## Signal Conditioning for Thermistors

In the last article we have seen some signal conditioning circuits for RTD. Today we are going to learn signal conditioning for thermistors. We know that thermistor is temperature measuring sensor made up of semiconducting material. The resistance of thermistor normally decreases as the temperature increases hence it is has negative temperature coefficient (NTC).

Change in resistance of thermistor due to change in temperature is given by the following equation,

Where

RT is the temperature at T(K),

R0 is the resistance at T0 normally at 298K,

? is the characteristic temperature constant of thermistor,

The characteristics of thermistor resistance vs. temperature is non linear therefore linearization circuit is also included along with amplifier in the signal conditioning circuits for thermistor.

### Signal Conditioning for Thermistors

Signal conditioning of thermistor includes bridge amplifier and linearization circuit. These circuits are explained as follows.

#### Bridge amplifier

Following circuit shows a bridge amplifier used for amplifying the output of thermistor. As the output range of thermistor is quite low and it not good to use such short range of output for getting good accuracy for operating any field devices.

Bridge amplifier consist of wheatstone bridge in which inverting amplifier with thermistor as feedback resistor is used in one of the arm as shown in the diagram. This operational amplifier produces output voltage proportional to the change in the resistance of the thermistor.

#### Linearization of thermistor

For linearization of thermistor characteristics there are several methods available.

• ##### Using parallel resistor:

In this method a parallel resistor is connected with thermistor. This method increases linearity but also decreases the sensitivity of the circuit.

the value of the equivalent reistance is given by,

where Rp is value of parallel resistor,

Rtm is thermistor temperature at mid scale temperature,

Tm is mid scale of temperature variation,

? is characteristic temperature constant.

• ##### Using serial resistor:

In this method a series resitance is used with thermistor. It reduces nonlinearity of conductance vs. temperature characteristics of thermistor.

The conductance Gs is given as,

Where Gtm is the conductance of thermistor at mid scale temperature Tm

• ##### Using op amp:

The third method for linearising the thermistor output is by using op amp.

Following circuit shows the linearization circuit for thermistor. Here we have used a thermistor along with series resistor connected to the inverting terminal of the op amp. An adjustable supply voltage is used to adjust the gain of the amplifier.

In this way we have seen different signal conditioning circuits for thermistor.

## Temperature Controlled DC Fan using Thermistor (Mini Project)

Hello friends, in this post we are going to make one simple mini project which is temperature controlled DC fan using a thermistor. In this mini project, we are going to control the speed of the DC fan automatically as the surrounding temperature changes. Thus when the temperature of surrounding increases speed of fan also increases and when temperature decrease speed of fan also decreases. This is achieved by using the principle of the thermistor.

## Components required

• R1 = 4.7K
• R2 = 47 Ohm
• NTC Thermistor = 4.7K

• Potentiometer (Vr) = 500K
• OP AMP IC 741
• Transistor T1 = BD140 (or other PNP transistor may work)
• 12V DC fan which is also used in computer (CPU)
• Diode 1N4007
• 12 V DC power supply

## Circuit diagram

The following figure shows a circuit diagram of temperature controlled DC fan using a thermistor.

## Working

The basic working principle of temperature controlled DC fan is based on the working principle of the thermistor. The 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 temperature controlled DC fan, we have used an NTC type thermistor. It is called NTC because its resistance increases when its temperature decreases and vice verse. Similarly, in PTC, its resistance increases when temperature increases and vice verse.

Op amp IC741 is used as a voltage comparator which compares the voltage between its two inputs i.e. inverting and non-inverting terminals. Pin number 2 is inverting terminal which is connected to the potentiometer and pin number 3 is a non inverting terminal which is connected in between thermistor and R1 which makes a voltage divider circuit. Thus the output of op amp is responsible for the speed of the fan.

When the temperature of surrounding increases, the temperature of thermistor also increases which causes its resistance to decrease, therefore voltage divider circuit causes more voltage across pin number 3. Thus the output voltage increases causing the speed of the fan to increase.