Ntc thermistor what is beta. Thermistor - characteristics and principle of operation. Application of RTS thermistors

Good day! Today in this article there will be a simple way to check thermistor. Probably all radio amateurs know that there are two types of thermistors NTC(Negative temperature coefficient) and PTC(Positive temperature coefficient). As their names suggest, the resistance of an NTC thermistor will be decrease with increasing temperature, and the resistance of the PTC thermistor is increase in temperature - will increase. You can roughly check NTC and PTC thermistors using any multimeter and soldering iron.

To do this, you need to switch the multimeter to resistance measurement mode and connect its terminals to the thermistor terminals (polarity does not matter). Remember the resistance and bring the heated soldering iron to the thermistor and at the same time watch the resistance, it should increase or decrease. Depending on what type of thermistor you have in front of you, PTC or NTC. If everything is as described above - the thermistor is working.

Now how will it be in practice, and for practice I took the first thermistor I came across, it turned out to be an NTC thermistor MF72. First of all, I connected it to the multimeter in order to film the testing process and due to the lack of alligator clips on the multimeter, I had to solder wires to the thermistor and then simply screw it to the contacts of the multimeter.

As you can see from the photo at room temperature, the thermistor resistance is 6.9 ohms, this value is unlikely to be correct, since the low battery indicator lights up. Then I brought the soldering iron to the thermistor and touched the terminal a little to quickly transfer heat from the soldering iron to the thermistor.

The resistance began to slowly decrease and stopped at a value of 2 Ohms, apparently at this temperature of the soldering iron this is the minimum value. Based on this, I am almost one hundred percent sure that this thermistor is working.

If the resistance change is not smooth or there is no change at all, then the thermistor is faulty.

Remember this is just a rough check. For an ideal test, you need to measure the temperature and the corresponding resistance of the thermistor, then compare these values ​​with the datasheet for this thermistor.

Temperature is one of the most common parameters recorded by an embedded system. There is a wide range of temperature sensors available for such measurements. Sensor types range from exotic black body detectors to simple resistive sensors, including all the many types in between. In this article, I will briefly discuss negative temperature coefficient thermistors (NTC thermistors), one of the most common temperature sensors used in various embedded systems.

Thermistors

A thermistor is a resistive element, usually made of a polymer or semiconductor, whose resistance changes with temperature. This type of device should not be confused with a resistive temperature sensor (RTD). RTDs are typically much more accurate, cost more, and cover a wider temperature range.

There are two types of thermistors, differing in the nature of the dependence of resistance on temperature. If the resistance value decreases with increasing temperature, we call this device a negative temperature coefficient (NTC) thermistor. If the resistance increases with temperature, the device is known as a positive temperature coefficient (PTC) thermistor. Typically, PTC devices are used as protection devices, and NTC devices are used as temperature sensors. Very often, NTC thermistors are used to control PN junctions of broadband laser diodes.

Another characteristic of a thermistor is cost. In small quantities, a typical thermistor typically costs between $0.05 and $0.10 per unit. Low price and ease of connection make these devices very attractive for embedded applications.

The typical thermistor temperature measurement range is -50°C to +125°C. Most applications using thermistors operate in the -10°C to +70°C range, or as it is called, the commercial ambient temperature range.

The typical error of the thermistor resistance is quite large. Most thermistors are manufactured with a resistance tolerance of ±5%.

However, their accuracy is quite acceptable. Typically, we can expect it to be in the range of ±0.5% to ±1.0%.

The expression relating temperature and resistance of a thermistor is known as the Steinhart-Hart equation. This nonlinear equation is shown below.

Figure 1 shows a graph of resistance versus temperature for the company's ERTJZET472 NTC thermistor. This graph shows that on a linear scale the dependence of resistance on temperature is very non-linear.

Typically, thermistors are rated by a parameter known as the R25 value. This is a typical thermistor resistance at 25°C. The R25 value for this thermistor is 4700 ohms.

We can easily connect the thermistor to a low power current source. We can then read the voltage using the ADC and compare the result with the corresponding row in the lookup table to find out the true temperature. We can also try to linearize the dependence of resistance on temperature.

