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Jan 22 2019

What Is Thermistor and How Does It Works?

Warm hints: This article contains about 4000 words and reading time is about 18 mins.

Introduction

The thermistor is a semiconductor ceramic component made of transition metal oxide as the main raw material. It belongs to the category of negative temperature coefficient thermistor, and it has the characteristic that the resistance value changes with the change of temperature, that is, the resistance value The temperature rises and falls. By using this characteristic, when it is connected in series in the power supply loop, the startup surge current can be effectively suppressed and the resistance value of the power type NTC thermistor can be utilized by the continuous action of the current after the completion of the suppression of the surge current. It drops to a very small extent and can also be used for temperature measurement and temperature compensation in metering devices, transistor circuits. The thermistor is connected in series in the circuit, mainly for "current insurance".

Article Core

Thermistor and its working principle.

Purpose

Introduce what the thermistor is and how it works.

Application

Semiconductor industry.

Keywords

Thermistor.

Catalog

Introduction


What Is A Thermistor?


The Shape and Symbol of the Thermistor


Thermistor Circuit Diagram Symbol


The Main Features of the Thermistor


 

The Role of the Thermistor

Temperature Measurement

Temperature Compensation

Overheating Protection

Liquid Level Measurement

Thermistor Symbol


Representation of the Thermistor In the Circuit Diagram


Thermistor Model


Working Principle of Thermistor


Working Principle of Thermistor


Thermistor Application Circuits



What Is A Thermistor?

In order to avoid the inrush current generated in the electronic circuit at the moment of power-on, a power type NTC thermistor is connected in series in the power supply circuit, which can effectively suppress the inrush current at the time of starting, and after the suppression of the surge current is completed, Due to the continuous action of its current, the resistance value of the power type NTC thermistor will drop to a very small extent, and the power consumed by it will be neglected and will not affect the normal operating current. Therefore, in the power supply circuit The use of power-type NTC thermistors is the easiest and most effective way to suppress surges during startup to protect electronic equipment from damage.

Thermistor for measuring/controlling temperature

Thermistors are sensitive components that are developed early, have many types, and are more mature. The thermistor is composed of a semiconducting ceramic material, and the principle of utilization is temperature-induced resistance change. If the electron and hole concentrations are n and p, respectively, and the mobility is μn, μp, then the conductance of the semiconductor is: σ=q(nμn+pμp) because n, p, μn, and μp are all functions dependent on temperature T. Therefore, the conductance is a function of temperature, so the temperature can be derived from the measured conductance and the resistance-temperature characteristic curve can be made. This is how semiconductor thermistors work. Thermistors include positive temperature coefficient (PTC) and negative temperature coefficient (NTC) thermistors, as well as critical temperature thermistors (CTR).

1. Rated zero-power resistor R25 Zero-power resistor refers to the extremely low power consumption of the PTC thermistor when measuring the PTC thermistor value at a certain temperature, as low as the PTC thermal due to its power consumption. The resistance change of the resistor is negligible. The rated zero power resistance refers to the zero power resistance measured at an ambient temperature of 25 °C.

2. Curie temperature Tc For PTC thermistor applications, the temperature at which the resistance value begins to increase steeply is important. We define it as the Curie temperature. The resistance of the PTC thermistor corresponding to the Curie temperature RTc = 2*Rmin.

3. Temperature coefficient The temperature coefficient of the α PTC thermistor is defined as the relative change in resistance caused by temperature changes. The greater the temperature coefficient, the more sensitive the PTC thermistor reacts to temperature changes. α = (lgR2-lgR1)/lge(T2-T1)

4. Rated voltage VN The rated voltage is the supply voltage below the maximum working voltage Vmax. Usually Vmax = VN + 15%

5. Breakdown voltage VD breakdown voltage refers to the highest voltage withstand capability of the PTC thermistor. The PTC thermistor will fail breakdown above the breakdown voltage.

6. Surface temperature Tsurf The surface temperature Tsurf refers to the temperature of the surface of the PTC thermistor when the PTC thermistor is in a state of thermal equilibrium with the ambient voltage for a long period of time.

7. The operating current Ik flows through the PTC thermistor enough to cause the PTC thermistor to rise above the Curie temperature. This current is called the operating current. The minimum value of the operating current is called the minimum operating current.

8. Non-operating current INk The current flowing through the PTC thermistor is insufficient to make the self-heating temperature of the PTC thermistor rise above the Curie temperature. Such a current is called a non-operating current. The maximum value of the non-operating current is called the maximum non-operating current.


The Shape and Symbol of the Thermistor

The Shape and Symbol of the Thermistor


Thermistor Circuit Diagram Symbol

A thermistor is a resistor whose resistance value is extremely sensitive to temperature, also called a semiconductor thermistor. It can be made of single crystal, polycrystalline, and semiconductor materials such as glass and plastic. This resistor has a series of special electrical properties. The most basic characteristic is that its resistance varies greatly with temperature and the volt-ampere curve is nonlinear.

