What is Negative Temperature Coefficient Thermistor
NTC stands for "Negative Temperature Coefficient". NTC thermistors are resistors with a negative temperature coefficient, which means that the resistance decreases with increasing temperature. They are primarily used as resistive temperature sensors and current-limiting devices. The temperature sensitivity coefficient is about five times greater than that of silicon temperature sensors (silistors) and about ten times greater than that of resistance temperature detectors (RTDs). NTC sensors are typically used in a range from −55 to +200℃.
Advantages of Negative Temperature Coefficient Thermistor
High sensitivity
NTC thermistors exhibit a high sensitivity to temperature changes, allowing for precise temperature measurements.
Wide temperature range
They can operate over a wide temperature range, making them suitable for both low and high-temperature applications.
Small size
These thermistors are compact in size, enabling easy integration into small electronic devices or circuits.
Fast response time
NTC thermistors have a fast response time, enabling quick and accurate temperature readings.
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Types of Negative Temperature Coefficient Thermistor
Bead thermistors
Bead formThese NTC thermistors are made from platinum alloy lead wires directly sintered into the ceramic body. They generally offer fast response times, better stability and allow operation at higher temperatures than Disk and Chip NTC sensors, however they are more fragile. It is common to seal them in glass, to protect them from mechanical damage during assembly and to improve their measurement stability. The typical sizes range from 0.075 – 5 mm in diameter.
Disk and Chip thermistors
Disk thermistorThese NTC thermistors have metalized surface contacts. They are larger and, as a result, have slower reaction times than bead type NTC resistors. However, because of their size, they have a higher dissipation constant (power required to raise their temperature by 1℃). Since the power dissipated by the thermistor is proportional to the square of the current, they can handle higher currents much better than bead type thermistors. Disk type thermistors are made by pressing a blend of oxide powders into a round die and then sintering at high temperatures. Chips are usually fabricated by a tape-casting process where a slurry of material is spread out as a thick film, dried and cut into shape. The typical sizes range from 0.25 to 25 mm in diameter.
Glass encapsulated NTC thermistors
These are NTC temperature sensors sealed in an airtight glass bubble. They are designed for use with temperatures above 150℃, or for printed circuit board mounting, where ruggedness is a must. Encapsulating a thermistor in glass improves the stability of the sensor and protects the sensor from the environment. They are made by hermetically sealing bead type NTC resistors into a glass container. The typical sizes range from 0.4 to 10 mm in diameter.
Negative Temperature Coefficient Thermistor Manufacturing Process
Raw Material Blend
NTC thermistor manufacturing begins with the precise blending of raw materials into an organic binder solution. These raw materials are powdered transition metal oxides such as manganese, nickel, cobalt, and copper oxides. Other stabilizing agents are added to the mix as well. The oxides and binders are combined using a wet process technique called ball milling. During the ball milling process, the materials are blended and the particle size of the oxide powders are reduced. The completed homogeneous mixture has the consistency of a thick slurry. The exact composition of the various metal oxides and stabilizing agents determines the resistance-temperature characteristics and resistivity of the fired ceramic component.
Tape Cast
The "slurry" is distributed over a moving plastic carrier sheet using a doctor blade technique. The exact material thickness is controlled by adjusting the height of the doctor blade above the plastic carrier sheet, the carrier sheet speed, and by adjustment of the slurry viscosity. The cast material is dried as it is carried through a long tunnel oven at elevated temperatures on a flat casting belt. The resulting "green" tape is ductile and easily formable. The tape is then subjected to quality inspection and analysis. This thermistor tape is cast in a wide range of thicknesses ranging from as thin as 0.001" to over 0.100" dependent upon the particular component specification.
Wafer Formation
The cast tape now is ready to be formed into wafers. When thin material is needed, the tape is simply cut into small squares. For thicker wafers, the tape is cut into squares which are then stacked one on top of another. These stacked wafers are then laminated together. This allows us to produce wafers to virtually any thickness required. The wafers then go through additional quality testing to ensure high uniformity and quality. Subsequently, the wafers are subjected to a binder burn out cycle. This process removes most of the organic binders from the wafer. Precise time/temperature controls are maintained during the binder burn out cycle in order to prevent unfavorable physical stresses on the thermistor wafers.
Sinter
The wafers are heated to very high temperatures in an oxidizing atmosphere. At these high temperatures, the oxides react with one another and fuse together forming a spinel ceramic matrix. During the sintering process, the material densifies to a predetermined degree and the grain boundaries of the ceramic are allowed to grow. A precise temperature profile is maintained during the sinter process in order to avoid fracturing of the wafers, and to ensure the production of finished ceramic capable of producing components with uniform electrical characteristics. After sintering, the wafers are again subjected to quality inspection and the electrical and physical characteristics are documented.
