Abstract: The thermistor is a sensor resistor whose resistance value changes as the temperature changes. The working principle of the thermistor is to use a sensor to help adjust the temperature. Its functions include voltage adjustment, volume control, time delay and circuit protection. The thermistor has the functions of temperature measurement, temperature compensation, overheating protection, and liquid level measurement. Let’s follow TOPCHIP ELECTRONICS to learn about thermistors.
What is a thermistor
The thermistor is a type of sensitive element. According to different temperature coefficients, it is divided into positive temperature coefficient thermistor (PTC) and negative temperature coefficient thermistor (NTC). 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 greater resistance value when the temperature is higher, and the negative temperature coefficient thermistor (NTC) has a lower resistance value when the temperature is higher. They are both semiconductor devices.
How the thermistor works
The thermistor is a sensor resistance. The resistance value of the thermistor changes with the change of temperature, which is different from the general fixed resistance. The resistance value of metal increases with the increase of temperature, but the opposite is true for semiconductors. Its resistance value decreases sharply with the increase of temperature and shows nonlinearity. When the temperature changes are the same, the resistance change of the thermistor is about 10 times that of the lead thermal resistor. Therefore, it can be said that the thermistor is particularly sensitive to changes in temperature. This temperature characteristic of semiconductors is because the conductive method of semiconductors is carrier (electron, hole) conduction. Since the number of carriers in a semiconductor is much smaller than the number of free electrons in a metal, its resistivity is very high. As the temperature increases, the number of conductive carriers in the semiconductor will increase, so the conductivity of the semiconductor will increase and its resistivity will decrease.
The thermistor is a thermal element made by utilizing the characteristic that the resistance value of semiconductors changes significantly with temperature. It is made from certain metal oxides in different formulations. Within a certain temperature range, the temperature change of the measured medium can be known by measuring the change in the resistance of the thermistor.
When the thermistor is installed in a circuit and used, when the ambient temperature is the same, the action time of the thermistor shortens sharply as the current increases; when the ambient temperature is relatively high, the thermistor has a shorter action time and smaller of holding current and operating current. When the circuit is working normally, the temperature of the thermistor is close to room temperature and the resistance is very small, so being connected in series in the circuit will not hinder the flow of current; when an overcurrent occurs due to a fault in the circuit, the temperature of the thermistor increases due to the increase in heating power. When the temperature exceeds the switching temperature, the resistance will increase sharply in an instant, and the current in the loop will quickly decrease to a safe value.
The role of thermistor
1. Temperature measurement. As a thermistor sensor for measuring temperature, the structure is generally simple and the price is relatively low;
2. Temperature compensation. Thermistor sensors can compensate for the humidity of certain components within a certain temperature range;
3. Overheating protection. When the temperature is greater than the mutation point, the current in the circuit can suddenly change from a few tenths of a milliampere to tens of milliamperes, so the relay operates to achieve overheating protection;
4. Liquid level measurement.
4. Thermistor model
There are three types of thermistors:
1. PTC refers to a thermistor phenomenon or material whose resistance increases sharply at a certain temperature and has a positive temperature coefficient.
2. NTC refers to the thermistor phenomenon and material whose resistance decreases exponentially as the temperature increases and has a negative temperature coefficient.
3. CTR (critical temperature thermistor) has negative resistance mutation characteristics.
Thermistor parameters
1. Nominal resistance Rc: Generally refers to the actual resistance value of the thermistor when the ambient temperature is 25°C.
2. Actual resistance RT: resistance value measured under certain temperature conditions.
3. Material constant: It is a parameter that describes the physical properties of the thermistor material and is also a thermal sensitivity index. The larger the B value, the higher the sensitivity of the thermistor. It should be noted that in actual operation, the B value is not a constant, but increases slightly as the temperature increases.
4. Resistance temperature coefficient αT: It represents the resistance change rate when the temperature changes by 1℃, the unit is %/℃.
5. Time constant τ: The thermistor has thermal inertia. The time constant is a parameter that describes the thermal inertia of the thermistor. It is defined as the time required for the temperature of the thermistor body to change by 63.2% of the difference between the two specific temperatures when the ambient temperature suddenly changes from one specific temperature to another specific temperature in a state of no power consumption. . The smaller τ is, the smaller the thermal inertia of the thermistor is.
6. Rated power PM: Under the specified technical conditions, the power dissipation allowed by the thermistor for long-term continuous load. The rated power must not be exceeded during actual use. If the ambient temperature where the thermistor operates exceeds 25°C, its load must be reduced accordingly.
7. Rated working current IM: The nominal current value specified by the thermistor under working conditions.
8. Measurement power Pc: At the specified ambient temperature, the electrical power consumed when the resistance change caused by the heating of the thermistor by the test current does not exceed 0.1%.
9. Maximum voltage: For NTC thermistors, it refers to the maximum DC voltage allowed to be continuously applied without causing thermal runaway of the thermistor under the specified ambient temperature; for PTC thermistors, it refers to the specified The ambient temperature and still air allow continuous application to the thermistor and ensure that the thermistor operates normally within the maximum DC voltage of the PTC characteristic part.
10. Maximum operating temperature Tmax: Under specified technical conditions, the maximum temperature allowed for long-term continuous operation of the thermistor.
11. Switching temperature tb: the temperature at which the resistance value of the PTC thermistor begins to increase.
12. Dissipation coefficient H: The power dissipated by the thermistor when the temperature increases by 1°C, in mW/°C.