On some memory-constrained systems, we simply don't have the luxury of creating a lookup table. Therefore, in such an application we will try to linearize the thermistor readings.

The first order approximation shows us that the thermistor resistance is approximately inversely proportional to temperature. With this in mind, we can create an inverse proportion circuit to try to linearize the resistance versus temperature curve. Figure 2 shows how this is done.

If we really wanted to save money, we could remove the voltage reference. This will require some additional filtering to remove any power supply noise. It is important that the ADC and the thermistor circuit share the same reference voltage. This allows us to use a ratiometric measurement method for the thermistor relative to the ADC reading. That is, the measurement will be independent of the excitation voltage of the thermistor interface circuit.

The temperature reading depends only on the bias resistance (RB) and thermistor resistance (RTH). We can call their ratio the division factor (D). The expression for the division factor is no different from the expression for a simple voltage divider (Equation 2).

Figure 3 shows a set of curves for various values ​​of the thermistor linearizing circuit bias resistance. These plots also show a reasonable degree of linearity over the range from 0 to 70 °C; however, the best linearity is achieved with a lower bias resistor value.

Another, better way to look at this is to graph the difference between the temperature values ​​taken from the documentation and the linearized values. Such a plot is shown in Figure 4. This figure also demonstrates that better linearity is achieved with a lower value of bias resistance. The graph shows that a 2 kΩ resistor will give linearity of approximately ±3 °C over a temperature range of 0 to 70 °C.

In this example, the linear expression for temperature versus resistance coefficient with a 2 kΩ bias resistor is given in Equation 3.

T - temperature in degrees Celsius,
D - division factor.

The same reference voltage is applied to the resistive divider and the ADC. Thus, we can easily deduce the dependence of the division coefficient on the ADC readings. Assuming that the converter is N-bit wide, we obtain the relationship shown in Equation 4.

D - division factor,
ADC - ADC readings,
N - ADC capacity (number of bits).

Substituting Equation 4 into Equation 3, we obtain an expression relating the ADC readings to temperature. It is represented by Equation 5.

conclusions

Sometimes, as embedded electronics designers, we have to solve the problem of connecting a sensor to a system. In this article, I looked at a simple thermistor-based temperature sensor circuit and showed how to linearize the temperature dependence of resistance.

One of the main advantages of using thermistors is their price. Typically, when purchased in small quantities, these sensors cost approximately $0.05 to $0.10. The accuracy for these sensors is quite decent. Typically the resistance tolerance or R25 tolerance for these devices is ±3% to ±5%. Therefore, a linearization scheme with a nonlinearity of ±3 °C can also be considered satisfactory.

Of course, we can always use a more expensive sensor, which will give a more accurate result. Similar types of sensors include:

1. Sensors with PN junction. Low cost, acceptable accuracy.
2. Temperature sensor microcircuits. They are usually some form of PN junction sensor.
3. Resistive temperature sensors (RTD). They are usually very accurate and significantly more expensive.
4. Thermocouples. Their measuring range is usually much larger and the price is relatively low.
5. Infrared sensors. They are most often used to measure thermal radiation, the levels of which are then converted into temperature.

These are just a few of the methods that can be used to measure temperature. I may be able to talk about some of them in a future article.

How do you measure the temperature in your embedded system? You see that I have shown a very cheap way to measure this physical parameter. But besides this, there are many other methods.

Do you know what an NTC thermistor is and what are its characteristics?

What are NTC thermistors?

The thermistor built into the stainless steel probe is a "negative temperature coefficient". NTC thermistors are negative temperature coefficient resistors, which means the resistance decreases as temperature increases. They are mainly used as resistive temperature sensors and current limiting devices. The temperature sensitivity coefficient is approximately five times greater than silicon temperature sensors (silistors) and approximately ten times greater than resistance temperature sensors (RTDs). NTC sensors are typically used between -55°C and 200°C.

