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Where is the thermistor used? What is a thermistor and its application in electronics

Semiconductor thermal resistors. Thermistors. Thermistors. Principle of operation and characteristics

Fundamentals of operation of semiconductor thermistors, their types, specifications, graph of the temperature dependence of the resistance.

The significant dependence of the resistance of semiconductors on temperature made it possible to design sensitive thermistors (thermistors, thermal resistances), which are bulk semiconductor resistances with a large temperature coefficient of resistance. Depending on the purpose, thermistors are made from substances with different resistivity values. For the manufacture of thermistors, semiconductors with both an electronic and a hole conductivity mechanism and pure substances can be used. The main parameters of the thermistor substance that determine its quality are: the value of the temperature coefficient, chemical stability and melting point.

Most types of thermistors operate reliably only within certain temperature limits. Any overheating above the norm has a detrimental effect on the thermistor (thermal resistance), and sometimes can even lead to its death.

To protect against the harmful effects of the environment, and primarily oxygen in the air, thermistors are sometimes placed in a cylinder filled with an inert gas.

The design of the thermistor is very simple. A piece of semiconductor is given the shape of a filament, a bar, a rectangular plate, a ball, or some other shape. Two terminals are mounted on opposite parts of the thermistor. The value of the ohmic resistance of the thermistor, as a rule, is noticeably greater than the resistance values of other elements of the circuit and, most importantly, depends sharply on temperature. So when a current flows in a circuit, its magnitude is mainly determined by the thermistor's ohmic resistance, or ultimately its temperature. As the temperature of the thermistor rises, the current in the circuit increases, and, conversely, as the temperature decreases, the current decreases.

The heating of the thermostat can be carried out by transferring heat from the environment, by the release of heat in the thermistor itself when an electric current passes through it, or, finally, with the help of special heating windings. The method of heating the thermistor is directly related to its practical use.

The resistance of the thermistor with a change in temperature can change by three orders of magnitude, i.e. 1000 times. This is typical for thermistors made of poorly conductive materials. In the case of well conducting substances, the ratio is in the range of ten.

Any thermistor has a thermal inertia, which in some cases plays a positive role, in others it either has practically no significance, or has a negative effect and limits the limits of the use of thermistors. Thermal inertia is manifested in the fact that the thermistor, subjected to heating, does not immediately take on the temperature of the heater, but only after a while. The characteristic of the thermal inertia of the thermistor can be the so-called time constantτ . The time constant is numerically equal to the amount of time during which the thermistor, previously at 0°C and then transferred to an environment with a temperature of 100°C, will reduce its resistance by 63%.

For most semiconductor thermistors, the dependence of resistance on temperature is non-linear (Fig. 1, A). The thermal inertia of a thermistor differs little from that of a mercury thermometer.

Under normal operation, the parameters of thermistors change little over time, and therefore their service life is quite long and, depending on the brand of the thermistor, varies in the range, the upper limit of which is calculated in several years.

For example, let's briefly consider three types of thermistors (thermal resistance): MMT-1, MMT-4 and MMT-5.

Figure 1(B) shows the principal arrangement and construction of these thermistors. The MMT-1 thermistor is coated on the outside with enamel paint and is designed to work in dry rooms; thermistors MMT-4 and MMT-5 are mounted in metal capsules and sealed. Therefore, they are not subject to the harmful effects of the environment, are designed to work in conditions of any humidity and can even be in liquids (not acting on the body of thermistors)

The ohmic resistance of thermistors is in the range from 1000 - 200000 ohms at a temperature of 20 ° C, and the temperature coefficientα about 3% per 1°C. Figure 2 shows a curve showing the percentage change in the ohmic resistance of a thermistor as a function of its temperature. In this graph, resistance at 20°C is taken as the initial value.

The described types of thermistors are designed to operate in the temperature range from -100 to + 120 ° C. Their overheating is unacceptable.

Thermal resistances (thermistors, thermistors) of the mentioned types are very stable, i.e., they retain their "cold" resistance practically unchanged, the value of which is determined at 20 ° C for a very long time. The high stability of MMT type thermistors determines their long service life, which, as indicated in the passport, is practically unlimited in their normal operation. Thermal resistances (thermistors, thermistors) of the MMT type have good mechanical strength.

In the figures: the designs of some thermistors, the characteristic temperature dependence of the resistance of the thermistor.

And consisting of a semiconductor material, which, with a small change in temperature, greatly changes its resistance. Typically, thermistors have negative temperature coefficients, meaning their resistance drops with increasing temperature.

General characteristics of the thermistor

The word "thermistor" is short for its full term: thermally sensitive resistor. This device is an accurate and easy-to-use sensor for any temperature changes. In general, there are two types of thermistors: negative temperature coefficient and positive temperature coefficient. Most often, the first type is used to measure temperature.

The designation of the thermistor in the electrical circuit is shown in the photo.

The material of thermistors are metal oxides with semiconductor properties. During production, these devices are given the following form:

disc-shaped;
rod;
spherical like a pearl.

The thermistor is based on the principle of a strong change in resistance with a small change in temperature. At the same time, for a given current in the circuit and a constant temperature, a constant voltage is maintained.

To use the device, it is connected to an electrical circuit, for example, to a Wheatstone bridge, and the current and voltage on the device are measured. According to Ohm's simple law R=U/I determine the resistance. Next, they look at the curve of dependence of resistance on temperature, according to which it is possible to say exactly what temperature the resulting resistance corresponds to. When the temperature changes, the resistance value changes sharply, which makes it possible to determine the temperature with high precision.

Thermistor material

The material of the vast majority of thermistors is semiconductor ceramics. The process of its manufacture consists in sintering powders of nitrides and metal oxides at high temperatures. The result is a material whose oxide composition has the general formula (AB) 3 O 4 or (ABC) 3 O 4, where A, B, C are metallic chemical elements. The most commonly used are manganese and nickel.

If the thermistor is expected to operate at temperatures less than 250°C, then magnesium, cobalt, and nickel are included in the ceramic composition. Ceramics of this composition shows the stability of physical properties in the specified temperature range.

An important characteristic of thermistors is their specific conductivity (the reciprocal of resistance). Conductivity is controlled by adding small concentrations of lithium and sodium to the composition of semiconductor ceramics.

Instrument manufacturing process

Spherical thermistors are made by applying them to two platinum wires at high temperature (1100°C). The wire is then cut to shape the thermistor contacts. For sealing, a glass coating is applied to the spherical device.

In the case of disk thermistors, the contact manufacturing process consists in applying a metal alloy of platinum, palladium and silver to them, and then soldering it to the thermistor coating.

Difference from platinum detectors

In addition to semiconductor thermistors, there is another type of temperature detector, the working material of which is platinum. These detectors change their resistance as the temperature changes in a linear fashion. For thermistors, this dependence of physical quantities has a completely different character.

