Thermistor: Characteristics, Classification, Symbol and Applications

Thermistors are a class of sensitive components. They are divided into positive temperature coefficient thermistors (PTC) and negative temperature coefficient thermistors (NTC) according to different temperature coefficients. The typical characteristic of a thermistor is that it is sensitive to temperature and exhibits different resistance values at different temperatures. The positive temperature coefficient thermistor (PTC) has a higher resistance value at higher temperatures, and the negative temperature coefficient thermistor (NTC) has a lower resistance value at higher temperatures. They both belong to semiconductor devices.

Catalog

I. Main characteristics

1. High sensitivity, its resistance temperature coefficient is 10 to 100 times greater than that of metal, and it can detect temperature changes of 10-6 °C;

2. Wide operating temperature range, normal temperature devices are suitable for -55 °C ～ 315 °C, high-temperature devices are suitable for temperatures higher than 315 °C (currently up to 2000 °C), low-temperature devices are suitable for -273 °C ～ -55 °C;

3. Small volume, can measure the temperature of voids, cavities, and blood vessels in the living body that cannot be measured by other thermometers;

4. Easy to use, the resistance value can be arbitrarily selected between 0.1 ～ 100kΩ;

5. Easy to be processed into complex shapes and can be produced in large quantities;

6. Good stability and strong overload capacity.

II. Classification

1. PTC Thermistors

PTC Thermistors

PTC (Positive Temperature Coefficient) Thermistor refers to a thermistor that has a sharp increase in resistance at a certain temperature and has a positive temperature coefficient. It can be used as a constant temperature sensor. The material is a sintered body with BaTiO3 or SrTiO3 or PbTiO3 as the main component. A small number of oxides such as Nb, Ta, Bi, Sb, Y, and La are doped to control the atomic valence to make it semiconductive. Semiconductorized materials such as BaTiO3 are referred to as semiconducting (body) ceramics; at the same time, oxides of Mn, Fe, Cu, Cr, and other additives increase their temperature coefficient of positive resistance are added, and they are formed by general ceramic technology. High-temperature sintering will semi-conductive the platinum titanate and its solid solution, to obtain a thermistor material with positive characteristics. Its temperature coefficient and Curie point temperature vary with the composition and sintering conditions (especially the cooling temperature).

Barium titanate crystals belong to a perovskite structure and are a ferroelectric material. Pure barium titanate is an insulating material. Adding trace rare earth elements to the barium titanate material, the resistivity increases sharply by several orders of magnitude near the Curie temperature after appropriate heat treatment, which results in the PTC effect. This effect is related to the ferroelectricity of the BaTiO3 crystal and its vicinity. Barium titanate semi-conductive ceramic is a polycrystalline material, and there is an intergranular interface between the grains. When the semi-conductive ceramic reaches a certain temperature or voltage, the grain boundary changes, and the resistance changes sharply.

The PTC effect of barium titanate semiconductive ceramics originates from the grain boundary. For conducting electrons, the interface between grains is equivalent to a potential barrier. When the temperature is low, due to the effect of the electric field in the barium titanate, the electrons easily cross the potential barrier, so the resistance value is small. When the temperature rises to near the Curie point temperature (ie, the critical temperature), the internal electric field is destroyed, and it cannot help conductive electrons cross the potential barrier. This is equivalent to the rise of the potential barrier and the sudden increase of the resistance value, resulting in the PTC effect. Physical models of the PTC effect of barium titanate semiconducting ceramics include the sea surface barrier model, Daniels's barium vacancy model, and the superimposed barrier model. They have explained the PTC effect from different aspects.

Experiments show that within the operating temperature range, the resistance-temperature characteristics of PTC thermistors can be approximated by experimental formulas:

RT = RT0 ex-pop (T-T0)

In the formula, RT and RT0 represent the resistance value when the temperature is T and T0, and Bp is the material constant of the material.

