Linear Temperature Sensor Usage Guide

1. What is a linear NTC temperature sensor ? A linear temperature sensor is a negative temperature coefficient (NTC) thermistor with linear output. It is actually a linear temperature-voltage conversion element, that is, under the condition of a working current (100uA), the voltage value of the element changes linearly with the temperature, thereby realizing a linear conversion of non-electrical quantity to electrical quantity. 2. What are the main features of a linear NTC temperature sensor? The main feature of this temperature sensor is that the temperature-voltage relationship is a straight line within the operating temperature range. For the design of secondary development temperature measurement and temperature control circuits, there is no need for linearization processing, and the design of temperature measurement or temperature control circuits can be completed, thereby simplifying the design and debugging of the instrument. 3. How is the temperature measurement range of a linear NTC temperature sensor specified? In general, the temperature measurement range can be between -200 and +200℃, but considering actual needs, such a wide temperature range is generally not required, so three different sections are specified to adapt to different packaging designs, and the selection of extension wires is also different. For linear thermistors used for temperature compensation, the operating temperature range is only set to -40℃～+80℃. This can fully meet the temperature compensation requirements of general circuits. 4. What principles should be followed when selecting extension cables? Generally, ordinary double-rubber cables should be used in the range of -200～+20℃ and -50～+100℃; high-temperature cables should be used in the range of 100～200℃. 5. What is the meaning of reference voltage? Reference voltage refers to the voltage value on the sensor when the sensor is placed in a temperature field (ice-water mixture) of 0℃ and the working current (100μA) is passed through it. It is actually the voltage at 0 point. Its symbol is V(0), which is calibrated at the factory. Since the temperature coefficient S of the sensor is the same, as long as the reference voltage value V(0) is known, the sensor voltage value at any temperature point can be obtained without grading the sensor. The calculation formula is: V(T)=V(0)+S×T Example: If the reference voltage V(0)=700mV; the temperature coefficient S=-2mV/℃, then at 50℃, the output voltage of the sensor V(50)=700-2×50=600(mV). This is the valuable point that linear temperature sensors are superior to other temperature sensors. 6. What is the meaning of temperature coefficient S? Temperature coefficient S refers to the ratio of the change in the output voltage value of the sensor to the change in temperature under specified working conditions, that is, the value of the output voltage change of the sensor for every 1℃ change in temperature: S=△V/△T (mV/℃). The temperature coefficient is the physical basis of the linear temperature sensor as a temperature measurement element. Its function is similar to the B value of the thermistor. This parameter is the same value in the entire operating temperature range, that is, -2mV/℃, and all types of sensors are also the same value. This is incomparable to traditional thermistor temperature sensors. 7. What is the significance of the parameter of interchangeability accuracy? Interchangeability accuracy refers to the maximum deviation of the voltage V (T) - temperature T curve of each sensor from the same ideal fitting straight line under the same working conditions (same working current, same temperature field). This deviation is usually expressed by converting the temperature-voltage conversion coefficient S of the sensor into temperature. Since the output linearization and temperature-voltage conversion coefficient of the sensor are the same, that is, they are interchangeable throughout the temperature measurement range, the interchangeability accuracy indicates the discrete degree of the reference voltage value, that is, the discrete value of the reference voltage value is converted into the size of the temperature value to describe the interchangeability between the entire batch of sensors. It is generally divided into three levels: the interchangeability deviation of level I is not greater than 0.3℃; level J is not greater than 0.5℃; level K is not greater than 1.0℃. 8. What is the meaning of linearity? Linearity describes the linearity of the output voltage value of the sensor with temperature changes. In fact, it is the maximum deviation of the output voltage of the sensor relative to the ideal fitting straight line within the working temperature range. Under normal circumstances, the typical value of its linearity is ±0.5%. Obviously, the higher the linearity of the sensor (the smaller its value), the simpler the design of the instrument is, and there is no need to use linearization processing at the input stage of the instrument. 9. Why is the linear temperature sensor a standardized output? The so-called standardized output means that at the temperature point of 0℃, under the specified working conditions, the output voltage value of the sensor is limited to a small range. Even if it is not interchangeable, its reference voltage value is limited to 690-710mV. In this way, it is easy to grasp the output of the sensor at a macro level during circuit design. Whether in bridge circuit design or temperature compensation, as long as it is considered between 690-710mV, it can be slightly adjusted during debugging. Unlike ordinary thermistors, which have different resistance values ​​due to different models, different design calculations are required for different models. Therefore, the standardized output of the linear temperature sensor can enable the instrument circuit to achieve standardized design. 10. How do users test the linear temperature sensor? After purchasing the sensor, the user can use two-point or three-point testing under constant current conditions according to the size of the temperature zone to test the interchangeability accuracy, linearity and temperature coefficient. In general, the simplest test method is to test the reference voltage value. All electrical parameters are delivered with a parameter table (certificate of conformity) to provide detailed parameter indicators of the batch of sensors. The test conditions are as follows: constant current source: 100μA±0.5%; constant temperature field: fluctuation: ≤±0.05℃; test instrument: 41/2 or 51/2 digital voltmeter. 11. Is it necessary to use a constant current source to power the temperature sensor in actual use? Generally speaking, it is not necessary. Bridge constant voltage power supply is completely acceptable. This is because under the current condition of about 100μA, the temperature-voltage conversion coefficient of the sensor changes very little. We can give an idea of ​​the measured order of magnitude: at 100μA, S=-2mV/℃; at 40μA, S=-2.1mV/℃; at 1000μA, S=-1.9mV/℃. When the actual bridge constant voltage power supply is used, the current change will not have such a large amplitude. When the constant voltage power supply is used, how to determine the load resistance value of the sensor? When the constant voltage is used for power supply, the load resistor is connected between the power supply and the positive electrode of the sensor, and the signal is output between the positive and negative electrodes of the sensor. When designing the resistance value R, the working current of the sensor can be 100μA at 0C. If the reference voltage of the sensor is V(0) (mV) and the constant voltage source is VDD (mV), then R = (VDD-V(0)) (mV)/0.1 (mA). For the calculated resistance value R, if the actual resistor does not have this resistance value, the nearest resistance value can be selected, which has no effect on the temperature measurement accuracy. 12. What are the advantages of using linear temperature compensation elements as circuit temperature compensation?

































































This mainly considers the output standardization of thermistor and the consistency of temperature coefficient, which is convenient for design. In addition, since the temperature coefficient is the same as the temperature coefficient of the base and emitter voltage of the transistor in the transistor circuit, it is very suitable as a base bias element to stabilize the working point of the transistor circuit. When several elements are used in series, different temperature coefficients can be adjusted by the potentiometer in parallel to achieve accurate temperature compensation. This kind of compensation element with adjustable temperature coefficient does not require complicated design and has no strict requirements on the working current of the element. This is also a major advantage of this linear thermistor element for temperature compensation.

13. What is the meaning of stability?

Stability refers to the annual drift of the reference voltage value of the sensor, which is converted into a temperature value according to the temperature-voltage conversion coefficient, that is, stability = ±△V/S/year. The stability of the linear temperature sensor is ±0.05℃/year. This parameter describes the ability of the sensor to maintain its original characteristics under various conditions of use.

14. Does long-line transmission have any effect on the sensor signal?

It should be said that the impact is not significant. Under normal circumstances, the transmission distance can reach more than 1000 meters. If the distance is further, you can consider converting the signal output by the sensor into a digital quantity locally, which can facilitate transmission over longer distances.