The development of electronics is gaining momentum every year. But, despite new inventions, devices designed at the beginning of the 20th century work reliably in electrical circuits. One such device is a thermistor. The form and purpose of this element are so diverse that only experienced electrical workers can quickly find it in the circuit. To understand what a thermistor is, you can only own knowledge about the structure and properties of conductors, dielectrics and semiconductors.
Content
- Description of the device
- Thermistor classification
- Technical characteristics and principle of operation
Description of the device
Temperature sensors are widely used in electrical engineering. Almost all mechanisms use analog and digital thermometer microcircuits, thermocouples, resistance sensors and thermistors. The prefix in the name of the device indicates that a thermistor is a device that depends on the influence of temperature. The amount of heat in the environment is the dominant indicator in its work. Due to heating or cooling, the parameters of the element change, a signal appears that is available for transmission to control or measurement mechanisms.
A thermistor is an electronic device in which the temperature and resistance values are inversely related.
There is also another name for it - thermistor. But this is not entirely correct, since in fact thermistor is one of the types of thermistor. A change in heat can affect the resistance of a resistive element in two ways: either increasing it or decreasing it.
Therefore, RTDs are subdivided by temperature coefficient into PTC (positive) and NTC (negative). РТС - resistors are called posistors, and NTC - thermistors.
The difference between RTS and NTC devices is the change in their properties when exposed to climatic conditions. The resistance of posistors is directly proportional to the amount of heat in the environment. When the NTC devices are heated, its value decreases.
Thus, an increase in the temperature of the posistor will lead to an increase in its resistance, and in the thermistor - to a drop.
View of a thermistor on electrical schematic diagrams similar to an ordinary resistor. A distinctive feature is a tilted straight line that crosses out the element. Thus, showing that the resistance is not constant, but can change depending on the increase or decrease in temperature in the environment.
The main substance for creating posistors is barium titanate. The manufacturing technology of NTC devices is more complex due to the mixing of various substances: semiconductors with impurities and glassy transition metal oxides.
Thermistor classification
The dimensions and design of thermistors are different and depend on their area of application.
The shape of the thermistors may resemble:
-
flat plate; - disk;
- kernel;
- washer;
- a tube;
- bead;
- cylinder.
The smallest bead-shaped thermistors. Their dimensions are less than 1 millimeter, and the characteristics of the elements are stable. The disadvantage is the impossibility of mutual substitution in electrical circuits.
Classification of thermistors by the number of degrees in Kelvin:
- super high temperature - from 900 to 1300;
- high-temperature - from 570 to 899;
- medium temperature - from 170 to 510;
- low temperature - up to 170.
The maximum heating, although admissible for thermoelements, affects their operation by deteriorating quality and the appearance of a significant error in the indicators.
Technical characteristics and principle of operation
The choice of a thermistor for a controlling or measuring mechanism is carried out according to the nominal passport or reference data. The principle of operation, basic characteristics and parameters of thermistors and posistors are similar. But there are some differences.
RTS - elements are assessed by three defining indicators: temperature and static volt-ampere characteristic, thermal coefficient of resistance (TCR).
The thermistor has a wider list.
In addition to the parameters similar to the posistor, the indicators are as follows:
- nominal resistance;
- dissipation factors, energy sensitivity and temperature;
- time constant;
- temperature and power to the maximum.
Of these indicators, the main ones that influence the selection and evaluation of a thermistor are:
- nominal resistance;
- thermal coefficient of resistance;
- power dissipation;
- operating temperature range.
The nominal resistance is determined at a specific temperature (most often twenty degrees Celsius). Its value in modern thermistors ranges from several tens to hundreds of thousands of ohms.
Some error in the value of the nominal resistance is acceptable. It can be no more than 20% and must be indicated in the passport data of the device.
TCS depends on heat. It sets the amount of change in resistance when the temperature fluctuates by one division. The index in its designation indicates the number of degrees Celsius or Kelvin at the time of measurement.
The release of heat on the part appears due to the flow of current through it when it is connected to an electrical circuit. The dissipated power is the value at which the resistive element heats up from 20 degrees Celsius to the maximum allowable temperature.
The operating temperature interval shows such a value at which the device works for a long time without errors and damage.
The principle of operation of thermal resistances is based on the change in their resistance under the influence of heat.
This happens for several reasons:
- due to phase transformation;
- ions with variable valence exchange electrons more vigorously;
- the concentration of charged particles in a semiconductor is distributed in a different way.
Thermistors are used in complex devices that are used in industry, agriculture, automotive electronics circuits. They are also found in appliances that surround a person in everyday life - washing machines, dishwashers, refrigerators and other equipment with temperature control.