The advantages of thermistors in comparison with platinum analogues are the following:

Higher resistance sensitivity to temperature changes over the entire operating range.
High level the stability of the instrument and the repeatability of the readings obtained.
Small size that allows you to quickly respond to temperature changes.

Thermistor resistance

This physical quantity decreases as the temperature increases, and it is important to consider the operating temperature range. For temperature limits from -55 °C to +70 °C, thermistors with a resistance of 2200 - 10000 ohms are used. For higher temperatures use devices with a resistance greater than 10 kOhm.

Unlike platinum detectors and thermocouples, thermistors do not have specific standards for resistance versus temperature curves, and there is a wide variety of resistance curves to choose from. This is due to the fact that each thermistor material, like a temperature sensor, has its own resistance curve.

Stability and accuracy

These instruments are chemically stable and do not degrade in performance over time. Thermistor sensors are among the most accurate temperature measuring instruments. The accuracy of their measurements over the entire operating range is 0.1 - 0.2 °C. Please note that most appliances operate within a temperature range of 0°C to 100°C.

Basic parameters of thermistors

The following physical parameters are basic for each type of thermistors (the decoding of the names is given on English language):

R 25 - resistance of the device in Ohms at room temperature (25 ° C). Checking this characteristic of the thermistor is simple using a multimeter.
Tolerance of R 25 - the value of the resistance deviation tolerance on the device from its set value at a temperature of 25 °C. As a rule, this value does not exceed 20% of R 25 .
Max. Steady State Current - The maximum value of current in amperes that can flow through the device for an extended period of time. Exceeding this value threatens with a rapid drop in resistance and, as a result, failure of the thermistor.
Approx. R of Max. Current - this value shows the value of resistance in Ohms, which the device acquires when the maximum current passes through it. This value should be 1-2 orders of magnitude less than the resistance of the thermistor at room temperature.
Dissip. Coef. - a coefficient that shows the temperature sensitivity of the device to the power absorbed by it. This factor indicates the amount of power in mW that the thermistor needs to absorb in order to increase its temperature by 1 °C. This value has importance, because it shows how much power you need to spend to heat the device to its operating temperatures.
Thermal Time Constant. If the thermistor is used as an inrush current limiter, it is important to know how long it will take to cool down after the power is turned off in order to be ready to turn it on again. Since the temperature of the thermistor after it is turned off decreases according to an exponential law, the concept of "Thermal Time Constant" is introduced - the time during which the temperature of the device decreases by 63.2% of the difference between the operating temperature of the device and the ambient temperature.
Max. Load Capacitance in μF - the amount of capacitance in microfarads that can be discharged through this device without damaging it. This value is indicated for a specific voltage, for example, 220 V.

How to check the thermistor for performance?

For a rough check of the thermistor for its serviceability, you can use a multimeter and a conventional soldering iron.

The first step is to turn on the resistance measurement mode on the multimeter and connect the output contacts of the thermistor to the multimeter terminals. In this case, the polarity does not matter. The multimeter will show a certain resistance in ohms, it should be recorded.

Then you need to plug in the soldering iron and bring it to one of the thermistor outputs. Be careful not to burn the device. During this process, you should observe the readings of the multimeter, it should show a smoothly decreasing resistance, which will quickly settle at some minimum value. The minimum value depends on the type of thermistor and the temperature of the soldering iron, usually it is several times less than the value measured at the beginning. In this case, you can be sure that the thermistor is working.

If the resistance on the multimeter has not changed or, on the contrary, has fallen sharply, then the device is unsuitable for its use.

notice, that this check is rough. For accurate testing of the device, it is necessary to measure two indicators: its temperature and the corresponding resistance, and then compare these values \u200b\u200bwith those stated by the manufacturer.

Areas of use

In all areas of electronics in which it is important to monitor temperature conditions, thermistors are used. Such areas include computers, high-precision equipment for industrial installations and devices for transmitting various data. So, the 3D printer thermistor is used as a sensor that controls the temperature of the heating bed or print head.

One common use for a thermistor is to limit inrush current, such as when turning on a computer. The fact is that at the moment the power is turned on, the starting capacitor, which has a large capacity, is discharged, creating a huge current in the entire circuit. This current is capable of burning the entire chip, so a thermistor is included in the circuit.

This device at the time of switching on had room temperature and a huge resistance. Such resistance can effectively reduce the current surge at the time of starting. Further, the device heats up due to the current passing through it and the release of heat, and its resistance decreases sharply. The thermistor's calibration is such that the operating temperature of the computer chip causes the thermistor's resistance to practically zero, and there is no voltage drop across it. After turning off the computer, the thermistor quickly cools down and restores its resistance.

Thus, using a thermistor to limit inrush current is both cost-effective and fairly simple.

Thermistor examples

Currently, there is a wide range of products on sale, here are the characteristics and areas of use of some of them:

The B57045-K nut mounted thermistor has a nominal resistance of 1 kΩ with a tolerance of 10%. Used as a temperature measurement sensor in consumer and automotive electronics.
The B57153-S disc instrument has a maximum current rating of 1.8 A at 15 ohms at room temperature. Used as an inrush current limiter.

A thermistor is a semiconductor component with a temperature-dependent electrical resistance. Invented back in 1930 by the scientist Samuel Ruben, to this day this component is widely used in technology.

Thermistors are made from various materials, which is quite high - significantly superior to metal alloys and pure metals, that is, from special, specific semiconductors.

Directly the main resistive element is obtained by powder metallurgy, processing chalcogenides, halides and oxides of certain metals, giving them various shapes, for example, the shape of discs or rods of various sizes, large washers, medium tubes, thin plates, small beads, ranging in size from a few microns to tens of millimeters .

By the nature of the correlation between the resistance of the element and its temperature, divide thermistors into two large groups - thermistors and thermistors. Thermistors have a positive TCR (for this reason, thermistors are also called PTC thermistors), and thermistors have a negative TCR (they are therefore called NTC thermistors).

Thermistor - a temperature-dependent resistor, made of a semiconductor material with a negative temperature coefficient and high sensitivity, a posistor -temperature-dependent resistor having a positive coefficient.So, with an increase in the temperature of the posistor case, its resistance also increases, and with an increase in the temperature of the thermistor, its resistance decreases accordingly.

The materials for thermistors today are: mixtures of polycrystalline transition metal oxides such as cobalt, manganese, copper and nickel, IIIBV-type compounds, as well as doped, glassy semiconductors such as silicon and germanium, and some other substances. Noteworthy are posistors made of solid solutions based on barium titanate.

Thermistors in general can be classified into:

Low temperature class (working temperature below 170 K);

Medium temperature class (operating temperature from 170 K to 510 K);

High-temperature class (operating temperature from 570 K and above);

A separate class of high-temperature (working temperature from 900 K to 1300 K).