The PTC effect originates from the nature of the ceramic grain boundaries and the precipitated phases between the grain boundaries and changes significantly with the type, concentration, and sintering conditions of impurities. Recently, practical thermistors are using silicon temperature-sensitive elements with silicon chips. This is a small and highly accurate PTC thermistor, which is composed of n-type silicon.

The PTC thermistor appeared in 1950, and then in 1954 the PTC thermistor with barium titanate as the main material appeared. PTC thermistors are used in industry for temperature measurement and control, and also for temperature detection and adjustment in certain parts of automobiles. They are also widely used in civilian equipment, such as controlling the water temperature of instant water heaters, the temperature of air conditioners and, using its heating for gas analysis and wind speed. The following introduces an example of the application of heaters, motors, transformers, high-power transistors, and other appliances in heating and overheating protection.

In addition to being used as a heating element, the PTC thermistor can also function as a "switch". It has three functions: a sensitive element, a heater, and a switch. It is called a "thermal switch". After the current passes through the element, the temperature of the heating element rises. When the temperature exceeds the Curie point temperature, the resistance increases, thereby limiting the current increase. Therefore, the decrease in current causes the component temperature to decrease, and the decrease in resistance value causes the circuit current to rise. As the temperature increases, the component temperature rinses and repeats. Therefore, it has the function of keeping the temperature in a specific range, and it also functions as a switch. Utilizing this temperature resistance characteristic to make a heating source. As heating elements, there are heaters, electric irons, drying cabinets, air conditioners, etc., and they can also protect the electrical appliances from overheating.

2. NTC Thermistors

 NTC Thermistors

NTC (Negative Temperature Coefficient) Thermistor refers to a thermistor with a negative temperature coefficient that decreases exponentially with temperature. Its material is a semi-conductor ceramic made of two or more metal oxides such as manganese, copper, silicon, cobalt, iron, nickel, zinc, etc., which are fully mixed, formed, and sintered. It can be made with a negative temperature coefficient (NTC) thermistor. Its resistivity and material constant vary with different material composition ratios, sintering atmosphere, sintering temperature, and structural state. Non-oxide NTC thermistor materials such as silicon carbide, tin selenide, and tantalum nitride have also appeared.

Most of the NTC heat-sensitive semi-conductive ceramics are oxide ceramics with spinel structures or other structures. They have a negative temperature coefficient. The resistance value can be approximated as:

Rt = RT * EXP (Bn * (1 / T-1 / T0)

Where RT and RT0 are the resistance values at temperature T and T0, respectively, and Bn is the material constant. The ceramic grain itself changes its resistivity due to temperature changes, which are determined by the characteristics of the semiconductor.

The development of NTC thermistors has gone through a long period. In 1834, scientists first discovered that silver sulfide has a negative temperature coefficient. In 1930, scientists discovered that cuprous oxide-copper oxide also has the performance of a negative temperature coefficient, and successfully used it in the temperature compensation circuit of aviation instruments. Subsequently, due to the continuous development of transistor technology, significant progress has been made in thermistor research. NTC thermistor was developed in 1960. NTC thermistors are widely used in temperature measurement, temperature control, and temperature compensation. under

The accuracy of the thermistor thermometer can reach 0.1 ℃, and the temperature sensing time can be less than 10s. It is not only suitable for granary thermometers, but also temperature measurement in food storage, medicine, and health, scientific farming, oceans, deep wells, high altitudes, glaciers, etc.

