Working principle of semiconductor thermistor

According to the temperature characteristics of thermistors can be divided into two types, with the increase of resistance with the temperature of positive temperature coefficient thermistors, conversely, negative temperature coefficient thermistors.

⑴ The working principle of positive temperature coefficient thermistor

This thermistor is sintered at high temperature by ceramic process with barium titanate (BaTio3) as the basic material and some rare earth elements added. Pure barium titanate is a kind of insulating material, but after adding the right amount of rare earth elements such as lanthanum (La) and niobium (Nb), it becomes a semiconductor material, known as barium titanate semiconductor. It is a polycrystalline material, and there is a grain interface between the grains, which is equivalent to a barrier for conducting electrons. When the temperature is low, due to the role of the electric field in barium titanate semiconductor, conductive electrons can easily cross the barrier, so the resistance value is small; When the temperature rises to the Curie point temperature (the critical temperature, the Curie point of this element is generally 120℃ for barium titanate), the internal electric field is destroyed and cannot help conductive electrons cross the barrier, so it shows a sharp increase in resistance. Because this component has not reached the Curie point resistance with temperature change is very slow, with constant temperature, temperature regulation and automatic temperature control function, only heat, not red, no flame, not easy to burn, voltage cross, DC 3 ~ 440V can, long service life, is very suitable for electric motors and other electrical devices overheating detection.

The working principle of negative temperature coefficient thermistor

Negative temperature coefficient thermistor is made of manganese oxide, cobalt oxide, nickel oxide, copper oxide, alumina and other metal oxides as the main raw material by ceramic process. These metal oxide materials have semiconductor properties, completely similar to germanium and silicon crystal materials, the number of carriers (electrons and holes) in the body is small, high resistance; As the temperature increases, the number of carriers in the body increases and the natural resistance decreases. A lot of negative temperature coefficient thermistor type, used to distinguish the low temperature (60 ~ 300 ℃), medium temperature (300 ~ 600 ℃), high temperature (> 600 ℃), three has a high sensitivity, good stability, fast response, long life, low price advantages, is widely applied in need point temperature automatic temperature control circuit, such as refrigerator, air conditioner, the greenhouse temperature control system.

Thermistor combined with a simple amplifier circuit, it can detect temperature changes in thousandths of a degree, so the electronic instrument and the composition of the thermometer, can complete high precision temperature measurement. General purpose thermistor working temperature is -55℃ ~ +315℃, special low temperature thermistor working temperature is lower than -55℃, up to -273℃.

Model of thermistor

Our domestic thermistor is made according to the ministry standard SJ1155-82 model, consists of four parts.

The first part: the main name, with the letter 'M' to indicate the sensitive element.

Part II: Categories, using the letter 'Z' for positive temperature coefficient thermistors, or using the letter 'F' for negative temperature coefficient thermistors.

Part III: Uses or features, represented by one digit (0-9). General numbers' 1 'for general use,' 2 'for voltage regulator (negative temperature coefficient thermistor),' 3 'for microwave measurement (negative temperature coefficient thermistor),' 4 'for para-thermal (negative temperature coefficient thermistor),' 5 'for temperature measurement,' 6 'for temperature control, '7' for demagnetization purposes (positive temperature coefficient thermistors), '8' for linear (negative temperature coefficient thermistors), '9' for constant temperature (positive temperature coefficient thermistors), and '0' for special (negative temperature coefficient thermistors)

Part FOUR: Serial number, also represented by numbers, on behalf of specifications, performance.

In order to distinguish the special needs of this series of products, the manufacturer often adds' derived serial number 'after the serial number, which is composed of letters, numbers and' - '.

Example: M Z 1 1

The serial number

General purpose

Positive temperature coefficient thermistor

Sensitive element

Main parameters of thermistor

The operating conditions of various thermistors must be within the allowable range of their factory parameters. The main parameters of thermistor are more than ten items: nominal resistance value, ambient temperature (maximum operating temperature), measured power, rated power, nominal voltage (maximum operating voltage), operating current, temperature coefficient, material constant, time constant, etc. The nominal resistance value is at 25℃ zero power resistance value, in fact there is always a certain error, should be within ±10%. Ordinary thermistor operating temperature range is large, can be selected from -55℃ to +315℃ according to need, it is worth noting that different types of thermistor maximum operating temperature difference is very large, such as MF11 sheet negative temperature coefficient thermistor for +125℃, and MF53-1 is only +70℃, students should pay attention to the experiment (generally not more than 50℃).