All these elements, both thermistors and posistors, can operate under various climatic external conditions and with significant physical external and current loads. However, in severe thermal cycling conditions, their initial thermoelectric characteristics change over time, such as the nominal resistance at room temperature and the temperature coefficient of resistance.

There are also combined components, for example thermistors with indirect heating. In the cases of such devices, both the thermistor itself and a galvanically isolated heating element are placed, which sets the initial temperature of the thermistor, and, accordingly, its initial electrical resistance.

These devices are used as variable resistors controlled by voltage applied to the thermistor heating element.

Depending on how the operating point is chosen on the IV characteristics of a particular component, the operating mode of the thermistor in the circuit is also determined. And the VAC itself is associated with design features and with the temperature applied to the component body.

To control temperature variations and to compensate for dynamically changing parameters, such as flowing current and applied voltage in electrical circuits that change following changes in temperature conditions, thermistors are used with the operating point set in the linear section of the I–V characteristic.

But the operating point is traditionally set on the falling section of the CVC (NTC thermistors), if the thermistor is used, for example, as a starting device, a time relay, in a system for tracking and measuring the intensity of microwave radiation, in fire alarm systems, in bulk solids flow control installations and liquids.

Most popular today medium temperature thermistors and posistors with TCR from -2.4 to -8.4% per 1 K. They operate in a wide range of resistances from units of ohms to units of megaohms.

There are posistors with a relatively small TCS from 0.5% to 0.7% per 1 K, made on the basis of silicon. Their resistance varies almost linearly. Such posistors are widely used in temperature stabilization systems and in active cooling systems for power semiconductor switches in a variety of modern electronic devices, especially in powerful ones. These components fit easily into the circuits and do not take up much space on the boards.

A typical posistor is in the form of a ceramic disk, sometimes several elements are installed in series in one housing, but more often in a single version in a protective enamel coating. Thermistors are often used as fuses for protection electrical circuits from voltage and current overloads, as well as thermal sensors and auto-stabilizing elements, due to their unpretentiousness and physical stability.

Thermistors are widely used in numerous areas of electronics, especially where precise temperature control is important. This is true for data transmission equipment, computer technology, high performance CPUs and high precision industrial equipment.

One of the simplest and most popular applications of a thermistor is effectively limiting inrush current. At the moment the voltage is applied to the power supply from the network, an extremely sharp, significant capacitance occurs, and a large charging current flows in the primary circuit, which can burn the diode bridge.

This current is limited here by the thermistor, that is, this component of the circuit changes its resistance depending on the current passing through it, since, in accordance with Ohm's law, it heats up. The thermistor then regains its original resistance after a few minutes, once it has cooled to room temperature.

The temperature sensor is one of the most commonly used devices. Its main purpose is to perceive the temperature and convert it into a signal. There are many different types of sensors. The most common of these are the thermocouple and the thermistor.

Kinds

Detection and measurement of temperature is a very important activity, it has many applications, from a simple household to an industrial one. A temperature sensor is a device that collects temperature data and displays it in a human-readable format. The temperature sensing market is showing continuous growth due to its R&D needs in the semiconductor and chemical industries.

Thermal sensors are mainly of two types:

Contact. These are thermocouples, filled system thermometers, thermal sensors and bimetal thermometers;
Contactless sensors. These infrared devices have a wide range of applications in the defense sector due to their ability to detect the thermal power of optical and infrared rays emitted by liquids and gases.

A thermocouple (bimetal device) consists of two different kinds of wires (or even twisted) together. The principle of operation of a thermocouple is based on the fact that the speeds at which two metals expand differ from each other. One metal expands more than the other and begins to bend around the metal that is not expanding.

A thermistor is a kind of resistor whose resistance is determined by its temperature. The latter is usually used up to 100°C whereas the thermocouple is designed for higher temperatures and is not as accurate. Thermocouple circuits provide millivolt outputs, while thermistor circuits provide high voltage output.

Important! The main advantage of thermistors is that they are cheaper than thermocouples. They can be bought literally for pennies, and they are easy to use.

Operating principle

Thermistors are usually sensitive and have different thermal resistance. In an unheated conductor, the atoms that make up the material tend to arrange themselves in the correct order, forming long rows. When a semiconductor is heated, the number of active charge carriers increases. The more charge carriers available, the more conductive the material has.

The curve of resistance and temperature always shows a non-linear characteristic. The thermistor works best in a temperature range of -90 to 130 degrees Celsius.

Important! The principle of operation of the thermistor is based on the basic correlation between metals and temperature. They are made from semiconductor compounds such as sulfides, oxides, silicates, nickel, manganese, iron, copper, etc., and can sense even slight temperature changes.

An electron pushed by an applied electric field can travel relatively long distances before colliding with an atom. The collision slows it down, so the electrical "resistance" will decrease. At higher temperatures, the atoms move more, and when a particular atom deviates somewhat from its usual "parked" position, it is more likely to collide with a passing electron. This "deceleration" manifests itself in the form of an increase electrical resistance.

For information. When the material cools, the electrons settle on the lowest valence shells, become unexcited and, accordingly, move less. In this case, the resistance to the movement of electrons from one potential to another drops. As the temperature of the metal increases, the resistance of the metal to the flow of electrons increases.

Design features

By their nature, thermistors are analog and are divided into two types:

metal (posistors),
semiconductor (thermistors).

posistors

Far from any current conductors can be used as a material for thermistors, since certain requirements are imposed on these devices. The material for their manufacture must have a high TCS.

Copper and platinum are suitable for such requirements, apart from their high cost. In practice, copper samples of TCM thermistors are widely used, in which the linearity of the dependence of resistance on temperature is much higher. Their disadvantage is low resistivity, rapid oxidation. In this regard, copper-based thermal resistances are of limited use, not more than 180 degrees.

PTC thermistors are designed to limit current when heated from higher power dissipation. Therefore, they are placed in series in an alternating current circuit in order to reduce the current. They (literally any of them) get hot from too much current. These devices are used in a circuit protection device, such as a fuse, as a timer in the degaussing circuit of CRT monitor coils.

For information. What is a posistor? A device whose electrical resistance increases with its temperature is called a posistor (PTC).

Thermistors

A device with a negative temperature coefficient (this is when the higher the temperature, the lower the resistance) is called an NTC thermistor.

For information. All semiconductors have varying resistance as temperature increases or decreases. This shows their hypersensitivity.

NTC thermistors are widely used as inrush current limiters, self-adjusting overcurrent protections, and self-regulating heating elements. Usually these devices are installed in parallel in the AC circuit.

They can be found everywhere: in cars, airplanes, air conditioners, computers, medical equipment, incubators, hair dryers, electrical outlets, digital thermostats, portable heaters, refrigerators, ovens, stoves and other various appliances.

The thermistor is used in bridge circuits.

