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1. What is a linear NTC temperature sensor?

A linear temperature sensor is a negative temperature coefficient (NTC) thermal element with linearized output. It is actually a linear temperature-voltage conversion element, which means that the voltage value of the element under the condition of working current (100uA) It changes linearly with temperature, thereby achieving a non-electrical-to-electrical linear conversion.

2. What are the main characteristics of a linear NTC temperature sensor?

The main characteristic of this temperature sensor is that the temperature-voltage relationship is in a straight line within the operating temperature range. For the secondary development of the temperature measurement and temperature control circuit design, the temperature measurement or temperature control circuit can be completed without linearization. Design, thereby simplifying the design and commissioning 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 ° C, but considering practical needs, such a wide temperature range is generally not necessary, so four different sections (ultra-low temperature section: -200- + 50 ℃; Section A: -50- + 120 ℃; Section B: -50- + 150 ℃; Section C: -50- + 200 ℃), in order to adapt to different package designs, the selection of extension cables is also different . As for the linear thermal element dedicated for temperature compensation, only the operating temperature range is set from -40 ° C to + 80 ° C. Can fully meet the temperature compensation for general circuits.

4． What principles should be followed in the selection of extension cables?

Generally, PTFE high temperature wire should be selected in the full temperature range.

5． What does the reference voltage mean?

The reference voltage refers to the voltage value of the sensor when it is placed in a temperature field (ice-water mixture) of 0 ° C, and the working current (100 μA) is passed. Actually it is the 0-point voltage. Its symbol is V (0). This value is calibrated when leaving 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 having to calibrate the sensor. Indexing. The calculation formula is:

V (T) = V (0) + S × T (its characteristic curve is shown below) (if not shown, please contact the company)

Example: if the reference voltage V (0) = 700mV; temperature coefficient S = -2mV / ℃, then at 50 ℃, the output voltage V (50) of the sensor = 700—2 × 50 = 600 (mV). This is why linear temperature sensors are more valuable than other temperature sensors.

6. What does the temperature coefficient S mean?

The temperature coefficient S refers to the ratio of the change in the output voltage of the sensor to the change in temperature under the specified operating conditions, that is, the value of the change in the output voltage of the sensor for each 1 ° C change in temperature: S = △ V / △ T ).

The temperature coefficient is the physical basis of the linear temperature sensor as a temperature measuring 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 various types of sensors It is also the same value, which is incomparable with traditional thermistor temperature sensors.

7. What is the meaning of the parameter of interchangeability?

The interchangeability accuracy refers to the maximum deviation of the voltage V (T) -temperature T curve of each sensor from the straight line under the same working conditions (same working current and same temperature field) for the same determined ideal fit line. This deviation is usually expressed by the temperature-voltage conversion coefficient S of the sensor. Because the sensor's output is linearized and the temperature-voltage conversion coefficient is the same, that is, it is interchanged throughout the temperature measurement range, the interchange accuracy indicates the degree of dispersion of the reference voltage value, that is, the discrete value of the reference voltage value is converted into the temperature value. Size to describe the degree of interchange between the entire batch of sensors. Generally, it is divided into three levels: the interchange deviation of level I is not greater than 0.3 ° C; level J is not greater than 0.7 ° C; level K is not greater than 1.5 ° C.

8. What does linearity mean?

Linearity is the degree of linearity that describes the output voltage of the sensor as a function of temperature. In fact, it is the maximum deviation of the output voltage of the sensor from the ideal fitted straight line within the operating temperature range. In general, the typical value of linearity is ± 0.5%. Obviously, the higher the linearity of the sensor (the smaller the value), the simpler the design of the meter is. There is no need to use linearization at the input stage of the meter.

9. Why is a linear temperature sensor a normalized output?

The so-called normalized output is that the sensor's output voltage value is limited to a small range at the 0 ° C temperature point under specified operating conditions. Even if it is not interchangeable, its reference voltage value is limited to 690-710mV. In the circuit design, it is easy to grasp the output of the sensor at a macro level. Regardless of the bridge design or temperature compensation, as long as it is considered between 690-710mV, it can be adjusted slightly during debugging. Unlike ordinary thermistors, because of different models, their resistance values ​​are also different. For different models, different design calculations are required. Therefore, the standardized output of the linear temperature sensor can enable the standardized design of the instrument circuit.

10． How do users verify linear temperature sensors?

