What is thermistor principle?

A thermistor is a special type of resistor whose resistance is more temperature-dependent than standard resistors.  The resistance of the thermistor decreases with increasing temperature. This is the basic principle of thermistors. This allows the thermistor circuit to detect very small temperature changes that cannot be observed with the Resistance temperature detectors (RTDs) or the thermocouples. Since the resistance of a thermistor is temperature dependent, it can be connected to an electrical circuit to measure body temperature. This high sensitivity to temperature changes makes thermistors very useful for accurately controlling and calibrating temperature measurements. In general, resistance increases with increasing temperature in most metals, but thermistors react negatively.

USE OF THERMISTOR:

• Temperature probe.
• Inrush current limiter.
• Self-regulating overcurrent protection device.
• Self-regulating heating element.

What are the types of thermistors?

There are two main types of thermistors: positive temperature coefficient (PTC) thermistors and negative temperature coefficient (NTC) thermistors.

Positive temperature coefficient thermistor

The PTC thermistor increases its resistance as the temperature rises. The relationship between resistance and temperature is linear and is expressed by the equation:

$R_t=R_0\left(kt+1\right)$

Here, R₀ is the initial value of the resistance, t is the temperature, k is the temperature coefficient. 

Uses of PTC

• A PTC thermistor can be used in place of a fuse to protect the circuit.
• As the circuit heats up, the resistance increases to prevent overloading.
• PTC thermistors are used as timers in the magnetic coil circuit of most cathode ray tube(CRT) displays. Device circuits using PTC thermistors are simple, reliable (because they are simple), and inexpensive.
• PTC thermistors can also be used as heaters in the automotive industry to further heat the diesel engine compartment or to heat diesel in cold climates prior to engine injection.
• PTC thermistors can be used as current-limiting devices for circuit protection for the replacement of fuses.

Negative temperature coefficient thermistor

Many NTC thermistors are made from an extruded disk or a cast semiconductor chip such as a sintered metal oxide. It works because increasing the temperature of the semiconductor increases the number of electrons that can move and carry a charge. This pushes the electrons into the conduction band. The more charge carriers available, the more current it can conduct. This is described by the formula.

$I=nAve$

From the formula, I = Current (amperes)
n = Carrier density (quantity/m³)
A = Cross-sectional area of ​​material (m²)
v = Carrier charge velocity (m/s)
e = Electron charge (coulombs)

Current is measured with an ammeter. Large temperature changes require compensation. For small changes in temperature, if the correct semiconductor is used, the resistance of a material is linearly proportional to its temperature. There are a variety of semiconductor thermistors ranging from 0.01 Kelvin to 2000 Kelvin (273.14 °C to 1700 °C)

Use of  NTC

• NTC thermistors are used as current limiters.
• Digital thermostats and temperature monitors in cars.
• An NTC thermistor can also be used to control the incubator temperature.
• Thermistors are also widely used in modern digital thermostats and to monitor the temperature of battery packs during charging.
• NTC thermistors are used throughout the consumer electronics industry to measure temperature. Toasters, coffee makers, refrigerators, freezers, hairdryers, and more all rely on thermistors to properly control the temperature.
• We can regularly use thermistors on the hot end of 3D printers. It controls the heat generated and allows the printer's control circuitry to maintain a constant temperature to melt the filament.
• NTC thermistors are used as a resistance thermometer for low-temperature measurements around 10 K.

Construction of Termistors

Thermistors are composed of a sintered mixture of metallic oxides such as manganese, cobalt, uranium. A thermistor is made from a semiconductor material. Thermistor beads can be as small as a few millimetres in diameter. In some thermistors, the ball is enclosed in a glass capsule.

Thermal Characterstics

• Thermistor exhibits high non-linear resistance characteristics as a function of temperature. PTC thermistors can be used as heating elements in small temperature-controlled ovens. NTC thermistors can be used as inrush current-limiting devices in power circuits.
• Inrush Current is the maximum instantaneous input current drawn by an electrical device when it is first turned on. The temperature of the thermistor can be changed externally by changing the ambient temperature and internally by self-heating as a result of the current flowing Through the device.

Resistance temperature characterstics

The resistance temperature is characteristic of platinum, which is often used as resistance material. thermometer. Thermistors measure the high sensitivity of temperature measurements. Thermistor performance is obviously non-linear, but a linear approximation of the resistance-temperature curve can be obtained. The changes in resistance temperature characteristics can be calculated easily.

