Speaking of the constant temperature circulator, it can only be discussed from the main parameters of the thermistor. It can be said that the precision control of the constant temperature circulator is not good, the error of the zero power resistance will be expanded, and the self-heating measurement is naturally affected, but the self-heating is controlled. Another key to the use of thermistors is the importance of a constant temperature circulator that affects the end of the chain.

1 NTC terminology and main parameters

In the field of home appliance development and development, in the process of using thermistor, engineers sometimes ambiguously detail the details of some main parameters. One of the reasons is that there is no uniform standard and specification for the definition and content of some parameters. With the implementation of the national standard "direct heating type negative temperature coefficient thermistor (Part I: General Specifications)" GB/T 6663.1-2007/IEC 60539-1:2002 (hereinafter referred to as "national standard") (September 2007) On the 1st), the situation began to change. Domestic thermistor manufacturers should label the thermistor parameters according to the “national standard”. Users can also request the parameters of the thermistor according to the “national standard”.

A thermistor is a thermally sensitive semiconductor component whose resistance value changes significantly as a function of (induction) temperature. A thermistor with a reduced resistance when the temperature rises is called a negative temperature coefficient thermistor (NTC). NTC is widely used in the field of home appliances.

Self-heating: When we measure and use the NTC, we always pass a certain amount of current, which causes the NTC to generate heat itself. The self-heating of NTC will cause its resistance to decrease, and it will change dynamically during measurement and application. Therefore, controlling self-heating is the key to using NTC. When NTC is used for temperature measurement, self-heating should be avoided as much as possible; when NTC is used for liquid level or wind speed measurement, self-heating is required.

Zero power resistance: See “national standard” for definition. The zero-power resistor is the most basic parameter of the thermal resistor. The resistance of the thermistor given by the manufacturer belongs to zero power, but the word “zero power” is easy to solve (because the zero-power detection in the physical sense is It does not exist, so understanding its engineering meaning is the content of the latter sentence in the definition "...The change in resistance caused by self-heating is negligible relative to the total measurement error." Usually, the zero power measurement of NTC is carried out in a constant temperature circulator. There are two main factors affecting the total measurement error: one is the current through the NTC, and the other is the constant temperature circulator accuracy. Generally speaking, there are many ways to reduce the current through the NTC. Once the current drops to a certain extent, the total error is often the accuracy of the constant temperature circulator.

Thermal time constant (τa) caused by changes in ambient temperature: Under normal conditions, NTC quickly enters the temperature environment of the set (and required medium) under stable room temperature conditions, and measures the time required for the temperature to rise by the specified amplitude T. The increase in temperature T is the time required from room temperature Ta to 63.2% of the difference in set temperature Tb. Τa reflects the response speed of the NTC when measuring temperature.

Dissipation coefficient (δ): The power consumed to increase the temperature of the NTC by 1K is called the dissipation coefficient. The δ calculation method given in “GB” 4.10.2 is as follows:

δ=UTH·ITH/(Tb- Ta)W/°C

Where: UTH is the terminal voltage of NTC; ITH is the current flowing through NTC; Tb is the self-heating stable temperature; Ta is the indoor temperature.

It can be seen that the rise in NTC temperature refers to the self-heating temperature. From another point of view, the temperature rise caused by self-heating can be calculated using δ.

For example, if δ is 0.1 W / ° C and the measured U TH·I TH is 0.5 W, then:

(T b- T a)=U TH·I TH /δ °C=0.5 /0.1 °C=5 °C

Self-heating causes NTC to be 5 ° C above ambient temperature.

2 characteristics of self-heating and dissipation coefficient

When measuring the dissipation factor δ, the “national standard” is required to be carried out in still air. Measurements are usually made in a glazing enclosure that defines the container. Some phenomena can be observed when we do the experiment. In a room where the air is relatively stable (the air that does not feel flowing), the temperature inside the glass frame is consistent with the room temperature. First measure the zero power resistance value. When the glass frame cover is removed, the resistance value does not change. Then, the dissipation coefficient is measured. When the self-heating reaches the thermal equilibrium, the current through the NTC and its terminal voltage are in a stable state. After the glass frame cover is removed, the current or terminal voltage fluctuates and loses its steady state. It shows that the weak temperature flow in the room affects the dissipation coefficient without affecting the zero power resistance value. Obviously, NTC produces a sensitive reflection of flowing air after self-heating, which is a useful feature.