Specifications

Thermistors are used in charging batteries. Their main characteristics are:

High sensitivity, temperature coefficient of resistance is 10-100 times that of metal;
Wide operating temperature range;
Small size;
Easy to use, the resistance value can be selected between 0.1~100kΩ;
Good stability;
Strong overload.

The quality of an instrument is measured in terms of standard characteristics such as response time, accuracy, and resistance to changes in other physical environmental factors. Service life and measuring range are just a few more important features which must be considered when considering usage.

Application area

Thermistors are not very expensive and can be readily available. They provide fast response and are reliable to use. The following are examples of how the devices can be used.

Air temperature sensor

An automotive thermal sensor is an NTC thermistor, which itself is very accurate when properly calibrated. The gauge is usually located behind the grille or bumper of the car and must be very accurate as it is used to determine the cut-off point. automatic systems climate control. The latter are adjustable in increments of 1 degree.

Automotive thermal sensor

The thermistor is built into the motor winding. Typically, this sensor is connected to a temperature relay (controller) to provide "Automatic Temperature Protection". When the motor temperature exceeds the set value set in the relay, the motor will automatically shut down. For less critical applications, it is used to trigger an overtemperature alarm with indication.

fire detector

You can make your own fire fighting device. Assemble a circuit from a thermistor or bimetallic strips borrowed from a starter. Thus, you can cause an alarm based on the action of a homemade temperature sensor.

In electronics, you always have to measure something, like temperature. This task is best handled by a thermistor - an electronic component based on semiconductors. The instrument detects a change in the physical quantity and converts it into an electrical quantity. They are a kind of measure of the rising impedance of the output signal. There are two types of devices: for posistors, resistance also increases with increasing temperature, while for thermistors, on the contrary, it decreases. These are elements that are opposite in action and identical in principle of operation.

Video

1.WHAT IS IT?
Thermistor is a semiconductor resistor, which uses the dependence of the resistance of a semiconductor on temperature.
Thermistors are characterized by a large temperature coefficient of resistance (TCR), the value of which exceeds that of metals by tens and even hundreds of times.
Thermistors are very simple and come in a variety of shapes and sizes.

In order to more or less imagine the physical basis of the operation of this radio component, you first need to get acquainted with the structure and properties of semiconductors (see my article “Semiconductor Diode”).
Brief reminder. Semiconductors contain free carriers electric charge two types: "-" electrons and "+" holes. At a constant ambient temperature, they spontaneously form (dissociation) and disappear (recombination). Average concentration of free carriers in a semiconductor remains unchanged - this is a dynamic balance. When the temperature changes, such an equilibrium is violated: if the temperature increases, then the carrier concentration increases (conductivity increases, resistance decreases), and if it decreases, then the concentration of free carriers also decreases (conductivity decreases, resistance increases).
The dependence of semiconductor resistivity on temperature is shown in the graph.
As you can see, if the temperature tends to absolute zero (-273.2 C), then the semiconductor becomes an almost perfect dielectric. If the temperature increases greatly, then, on the contrary, an almost ideal conductor. But the most important thing is that the R(T) dependence of a semiconductor is strongly pronounced in the range of conventional temperatures, say, from -50C to +100C (you can take it a little wider).

The thermistor was invented by Samuel Ruben in 1930.

2. MAIN PARAMETERS
2.1. Nominal resistance - thermistor resistance at 0°C (273.2K)
2.2. TKS is physical a value equal to the relative change in the electrical resistance of a section of an electrical circuit or the specific resistance of a substance with a change in temperature by 1 ° C (1 K).
There are thermistors with negative ( thermistors) and positive ( posistors) TCS. They are also called NTC thermistors (Negative temperature coefficient) and PTC thermistors (Positive temperature coefficient), respectively. For posistors, the resistance also increases with increasing temperature, while for thermistors, on the contrary: as the temperature increases, the resistance decreases.
The TCR value is usually given in reference books for a temperature of 20 ° C (293 K).

2.3. Operating temperature range
There are low temperature thermistors (designed to operate at temperatures below 170 K), medium temperature (170–510 K) and high temperature (above 570 K). In addition, there are thermistors designed for operation at 4.2 K and below and at 900–1300 K. The most widely used medium temperature thermistors with TCR from -2.4 to -8.4% / K and a nominal resistance of 1–106 Ohm .

Note. In physics, the so-called absolute temperature scale (thermodynamic scale) is used. According to it, the lowest temperature in nature (absolute zero) is taken as the starting point. On this scale, the temperature can only be with the “+” sign. There is no negative absolute temperature. Designation: T, unit of measure 1K (Kelvin). 1K=1°C, so the formula for converting temperature from the Celsius scale to the thermodynamic temperature scale is very simple: T=t+273 (approximately) or, respectively, vice versa: t=T-273. Here t is the temperature on the Celsius scale.
The ratio of the Celsius and Kelvin scales is shown in

2.4. The rated power dissipation is the power at which the thermistor maintains its parameters in the given specifications limits during operation.

3. MODE OF OPERATION
The operating mode of the thermistors depends on which section of the static volt-ampere characteristic(VAC - ) the operating point is selected. In turn, the I–V characteristic depends both on the design, dimensions and basic parameters of the thermistor, and on the temperature, thermal conductivity of the environment, and thermal coupling between the thermistor and the medium. Thermistors with an operating point in the initial (linear) section of the CVC are used to measure and control temperature and compensate for temperature changes in parameters electrical circuits and electronic appliances. Thermistors with a working point on the downward section of the CVC (with negative resistance) are used as starting relays, time relays, microwave electromagnetic radiation power meters, temperature and voltage stabilizers. The mode of operation of the thermistor, in which the operating point is also on the descending section of the I–V characteristic (in this case, the dependence of the thermistor resistance on the temperature and thermal conductivity of the environment is used), is typical for thermistors used in thermal control and fire alarm, regulation of the level of liquid and granular media; the operation of such thermistors is based on the occurrence of a relay effect in the circuit with the thermistor when the ambient temperature or the conditions of heat exchange between the thermistor and the medium change.
There are thermistors of a special design - with indirect heating. Such thermistors have a heated winding isolated from the semiconductor resistive element (if the power released in the resistive element is small, then the thermal regime of the thermistor is determined by the temperature of the heater, and, consequently, by the current in it). Thus, it becomes possible to change the state of the thermistor without changing the current through it. Such a thermistor is used as a variable resistor controlled electrically from a distance.
Of the thermistors with a positive temperature coefficient, the most interesting are the thermistors made from solid solutions based on BaTiO. They are called posistors. Known thermistors with a small positive TCR (0.5–0.7% / K), made on the basis of silicon with electronic conductivity; their resistance varies with temperature approximately linearly. Such thermistors are used, for example, for temperature stabilization electronic devices on transistors.
On fig. The dependence of the resistance of the thermistor on temperature is shown. Line 1 - for TCS< 0, линия 2 - для ТКС > 0.

