Thermistor – A temperature measuring transducer

A thermistor is basically a resistor whose resistance changes with the change in the temperature. A thermister offers a non-linear curve between temperature and resistance graph.

Thermistors are of two types:

1. NTC - Which stands for negative temperature coefficient which means that the resistance of the thermistor decreases with the increase in temperature.
2. PTC - Which stands for positive temperature coefficient. The resistance of the PTC thermistor increases with the increase in temperature.

NTC thermistors are used most widely.

A thermistor is a transducer that can convert temperature into an electrical signal. The thermistor is used in temperature measurement. The range of a thermistor is from -50 to 200 degree Celsius.

Using thermistor for temperature measurement

A voltage divider circuit is used for measuring temperature with the help of the thermistor. The circuit diagram is shown below:

It consists of a resistor and a thermistor connected in a series with positive supply. As the resistors are connected in a series, the voltage divides in both oh the resistor. Output voltage Vout is given by:

$$V_{out}=V_{in} \times {{R_t} \over {R+R_t}}$$

Where Vin is supply voltage which is +Vcc in this circuit diagram. As Rt is temperature dependent, the output voltage depends upon temperature. In the case of NTC thermistor, the output voltage will decrease with an increase in the temperature because resistance Rt is decreasing as per circuit diagram.

We can also interchange the position of the thermistor and resistor as shown below:

In this case, the output voltage is given by:

$$V_{out}=V_{in} \times {{R} \over {R+R_t}}$$

Clearly, from the above equation, for NTC thermistor the output voltage increases with the increase in temperature as the resistance of the thermistor decreasing.

What to do with output voltage?

We can connect Vout terminal to a microcontroller for processing of the voltage signal. Microcontroller consists of an analog to digital converter that converts the analog voltage into a digital signal which can be used by the microcontroller for processing the data.

After processing of the voltage signal, we can use the microcontroller for measuring the temperature by displaying the processed data in a display and also we can use that data for controlling.

My favourite circuit using a thermistor

Let us replace the resistor R with a LED.

By proper calibration, the intensity of the LED will change with the change in temperature. When temperature increases the resistance of the NTC thermistor decreases and the current through the LED will increases. As the current increases, the intensity of the LED also increases.

Applications

1. Because of the low-temperature range, the thermistor is used in Li-ion battery for overheating protection.
2. It is used in the smartphone to measure the temperature of the smartphone.
3. It is used in LCD temperature measurement for brightness or contrast control.