Introduce thermistor and related applications

2019-09-27  672

The resistance value of thermistor increases linearly with the increase of temperature. The shape of thermistor is an axial lead glass encapsulation structure. It has the advantages of small size, strong structure, standardized shape, high precision and fast response. It also has strong stability and repeatability. The millions of times characteristic curve of thermistor is unchanged. It can be used in high temperature and humidity. It is often used in temperature measurement and control in communication, automobile, instrumentation, calculator, household appliances and other industries. This paper will introduce the thermistor and its related applications.

I. Introduction

Thermistor (Thermal Resistor), abbreviated as TSR, is a temperature-sensitive resistor. Its main function is to provide power-off function, similar to fuse. The difference between fuse and fuse is that the fuse can not restore power-on function after the current is too large to be cut off, and new insurance needs to be replaced. Wire; Thermistor will automatically restore the power-on function if the current and temperature return to normal again after the power-off due to abnormal current. It can be reused without replacing parts to achieve the purpose of protecting electronic components. [2] Thermistors have a wide range of applications, such as measuring equipment for consumer electronics, toys, household appliances, industrial measuring products and personal computers. Thermistors are the most easily accessible temperature sensing components. Thermistor can change the value of resistor according to the ambient temperature. If we can understand its characteristics, improve its shortcomings, reduce reading temperature errors and enhance its accuracy, it will reduce circuit damage, not only reduce the cost of manufacturers, but also expand its application areas, widely used in electronics, information, communications, household appliances, automobiles, biomedical, aerospace and other related industries. [3] Thermistors, like RTD, are thermosensitive semiconductors, and their resistance varies with temperature. Thermistors consist of metal oxide semiconductor (MOS) materials and are enclosed in glass or epoxy resin. Meanwhile, the Nominal resistance of thermistors is generally higher than RTD (from 2,000 to 10,000 &Ugrave); and can be used for lower current.

Types and characteristics of thermistors

Thermistor is a semiconductor module whose resistance value is very sensitive to temperature. It can be divided into positive temperature coefficient (PTC) and negative temperature coefficient (NTC) thermistors according to different materials. [3]

(1) Positive Temperature Coefficient (PTC): According to the raw materials used, it can be divided into two categories: Ceramic PTC thermistor and Polymeric PTC thermistor. CPTC thermistor is made of barium titanate, titanium dioxide and other materials with a small amount of rare earth elements added and sintered at high temperature. This kind of component will maintain a stable low resistance value in a certain temperature range, until the temperature is higher than the temperature of the material, its resistance value will increase substantially.

PPTC thermistors are mainly made of polyethylene and conductive carbon black particles. When there is too much current flowing through the device, it will expand due to heating; its expansion will disperse the carbon particles and increase the impedance. This causes the device to heat faster and expand more, the impedance increases again, and the current in the circuit decreases significantly. When the power supply and fault are removed, it will shrink to its original shape and return to a low impedance state.

(2) Negative Temperature Coefficient (NTC): NTC is a semiconductor ceramic module made of metal oxides such as manganese, cobalt, nickel and copper. Because these metal oxides have semiconductor properties, their conductive modes are similar to those of semiconductors such as germanium and silicon. At low temperatures, the number of carriers (electrons) of these oxide materials is small. With the increase of temperature, the number of carriers increases and the resistance decreases. According to different applications, the shape of NTC thermistors can be divided into spherical, wafer, baseball, diode or wafer shapes. Most of them have high heat resistance, high reliability and high precision. Used as temperature measurement, temperature control or circuit temperature compensation.

III. Matters for Attention in Use

(1) Self-heating

Because a thermistor is a resistor, the current flowing through it will generate a certain amount of heat, so circuit designers should ensure that the series resistor is large enough to prevent the thermistor from overheating. Otherwise, the system measures the heat emitted by the thermistor rather than the temperature of the surrounding environment.

(2) Thermal runaway phenomenon

For example, when NTC is used, the resistance decreases with the increase of temperature, which will increase the current flowing through NTC. At this time, the power loss and current of NTC will increase rapidly by square times. When the power loss increases, the thermal effect will increase, the temperature will rise, and the resistance value of NTC will decrease again, and the current will follow. It increases again, so that the cycle is finally burned out because it exceeds the watt number of NTC loads.

(3) Cumulative error

The resistance of thermistors varies with temperature. In order to obtain accuracy, attention should be paid to the accuracy of components, including resistors, reference voltages and thermistors themselves, and the cumulative errors of all components should be calculated carefully in order to match the accuracy of thermistors.

(4) Different reaction rates

Different types of thermistors have different changing speeds because of different temperatures. Therefore, different types of thermistors must be selected according to different uses.