Simple Temperature Controlled DC Fan Circuit is like smart fan, which uses special sensor to feel heat around; so when place, it get too hot then circuit makes fan run fast like turning AC high.
Therefore, if the place cools again then fan slow down or stop to save power, hence, this circuit uses parts like thermistor which change with temperature and control part to do all this by itself.
So the electronics stay cool and works good.
Circuit Working:
Parts List:
| Components | Values | Quantity |
|---|---|---|
| Resistors | 4.7k 1/4 watt | 1 |
| 47Ω 1/4 watt | 1 | |
| Preset 500k | 1 | |
| NTC Thermistor 4.7k | 1 | |
| Semiconductors | IC 741 | 1 |
| Transistor BD140 | 1 | |
| Diode 1N4007 | 1 | |
| Fan 12V | 1 |
To begin with, this circuit cools electronic devices that become hot, and it turns ON the DC fan when the nearby temperature rises above the set level.
When temperature go back normal fan turns OFF by itself and this fan used is small 12V DC brushless fan like in computer.
Also, the circuit uses thermistor which is a resistor that change its resistance with heat.
There are two types of thermistors: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient).
NTC type the resistance go down when temperature goes up and PTC type the resistance go up when temperature go up.
Furthermore, thermistors come in many values like 100 ohms to 10k and more and in this circuit it is 4.7k ohm NTC thermistor.
Now, IC 741 work like voltage checker comparator to control the fan.
Pin 2 minus input get voltage from VR adjustable one and pin 3 plus input get voltage from R1 and thermistor as they work like divider.
So, the voltage at pin 3 changes according to the thermistor resistance and if the temperature stays at the level set by VR, the voltage at pin 3 becomes higher than the voltage at pin 2 and the IC output goes high.
When output high transistor T1 stay OFF because base get positive then fan wont work; but if temperature go higher then thermistor resistance goes down and this make voltage at pin 3 go low.
Now IC output goes low when turn ON transistor T1 then fan start to spin and make air flow and when temperature come back to normal then fan stops again.
Lastly, diode 1N4007 is important as it removes back EMF when T1 turn OFF so no damage happen.
Formulas:
Simple Rules and Formulas for Making Temperature Controlled DC Fan Circuit Using IC 741 Comparator
Voltage Divider Output goes to Pin 3 of IC 741 Non-Inverting Input formula:
Vin- = VCC * Rbias / (RNTC + Rbias)
where,
- VCC is power supply for 12V
- RNTC is resistance of thermistor that change with heat
- Rbias is resistor in series with thermistor
This formula gives voltage from thermistor side and IC 741 Output Logic Comparator Behavior:
If Vin- > Vref → Output of IC (Vout) is Low near 0V
If Vin- < Vref → Output of IC (Vout) is High near VCC like 12V
This output go to base of transistor.
Transistor Base Resistor Formula:
Rbase = (VCC – Vout) / Ibase
where,
- Ibase the current needed for the transistor to work, which depends on the transistor gain and fan current.
This resistor limit current going into transistor base.
Fan Current (Transistor Collector Current) formula:
Icollector = (VCC – VCE(sat)) / Rfan
where,
- VCE(sat) is small voltage drop across transistor when it is ON
- Rfan is resistance of the fan one can measure with multimeter
Note:
This circuit uses:
- IC 741 as comparator
- NTC Thermistor for sensing heat
- Transistor for switching fan ON/OFF
This is easy way to control fan with temperature and one can change resistor, thermistor or VR values to fit their own project or temperature limit.
Also, let me know if anyone want help with choosing actual parts or drawing a simple circuit diagram in this same style.
How to Build:
To build a Simple Temperature Controlled DC Fan Circuit below are the steps:
- First, connect to a voltage divider made with NTC thermistor and resistor R1.
- Then one side of R1 will connect to +12V positive supply and one side of thermistor will connect to GND
- Join the middle point between R1 and thermistor of pin 3
- After that, connect to middle pin wiper of variable resistor V, one side of VR connects to +12V and other side will connect to GND
- Now connect to base of PNP transistor BD140 using a resistor R2 and emitter connect to GND and collector connect to negative (-) terminal of DC fan
- Positive (+) terminal of DC fan connect to +12V supply
- Also, cathode strip side connect to +12V and anode to emitter of transistor and fans negative side,
Notes:
- Double check all wires and parts before turning power ON and then turn the VR to set temperature level where fan should start.
- When temperature go above that level then fan will turn ON automatically and when temperature goes back down then fan turns OFF
Conclusion:
Overall, Simple Temperature Controlled DC Fan Circuit is very useful; as it turn fan ON or OFF by itself, depending on how hot the place is.
Moreover, this help keep electronic devices like computers and other things cool so they do not get too hot and stop working and it also ensures everything run smooth and safe.