Electrode
Ohmic contact to the ceramic wafer is obtained using a thick film electrode material. The material is typically silver, palladium-silver, gold, or platinum depending upon the application. The electrode material is comprised of a mixture of metal, glass, and various solvents and is applied to the two opposing surfaces of the wafer or chip by screen printing, spraying, or brushing. The electrode material is fired onto the ceramic in a thick film belt furnace and an electrical union and a mechanical bond forms between the ceramic and the electrode. The metalized wafers are then inspected and the attributes documented. Precise controls during the electrode process ensures that the components produced from the wafers will have exceptional long-term reliability.
Resistance Test
All thermistors are tested for proper resistance value, usually 25°C. The chips are normally tested automatically, but can also be tested manually depending upon the quantity produced and specification. The automatic chip handlers are interfaced with resistance test equipment and computers which are programmed by the operator to place the chips into various bins dependent upon their resistance value.
Encapsulation
In order to protect the thermistor from the operating atmosphere, humidity, chemical attack, and contact corrosion, the leaded thermistor is often coated with a protective conformal coating. The encapsulant is typically a high thermally conductive epoxy resin. Other encapsulants include silicone, ceramic cement, lacquer, urethane, and shrink sleeving. The encapsulant also assists in assuring good mechanical integrity of the device. Thermal response of the thermistor is taken into consideration when choosing an encapsulating material. In applications where fast thermal response is essential, a thin coat of a high thermally conductive encapsulant is utilized. Where environmental protection is more important, another encapsulant may be chosen. Encapsulants such as epoxy, silicone, ceramic cement, lacquer, and urethane are normally applied using a dip process and the material is either allowed to cure at room temperature or placed in an oven at an elevated temperature. Precise time, temperature, and viscosity controls are used throughout the process in order to ensure that pinholes or other deformities do not develop.
Termination
Thermistors are frequently supplied with terminals attached to the ends of its lead wires. Before the terminals are applied, the insulation on the lead wires is stripped appropriately to accommodate the specified terminal. These terminals are attached to the lead wires using specially tooled application machines. Subsequently, the terminals may be inserted into plastic or metal housings before being shipped to the customer.
Probe Assembly
For environmental protection or for mechanical purposes, thermistors are often potted into probe housings. These housing can be made of materials including epoxy, vinyl, stainless steel, aluminum, brass, and plastic. In addition to providing a suitable mechanical mounting for the thermistor element, the housing protects it from the environment to which it will be subjected. The proper selection of lead wire, lead wire insulation material, and potting material will result in a satisfactory seal between the thermistor and the outside environment.
Marking
The completed thermistor can be marked for easy identification. This can be as simple as a color dot or more complex such as a date code and part number. In certain a
Temperature range
When choosing a temperature sensor, the first consideration should be the temperature range of the application.
Since NTC thermistors perform well in an operating range between -50℃ and 250℃, they are well suited for a wide range of applications in many different industries.
Accuracy
Of the basic sensor types, an NTC thermistor's ability to achieve the highest accuracy is within the -50℃ to 150℃ range, and up to 250℃ for glass encapsulated thermistors.
Accuracy ranges from 0.05℃ to 1.00℃.
Stability
Stability is important in applications where long-term operation is the goal. Temperature sensors can drift over time, depending upon their materials, construction, and packaging.
An epoxy-coated NTC thermistor can change by 0.2℃ per year while a hermetically sealed one changes by only 0.02℃ per year.
Packaging
Packaging requirements are dictated by the environment the sensor will be used in.
NTC Thermistors can be customized and potted into various housings dependent on application requirements. They can also be epoxy coated or glass encapsulated for further protection.
Noise immunity
NTC Thermistors offer excellent immunity to electrical noise and lead resistance.
More considerations
NTC thermistors have specific electrical properties:
- Current-time characteristic
- Voltage-current characteristic
- Resistance-temperature characteristic
Type and size of product
The thermistor user will usually know what is needed in terms of size, thermal response, time response, and other physical features that go into the configuration of the thermistor. It should be easy to narrow down the choice of NTC thermistors even when data is lacking, but a careful analysis of the intended application of the thermistor must be made.
Resistance-temperature curves
Data sheets contain a table or matrix of resistance ratios versus temperature for each of their NTC thermistor products. Coefficients α and β are also provided for particular equations to help the user or designer to translate resistance tolerance in terms of accuracy in temperature, as well as calculate the temperature coefficient for each curve.
There is quite a wide range of materials that can be used to manufacture thermistors, but there are limitations involved depending on the size, operating and storing temperature range and nominal resistance values.