4. APPLICATION
When using thermistors as sensors, two main modes are distinguished.
In the first mode, the temperature of the thermistor is practically determined only by the ambient temperature. The current passing through the thermistor is very small and practically does not heat it.
In the second mode, the thermistor is heated by the current passing through it, and the temperature of the thermistor is determined by changing heat transfer conditions, for example, airflow intensity, density of the surrounding gaseous medium, etc.
Since thermistors have a negative coefficient (NTC), and posistors have a positive coefficient (PTC), they will also be indicated on the diagrams accordingly.

NTC thermistors are temperature-sensitive semiconductor resistors whose resistance decreases with increasing temperature.

Application of NTC thermistors

PTC thermistors are ceramic components whose resistance instantly rises when the temperature exceeds an acceptable limit. This feature makes them ideal for various applications in modern electronic equipment.

Application of PTC thermistors

Illustrations for the use of thermistors:

- temperature sensors automobiles, in systems for adjusting the speed of rotation of coolers, in medical thermometers

- in home weather stations, air conditioners, microwave ovens

- in refrigerators, kettles, heated floors

- in dishwashers, car fuel flow sensors, water flow sensors

- in cartridges laser printers, degaussing systems for CRT monitors, ventilation and air conditioning systems

5. Examples of amateur radio designs using thermistors

5.1. Thermistor protection device for incandescent lamps
To limit the initial current, it is sometimes enough to connect a constant resistor in series with the incandescent lamp. In this case right choice the resistance of the resistor depends on the power of the incandescent lamps and on the current consumed by the lamp. The technical literature contains information on the results of measurements of current surges through the lamp in its cold and heated states when a limiting resistor is connected in series with the lamp. The measurement results show that the current surges through the filament of an incandescent lamp are 140% of the rated current flowing through the filament in a heated state and provided that the resistance of the series-connected limiting resistor is 70-75% of the nominal resistance of an incandescent lamp in working condition. And from this it follows that the preheating current of the lamp filament is also 70-75% of the rated current.

The main advantages of the circuit include the fact that it eliminates even small current surges through the filament of an incandescent lamp when turned on. This is ensured by the thermistor installed in the protection device. R3. At the initial moment of inclusion in the network, the thermistor R3 has a maximum resistance limiting the current flowing through this resistor. With gradual heating of the thermistor R3 its resistance gradually decreases, causing the current through the incandescent lamp and resistor R2 also gradually increases. The device circuit is designed in such a way that when a voltage of 180-200 V is reached on the incandescent lamp, the resistor R2 voltage drops, which leads to the operation of the electromagnetic relay K1. In this case, the relay contacts KL1 and K1.2 are closed.
Please note that another resistor is connected in series in the circuit of incandescent lamps - R4, which also limits inrush currents and protects the circuit from overloads. When the contacts of the relay KL1 are closed, the control electrode of the thyristor is connected VS1 to its anode, and this in turn leads to the opening of the thyristor, which ultimately shunts the thermistor R3, turning it off. Relay contacts K1.2 shunt resistor R4, which leads to an increase in voltage on incandescent lamps H2 and H3, and their filaments begin to glow more intensely.
The device is connected to an alternating current network with a voltage of 220 V, a frequency of 50 Hz using an electrical connector X1 type "fork". Turning the load on and off is provided by a switch S1. The fuse F1 is installed at the input of the device, which protects the input circuits of the device from overloads and short circuits with incorrect installation. The inclusion of the device in the AC mains is controlled by the indicator lamp HI glow discharge, which flares up immediately after switching on. In addition, a filter is assembled at the input of the device, which protects against high-frequency interference that penetrates into the power supply network of the device.
In the manufacture of an incandescent lamp protection device H2 and NZ used the following components: thyristor VS1 type KU202K; rectifier diodes VD1-4 type KDYU5B; indicator light H1 type TH-0.2-1; incandescent lamps H2, NC type 60W-220-240V; capacitors C1-2 type MBM-P-400V-0.1 μF, SZ - K50-3-10B-20 μF; resistors R1 type ВСа-2-220 kOhm, R2 - VSa-2-10 Ohm, R3 - MMT-9, R4 - homemade wire with a resistance of 200 ohms or type C5-35-3BT-200 ohms; electromagnetic relay K1 type RES-42 (passport RS4.569.151); electrical.connector X1 plug type with electric cable; switch S1 type P1T-1-1.
When assembling and repairing the device, other components can be used. Resistors of type BC can be replaced by resistors of types MLT, MT, S1-4, ULI; MBM type capacitors - on K40U-9, MBGO, K42U-2, K50-3 type capacitor - on K50-6, K50-12, K50-16; electromagnetic relay type RES-42 - for relay types RES-9 (passport RS4.524.200), RVM-2S-110, RPS-20 (passport RS4.521.757); thyristor type KU202K - on KU202L, KU202M, KU201K, KU201L; thermistor of any series.
To adjust and adjust the incandescent lamp protection device, you will need a power supply and an autotransformer that allows you to increase the AC supply voltage to 260 V. The voltage is applied to the input of the X1 device, and it is measured in points BUT and B, setting the voltage on the incandescent lamps to 200 V with an autotransformer. Instead of a constant resistor R2 install a wire variable resistor type PZVt-20 Ohm. Gradually increasing the resistance of the resistor R2 mark the moment of operation of the relay K1. Before making this adjustment, the thermistor R3 is shunted with a short-circuited jumper.
After checking the voltage on incandescent lamps with temporarily closed resistors R2 and R3 remove the jumpers, install the resistor in place R2 with the appropriate resistance, check the delay time of the electromagnetic relay, which should be within 1.5-2 s. If the relay operation time is much longer, then the resistance of the resistor R2 must be increased by a few ohms.
It should be noted that this device has a significant drawback: it can be turned on and off only after the thermistor R3 has completely cooled down after heating and is ready for a new switching cycle. The thermistor cooling time is 100-120 s. If the thermistor has not yet cooled down, then the device will operate with a delay only due to the resistor included in the circuit R4.

5.2. Simple thermostats in power supplies
First, the thermostat. When choosing a circuit, factors such as its simplicity, the availability of the elements (radio components) necessary for assembly, especially those used as temperature sensors, the manufacturability of assembly and installation in the PSU case, were taken into account.
According to these criteria, V. Portunov's scheme turned out to be the most successful. It reduces the wear of the fan and reduces the noise level generated by it. The diagram of this automatic fan speed controller is shown in fig. . The temperature sensor is diodes VD1-VD4, connected in the opposite direction to the base circuit of the composite transistor VT1, VT2. The choice of diodes as a sensor led to the dependence of their reverse current on temperature, which is more pronounced than the similar dependence of the resistance of thermistors. In addition, the glass case of these diodes makes it possible to do without any dielectric spacers when installing power supply transistors on the heat sink. An important role was played by the prevalence of diodes and their availability for radio amateurs.

