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Negative temp coefficient thermistor, also known as NTC thermistor, is a kind of sensor resistance whose resistance value decreases with increasing temperature. It uses semiconductor materials with a large negative temp coefficient. The resistors are widely used in switching power supplies, UPS power supplies, electronic ballasts, temperature sensors, automatic adjustment heaters and others. The following describes the functions of the thermistor in three aspects: surge absorption, temperature measurement and temperature compensation.
To suppressing surge current. This thermistor is a power type, although it is small in size, it has a large power, generally connected in series on the mains input lines. It has a rated zero-power resistance value, and this resistance value is very small in general. Zero power resistance is the basic parameter of thermistor, which is usually given by manufacturers. When connected in series in the power circuit, it can effectively suppress the startup surge current, and its power consumption can be almost ignored.
The characteristics of power thermistors are: ① Small size and high power; ②Strong surge current suppression capability and fast response; ③ Large material constant (b value); ④ Long service life, high reliability, and wide working range.
As a temperature sensor for measuring temperature, and as a thermistor for measuring temperature, it uses the change of outside temperature to change the resistance value, because when the negative temp coefficient thermistor resistor is connected to the circuit, it will always pass a certain amount of current to make the NTC heat up, and resistance value drop, which will have a great influence on the measurement. Therefore, it is necessary to control its own heating to avoid excessive current flowing through the thermistor and measurement errors are caused by the heating of the component itself. Write all or part of the resistance value corresponding to the temperature into the CPU, so that when the external temperature changes, the change in resistance is realized as a change in voltage. This kind of package includes chip type, epoxy head type, glass sealing type, straw hat type and so on. According to the principle of resistor divider, when the NTC changes, the voltage also changes. The A/D port is used to detect voltage changes, and all the data used by the thermistor is written into the single-chip microcomputer. Different resistance values correspond to different temperatures, and the program is constantly checking.
If the temperature accuracy is higher, NTC with higher accuracy can be used. At the same time, a bridge circuit and an amplifier are used. The bridge circuit is composed of thermistors NT1/NT2, resistors R1 and R2. As long as the thermistor has a temperature difference, the amplifier will output the corresponding signal.
Temperature compensation requires high accuracy in some electrical appliances, especially in meters, where many parts are made of metal wires, such as wire-wound resistors. Metals generally have a positive temp coefficient, which can be compensated with a negative temp coefficient thermistor. One positive and one negative can offset errors caused by temperature changes and improve measurement accuracy. As temperature compensation, the alloy copper wire resistance is usually connected in parallel with the NTC thermistor, and then connected in series with the compensation element.