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How Do Negative Temperature Coefficient Sensors Work?

Mar. 05, 2021

How Do Negative Temperature Coefficient Sensors Work?

How Do Negative Temperature Coefficient Sensors Work?cid=11

Negative Temperature Coefficient (NTC) Sensors provide solid-state temperature sensing for a range of applications and are available in custom-engineered probe package configurations for a variety of mounting and connectivity options. Thermistors are ceramic semiconducting elements made from an exorbitant oxide material. It has the feature that the resistance changes according to the ambient temperature. Namely, their resistance declines with the rising of ambient temperature at a determinate measuring power. With this feature, the NTC thermistor and temperature sensor can be applied in the situation of temperature measurement and control, compensation, and surge current protection. The linear temperature sensor is a linearized output negative temperature coefficient (NTC) thermosensitive element. It is actually a linear temperature-voltage conversion element. That is to say, under the working current (100uA), the voltage value of the element changes linearly with temperature, realizing the linear conversion from non-electric quantity to electric quantity.

How Do Negative Temperature Coefficient Sensors Work?cid=11

The main characteristic of linear NTC temperature sensor is that the temperature - voltage relationship within the operating temperature range is a straight line, the secondary development of temperature measurement and temperature control circuit design, will not need to linearize processing, you can complete the temperature measurement or temperature control circuit design, simplify instrument design and debugging.

How Do Negative Temperature Coefficient Sensors Work?cid=11

Selection of extension line should follow the principles:

General -200 ~ +20℃, -50 ~ +100℃ should choose ordinary double glue line; High-temperature line should be selected within the range of 100 ~ 200℃.

The principle and application of linear NTC temperature sensor are introduced

The meaning of reference voltage:

Reference voltage refers to the sensor in the temperature field of 0℃ (ice water mixture), through the working current (100μA) conditions, the voltage value of the sensor. It's actually zero voltage. Its symbol is V (0), which is calibrated when the sensor leaves the factory. If the temperature coefficient of the sensor is the same as S, then the reference voltage value V (0) can be known, and the sensor voltage value at any temperature point can be obtained without dividing the sensor. Its calculation formula is as follows:

V of T is equal to V of 0 plus S times T

Example: such as reference voltage V (0) = 700mV;Temperature coefficient S = -2mV /℃, then at 50℃, the output voltage of the sensor V (50) = 700-2 ×50=600 (mV).This is the value of the linear temperature sensor over other temperature sensors.

Linear NTC temperature sensor temperature measurement range specification:

In general, the temperature range can be -200 ~ +200℃, but considering the actual needs, generally do not need such a wide temperature range, stipulate three different sections, in order to adapt to different package design, while the extension line selection is also different. And the temperature compensation special linear thermal element only set the working temperature range is -40℃ ~ +80℃. It can be used for the temperature compensation of the general circuit.

The meaning of temperature coefficient S:

Temperature coefficient S refers to the specified working conditions, the ratio of the sensor output voltage change to the temperature change, that is, the value of the sensor output voltage change for each temperature change of 1℃: S =△ V /△ T (mV /℃).

The temperature coefficient is the physical basis of the linear temperature sensor as the temperature measuring element, and its role is similar to the thermistor B value. This parameter is the same value in the entire operating temperature range, that is -2mV /℃, and various types of sensors are the same value, which is incomparable to the traditional thermistor temperature sensor.

The meaning of the parameter of interchange precision is as follows:

Interchange accuracy refers to the maximum deviation between the voltage V (t) -temperature T curve of each sensor and the same defined ideal fitting line under the same working condition (same working current and same temperature field). This deviation is usually expressed by converting the temperature of the sensor temperature-voltage conversion coefficient S into the temperature. The sensor output is linearized and the temperature-voltage conversion coefficient is the same, that is, the whole process is interchanged within the temperature measurement range. The interchange precision represents the degree of discreteness of the reference voltage value, that is, the discreteness of the reference voltage value is converted into the temperature value to describe the degree of interchange among the whole batch of sensors. Generally divided into three levels: I level exchange deviation is not greater than 0.3℃; Grade J is not greater than 0.5℃; Grade K is not greater than 1.0℃.