Resistor R1 eliminates the possibility of failure of transistors VTI, VT2 in the event of thermal breakdown of the diodes (for example, when the fan motor is jammed). Its resistance is chosen based on the maximum permissible value of the base current VT1. Resistor R2 determines the threshold for the regulator.
It should be noted that the number of temperature sensor diodes depends on the static current transfer coefficient of the composite transistor VT1, VT2. If, with the resistance of the resistor R2 indicated in the diagram, room temperature and the power on, the fan impeller is stationary, the number of diodes should be increased. It is necessary to ensure that after applying the supply voltage, it confidently begins to rotate at a low frequency. Naturally, if the speed is too high with four sensor diodes, the number of diodes should be reduced.

The device is mounted in the power supply housing. The terminals of the same name of the diodes VD1-VD4 are soldered together, placing their cases in the same plane close to each other. The resulting block is glued with BF-2 glue (or any other heat-resistant, for example, epoxy) to the heat sink high voltage transistors with reverse side. Transistor VT2 with resistors R1, R2 soldered to its terminals and transistor VT1 (Fig. 2) are installed with the emitter output into the “+12 V fan” hole of the power supply board (the red wire from the fan was previously connected there). The adjustment of the device is reduced to the selection of the resistor R2 after 2 .. 3 minutes after turning on the PC and warming up the PSU transistors. Temporarily replacing R2 with a variable (100-150 kOhm), such a resistance is selected so that at rated load the heat sinks of the power supply transistors heat up no more than 40ºС.
To avoid defeat electric shock(heat sinks are under high voltage!) You can "measure" the temperature by touch only by turning off the computer.
A simple and reliable scheme was proposed by I. Lavrushov. The principle of its operation is the same as in the previous circuit, however, an NTC thermistor is used as a temperature sensor (nominal value of 10 kOhm is not critical). The transistor in the circuit is selected type KT503. As determined by experience, its operation is more stable than other types of transistors. It is desirable to use a multi-turn tuning resistor, which will allow you to more accurately adjust the temperature threshold of the transistor and, accordingly, the fan speed. The thermistor is glued to the 12 V diode assembly. If not available, it can be replaced with two diodes. More powerful fans with a current consumption of more than 100 mA should be connected through a composite transistor circuit (the second KT815 transistor).

Diagrams of two other, relatively simple and inexpensive PSU cooling fan speed controllers are often provided on the Internet (CQHAM.ru). Their peculiarity is that the integral stabilizer TL431 is used as a threshold element. It is quite easy to “get” this microcircuit when disassembling old ATX PC PSUs.
The author of the first scheme is Ivan Shor. When repeated, it turned out to be expedient to use a multi-turn resistor of the same rating as a tuning resistor R1. The thermistor is attached to the radiator of the cooled diode assembly (or to its body) through the KPT-80 thermal paste.

A similar circuit, but on two KT503 connected in parallel (instead of one KT815) in Fig.5. With the specified ratings of parts, 7V is supplied to the fan, increasing when the thermistor is heated. KT503 transistors can be replaced with imported 2SC945, all resistors with a power of 0.25W.

More complex scheme cooling fan speed controller is successfully used in another PSU. Unlike the prototype, it uses "television" transistors. The role of the radiator of the regulated transistor T2 on it is performed by the free section of the foil left on the front side of the board. This scheme allows, in addition to automatically increasing the fan speed when the radiator of the cooled PSU transistors or diode assembly is heated, to set the minimum threshold speed manually, up to the maximum.

5.3. Electronic thermometer with an accuracy of at least 0.1 °C.
It is easy to assemble it yourself according to the diagram below. Compared to a mercury thermometer, an electric thermometer is much safer, in addition, if a non-inertial thermistor of the STZ-19 type is used, the measurement time is only 3 s.

The basis of the circuit is the DC bridge R4, R5, R6, R8. Changing the resistance value of the thermistor leads to unbalance of the bridge. The unbalance voltage is compared with the reference voltage taken from the divider-potentiometer R2. The current flowing through R3, PA1 is directly proportional to the unbalance of the bridge, and hence the measured temperature. Transistors VT1 and VT2 are used as low-voltage zener diodes. They can be replaced by KT3102 with any letter index. Setting up the device begins with measuring the resistance of the thermistor at a fixed temperature of 20°C. After measuring R8 from two resistors R6 + R7, it is necessary to select the same resistance value with high accuracy. After that, potentiometers R2 and R3 are set to 1h middle position. You can use the following procedure to calibrate a thermometer. As a source of reference temperature, a container with heated water is used (it is better to choose a temperature closer to the upper limit of measurement), the temperature of which is controlled by a reference thermometer.
After turning on the power, perform the following operations:
a) we switch the switch S2 to the "CALIBRATION" position and with the resistor R8 we set the arrow to the zero mark of the scale;
b) place the thermistor in a container with water, the temperature of which should be within the measured range;
c) set the switch to the "MEASUREMENT" position and with the resistor R3 set the instrument pointer to the scale value, which will be equal to the measured value in accordance with the readings of the reference thermometer.
Operations a), b), c) are repeated several times, after which the setting can be considered complete.

5.4. Attachment to the multimeter for measuring temperature

A simple attachment containing six resistors allows you to use a digital voltmeter (or multimeter) to measure temperature with a resolution of 0.1 ° C and a thermal inertia of 10 ... 15 s. With such speed, it can also be used to measure body temperature. AT measuring device no changes are required, and the manufacture of the set-top box is also available to novice radio amateurs.
A semiconductor thermistor STZ-19 with a nominal resistance of 10 kOhm at t = 20°C was used as a sensor. Together with an additional resistor R3, it forms one half of the measuring bridge. The second half of the bridge is a voltage divider of resistors R4 and R5. the last during calibration set the initial value of the output voltage. The multimeter is used in the DC voltage measurement mode within 200 or 2000 mV. An appropriate choice of the resistance of the resistor R2 changes the sensitivity of the measuring bridge.
Immediately before measuring the temperature with a variable resistor R1, the supply voltage of the measuring circuit is set equal to that at which the initial calibration was performed. The attachment for reading the measured temperature is turned on with the SB1 push-button switch, and the transfer from the measurement mode to the voltage setting mode is switched on by the SB2 switch.
The calculation of an additional resistor R3 connected in series with the thermistor is carried out according to the formula R3 = Rtm (B - 2Tm) / (B + 2Tm), where RTm is the resistance of the thermistor in the middle of the temperature range; B is the thermistor constant; Tm - absolute temperature in the middle of the measuring range Т = t° + 273.
This value of R3 ensures the minimum deviation of the characteristic from linear.
The thermistor constant is determined by measuring the resistances RT1 and RT2 of the thermistor at two temperatures T1 and T2 and then calculating by the formula B = ln(RT1/RT2)/(1/T-1/T2).
On the contrary, with known parameters of a thermistor with a negative TCR, its resistance for a certain temperature T can be determined by the formula
The attachment is calibrated at two points: Tk- \u003d Tm + 0.707 (T2-T.) / 2 and TK2 \u003d Tm-0.707 (12-10 / 2, where Tm \u003d (Tm + T2) / 2, Ti and T2 - the beginning and the end of the temperature range.
During the initial calibration with a fresh battery, the resistance of the variable resistor R1 is set to the maximum so that as the capacitance is lost and the cell voltage decreases, the voltage on the bridge can be kept unchanged (the prefix consumes a current of about 8 mA). By adjusting the trimmer resistors R2, R5, the readings of the digital indicator of the multimeter are matched in three digits to the temperature values of the thermistor T "1 and T" 2, controlled by an accurate thermometer. If it is not available, use, for example, a medical thermometer to control the temperature within its scale and a stable melting temperature of ice - 0 ° C.
The author used M-830 from Mastech as a multimeter. Resistors R2, R5 are better to use multi-turn (SP5-1V, SP5-14). a R1 - single-turn, for example PPB: resistors R3 and R4 - MLT-0.125. To turn on the power and switch the set-top box mode, you can take the P2K pushbutton switches without fixing.
In the manufactured attachment, the boundaries of the measured temperature range were set - Т1 = 15°С: Т2 = 45°С. In the case of measurements in the range of positive and negative temperature values on the Celsius scale, the sign indication is obtained automatically.