3 parameters affecting the measured temperature

NTC is widely used for temperature measurement because of its low price and significant resistance change with temperature. Usually, a precision resistor is connected in series with the NTC. The change of the NTC resistance value is converted into a voltage change and directly enters the input interface of the A/D of the comparison circuit or the single chip microcomputer. It does not have to be amplified, and the circuit configuration is extremely simple. In addition to selecting the appropriate R and B values ​​when using NTC, measurement speed and accuracy should also be considered.

Choosing the appropriate Ï„a:Ï„a value directly reflects the response speed of the NTC measurement temperature, but not as small as possible, and the Ï„a value needs to be compared and traded off. Since the value of Ï„a is related to its package size, the package size of NTC is small, the value of Ï„a is small, and the mechanical strength is low; when the package size is large, the value of Ï„a is large and the mechanical strength is high.

Determine the current range: The range of operating current can be determined according to the non-self-heating maximum power provided by the manufacturer or by using the dissipation coefficient. ,

However, it is necessary to pay attention to the fact that the δ value provided by many manufacturers is the parameter before NTC secondary packaging, but the current determined by this δ parameter does not generate self-heating, but is too conservative, affecting the looseness of the selection parameter, because two The non-self-heating maximum power after the sub-package has been increased. The method of determining the current range using the dissipation coefficient is to first determine the accuracy of the NTC and then determine the allowable self-heating power consumption. For example, if the accuracy of the NTC is 0.1 ° C, the self-heating temperature does not exceed 0.1 ° C to meet the accuracy requirement, that is, the power less than 0.1 δ is the power that does not generate self-heating.

Other factors to be aware of: After 1NTC secondary packaging, the parameter value of τa is increased compared to before packaging. 2 NTC of the same model and specification in different media, the values ​​of δ, τa and other parameters vary greatly, and the media of the parameters should be noted. 3 In the flowing air, the NTC slightly produces a little self-heating, which has little effect on the accuracy. The 4NTC temperature sensor can't touch non-detected objects. For example, in a home air conditioner, the temperature sensor head measuring the room temperature in front of the fins should not touch the fins.

4 Wind speed measurement principle

Based on the above description of the measurement of the dissipation coefficient δ, the NTC is sensitive to air flow in the self-heating state, indicating that it has the potential to measure wind speed. The NTC is supplied with a constant current that generates self-heating under the same temperature and electrical conditions, such as in a stable room temperature environment. First, the NTC is placed in still air. At this time, the terminal voltage is minimum, and then the wind speed is gradually increased from small to large, and accordingly, the terminal voltage is gradually increased. Because the flowing air makes the self-heating temperature of NTC decrease, the resistance increases, the air flow rate increases, the temperature drops more obviously, and the resistance increases more significantly. Conversely, when we know the degree of NTC self-heating decline (the value of the terminal voltage) ) You can know the speed of the wind, which is the basic principle of NTC measuring wind speed.

The temperature of the air is actually different when it is actually measured, because the drop in the air temperature also causes the self-heating temperature to drop, so the air temperature is measured at the same time when measuring the wind speed. Once the air temperature is known, and at the same time, the parameter (the magnitude of the terminal voltage value) that decreases with the increase of the wind speed and the temperature of the self-heating temperature is obtained, the measurement of the wind speed can be completed after the processing of the two data.

As with level measurement, wind speed measurements also require some basic work. However, the basis of the wind speed measurement or the calculation workload is many times more than the liquid level measurement. The liquid level measurement only needs to obtain the parameters of the two media at different temperatures, that is, two sets of data, and the wind speed measurement must obtain the measurement (wind speed, temperature). The data of different wind speeds at each temperature point in the range is a family series.