The symbols in the circuit are as follows:

Thermistor Circuit Diagram Symbol


The Main Features of the Thermistor

1. The sensitivity is higher, and the temperature coefficient of resistance is 10~100 times larger than that of metal, and the temperature change of 10-6 °C can be detected;

2. Wide operating temperature range, room temperature device is suitable for -55 ° C ~ 315 ° C, high temperature device temperature is higher than 315 ° C (currently up to 2000 ° C), low temperature device is suitable for -273 ° C ~ 55 ° C;

3. Small size, able to measure the temperature of the voids, cavities and blood vessels in the body that other thermometers cannot measure;

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

5. Easy to process into complex shapes, can be produced in large quantities;

6. Good stability and strong overload capability.


The Role of the Thermistor

Temperature Measurement

As a thermistor sensor for measuring temperature, the structure is relatively simple and inexpensive. Thermistors without an outer protective layer can only be used in dry places; sealed thermistors are not afraid of moisture and can be used in harsh environments. Since the resistance value of the thermistor sensor is large, the resistance and contact resistance of the connecting wire can be neglected, so the thermistor sensor can be applied in the long-distance measurement temperature of several kilometers, and the measuring circuit adopts a bridge.

Temperature Measurement


Temperature Compensation

The thermistor sensor compensates for the humidity of certain components over a range of temperatures. For example, the moving coil in the head of the moving coil type meter is wound by a copper wire, and the temperature rises and the resistance increases, causing a temperature error. Therefore, the negative temperature coefficient thermistor and the manganese copper wire resistance can be connected in parallel in the loop of the moving coil and then connected in series with the compensated component, thereby offsetting the error caused by the temperature change.


Overheating Protection

Overheat protection is directly protected against indirect protection. For small current applications, the thermistor sensor can be directly connected to the load to prevent overheating damage to protect the device. For large current applications, it can be used to protect relays and transistor circuits. For example, a sudden-type thermistor sensor is embedded in the stator winding of the motor and connected in series with the relay. When the motor is overloaded, the stator current increases, causing heat. When the temperature is greater than the sudden change point, the current in the circuit can be mutated to a few tens of milliamps within a few tenths of milliamperes, so the relay acts to achieve overheat protection.

Overheating Protection



Liquid Level Measurement

A certain heating current is applied to the NTC thermistor sensor, and its surface temperature will be higher than the ambient air temperature, at which time its resistance is small. When the liquid is higher than its installation height, the liquid will take away its heat, causing the temperature to drop and the resistance to rise. Judging its resistance change, you can know whether the liquid level is lower than the set value. The oil level alarm sensor in the car fuel tank is made using the above principle.

Liquid Level Measurement


Thermistor Symbol

What does the letter in the electrical symbol of the thermistor mean, some are vm, and the one with O is a thermistor. With U is the varistor thermistor symbol thermistor resistance value varies with the outside temperature. Some are negative temperature coefficients, expressed by NTC; some are positive temperature coefficients, expressed by PTC. The temperature is expressed by θ or t°. Its text symbol is "RT". In the circuit diagram, the signs of the photoresistor and the thermistor are expressed as:

Thermistor Symbol


Representation of the Thermistor In the Circuit Diagram

Representation of the Thermistor In the Circuit Diagram


Model

The positive temperature coefficient thermistor is abbreviated as PTC (short for Positive Temperature Coefficient), which exceeds a certain temperature (Curie temperature - Curie temperature refers to the temperature at which the material can change between ferromagnet and paramagnet. Below Curie At the temperature, the substance becomes a ferromagnetic body, and the magnetic field associated with the material is difficult to change. When the temperature is higher than the Curie temperature, the substance becomes a paramagnetic body, and the magnetic field of the magnet easily changes with the change of the surrounding magnetic field. When the magnetic sensitivity is about 10 negative 6th power.), its resistance value increases stepwise with increasing temperature. The principle is to introduce trace rare earth elements such as La, Nb. into ceramic materials. . Etc., the resistivity can be reduced to below 10 Ω·cm, which is a good semiconductor ceramic material. This material has a large positive temperature coefficient of resistance, and its resistivity can be increased by 4 to 10 orders of magnitude in the temperature range of several tens of degrees above the Curie temperature, that is, a so-called PTC effect is produced. A large number of PTC thermistors currently used: PTC thermistors for constant temperature heating; PTC thermistors for low voltage heating; thermistors for air heating; PTC thermistors for overcurrent protection; PTC thermistors for overheat protection Resistance; PTC thermistor for temperature sensing; PTC thermistor for delay start; NTC (short for Negative Temperature Coefficient), whose resistance decreases with increasing temperature of. It is mainly made of metal oxides such as manganese, cobalt, nickel and copper, and is made by ceramic technology. These metal oxide materials all have semiconductor properties because they are completely similar in electrical conductivity to semiconductor materials such as germanium and silicon. NTC thermistor temperature coefficient -2% ~ -6.5%, can be widely used in temperature measurement, temperature compensation, suppression of surge current and other occasions.