A thermistor has a negative coefficient of temperature resistance, which is typically around 0.05/$C_{\circ }$.The Stein-Hart equation is

$T=\frac{1}{A+B\ln \left(R\right)+C{\left[\ln \left(R\right)\right]}^3}$

Here, A, B, and C are found by choosing three (R, T) data points and solving three simultaneous equations.

Voltage time characterstics

The thermistor voltage drop increases with increasing current until it reaches a peak value beyond which the voltage drop increases as the current increases. In this proportion of the curve, the thermistor exhibits negative resistance characteristics. If a very small voltage is applied to the thermistor, the resulting small current does not produce sufficient heat to raise the temperature above the ambient temperature. These characteristics of self-heat provide an entirely new field of uses for thermistors.

Current time characterstics

The curve indicates the time delay to reach maximum current as a function of the applied voltage. When the heating effect just described occurs in a thermistor network, a certain finite time is required for the thermistor to heat and the current to build up to a maximum steady-state value. This characteristic provides a simple and accurate means of achieving time delays from milliseconds to minutes.

Thermistor characterstics

• Thermistors are small, rugged, and inexpensive.
• Thermistors have good stability after aging.
• The upper operating temperature limit of a thermistor depends on the physical change in the solder or material used in the electrical connection.
• The response time of a thermistor can vary from a few seconds to a few minutes, depending on the size of the sensed mass and the thermal capacity of the thermistor.
• The test current should be kept as low as possible to prevent the thermistor from self-heating.

Common Mistakes

While going through this topic the student may get confused in the temperature measuring devices and the thermistors as there are some common features in the temperature measuring devices and the thermistors.

Context and Applications

The topic Thermsitors is basically studied in various courses, some of them are mentioned below:

• Bachelor of Technology in electrical engineering
• Master of Technology in electrical engineering

There are many applications of the topic that are mentioned below:

• Home Appliances (Home Appliances)-thermistors are very common, but you might be surprised to learn how many appliances in your home use thermistors to control and measure temperature. Some of these applications are:
• The thermistor in the protective device monitors the surge and ensures that the correct amount flows through the connected device.
• Digital Thermometers Because thermometers are fast and accurate, they often use a thermistor as the temperature sensing element.
• Microwave/boiler thermistors monitor internal temperature and prevent overheating.
• Thermistors in 3D printers are used for temperature control because they require precise control.

Related Concepts

• Temperature measuring devices
• Thermostats
• Electrical appliances

Practice Problems

Q1. How many types of Thermistors are there?
a) One
b) Two
c) Three
d) Four
Correct option: (b)

Explanation: There are two main types of thermistors: positive temperature thermistors and negative temperature thermistors.

Q2. Thermistor has a resistance of ________
a) 0.5 Ω to 0.75 MΩ
b) 20 Ω to 60 kΩ
c) 10 Ω to 20 kΩ
d) 70 Ω to 100 kΩ
Correct option: (a)  

Explanation:  Thermistor has a resistance range of 0.5 Ω to 0.75 MΩ. The thermistor consists of a mixture of metallic oxides of manganese, nickel, cobalt, copper, iron, and uranium.

Q3. Thermistors can be used as
a) Vacuum measurement
b) To measure the percent of CO₂ in air
c) Temperature compensations
d) All of the above

Correct option: (d)

Explanation: The thermistor is normally used in vacuum measurement, to measure the percent of CO₂ in air, Temperature compensations.

Q4. What is the value of the thermistor senstivity?

$$\begin{gathered} \mathrm{a}) 0.05 \mathrm{\Omega }/°\mathrm{C} \\ \mathrm{b}) 10 \mathrm{\Omega }/°\mathrm{C} \\ \mathrm{c}) 15 \mathrm{\Omega }/°\mathrm{C} \\ \mathrm{d}) 20 \mathrm{\Omega }/°\mathrm{C} \end{gathered}$$

Correct option: (b)

Explanation: The senstivity of a thermistor is $10 \mathrm{\Omega }/°\mathrm{C}$

Q5. A thermisttor has a resistance temperature coefficient of $-5\%$ over a temperature range of  $30°\mathrm{C}$ and $50°\mathrm{C}$. If the resistance of the thermistor is $200 \mathrm{\Omega }$ at $30°\mathrm{C}$. What is the resistance at $40°\mathrm{C}$?

1. $100 \mathrm{\Omega }$
2. $400 \mathrm{\Omega }$
3. $300 \mathrm{\Omega }$
4. $500 \mathrm{\Omega }$

Correct option: (a)

Explanation: Resistance at a temperature of $40°\mathrm{C} \mathrm{is}: {\mathrm{R}}_{40}=200[(1-0.05)(40-30)]=100 \mathrm{ῼ}$.