5.5. Thermal relay
The thermal relay circuit is shown in. The heat-sensitive element of this machine is a semiconductor thermistor, the resistance of which increases sharply with decreasing temperature. So at room temperature (20 C) its resistance is 51 kOhm, and at 5-7 C it is already almost 100 kOhm, that is, it almost doubles. It is this property that is used in the automatic temperature controller.

At normal temperatures, the resistance of the thermistor R1 is relatively small, and a constant bias is applied to the base of the transistor VT1, which keeps it in the open state. As the temperature decreases, the resistance of the thermistor increases, the base current decreases, and the transistor begins to close. Then the Schmidt trigger, assembled on transistors VT2 and VT3, "overturns" (VT2 opens and VT3 closes) and supplies a bias to the base circuit of transistor T4, in the emitter circuit of which an electromagnetic relay is included. Transistor VT4 opens and turns on relay K1. Trimmer R3, you can select the trigger thresholds and, therefore, the temperature that the device will automatically maintain. Diode VD2, connected in the opposite direction, shunts the relay winding and protects the transistor from breakdown when the relay is turned on, when self-induction EMF occurs in its winding. Simultaneously with the operation of the relay, the HL1 LED starts to glow, which is used as an indicator of the operation of the entire device. Zener diode VD1 and resistor R9 form the simplest parametric voltage regulator for power supply electronic circuit devices, and capacitors C1 and C2 filter the alternating voltage rectified by the diode bridge VD3-VD6.
You can easily buy all the parts for assembling the device in a radio store. MLT type resistors, transistor VT1 -MP41; VT2, VT3 and VT4 - MP26. Instead, you can use any p-n-p transistors rated for voltages of at least 20 V. Relay K1 - type RES-10 or similar, operating at a current of 10-15 mA with switching or breaking contacts. If you cannot find the relay you need, do not despair. By replacing the VT4 transistor with a more powerful one, for example GT402 or GT403, you can include almost any relay used in transistor equipment in its collector circuit. LED HL1 - any type, transformer T1 - TVK-110.
All parts, with the exception of the thermistor R1, are mounted on printed circuit board, which is located in the room along with the electronic switch. When, when the temperature drops, the relay is activated and closes contacts K 1.1, a voltage appears on the control electrode of the triac VS1, which unlocks it. The circuit is closed.
Now about the establishment of an electronic circuit. Before connecting the contacts of relay 4 to the thyristor VS1, the thermostat must be tested and adjusted. You can do it like this.
Take a thermistor, solder a long wire in two-layer insulation to it and place it in a thin glass tube, sealing both ends with epoxy for tightness. Then turn on the power of the electronic regulator, lower the tube with the thermistor into a glass of ice and, by rotating the trimming resistor, achieve the relay operation.

5.6. Thermostat circuit for stabilizing the heater temperature (500 W)

The thermostat, the diagram of which is shown below, is designed to maintain a constant temperature of the air in the room, water in vessels, in thermostats, as well as solutions in color photography. A heater with a power of up to 500 W can be connected to it. The temperature controller consists of a threshold device (based on transistors T1 and T2), an electronic relay (based on transistor TZ and thyristor D10) and a power supply. temperature sensor the thermistor R5 is used, which is included in the voltage supply circuit to the base of the transistor T1 of the threshold device.
If the environment is at the required temperature, the threshold device transistor T1 is closed and T2 is open. Transistor TZ and thyristor D10 of the electronic relay are closed in this case, and the mains voltage is not supplied to the heater. When the temperature of the medium decreases, the resistance of the thermistor increases, as a result of which the voltage at the base of the transistor T1 increases. When it reaches the threshold of the device, the transistor T1 will open, and T2 will close. This will open the transistor TK. The voltage that occurs across the resistor R9 is applied between the cathode and the control electrode of the thyristor D10 and will be enough to open it. The mains voltage through the thyristor and diodes D6 - D9 will go to the heater.
When the temperature of the environment reaches the required value, the thermostat will turn off the voltage from the heater. The variable resistor R11 is used to set the limits of the maintained temperature.
Thermistor MMT-4 is used in the thermostat. The Tr transformer is made on the Ш12Х25 core. Winding I contains 8000 turns of wire PEV-1 0.1, winding II - 170 turns of wire PEV-1 0.4.

5.7. THERMOREGULATE FOR THE INCUBATOR
A scheme of a simple and reliable thermal relay for an incubator is proposed. It is characterized by low power consumption, the heat generation on the power elements and the ballast resistor is negligible.
I propose a scheme for a simple and reliable thermal relay for an incubator. The scheme has been manufactured, tested, verified in continuous operation for several months of operation.
Technical data:
Supply voltage 220 V, 50 Hz
Switched active load power up to 150 W.
Temperature maintenance accuracy ±0.1 °С
Temperature control range from + 24 to 45°С.
circuit diagram devices

