Last Updated on October 22, 2023 by Kevin Chen
If you’ve ever used a thermometer or thermostat, then you probably know about NTC (negative temperature coefficient) thermistors. They are used in a variety of devices and appliances due to their ability to measure temperature and react to it.
In other words, they have the potential to detect when something is overheating and prevent it from getting out of hand. In this blog post, we will discuss everything you need to know about NTC thermistors.
What is an NTC thermistor?
A thermistor is a type of resistor whose electrical resistance varies with temperature. There are many different types of thermistors, each one with a particular set of properties. One of the most common types is the negative temperature coefficient (NTC) thermistor.
The name comes from the fact that the resistance of an NTC thermistor decreases as the surrounding temperature increases. This is in contrast to the positive temperature coefficient (PTC) thermistors which increase their resistance as the surrounding temperature increases.
As the temperature of an NTC thermistor increases, its electrical resistance decreases, and its ability to conduct electricity also decreases. This means that NTC thermistors are used in applications where a temperature rise is expected, for example, in automobile exhaust systems and in the heating elements in electrical stoves.
Characteristics of NTC Thermistor
The best way to understand how a thermistor works is to outline its main characteristics. They define how these transistors work and also their performance.
Response to the temperature
NTC thermistors are designed to operate at the temperature range of between -55 degrees to 200 degrees Celcius. However, there are some NTC thermistors that can operate beyond the above ranges.
The power rating of a thermistor is determined by its maximum operating temperature. The higher the maximum operating temperature, the higher will be its power rating. Most of them can handle up to 1W of heat, although there are some that can handle up to 5W or even more.
Maximum current handling capacity
It is also determined by its maximum operating temperature just like its power rating. When it comes to maximum current handling capacity, there are two main types of thermistors: high current thermistors and low current thermistors. The former type can handle up to 15mA while the latter type has a maximum current handling capacity of 0.5mA or less than that at room temperatures. However, these limits are set for room temperatures only and they change as temperatures rise or fall due to other circumstances.
The heat capacity of an NTC thermistor is the amount of heat that is required to bring about the increasing effect on the temperature of the thermistor by one degree. The heat capacity is usually measured in joules per kelvin.
Maximum temperature limit
The maximum temperature limit is the highest temperature that the thermistor is capable of handling without failing. Usually, it is measured in degrees Celsius.
Working voltage limit (WVL)
The working voltage limit or WVL is the maximum voltage that can be applied to a thermistor without damaging it. The rating of this parameter also depends on its maximum operating temperature, as well as its resistance at room temperatures. For example, most thermistors have a WVL rating of up to 50V at room temperatures. However, if they have a high maximum operating temperature and/or low resistance under normal conditions, their WVL rating will be lower than this value. For example, some high-temperature NTC thermistors have a maximum operating temperature of up to 200 °C and their working voltage limits are only around 10–20V at room temperatures.
Construction and components of NTC Thermistor
What does a typical NTC thermistor look like and which components make up this device? Let’s find out!
NTC thermistors are usually made up of a ceramic body, which is formed from sintered oxides of silicon (SiO) and sometimes boron (B) as well. A thin wire of nichrome or platinum-rhodium alloy, called the sensing element, is embedded in this body. This sensing element consists of a thin wire that has been coated with a metal layer that acts as an electrical conductor. The combination of the ceramic body and the metal-coated wire constitutes the sensing element or thermistor.
The NTC thermistor is attached to one end of a fixed resistance network called the lead wires and they are connected to a temperature controller. The other end of this fixed resistance network is attached to ground via another lead wire, which acts as an output terminal for the device.
In terms of size, NTC thermistors are usually cylindrical in shape and have a diameter of 0.5 to 6mm.
The thickness of the ceramic body is usually chosen between 0.2 and 3mm, depending on the power rating of the device.
The sensing element is coated with an insulator, called the coating, which is made from a material with high thermal conductivity such as alumina or silicon dioxide (SiO). The coating serves as an electrical insulator to prevent short circuits caused by high temperatures and also helps to protect the sensing element from oxidation or corrosion.
Classification of NTC thermistor
NTC thermistors are grouped into different groups. They include:
Beard NTC thermistor
These NTC thermistors have platinum lead wires that are embedded into the ceramic body of the thermistor. This construction guarantees the stability of the thermistor in all sorts of applications. The construction also allows them to operate at a high temperature range.
Glass-sealed NTC thermistor
As the name suggests these ones are encapsulated by an airtight glass. The glass shell protects the ceramic body and prevents the oxidation of the sensing element. Also, the glass prevents the evaporation of the liquid refrigerant to a large extent.
Disc and Chip thermistors
These types of thermistors are known for their metal surface contacts. They are larger than the beard NTCs, a feature that slows down their response rate. On the flip side, they tend to have a better heat dissipation rate than the other classes of thermistors.
How do NTC thermistors work?
When the temperature of an NTC thermistor increases, the atoms inside the thermistor get closer together, making the thermistor smaller. This means that the volume of the thermistor decreases, bringing the atoms closer together.
