NTC Thermistors NTC (Negative Temperature Coefficient) refers to the phenomenon and materials of a thermistor whose resistance decreases exponentially with increasing temperature, exhibiting a negative temperature coefficient. These materials are semiconductor ceramics produced by thoroughly mixing, molding, and sintering two or more metal oxides such as manganese, copper, silicon, cobalt, iron, nickel, and zinc. Their resistivity and material constant vary depending on the proportion of material components, sintering atmosphere, sintering temperature, and structural state. Currently, non-oxide-based NTC thermistor materials, such as silicon carbide, tin selenide, and tantalum nitride, have also emerged.
Most NTC thermistor ceramics are spinel-structured or other oxide ceramics with a negative temperature coefficient. Their resistance can be approximated as:
Where RT and RT0 are the resistance values at temperatures T and T0, respectively, and Bn is the material constant. The resistivity of the ceramic grains themselves changes due to temperature variations, which is determined by the semiconductor properties. The development of NTC thermistors has undergone a long process. In 1834, scientists first discovered that silver sulfide has a negative temperature coefficient. In 1930, scientists discovered that cuprous oxide-copper oxide also has a negative temperature coefficient and successfully applied it to temperature compensation circuits in aviation instruments. Subsequently, due to the continuous development of transistor technology, the research on thermistors made significant progress. In 1960, the N1C thermistor was developed. NTC thermistors are widely used in temperature measurement, temperature control, and temperature compensation.
Its measurement range is generally -10 to +300℃, but can also be -200 to +10℃, and even used in environments of +300 to +1200℃. RT is the NTC thermistor; R2 and R3 are bridge balancing resistors; R1 is the starting resistor; R4 is the full-scale resistor, used to calibrate the meter, also called the calibration resistor; R7, R8, and W are voltage divider resistors, providing a stable DC power supply to the bridge. R6 is connected in series with the meter (microammeter) to correct the meter scale and limit the current flowing through it. R5 is connected in parallel with the meter for protection. A thermistor RT is connected as a temperature sensing probe in the unbalanced bridge arm (i.e., R1, RT). Because the resistance of the thermistor changes with temperature, the meter reading across the diagonal of the bridge changes accordingly. This is the working principle of a thermistor thermometer.
Thermistor thermometers can achieve an accuracy of 0.1℃ and a sensing time as short as 10 seconds. They are suitable not only for grain storage temperature measurement but also for temperature measurement in food storage, medicine and hygiene, scientific farming, oceans, deep wells, high altitudes, glaciers, and other fields.
