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.




[caption id="attachment_846" align="aligncenter" width="1022"]Bridge amplifier for thermistor Bridge amplifier for thermistor[/caption]



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.

[caption id="attachment_847" align="aligncenter" width="341"]Parallel Resistor with thermistor Parallel Resistor with thermistor[/caption]

the value of the equivalent reistance is given by,

[caption id="attachment_848" align="aligncenter" width="155"]parallel resistor output parallel resistor output[/caption]

 

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.

[caption id="attachment_849" align="aligncenter" width="341"]Series resistor with thermistor Series resistor with thermistor[/caption]

The conductance Gs is given as,

[caption id="attachment_850" align="aligncenter" width="207"]series output series output[/caption]

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.

[caption id="attachment_851" align="aligncenter" width="446"]Thermistor linearization using op amp Thermistor linearization using op amp[/caption]

 

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

See also: Signal Conditioning for RTD

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