The nonlinearity of the relationship between resistance and temperature exhibited by NTC resistors has been a major problem when using analog circuits to accurately measure temperature, but the rapid development of digital circuits has solved this problem, allowing precise values ​​to be calculated by interpolating lookup tables or by solving equations that approximate the typical NTC curve.

NTC Thermistor Definition

An NTC thermistor is a temperature-sensitive resistor whose resistance exhibits a large, precise, and predictable decrease as the temperature of the resistor core increases over a range of operating temperatures.

Characteristics of NTC thermistors

Unlike RTDs (resistance temperature detectors) that are made of metals, NTC thermistors are usually made of ceramics or polymers. Different materials used result in different temperature responses as well as other characteristics.

Temperature reaction

Although most NTC thermistors are generally suitable for use in the temperature range of -55°C to 200°C, where they give the most accurate readings, there are special families of NTC thermistors that can be used at temperatures approaching absolute zero (-273.15° C), as well as those specifically designed for use above 150 °C.

The temperature sensitivity of an NTC sensor is expressed as "percentage change per degree C". Depending on the materials used and the manufacturing process, typical temperature sensitivity values ​​range from -3% to -6% per °C.

NTC characteristic curve

As can be seen from the figure, NTC thermistors have a much steeper resistance-temperature slope compared to platinum alloy RTDs, resulting in better temperature sensitivity. However, RTDs remain the most accurate sensors, accurate to within ±0.5% of the measured temperature, and are useful over a temperature range of -200°C to 800°C, much wider than NTC temperature sensors.

Comparison with other temperature sensors

Compared to RTDs, NTCs are smaller in size, faster in response, more resistant to shock and vibration, and have a lower cost. They are slightly less accurate than RTDs. Compared to thermocouples, the accuracy obtained from both is similar; however, thermocouples can withstand very high temperatures (on the order of 600 °C) and are used in place of NTC thermistors, where they are sometimes called pyrometers. However, NTC thermistors provide greater sensitivity, stability, and accuracy than thermocouples at lower temperatures, and are used with less power and therefore have lower overall costs. The cost is further reduced by eliminating the need for signal conditioning circuits (amplifiers, level translators, etc.) that are often needed when working with RTDs and are always needed for thermocouples.

Self-heating effect

The self-heating effect is a phenomenon that occurs when current flows through an NTC thermistor. Since a thermistor is basically a resistor, it dissipates energy as heat when current flows through it. This heat is generated in the thermistor core and affects the accuracy of the measurements. The extent to which this occurs depends on the amount of current flowing, the environment (whether it is liquid or gas, whether there is any flow above the NTC sensor, etc.), temperature coefficient of the thermistor, total amount of thermistor area, etc. The fact that the resistance of the NTC sensor, and therefore the current flowing through it, depends on the environment and is often used in liquid storage tanks.

Heat capacity

Heat capacity is the amount of heat required to raise the temperature of the thermistor by 1°C and is usually expressed in mJ/°C. Knowing the exact heat capacity is important when using an NTC thermistor sensor as an inrush current limiter because it determines the speed of response of the NTC temperature sensor.

Curve selection and calculation

A careful selection process should consider the thermistor dissipation constant, heat treatment time constant, resistance value, resistance-resistance curve and tolerances to account for the most important factors.
Because the relationship between resistance and temperature (R-T curve) is highly nonlinear, certain approximations must be used in practical system designs.

First order approximation

One approximation, and the simplest to use, is the first-order approximation, which states that:

first order approximation formula: dR = k * dT

Where k is the negative temperature coefficient, ΔT is the temperature difference, ΔR is the change in resistance resulting from the change in temperature. This first-order approximation is valid only for a very narrow temperature range and can only be used for temperatures where k is almost constant over the entire temperature range.

Beta formula

Another equation gives satisfactory results with an accuracy of ±1°C over the range from 0°C to +100°C. It depends on a single material constant β, which can be obtained by measurement. The equation can be written as:

Beta equation: R(T) = R(T0) * exp(beta * (1/T-1/T0))

Where R(T) is the resistance at temperature T in Kelvin, R(T0) is the reference point at temperature T0. The beta formula requires two-point calibration and is typically no more than ±5°C over the entire useful range of the NTC thermistor.

