A thermistor is a thermally sensitive resistor. It comes in different shapes and types, and it’s very common in many applications around us.
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What does a thermistor do, exactly? How to choose the best thermistor for your needs? Where’s it used? And what are the different types of thermistors out there?
This post answers all of these questions and more with a complete overview of thermistors, their functions, types, and uses.
What Is a Thermistor in a refrigerator?
Simply put, thermistors are semiconductors. The material from which a thermistor is made has a great influence on its temperature and resistance.
They’re typically made of metallic oxides, stabilizers, and binders pressed together and molded into discs or cylinders. Thermistors can take many shapes, but they mostly remain in disc form.
The shape of the thermistor depends mainly on its environment, whether it’s embedded within the system or surface-mounted.
Thermistors work in a non-linear way. Their resistance increases or decreases according to a curve. The material of the thermistor determines the specifics of this curve.
In other words, this resistance/temperature curve is measured by the ratio of these materials. Thermistor manufacturers usually control the ratio and display it accurately for buyers.
What Does a Thermistor Do?
The main function of a thermistor is to show changes in temperature. It reflects any change in body temperature through a corresponding change in resistance.
Thermistors are usually used with appliances that require reliability, stability, and ruggedness. They can work under extreme conditions.
Thermistors can be coated with epoxy for lower temperatures (-58 to 316°F) or glass for higher temperatures (-58 to 572°F).
The coatings help protect the thermistor and its wires against mechanical stress, humidity, and corrosion.
There are two quality classes for thermistors. Class A thermistors have the highest accuracy. Class B, on the other hand, can be used if great accuracy isn’t a must.
The good thing is that your thermistor remains chemically stable and maintains the same level of accuracy for as long as you have it. It never changes with age.
Moreover, thermistors are cheap, durable, and easy to use. They’re common and can be closer to you than you think.
Common Thermistor Applications
Thermistors are typically used in both simple and complex applications such as:
- Vehicles
- Digital thermometers
- Fire alarms
- Ovens
- Refrigerators
- Optical blocks
- Charge-coupled devices
- Laser stabilization detectors
- Ceramic heaters
- Thermoelectric actuator valves
- Bimetallic switches
- Coffee pot heaters
Types of Thermistor
There are two types of thermistors on the market: NTC and PTC.
1. Negative Temperature Coefficient (NTC)
This type presents a decrease in electrical resistance when there’s an increase in temperature.
NTC thermistors can be used in devices that measure and control temperatures. They’re used in electrical circuits that need temperature compensation, including LCDs and oscillators.
Another function of NTCs is they help limit inrush current because they go through a time delay before reaching lower resistance.
NTC thermistors can also function as sensors for the presence of liquids. They can detect a liquid when they get into contact with it.
2. Positive Temperature Coefficient (PTC)
Unlike NTCs, this type shows an increase in electrical resistance when there’s an increase in temperature.
PTC thermistors can be used both as heaters or self-resettable fuses. They function effectively in heating an item to a certain temperature and maintaining that temperature.
When they reach a state of high resistance, PTCs tend to self-regulate at a set temperature. If the temperature decreases, the resistance decreases, allowing more current to flow. This uses more power and raises the temperature again.
On the other hand, if the temperature increases, the resistance increases. This limits the current flow and causes a decrease in temperature.
PTC thermistors can function as heating objects in complex applications. They’re found in diesel fuel heaters to heat fuel and facilitate the starting of cold engines.
They’re also utilized in the operation of dishwashers, soap dispenser doors, the angle-of-attack indicators in aircraft, and engine block heaters.
The Configurations of Thermistors
The following are the three commonly used thermistor configurations:
1. Hermetically Sealed Flexible Thermistor
HSFH thermistors are sealed with PFA (plastic polymer) coats to shield the sensing part from corrosion and moisture.
These are usually employed to measure the temperature of liquids such as chemicals, oils, or even food.
2. Bolt-On/Washer-Mounted Sensors
This type of thermistor can be installed within holes or narrow openings. They have a small thermal mass, which makes them work faster.
They’re usually used in household appliances, pipes, and water tanks.
3. Self-Adhesive Surface-Mount Style
These thermistors have adhesive exteriors that easily stick on flat or curved surfaces. It’s easy to remove or reapply them, and they’re used in many commercial and industrial applications.
There are heavy-duty types called probe-mounted thermistors. These are specially designed to be safely immersed in corrosive fluids. They’re typically used in industrial applications.
Mount thermistors can be vinyl-tipped to be safely used in biological operations or outdoors as well.
Thermistors vs. Resistance Temperature Detectors (RTDs)
A resistance temperature detector (RTD) is another sensor that performs the same function as a thermistor. It even measures temperature in the same way—through resistance.
The question is, which is better and why?
While there’s no general answer to that question, your purpose determines which device to use.
The main difference between a resistance temperature detector and a thermistor is in their mechanism.
RTDs work linearly, unlike thermistors that work according to a temperature/resistance curve. Moreover, RTDs cover a wider temperature range and are, therefore, more accurate.
A thermistor, on the other hand, is smaller, faster, cheaper, and more durable. It works best when measuring a single point temperature. However, it has a limited temperature range.
In Conclusion
Thermistors measure temperature through resistance. They increase or decrease in resistance according to the changes that happen in temperatures.
Thermistors come in two different types and a variety of configurations. They’re used in several operations, from vehicles and laser stabilization detectors to ovens and refrigerators.
Other devices, like RTDs, perform the same function as thermistors, but thermistors are faster, cheaper, smaller, and more durable.