3. CTR 

The critical temperature thermistor CTR (Critical Temperature Resistor) has a sudden change in negative resistance. At a certain temperature, the resistance value decreases sharply with increasing temperature and has a large negative temperature coefficient. The constituent material is a mixed sintered body of element oxides such as vanadium, barium, strontium, and phosphorus. It is a semi-glassy semiconductor, also known as a glass thermistor. The sudden temperature changes with the addition of germanium, tungsten, molybdenum, and other oxides. This is due to the difference in the lattice spacing of vanadium oxide due to the incorporation of different impurities. If the vanadium pentoxide becomes vanadium dioxide in an appropriate reducing atmosphere, the temperature of the electrical resistance changes rapidly; if it is further reduced to vanadium trioxide, the rapid change disappears. The temperature at which a sudden change in resistance occurs corresponds to the location of the semi-glass semiconductor's sudden change in physical properties, so a semiconductor-metal phase shift occurs. CTR can be used as a temperature control alarm and other applications.

III. Thermistor symbol

What does the letter in the electrical symbol of the thermistor mean, some are o and some are VM. Those with o are thermistors, and U is the varistor. The resistance of the thermistor changes with the outside temperature. Some have a negative temperature coefficient and are represented by NTC; some have a positive temperature coefficient and are represented by PTC. Use θ or t ° to express temperature. Its text symbol is "RT". In the circuit diagram, the symbols of the photoresistor and thermistor are expressed as:

symbols of the photoresistor and thermistor 

Representation of thermistor in the circuit diagram：

the thermistor in the circuit diagram

IV. Thermistor Test

When testing, use the multimeter ohm range (depending on the nominal resistance value to determine the range, generally R × 1 range), which can be divided into two steps: First, the normal temperature test (indoor temperature is close to 25 °C), use the crocodile clip instead of the test lead. Measuring the actual resistance of the two pins of the PTC thermistor and compare it with the nominal resistance. It is normal for the difference between the two to be within ± 2Ω. If the actual resistance value is above ± 2Ω; from the nominal resistance value, it indicates that its performance is poor or damaged. Secondly, based on the normal temperature test, the second step test can be a performed-heating test, heating a heat source (such as an electric soldering iron) close to the thermistor, and observing the universal indicator. It is seen that the universal indicator changes with increasing temperature, which indicates that the resistance value is gradually changing (the resistance value of the NTC of the negative temperature coefficient thermistor will become smaller, and the resistance value of the PTC of the positive temperature coefficient thermistor will become larger). When the resistance value changes to a certain value, the display data will gradually stabilize, indicating that the thermistor is normal. If the resistance value does not change, it indicates that its performance is degraded and cannot be used further.

The following points should be paid attention to during the test: 

(1) Rt is measured by the manufacturer when the ambient temperature is 25 °C, so when measuring Rt with a multimeter, it should also be performed when the ambient temperature is close to 25 °C to ensure that the test can be performed. 

(2) The measurement power must not exceed the specified value, to avoid measurement errors caused by current thermal effects. 

(3) During the test, do not pinch the thermistor with your hand to prevent the human temperature from affecting the test. 

(4) Be careful not to place the heat source too close to the PTC thermistor or directly contact the thermistor to prevent it from being burned.

V. Applications

gas analyzer

The use of thermistors is very wide, the main applications are: Use non-linear characteristics to complete the functions of voltage stabilization, limiting, switching, and overcurrent protection; Use the difference of heat dissipation characteristics in different media to measure flow rate, flow rate, liquid level, thermal conductivity, vacuum degree, etc.; Use thermal inertia as a time delay.

Thermistors can also be used as electronic circuit components for instrument line temperature compensation and temperature difference cold junction temperature compensation. The self-heating characteristic of the NTC thermistor can be used to realize automatic gain control, which forms the RC oscillator amplitude stabilization circuit, delay circuit, and protection circuit. When the self-heating temperature is much greater than the ambient temperature, the resistance value is also related to the heat dissipation conditions of the environment. Therefore, the characteristics of the thermistor are often used in flow meters, flow meters, gas analyzers, and thermal analysis to make special detection elements. PTC thermistors are mainly used for overheat protection of electrical equipment, non-contact relays, constant temperature, automatic gain control, motor startup, time delay, automatic demagnetization of color TVs, fire alarms, and temperature compensation.

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