## A Practical Study Of NTC Thermistors and Thermal Time Constant

### What are NTC Thermistors?

The thermistor is generally a semi-conductor material that has been specially made to resist and sustain the large changes that happen in the temperature. The resistance would be measured by the usage of small DC Current, which is being passed through the thermistor.

Solid state temperature sensors act like the electrical resistors which are temperature sensitive. In other words, the thermistors are made up of ceramics that consists of highly sensitive materials that show resistance with the temperature.

### What is Thermal Time Constant?

Thermal Time Constant, is the time that thermistor temperature sensor would take to change the 63.2% of difference between the initial and final temperature, when put under zero conditions.

When you are about the Thermal Time Constant, you would need to apply a temperature change. However, if the temperature change is not spontaneous, then the Thermistors would only be measuring an ambient rate of change. That is the reason for changing the temperature instantaneously.

### How does Thermal Time Constant work with A Thermistor?

The thermal time constant is the fundamental factor for a thermistor. Larger the thermistor, slower would be the response time. Vice-versa, the smaller the size of the thermistor, the faster it would be.

So, while you are selecting a NTC thermistor for your application, you need to consider the following:

• Response time which is needed to meet your requirements, and
• Thermistor type you would use.

If you end up selecting a wrong thermistor, then it will result in the response time being shown either too slow or too fast, which would of no use to your application.

### How Thermal Time Constant Can Be Measured?

There are several parameters that affect the measurement of the Thermal Time Constant. Some of the variables are mentioned below:

• Shape of the NTC Thermistor
• Nature of the surroundings of the Thermistor, whether it is a liquid or gas medium in which the thermistor operates.
• The material that is being used for encapsulation of the Thermistor.
• The constant mass of the NTC Thermistor.
• Now comes the major variable, Sintering. It affects the resistivity and stability as well if you are closing the pores existing between different oxide particles.

### Factors to be Considered While Measurement of the NTC Thermistor

• The surrounding medium in which the readings are being taken.
• The flow of liquid or gas in the ambient, which would be directly impacting the measurement of the thermistor.

### Why is Thermal Time Constant Considered as a Critical Factor?

The Thermal Time Constant determines the time it will take for the circuit to respond. When you are practically implementing this system, then it would need you to silence the alarm system while the current is still being supplied.

### The Bottom Line

Using wrong thermistor, especially a wrong NTC thermistor, could be fatal for lives as well as property. So why not contact Analog Technologies for proper functioning of the circuit and the entire application.