The following are some examples of models for reference only:

Thermistor CWF2-5K±3%40CM; Thermistor CWF2-10K±3%1.5M; Thermistor 5D-9

Thermistor 16D-9; Thermistor 5D-11; Thermistor 16D-11; Thermistor 5D-13

Thermistor 16D-13; thermistor 5D-15; thermistor CWF2-5K±5%160CM,

Thermistor 5D-20; Thermistor CWF2-10K±1%2M; Thermistor CWF52-5K±5%60CM


Working Principle of Thermistor

The thermistor will be in a non-operating state for a long time; when the ambient temperature and current are in the c-zone, the heat-dissipating power of the thermistor is close to the heating power, and thus may or may not operate. When the thermistor has the same ambient temperature, the operating time is sharply shortened as the current increases. The thermistor has a shorter operating time and a smaller holding current and operating current when the ambient temperature is relatively high.

1. The ptc effect is a material with a PTC (positive temperature coefficient) effect, that is, a positive temperature coefficient effect, which only means that the resistance of the material increases with increasing temperature. For example, most metal materials have a ptc effect. Among these materials, the ptc effect appears as a linear increase in electrical resistance with increasing temperature, which is known as the linear ptc effect.

2. Non-linear ptc effect through the phase change material will appear a sharp increase in resistance along the narrow temperature range of several to more than a dozen orders of magnitude, that is, nonlinear ptc effect, a considerable number of types of conductive polymers will present this Effects such as polymer ptc thermistors. These conductive polymers are very useful for making overcurrent protection devices.

3. Polymer ptc thermistor for overcurrent protection polymer ptc thermistor is often referred to as self-recovery fuse (hereinafter referred to as thermistor), because of its unique positive temperature coefficient resistance characteristics, it is very suitable Used as an overcurrent protection device. The thermistor is used in the same way as a normal fuse and is used in series in the circuit.

When the circuit works normally, the thermistor temperature is close to room temperature and the resistance is very small. The series connection in the circuit does not hinder the current from passing through; and when the circuit has an overcurrent due to the fault, the thermistor increases the temperature due to the increase of the heating power. When the temperature exceeds the switching temperature , the resistance will increase sharply, and the current in the loop will quickly decrease to a safe value. It is a schematic diagram of the current change during the protection of the thermistor to the AC circuit. After the thermistor is activated, the current in the circuit is greatly reduced. In the figure, t is the operating time of the thermistor. Since the polymer ptc thermistor has good designability, it can adjust its sensitivity to temperature by changing its switching temperature , so it can play both over-temperature protection and over-current protection, such as kt16. The -1700dl specification thermistor is suitable for overcurrent and overtemperature protection of Li-Ion and NiMH batteries due to its low operating temperature. The influence of ambient temperature on polymer PTC thermistor The polymer PTC thermistor is a direct-heating, step-type thermistor whose resistance change process is related to its own heat and heat dissipation, so its current is maintained (ihold ), the action current (itrip) and the operating time are affected by the ambient temperature. When the ambient temperature and current are in the a zone, the thermistor heating power is greater than the heat dissipation power and will act; when the ambient temperature and current are in the b zone, the heating power is less than the heat dissipation power, and the polymer ptc thermistor can be recovered due to the resistance, so Repeat multiple uses. Figure 6 is a schematic diagram of the resistance change with time during the recovery process after the thermistor is activated. The resistance is generally restored to a level of about 1.6 times the initial value in a few ten seconds to several tens of seconds. At this time, the holding current of the thermistor has returned to the rated value and can be used again. Thermistors with smaller areas and thicknesses recover relatively quickly; while thermistors with larger areas and thicknesses recover relatively slowly.


Main Parameters of the Thermistor

Room temperature resistance value: also known as nominal resistance value, refers to the resistance value at 250C temperature.

Minimum resistance value: The resistivity at zero power of the component is the resistance value at the lowest point of the temperature characteristic curve.

Maximum resistance value: The resistivity at zero power of the component is the resistance value at the highest point of the temperature characteristic curve.

Temperature coefficient: The coefficient of change in resistance caused by temperature change. The larger the temperature coefficient, the more sensitive the thermistor reacts to temperature changes.

Rated voltage: The voltage required for the thermistor to operate stably.

Curie temperature: For a thermistor, the temperature at which the resistance value begins to rise steeply is important. This temperature is called the Curie temperature.


Thermistor Application Circuits

Thermistor Application Circuit (1)

The figure is a temperature compensation circuit using a thermistor. It is a temperature compensation circuit of infrared light-emitting diode VD1. VD1 is used as a photoelectric detector for modulating light. The maximum current is 5OmA and the temperature range is 10-55 °C. RT is a negative temperature coefficient thermistor connected to the base of VT1 to reduce the self-heating effect of the thermistor. RP2 and RP3 are used to adjust the temperature compensation characteristics. VD2 is the receiver, and the received signal is amplified by the TAA761.

Thermistor Application Circuit (1)


Thermistor Application Circuit (2)

Thermistor Application Circuit (2)


Thermistor Application Circuit (3)

Thermistor Application Circuit (3)


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