Steinhart-Hart equation

The best approximation known to date is the Steinhart-Hart formula, published in 1968:

Steinhart equation for exact approximation: 1/T = A + B * (ln(R)) + C * (ln(R))^3

Where ln R is the natural logarithm of the resistance at temperature T in Kelvin, and A, B and C are coefficients obtained from experimental measurements. These factors are usually published by thermistor suppliers as part of a data sheet. The Steinhart-Hart formula is typically about ±0.15°C over the -50°C to +150°C range, which is large for most applications. If high accuracy is required, the temperature range should be reduced and the accuracy is better than ±0.01°C in the range from 0°C to +100°C.

Choosing the Right Approximation

The choice of formula used to derive temperature from resistance measurements should be based on available computing power as well as actual tolerance requirements. In some applications the first order approximation is more than sufficient, while in other cases even the Steinhart-Hart equation suffices and the thermistor must be calibrated point by point, making a large number of measurements and creating a lookup table.

Design and properties of NTC thermistors

Materials commonly used in the manufacture of NTC resistors are platinum, nickel, cobalt, iron and silicon oxides, used as pure elements or ceramics and polymers. NTC thermistors can be divided into three groups, depending on the manufacturing process used.

Bead or ball shape. These NTC thermistors are made of platinum alloy lead wires directly sintered into a ceramic body. They typically provide faster response times, better stability, and can operate at higher temperatures than NTC disk and chip sensors, but are more fragile. They usually seal them in glass to protect them from mechanical damage during assembly and improve their measurement stability. Typical sizes range from 0.075 to 5 mm in diameter.

Disk and chip thermistors

Thermistor in the form of a disk. NTC thermistors have metallized surface contacts. They are larger and, as a result, have slower response times than ball-type NTC resistors. However, due to their size, they have a higher dissipation constant (the power required to raise their temperature by 1 °C), and since the power dissipated by a thermistor is proportional to the square of the current, they can handle higher currents much better than ball-type thermistors . Disc-type thermistors are produced by pressing a mixture of oxide powders into a circular matrix, which is then sintered at high temperatures. Chips are typically made by injection molding, where a suspension of material is spread into a thick film, dried, and cut into a shape. Typical sizes range from 0.25 to 25 mm in diameter.

Fiberglass with NTC thermistor

These are NTC temperature sensors sealed in an airtight glass bubble. They are designed for use at temperatures above 150°C or for PCB mounting where durability is required. Encapsulating the thermistor in glass increases the stability of the sensor as well as protecting the sensor from the environment. They are manufactured by hermetically sealing NTC type resistors into a glass container. Typical sizes range from 0.4 to 10 mm in diameter.

NTC thermistors are used in a wide range of applications. They are used for temperature measurement, control temperature and temperature compensation. They can also be used to detect the absence or presence of liquid, as current limiting devices in power circuits, temperature monitoring in automotive components, and many others. NTC sensors can be divided into three groups, depending on the electrical characteristics used in units and devices.

Resistance-temperature characteristic

Applications based on the resistance-time characteristic include temperature measurement, control and compensation. This also includes situations in which an NTC thermistor is used, so that the temperature of the NTC temperature sensor is related to some other physical phenomena. This group of units requires the thermistor to operate under zero power conditions, which means that the current passing through it is kept as low as possible to avoid heating the probe.

Current time characteristic

Devices based on the current time characteristic are: time delay, inrush current limiting, surge suppression and much more. These characteristics are related to the heat capacity and dissipation constant of the NTC thermistor used. The circuit typically relies on an NTC thermistor, heating up due to the current passing through it. At some point this will cause some kind of change in the circuit, depending on the device in which it is used.

Voltage characteristic

Devices based on the voltage and current characteristics of the thermistor typically involve changes in environmental conditions or circuit changes that result in changes in the operating point on a given curve in the circuit. Depending on the application, this can be used for current limiting, temperature compensation or temperature measurement.

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