Nominal resistance value
The next factor to consider is whether the application needs to be curve matched or point matched. This will allow for the calculation of the needed nominal resistance value at a given temperature.
offers an entire range of nominal resistance values for their NTC thermistors. The standard reference temperature is 25℃, but buyers and designers can request different temperatures.
- A word of caution: If the desired resistance is not available in the combination of product type and material component, then a decision must be made as to which characteristic takes priority: product type/size, material preference or resistance ratio.
Resistance tolerance
When looking at product specification sheets, provides standard tolerances. For example, disc or chip thermistors usually have a zero-power resistance distribution of ± 1% to ± 20%.
To save on costs, Recommends specifications of the broadest possible tolerance that is relevant to intended use.
The working principle of the NTC thermistor is mainly dependent on the ambient temperature. Once the thermistor's temperature enhances then its resistance will be decreased. For every 1-degree centigrade rise of temperature, 5% resistance will be decreased.
There are two factors that affect a material's resistance to electrical flow: the number of free electrons in the material and the ease with which they can move through it. The latter is affected by the crystal structure of the material, which will have more or fewer “free-electron paths” for the current to flow through.
NTC thermistors are made from ceramics containing metal oxides, including Mn-Ni-Co oxide, Ni-Cr oxide and Cu-Ni oxide with additives. When these metals are combined with oxygen, they form bonds that limit the number of free-electron paths in the crystal structure, increasing resistance.
At higher temperatures, however, collisions between atoms cause the crystal structure to break down slightly, releasing some electrons and creating free-electron paths where they didn't exist before. The more free-electron paths there are, the less resistance there is to electrical flow. That's how NTC thermistors exhibit a drop in resistance as temperature increases.
Difference between NTC Thermistor and PTC Thermistor
| NTC Thermistor | PTC Thermistor |
| In NTC thermistor, the term “NTC” stands for negative temperature co-efficient. | In PTC thermistor, the term “PTC” stands for positive temperature co-efficient. |
| In this thermistor, when the temperature increases the resistance will be decreased. | In this thermistor, when the temperature increases the resistance will be increased. |
| The materials used to make NTC thermistors are; cobalt, oxides of nickel, manganese, copper, etc. | The material used to make a PTC thermistor is barium titanate. |
| These are used in the measurement & control of temperature-based applications. | These are used to protect different circuits from high temperatures. |
| These are applicable for temperature ranges from -55oC to 200oC. | These are applicable for temperature ranges from 0oC to 200oC. |
| An example is the SMD KT series NTC thermistor designed by ATC Semitec Limited. | An example is; SMD PTC thermistors designed by ATC Semitec Limited. |
What are the applications of Negative Temperature Coefficient Thermistor
To suppressing surge current. This thermistor is a power type, although it is small in size, it has a large power, generally connected in series on the mains input lines. It has a rated zero-power resistance value, and this resistance value is very small in general. Zero power resistance is the basic parameter of thermistor, which is usually given by manufacturers. When connected in series in the power circuit, it can effectively suppress the startup surge current, and its power consumption can be almost ignored.
The characteristics of power thermistor types are: ① Small size and high power; ②Strong surge current suppression capability and fast response; ③ Large material constant (b value); ④ Long service life, high reliability, and wide working range.
As a temperature sensor for measuring temperature, and as a thermistor for measuring temperature, it uses the change of outside temperature to change the resistance value, because when the negative temp coefficient thermistor resistor is connected to the circuit, it will always pass a certain amount of current to make the NTC heat up, and resistance value drop, which will have a great influence on the measurement. Therefore, it is necessary to control its own heating to avoid excessive current flowing through the thermistor and measurement errors are caused by the heating of the component itself. Write all or part of the resistance value corresponding to the temperature into the CPU, so that when the external temperature changes, the change in resistance is realized as a change in voltage. This kind of package includes chip type, epoxy head type, glass sealing type, straw hat type and so on. According to the principle of resistor divider, when the NTC changes, the voltage also changes. The A/D port is used to detect voltage changes, and all the data used by the thermistor is written into the single-chip microcomputer. Different resistance values correspond to different temperatures, and the program is constantly checking.
If the temperature accuracy is higher, NTC with higher accuracy can be used. At the same time, a bridge circuit and an amplifier are used. The bridge circuit is composed of thermistors NT1/NT2, resistors R1 and R2. As long as the thermistor has a temperature difference, the amplifier will output the corresponding signal.
Temperature compensation requires high accuracy in some electrical appliances, especially in meters, where many parts are made of metal wires, such as wire-wound resistors. Metals generally have a positive temp coefficient, which can be compensated with a negative temp coefficient thermistor. One positive and one negative can offset errors caused by temperature changes and improve measurement accuracy. As temperature compensation, the alloy copper wire resistance is usually connected in parallel with the NTC thermistor, and then connected in series with the compensation element.
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