting a circuit on how to control the fan speed as speed control of the fan is not discussed in this thermistor applications circuit.
Circuit Diagram of Automatic Temperature Control Fan
- 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
- The 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 unwanted AC ripples.
- IC LM324 has four comparators each having an inverting and noninverting terminal. Here we use the first comparator, inverting terminal which is connected to a reference voltage made by two voltage divider resistors R1, R2, and a variable resistor RV2.
- The non-inverting terminal is connected to the thermistor and resistor RV1. The temperature at which the 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 the relay will switch ON the fan as a phase of the AC supply is connected to the NO (Normally Open) connection of the relay.
- When the transistor is ON, the resistors R1 and RV2 become parallel decreasing the voltage drop(reference voltage). Hence fan will not be OFF through the fan ON time the temperature is reached. This difference in temperature can be varied by using the variable resistor RV2.
Components Pin Out
No one likes to left the bed at night when it is too cold. This easy-to-make circuit will help you to automatically control your fan speed as well turning it On and Off depending on the surrounding temperature.