The meaning of linearity:

Linearity describes the linearity of the sensor output voltage with temperature, in fact, it is the maximum deviation of the phase ideal fitting line within the working temperature range of the sensor output voltage. In general, the typical value of linearity is ±0.5%. Obviously, the higher the linearity of the sensor (the lower the value), the simpler the meter design. The input stage of the meter does not need to be linearized at all.

Linear temperature sensors are normalized output for the following reasons:

So-called standardized output is the point of 0 ℃ temperature sensor on the working conditions, the output voltage is limited to a small range, not swap, with the benchmark voltage limit between 690-710 mv, only such circuit design, easy on the macro grasp the sensor output, bridge road design temperature compensation, consider between 690-710 mv, debugging can be a little adjustment. Unlike the ordinary thermistor model, its resistance value is the same, for different models, need to carry out different design calculation. The standardized output of the linear temperature sensor can standardize the design of the instrument circuit.

Whether the actual use of temperature sensor must use constant current source power supply analysis:

In general, it is not necessary, and the constant voltage power supply of the bridge circuit is completely acceptable (see 16 sensor signal processing circuits). This is about 100 μA current condition, the sensor temperature - voltage conversion coefficient change is very small, can give a measured order of magnitude concept:

100 mu as = 2 mv / ℃

40 mu a when s = 2.1 mv / ℃

1000 mu as = 1.9 mv / ℃

But the actual bridge constant voltage power supply, its current change will not have so large.

A criterion for determining the resistance value of the sensor load in constant voltage power supply:

Constant voltage power supply, the load resistance between the power supply and the positive electrode of the sensor, the signal output from the positive electrode of the sensor and the negative electrode, design resistance value R, at 0C to make the sensor working current is 100 μA.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).Calculate the resistance value R, the actual resistance has no such resistance value, can choose the nearest resistance value, has no effect on the temperature measurement accuracy.

The advantages of linear temperature compensation element as circuit temperature compensation:

This mainly considers the output normalization of the thermal sensor and the consistency of the temperature coefficient, which is convenient for design. In addition, the temperature coefficient is the same as the temperature coefficient of the transistor base and emitter voltage in the transistor circuit, so it is very suitable to be used as the base bias element of the working point of the transistor circuit. When several components are used in series, a parallel potentiometer can be used. The potentiometer adjusts different temperature coefficients to achieve accurate temperature compensation (see Figure 3). This kind of temperature coefficient adjustable compensation element does not need a complicated design and does not have strict requirements on the working current of the element, which is also a major advantage of this kind of linear thermal element for temperature compensation.

The difference between civilian grade and industrial grade:

The main interchange precision is different, a single instrument for mass group testing applications, and the test precision requirements are higher in the industrial environment, industrial-grade is recommended; And a table with only one sensor batch large reliability requirements of high civil products, the use of civilian grade is recommended.

Sensor signal processing circuit:

Note: The bridge circuit is R2 to offset the sensor reference voltage value V (0), that is, adjust the R2 voltage equal to the sensor reference voltage value so that the bridge output 0C is 0V, and then output to the amplifier or the next level of the circuit by -2mV/C.As a temperature control circuit design, the r2 on voltage output to the phase comparator with side, the sensor output connected to the comparator reverse side, the r2 selection depends on the temperature control point voltage, can use the formula to calculate v (t) = v (0) + s * t that the v (0) is a sensor (factory) given reference voltage value, s for the sensor voltage temperature coefficient (factory) given, t temperature value for the temperature control point. It is suggested that R2 adopt a multi-turn potentiometer to set the temperature control point more accurately.

Whether linear NTC temperature sensor can replace thermistor, thermocouple, and other thermal resistance analysis:

-200 ~ + 200C temperature range can be completely replaced, without major changes to the original circuit, do not need to do linearization of the sensor, the reference voltage value and voltage temperature coefficient of these two parameters can be designed circuit, these two parameters factory to give calibration, for the same user, different batches of products the parameters unchanged.

The meaning of stability:

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

Analysis of the influence of long line transmission on sensor signal:

It should be said that the impact is not large, under normal circumstances, the transmission distance can reach more than 1000 meters. If the distance is too far, you can consider converting the output signal of the sensor into digital quantity, which can facilitate the realization of long-distance transmission.

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