Thermistor is a special type of resistor whose resistance depends on the temperature. This post will discuss in detail about what is Thermistor, its different types, how it works, various applications, advantages and disadvantages.
What is Thermistor
A Thermistor is a two terminal, solid-state electrical component whose resistance depends on temperature. “Thermal Resistors” are shortly called Thermistors. It is made of Metal Oxide Semiconductor and constructed in the shape of a bead, disk, or a cylinder and then encapsulated with material such as epoxy or glass.
Fig. 1 – Introduction to Thermistor
Thermistor was discovered by Michael Faraday, when during his experiments he noticed that the resistance of Silver Sulphide decreased as the temperature increased. Through this research, Samuel Ruben invented the first commercial Thermistor in 1930’s. It also called as Resistance Thermometer and hence they are widely used as Temperature Sensors.
Fig. 2 – Thermistor Symbol (a) International Standard (b) American Standard
Types of Thermistors
There are two kinds of Thermistors namely:
- NTC (Negative Temperature Coefficient) Thermistors
- PTC (Positive Temperature Coefficient) Thermistors
NTC (Negative Temperature Coefficient) Thermistors
This type of Thermistor is defined as a resistor that is thermally sensitive. Its resistance decreases in an exact and predictable manner as the core temperature increases over a temperature range.
These devices are used as resistive temperature sensors. They have a very large temperature sensitivity coefficient. NTC’s can be used in large variations of temperatures between -50°C and 200°C.
Fig. 3 – NTC Curve vs RTD curve
The characteristic NTC curve as shown in the Fig. 3, shows that NTC Thermistors have a distinctly steep Resistance-Temperature slope when compared to Resistance Temperature Detectors (RTD).
NTC Thermistors are classified into two types based on the material used. They are:
- Bead Thermistors
- Disk and Chip Type Thermistors
- Made of Platinum alloy lead wires which are sintered into a ceramic body.
- They respond fast and work at higher temperatures than other sensors.
- They are very fragile.
- They are often sealed in glass so that they are protected from damage when they are being assembled and improves their measurement stability.
Disk and Chip Type Thermistors
- Larger than Bead Thermistors.
- They both have metal surfaces.
- Their reaction time is slower than that of Bead Thermistors.
- Their size enables them to have greater power when raising their temperature by 1°C.
- In fact, they can handle higher currents better than Bead Thermistors.
- Their sizes range from 0.25 mm – 25 mm diameter.
Fig. 4 – Physical view of Thermistors
PTC (Positive Temperature Coefficient) Thermistors
The PTC Thermistor is defined as a thermally sensitive resistor. Its resistance increases substantially with temperature. They are often made of poly-crystalline ceramic materials.
PTC’s are very resistant in their original state so dopants are added to make them semi-conductive. Their transition temperature falls between 60°C and 120°C.
PTC Thermistors are categorized into groups according to the materials they are made of and their structure. They are:
- Silistor PTC Thermistors
- Switching Type PTC Thermistors
Fig. 5 – Classification of Thermistors
Silistor PTC Thermistors
- These Thermistors use doped silicon for their semi-conductivity.
- Doped silicon is made by adding impurities to silicon to change it from an insulator to a conductor.
- They are manufactured from extremely thin sheets of silicon and then shaped differently.
- They are used as temperature sensors.
Switching Type PTC Thermistors
- These are made of poly-crystalline materials.
- These materials are then ground, mixed and shaped; then they are sintered.
- The manufacturing process must be clean since impurities in the consistency of the materials will result in great changes in their thermal and electrical properties.
How does Thermistor Work
Let us understand working principle of Temperature Control System which uses Thermistor. The system consists of:
- Temperature Detection Unit
- Temperature Control Unit
- GUI Unit
Temperature Detection Unit
A Thermistor along with a voltage divider circuit is used to detect the current temperature of the system. Thermistor produces different resistance for different temperatures which results in different Voltage drop in the divider circuit. ADC converts these analog signals to Digital samples.
Temperature Control Unit
This unit consists of Microcontroller, DAC, Difference/ Power Amplifier. The Microcontroller receives the digital samples continuously from ADC and determines the temperature of the system. It compares the actual temperature against the temperature required by the system to run smoothly.
Fig. 6 – Block Diagram of Temperature Control Unit
The 8-bit output of the Microcontroller is then converted back to Analog signal by DAC. This signal is then fed to Difference Amplifier/Power Amplifier to either increase or decrease the temperature of the system.
The GUI (Graphical User Interface) unit is designed for the end-user to communicate with the system. User can read the different settings of the system and can make the desired changes.
Applications of Thermistor
Below, is a list of some of the applications of Thermistor:
- Digital thermometers use Thermistors to record body temperature and is displayed on the thermometer’s small screen.
- They are widely used in Automobiles to determine and record the temperature of the oil and coolant found in the engine which helps us to realize when the car is overheating.
- Any kind of rechargeable battery has a Thermistor built in it. It regulates the temperature and helps the battery not to get burnt.
- They are also used in computer electronics like CPU, Hard Disk Drives, LCD’s etc.
- They are extensively used in household electronics like refrigerators, washing machines and electric cookers, microwave oven etc.
Advantages of Thermistor
The advantages of Thermistor include:
- They are small and occupy very less space.
- Production is high making manufacturing costs low and so they are cheap to use.
- NTC Thermistors have excellent sensitivity. They are made of purest materials which contribute to this sensitivity.
- They respond quickly and efficiently over a small temperature range.
Disadvantages of Thermistor
Below is a list of disadvantages of thermistor:
- The resistance to temperature characteristics is non-linear.
- Some Thermistors cannot take the large vast change in temperatures that certain motors or engines need.
- They need to have better shielding power lines to make them more connected to earth ground.
- They need an excitation current, or more commonly known as a voltage source.