After purchasing the sensor, users can use a two-point or three-point test under constant current conditions and depending on the size of the temperature zone to check the interchangeability, linearity, and temperature coefficient. In general, the simplest test method is to check the reference voltage value. All electrical parameters are delivered with a parameter list (certificate of qualification) at the time of delivery to provide detailed parameter indicators of the batch of sensors.

The test conditions have the following requirements:

Constant current source: 100μA ± 0.5%;

Constant temperature field: Fluctuation: ≤ ± 0.05 ℃;

Test instrument: 41/2 or 51/2 digital voltmeter.

11. Must the temperature sensor be powered by a constant current source in actual use?

Under normal circumstances, it is not necessary, and constant voltage power supply of the bridge is completely possible (see 16 sensor signal processing circuits). This is because under the current condition of about 100 μA, the temperature-voltage conversion coefficient of the sensor has a small amount of change, which can give a concept of measured magnitude:

S = -2mV / ℃ at 100μA

S = -2.1mV / ℃ at 40μA

S = -1.9mV / ℃ at 1000μA

In the actual constant voltage supply of the bridge circuit, the current change will not have such a large amplitude.

How to determine the load resistance value of the sensor when the voltage is constant?

When constant voltage power is supplied, the load resistor is connected between the power source and the positive electrode of the sensor, and the signal is output between the positive electrode and the negative electrode of the sensor. When designing the resistance value R, the working current of the sensor can be 100 μA 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 resistance does not have such resistance value, the nearest resistance value can be selected, which has no effect on the temperature measurement accuracy.

12. What are the advantages of linear temperature compensation components as circuit temperature compensation?

This mainly considers the output standardization of the thermal element and the consistency of the temperature coefficient, which is convenient for design. In addition, since the temperature coefficient is the same as the temperature coefficient of the transistor base and the emitter voltage in the transistor circuit, it is very suitable as a base bias current element that stabilizes the operating point of the transistor circuit. When several components are used in series, different temperature coefficients can be adjusted by the potentiometer in parallel to achieve accurate temperature compensation (see Figure 3). This type of compensation element with adjustable temperature coefficient does not need complicated design, and there is no strict requirement on the working current of the element. This is also a great advantage of this linear thermal element for temperature compensation.

13. What does stability mean?

Stability refers to the annual drift of the reference voltage value of the sensor. This drift is then converted to 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 ° C / year. This parameter describes the sensor's ability to maintain its original characteristics under various conditions of use.

14. Does the long-distance transmission affect the sensor signal?

It should be said that the impact is not large, and the transmission distance can reach more than 1,000 meters under normal circumstances. If the distance is longer, you can consider converting the signal output by the sensor into a digital quantity locally, so that it can easily achieve longer distance transmission.

15. What is the difference between civilian and industrial use?

The main reason is that the interchangeability is different. For applications where a single instrument is used for large-scale group measurement and the test accuracy requires a high industrial environment, it is recommended to use industrial grade; while a meter uses only one sensor in large batches. High civilian products, it is recommended to use civilian grade.

16. Sensor signal processing circuit

Note: The bridge circuit uses R2 to offset the sensor's reference voltage value V (0), that is, adjust the voltage on R2 to be equal to the sensor's reference voltage value, so that the bridge circuit output is 0V at 0C, and then -2mV / C is output to the amplifier or the next stage circuit. If it is designed as a temperature control circuit, the voltage on R2 is output to the non-inverting terminal of the comparator, and the output of the sensor is connected to the inverting terminal of the comparator. T) = V (0) + S × T, where V (0) is the reference voltage value of the sensor (given at the factory), S is the voltage temperature coefficient of the sensor (given at the factory), and T is the temperature control point Temperature value. It is recommended that R2 use a multi-turn potentiometer in order to set the temperature control point more accurately.

17. Linear NTC temperature sensor can be used as linear PTC temperature sensor (if no picture is shown, please contact the company)

In many occasions for temperature measurement and control, a temperature sensor with a positive temperature coefficient (ie, a PTC temperature sensor) is needed. A linear NTC temperature sensor can be conveniently used as a linear PTC temperature sensor.

18. Can linear NTC temperature sensors replace thermistors, thermocouples, and other thermal resistors?

It can be completely replaced in the temperature range of -200 ～ + 200C, without major changes to the original circuit, and without linearizing the sensor, as long as the two parameters of the reference voltage value and the voltage temperature coefficient can design the circuit, this The two parameters are calibrated by the manufacturer when leaving the factory, and for the same user, the parameters of different batches of products are unchanged.