Since the volume of the thermistor has decreased, there are fewer free electrons present in the thermistor which results in a lower resistance. This decrease in resistance is caused by the voltage placed across the terminals of the thermistor because the electrons can flow freely through the thermistor.
When the surrounding temperature decreases, the volume of the thermistor increases, which means that the atoms are further apart. This increase in volume brings more free electrons into the thermistor which results in higher resistance. This increase in resistance is caused by the increase in the voltage across the terminals of the thermistor, which brings more electrons into the thermistor.
Advantages of using NTC Thermistor
– Temperature Stability – This refers to the stable temperature range of the thermistor. When the temperature of the NTC thermistor is within the specified range, its resistance will remain constant. This allows the NTC thermistor to maintain a stable output voltage that can be used for various applications.
– Affordable – These thermistors are comparatively more affordable compared to other types of thermistors. This is because NTC thermistors use less power than RTDs. Also, the most accurate NTC thermistor is available in a 0 – 100 degree Celsius range, which makes it useful for a wider range of applications.
– Safety – The NTC thermistor has a safety feature that prevents it from over-heating. When the temperature reaches its maximum value, the circuit will not be able to deliver electricity and will shut down automatically.
– Compact Size – The compact size of this type of thermistor allows them to be used in almost any application that requires a temperature sensing device.
-Fast response: NTC thermistors are very responsive and can be used for a wide range of applications. The response time is about 1-3 minutes.
-Applications – These thermistors are suitable for a wide range of applications, such as temperature control of circuits, industrial processes, and more.
-Compatibility – NTC thermistors can be used with almost any electronic device that requires measuring temperature. They are also compatible with most electronic components that require a NTC thermistor.
-High Accuracy – This type of thermistor is very accurate and has an accuracy rating between 0 to 50 degrees Celsius. This means that it can be used in a wide range of applications that require accurate temperature sensing devices.
-Low Power Consumption – The power consumption rate of NTC thermistors is very low when compared to RTDs. Also, the output voltage will remain constant even when they are subjected to high temperatures and variations in the supply voltage.
-Wide Temperature Range – The temperature range of this type of thermistor is from 0 degrees Celsius to 100 degrees Celsius, which makes it suitable for most applications where stable temperature sensing devices are required.
-Long life span – These types of thermistors have a long life span, which represents their ability to withstand adverse environmental conditions including high temperatures and extreme fluctuations in the supply voltage without failing or deforming over time.
Applications of NTC Thermistors
Here are some of the most common real-life applications of the NTC thermistors:
-In the medical field: thermistors are used in thermometers, blood temperature probes.
-In heating and cooling systems: NTC thermistors are used in temperature-controlled oscillators that regulate the speed of blowers, fans, and pumps.
-In electric power systems: NTC thermistors are used for voltage regulation in power converters.
-In motor control: NTC thermistors are used for overcurrent protection and as a current sensor.
-In electric vehicles: NTC thermistors are used to protect batteries from overcharging, to control charging current, and as a temperature sensor in HVAC applications.
-In industrial processing: NTC thermistors are used as a temperature sensor for measuring process temperatures and as an overcurrent protection device.
-In chemical processes: NTC thermistor is used in chemical processes like distillation, refrigeration, and boiling.
-In heat exchanger controls: an NTC thermistor is used to control the flow of steam or hot water in heat exchangers. The flow is controlled by changing the resistance between the two ends of a heat exchanger which has a fixed temperature difference between the two ends.
Factors affecting NTC thermistor behavior
– Ambient temperature – The surrounding temperature will affect the operating range of the thermistor. The higher the ambient temperature, the lower the operating range of the thermistor.
– Load current – A heavy load will increase the temperature of the thermistor, which will shorten the operating range. A low load current will increase the life of the thermistor, but it will also increase the operating range.
– Temperature coefficient – The temperature coefficient is the amount the resistance of the thermistor changes with temperature. The larger the temperature coefficient, the greater the resistance changes with temperature.
How to select an NTC Thermistor, and where to find them
When you select an NTC thermistor, be sure that it has the correct voltage, power rating, and operating temperature range.
Next, you will need to consider the length, width, and thickness of the thermistor. The length of the thermistor is measured from the end of one lead to the end of the other, while the width is the distance between the leads, and the thickness is the distance from the leads to the center of the thermistor where the core is located.
Finally, it is important to consider the packaging of the thermistor. The most common packaging types are axial and radial.
Choosing NTC thermistor manufacturer
Choosing NTC thermistor manufacturer in China When you decide on an NTC thermistor manufacturer, you will want to make sure that they are capable of producing the type of thermistor that you need.
It is important to make sure that the NTC thermistor that you select is manufactured by a reputable company.
You also want to make sure that the NTC thermistor manufacturer has the capability to produce an NTC thermistor of your type and size, and at a reasonable price.
The easiest way of finding these manufacturers is by going through a sourcing agent. And this is where ICRFQ comes in. We are a reputable sourcing agent for the electrical components in China.
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