5 Liquid level measurement principle

Gases and liquids are distinctly different media. When using NTC to measure them, if the two media can be distinguished, the problem of liquid level measurement is solved. When NTC measures temperature in a non-self-heating state or a zero-power state, it is impossible to judge what medium of the object to be measured based on the measurement result. When the NTC is in a self-heating state, the NTC dissipates the coefficient (δ) in different media at the same medium temperature. When the NTC is placed in a different medium, the same electrical conditions will occur. The electrical performance reflects that this is the basic basis for measuring the liquid level.

Taking water and air at the same temperature as an example, under the same electrical conditions, for example, a constant current is supplied to the NTC to cause self-heating in the air. After the heat balance, the voltage across the NTC is relatively stable, and then, it is placed in the water. The voltage at both ends rises. Because NTC enters the water from the air, the temperature drops, causing the resistance to rise and the terminal voltage to rise. The heat capacity of water is 2.5 times that of air. The self-heating temperature of NTC in water needs to reach 2.5 times the self-heating temperature of air.

In the actual liquid level measurement, the temperature of water and air tends to be inconsistent. When the air temperature is low and the water temperature is high, it is impossible to judge whether the NTC is in water or air according to the magnitude of the voltage. However, for a temperature point, NTC has two voltage values ​​in water and air respectively. In other words, when we know a temperature point and know the voltage value of water and air at this temperature point in advance, we can The measured voltage value determines whether the NTC is in the water or in the air. That is to say, the temperature must be measured simultaneously in the process of measuring the liquid level. In general, the NTC cannot measure the temperature in the self-heating state, which requires an NTC to measure the temperature. With two NTCs, one is in a non-self-heating state and the other is in a self-heating state, and the water level can be measured by electronic circuit processing. For the same reason, the problem of liquid level measurement of other gas and liquid media can be solved.

It should be pointed out that the design of the liquid level measuring circuit needs to complete some basic work, because the self-heating state of the NTC of different circuits is not necessarily the same, and it is necessary to obtain two kinds of media at each temperature point within the measuring temperature range through testing or calculation. The electrical parameters are for the two corresponding series. Usually, the measurement scheme is determined first, then the circuit is determined, and then the data of the two media under each temperature condition is measured or calculated according to the circuit requirements. Sometimes analog circuits need to plot the temperature and voltage curves of NTC in two media (curves in the same temperature reference frame), while digital and microcontroller circuits require a list of electrical parameters for both media.

6 other applications

In addition to temperature measurement, NTC has many comparable advantages in measuring liquid level and wind speed, and has the potential to replace other measurement and control methods.

An example of the application of NTC in water level measurement is described in the 21st issue of Home Appliance Technology magazine in 2008 (more details are not mentioned here). Other water shortage alarms such as hot water bottles, coffee makers, humidifiers, etc. can be considered to use NTC's liquid level measurement technology.

NTC can also be widely used in places where wind speed and air volume are measured. It is characterized by low cost and extremely simple circuit structure. For example: 1 filter screen for household air conditioners. The NTC installed at the air outlet detects the wind speed. When the detected wind speed is reduced to the specified range compared with the wind speed of the wind volume, the user is prompted to clean the filter; 2 the same idea can also realize the dust removal prompt of the vacuum cleaner; 3 gas water heater Exhaust air monitoring. When the NTC detects the failure of the exhaust air stop (or blocked), cut off the air supply and alarm; 4 cold air metering, separately measure the centralized air supply system, and install the NTC metering wind speed at the air outlet (reconside the air outlet area and average wind speed) And other factors), can achieve centralized cooling for separate billing.
Here is a summary, whether it is a thermistor or a constant temperature circulator. If a device is to be operated well, every step involved is critical, and the efficacy of the thermistor may be The temperature control accuracy of a constant temperature circulator is exhausted, and factors such as a constant temperature circulator are too much in a large device. The spirit of the butterfly effect is very good. I hope everyone can understand it!