Introduction

In this project we are making an automatic cooling system in which the fan is activated when the  temperature is increased and the fan is deactivated when the  temperature is decreased. Fan speed is proportional to temperature. This is a very simple and exciting approach towards the thermistor based technology. In this process we can operate a small dc motor directly. The extension of the project is to further add a relay and a relay driver circuit and this will support  a table fan or a ceiling fan whatever you choose. But if we use a relay fan speed will not be proportional to temperature. So, in this context our focus will be to get a proportional fan speed without using any relay. Here we will use some simple and easily available components to construct the circuit.

Components required

Vero board

Red LED

Yellow LED

Resistors 470 Ω, 1.5 k Ω, 1 Ω, 270 Ω

Thermistor 10 k Ω

DC Motor

4V Battery

Transistors BC 547, BD 140

Switch

Soldering iron

Components Description

Resistor

A resistor restricts the flow of current. The resistors used here are carbon film and quarter watt type.

LED (Light Emitting Diode)

It is a transducer which converts electrical energy into light energy.

Transistor

A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit. BC 547 is a general purpose transistor. It is an npn epitaxial silicon transistor. Its maximum collector current (DC) is 100mA.

BD 140 is a pnp epitaxial silicon transistor produced by Fairchild Semiconductor. It is suitable for medium power linear and switching applications. Its maximum collector current (DC) is 1.5 A.

Thermistor

Thermistor is a component which changes its resistance as its temperature changes. There are two types of thermistor available which are NTC i.e. negative temperature coefficient and other is PTC which is positive temperature coefficient. In this project, we have used one 10 kΩ thermistor with NTC. A 10 kΩ thermistor has a resistance of 10 kΩ at 25 degree Celsius. If there is a drop in temperature, for example to 20 degree Celsius, the resistance goes up to 12.5 kΩ. If the temperature rises to 30 degree Celsius, the resistance falls to 8 kΩ. Depending on the variable value of resistance of thermistor the circuit exhibits its behaviour accordingly.

Thermistors are nonlinear, that means the points on a graph representing the relationship between resistance and temperature will not make a straight line. Thermistors are easy to use, inexpensive, sturdy, and respond predictably to changes in temperature. Sometimes they do not work properly with excessively hot or cold temperature, they are the sensor of choice for applications that measure temperature at a desired reference point. They are ideal when very precise temperatures are required. Some of the most common applications of thermistors are in digital thermometers, in cars to measure oil and coolant temperatures, and in household appliances such as ovens and refrigerators, These are widely used in almost any application that requires heating or cooling protection circuits for safe operation.

Working

The basic working principle of this project is based on the working principle of thermistor. Here we have used NTC type. It is called NTC because its resistance increases when its temperature decreases and vice versa. Similarly in PTC its resistance increases when temperature increases and vice versa. It is used for automatic cooling system getting feedback from surrounding system. Its best application is for protection of electrical and electronics equipments from overheating. It has also a wide use in automatic air blast cooling for power transformers

Here we have designed simple circuit based on two transistors that can be used to control the speed of a 6V DC fan depending on the temperature. The left LED series with a resistance in the circuit diagram is just a power indicator. A thermistor  is used to sense the temperature. This project runs on 4 volt dc. Here we have used a 4 volt rechargeable lead acid battery.

For heating purpose we are using a heated soldering iron near the thermistor. You can use a burning matchstick or a glowing candle also. When the temperature increases, the resistance of the thermistor decreases which increases the base current of transistor BC 547 which in turn decreases the collector voltage of the same transistor. Since the collector of BC 547 is coupled to the base of transistor BD 140, the decrease in collector voltage of BC 547  forward biases  BD 140 more and so do the speed of the motor. There is an LED series with a resistance which is connected to collector of BD140. The brightness of the LED will be proportional to the speed of the motor.

Applications

• It can be used for automatic cooling system getting feedback from surrounding system.

• It is very useful for protection of electrical and electronics equipments like inverters, computers, laptops from overheating. Increases  life span of the machine without damaging it from overheating.

• It has a wide use in automatic air blast cooling for power transformers. Power transformers are very large and bulky in size. Oil content is also large in volume. Those are generally installed at outdoor. During day time they absorb extra heat from the sun due to the huge metal body and over heated and there may be also overloading phenomenon. So automatic air blast cooling is preferred.

• It can be used at home to get maximum speed of the fan during noon and for this we have to fix the thermistor with short metal piece attached to it at the roof top of the house.

• When temperature increases beyond a specific value, the green vegetables start damaging. In a very simple way and using such low cost approach, several air cooling modules can be setup for maintaining a comparatively low temperature. It has a very useful application in a vegetable storing room to control temperature in an effective way.

Precaution

Use a proper heat sink for the transistor BD140.

****************************************************************************

Watch the video in our YOU TUBE channel