What is a thermistor?

A thermistor is a resistor and its resistance would vary if its temperature changes, so basically a thermistor can be described as a temperature-sensitive electrical resistor. The thermistor is highly sensitive to temperature, it is a class of metal oxide. By using a thermistor we can do accurate temperature measurement and it is inexpensive too. The thermistors are constructed from the metal oxide semiconductor material and will be encapsulated in a glass or epoxy bead. The thermal mass of the thermistor is very low and because of this, the thermistor has a fast response time. The thermistor has a negative temperature range and also a positive temperature range. The NTP thermistors resistive property is inversely proportional to the temperature so if the temperature increases then the resistance would decrease. The thermistor which has a positive temperature coefficient would increase its resistance if there is an increase in temperature.

The thermistors are used as inrush current limiters, temperature sensors, and also as a self-regulating heat elements.

What are the advantages of thermistors over other temperature sensors?

  • Thermistors are very cheaper when compared to other sensors
  • It is waterproof
  • This device would work at any voltage
  • Accuracy is very high when compared to other temperature sensors
  • It can read very low voltage without using an amplifier
  • Thermistors are reliable when compared to other types of temperature sensors

How thermistors are constructed?

NTC thermistor

The NTC thermistor is mostly made from metal oxides such as cobalt, nickel, and copper. By permitting a chemical reaction the metals are oxidized and then it will be converted into powder and after that, it will be subjected to heat. By using certain semiconducting material we can crystalize the NTC thermistor.

PTC thermistors

This type of thermistors is mostly constructed by adding a little amount of semiconducting material into polycrystalline ceramic. So if the temperature reaches a certain point the semiconducting material would act as a barrier to the electricity flow and the resistance would suddenly increase.

How does a thermistor work and what are the types of thermistors?

The working of the thermistor is depended on the resistance it provides according to the change in temperature. The amount of resistance would be related to the material which is used for the construction of the thermistor.

NTC thermistor

In this type, the resistance would decrease if there is an increase in temperature so we can say that the resistance is inversely proportional to the temperature. So the characteristic of the material would affect the measurement and according to the material used for the construction, the relationship between the temperature and the resistance would vary. NTC can be used as a temperature controller, the NTC thermistors are used when we need to increase the current in a circuit when the temperature is increased. This type of thermistor is used for low-temperature measurement and they are also used as sensors in automotive applications to check the temperature of oil and coolant. Most of the NTC thermistors are constructed by using a pressed disc, rod plate, bead, etc.

PTC thermistor

The PTC thermistor working is the opposite of the NTC thermistor, it would increase the resistance according to the increase in temperature. So the temperature and resistance are directly proportional.

The application of the PTC thermistors is not as much as an NTC thermistor, but these thermistors are used for circuit protection, and these thermistors are also used as a current limiting device. In this thermistor if the temperature is increased then the resistance will be increased too, so because of this the resistance would increase and the current will be decreased and because of this, it can be used as a current limiting device. We can use this type of thermistor when we need to stop the flow of current during high temperatures. Most of the PTC thermistors are constructed from doping a polycrystalline ceramic and this material would increase its resistance suddenly at a certain critical temperature.

What is the difference between a thermistor and an RTD?

The major difference between the thermistor and RTD is the material that they are made of. The RTDs are made of metals and the thermistors are constructed by using polymer materials or by ceramic materials. The thermistor has better performance than an RTD, when compared to the RTD the thermistors are cheaper, accurate, and also have good response. The only disadvantage of a thermistor, when compared to an RTD, is that the RTD has a good temperature range, it has a wider range for the temperature measurement.

How can we use the self-heating effects of thermistors?

So if there is a current circulation through the thermistor then there would be heat and thus the thermistor’s temperature will be more than its surrounding. So if the thermistor is used to measure the environmental temperature then there would be an error due to this electrical heating and we can correct this error. We can use this effect to create a sensitive airflow device, and it can be used as timers and also as relays too.

How to select a thermistor?

  • Zero power resistance -This is the DC resistance value of the thermistor which is measured at a certain temperature and the power which is dissipated by the thermistor would be low and if any further decrease in power would change the resistance
  • Resistance ratio characteristics
  • Negative temperature coefficient
  • Positive temperature coefficient
  • Maximum operating temperature
  • Maximum power rating
  • Dissipation constant – This is the ratio of the power dissipation in a thermistor according to the change in body temperature
  • Thermal time constant
  • Resistance temperature characteristics – This is the relationship between the zero power resistance of a thermistor and its body temperature
  • It must be selected according to the temperature or resistance tolerance characteristics
  • Measurement accuracy
  • Sensing temperature
  • Insulation test voltage
  • Current time characteristics
  • Stability – This is the ability of the thermistor to regain its characteristics after it is used for electrical testing

What are the advantages of thermistor?

  • High resistance so there won’t be any problem with lead wires
  • The relation between the resistance and temperature is highly nonlinear
  • It is so small
  • Low cost
  • Quick response
  • It can be easily manufactured in bulk
  • Wide temperature range
  • High sensitivity and resolution
  • Really accurate
  • It is capable to withstand shock and vibration

What are the disadvantages of a thermistor?

  • Extra circuitry is required for its non-linear response
  • Due to this additional circuitry linear response is limited
  • The operating range of a single sensor will be narrow linear
  • The glass could break if it is not handled properly
  • The excitation current is needed
  • Thermistors are not stable as RTD
  • High resistance could lead to self-heating errors
  • We can’t use thermistors for extremely hot or cold temperatures

What are the applications of thermistor?

  • It is used in a digital thermometer
  • It is used in automobiles to determine the oil and coolant temperature
  • It can be seen in household appliances such as microwave and fridge
  • This device is also used to check the thermal conductivity of the electrical material
  • It is used in rechargeable batteries to maintain the temperature range of the battery
  • It is used in the medical field – Blood analysis equipment, blood dialysis equipment, internal body temperature monitor, temperature regulators skin temperature monitor, and a lot more.
  • Military and aerospace- aircraft temperature, fire control equipment, missiles and spacecraft temperature
  • Industrial electronics – Measurement of fluid flow, gas flow indicators, HVAC, Thermostats, Thermocouple compensation
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