A comparator is assembled on the DA1 chip. Adjustment of the set temperature is made by a variable resistor R4. The temperature sensor R5 is connected to the circuit with a shielded wire in PVC insulation through a C1R7 filter to reduce interference. You can use a double thin wire twisted into a bundle. The thermistor must be placed in a thin PVC tube.
Capacitor C2 creates a negative feedback by alternating current. The circuit is powered through a parametric stabilizer made on a VD1 zener diode of the D814A-D type. Capacitor C3 is a power filter. Ballast resistor R9 to reduce power dissipation is composed of two resistors connected in series 22 kOhm 2 W. For the same purpose transistor key on VT1 type KT605B, KT940A is connected not to the zener diode, but to the anode of the thyristor VS1.
The rectifier bridge is assembled on VD2-VD5 diodes of the KD202K, M, R type, installed on small U-shaped aluminum radiators 1-2 mm thick with an area of 2-2.5 cm2. The VS1 thyristor is also installed on a similar radiator with an area of 10- 12 cm2
As a heater, lighting lamps HL1...HL4 are used, connected in series-parallel to increase the service life and eliminate emergencies in the event of a burnout of the filament of one of the lamps.
Schema work. When the temperature of the temperature sensor is less than the specified level set by the potentiometer R4, the voltage at pin 6 of the DA1 chip is close to the supply voltage. The key on the transistor VT1 and thyristor VS1 is open, the heater on HL1...HL4 is connected to the network. As soon as the temperature reaches a predetermined level, the DA1 chip will switch, the voltage at its output will become close to zero, the thyristor key will close, and the heater will turn off the mains. When the heater is turned off, the temperature will begin to decrease, and when it falls below the set level, the key and heater will turn on again.
Parts and their replacement. As DA1, you can use K140UD7, K140UD8, K153UD2 (Editor's note - almost any operational amplifier or comparator will do). Capacitors of any type for the corresponding operating voltage. Thermistor R5 type MMT-4 (or another with negative TKS). Its value can be from 10 to 50 kOhm. In this case, the value of R4 should be the same.

A device made from serviceable parts starts working immediately.
During testing and operation, safety regulations must be observed, since the device has a galvanic connection with the network.

5.8. THERMOSTAT
The thermostat is designed to maintain the temperature in the range of 25-45°C with an accuracy of no worse than 0.05C. With the obvious simplicity of the circuit, this thermostat has an undoubted advantage over similar ones: there are no elements in the circuit that operate in a key mode. Thus, it was possible to avoid impulse noise that occurs when switching loads with a significant current consumption.

The heating elements are wire resistors (10 Ohm, 10 W) and a P217V control transistor (can be replaced by any modern silicon transistor p-p-r structures). Refrigerator - radiator. The thermistor (MMT-4 3.3 Kom) is soldered to a copper cup, into which a temperature-controlled jar is inserted. It is necessary to wrap several layers of thermal insulation around the cup and make a thermally insulating lid over the jar.
The circuit is powered by a stabilized laboratory block nutrition. When the circuit is turned on, heating begins, which is signaled by a red LED. When the set temperature is reached, the brightness of the red LED decreases and the green light starts to glow. After the end of the process of "running out" of the temperature, both LEDs glow at full intensity - the temperature has stabilized.
The whole circuit is located inside a U-shaped aluminum radiator. Thus, all elements of the circuit are also temperature-controlled, which increases the accuracy of the device.

5.9. Temperature, light or voltage regulator
This simple electronic regulator depending on the sensor used, it can act as a temperature, light or voltage regulator. The device was taken as a basis, published in the article by I. Nechaev "Temperature regulators of the tip of network soldering irons" ("Radio", 1992, No. 2 - 3, p. 22). The principle of its operation differs from the analogue only in that the threshold of the transistor VT1 is regulated by the resistor R5.

The regulator is not critical to the ratings of the applied elements. It operates at a stabilization voltage of the zener diode VD1 from 8 to 15 V. The resistance of the thermistor R4 is in the range from 4.7 to 47 kOhm, the variable resistor R5 is from 9.1 to 91 kOhm. Transistors VT1, VT2 are any low-power silicon structures p-p-p and p-p-p, respectively, for example, the KT361 and KT315 series with any letter index. Capacitor C1 can have a capacity of 0.22 ... 1 microfarad, and C2 - 0.5 ... 1 microfarad. The latter must be designed for an operating voltage of at least 400 V.
A properly assembled device does not need to be adjusted. In order for it to act as a dimmer, the thermistor R4 must be replaced with a photoresistor or photodiode connected in series with a resistor whose value is selected experimentally.
The author's version of the design described here is used to control the temperature in a home incubator, therefore, to increase reliability, when the VS1 trinistor is open, the lighting lamps connected to the load (four parallel-connected lamps with a power of 60 W for a voltage of 220 V) burn at full heat. When operating the device in the dimmer mode, a bridge rectifier VD2-VD5 should be connected to points A-B. Its diodes are selected depending on the regulated power.
When working with the regulator, it is important to observe electrical safety measures: it must be placed in a plastic case, the handle of the resistor R5 should be made of insulating material and ensure good electrical insulation of the thermistor R4.

5.10. Lamp power daylight direct current
In these devices, a pair of connector contacts of each filament can be connected together and connected to “its own” circuit - then even a lamp with burnt filaments will work in the lamp.

A diagram of a device variant designed to power a fluorescent lamp with a power of 40 W or more is shown in fig. . Here, the bridge rectifier is made on diodes VD1-VD4. And the "starting" capacitors C2, C3 are charged through thermistors R1, R2 with a positive temperature coefficient of resistance. Moreover, in one half-cycle, the capacitor C2 is charged (through the thermistor R1 and the diode VD3), and in the other - C3 (through the thermistor R2 and the diode VD4). Thermistors limit the charging current of capacitors. Since the capacitors are connected in series, the voltage across the EL1 lamp is sufficient to ignite it.
If the thermistors are in thermal contact with the bridge diodes, their resistance will increase when the diodes are heated, which will reduce the charging current.

The inductor, which serves as a ballast resistance, is not necessary in the considered power devices and can be replaced by an incandescent lamp, as shown in fig. . When the device is connected to the network, the lamp EL1 and the thermistor R1 heat up. The alternating voltage at the input of the diode bridge VD3 increases. Capacitors C1 and C2 are charged through resistors R2, R3. When the total voltage on them reaches the ignition voltage of the EL2 lamp, the capacitors will quickly discharge - this is facilitated by the diodes VD1, VD2.
Complementing an ordinary lamp with an incandescent lamp with this device with fluorescent lamp, general or local lighting can be improved. For a 20W EL2 lamp, EL1 should be 75W or 100W, if EL2 is 80W, EL1 should be 200W or 250W. In the latter version, it is permissible to remove charge-discharge circuits from resistors R2, R3 and diodes VD1, VD2 from the device.

This concludes my review of THERMORESTORS.
A few words about another radio component - varistor.
I do not plan to do a separate article about him, so - briefly:
A VARISTOR is also a semiconductor resistor whose resistance depends on the applied voltage. Moreover, as the voltage increases, the resistance of the varistor decreases. Everything is elementary. The greater the strength of the external electric field, the more electrons it "rips off" from the shells of the atom, the more holes are formed - the number of free charge carriers increases, the conductivity also increases, and the resistance decreases. This is if the semiconductor is pure. In practice, everything is much more complicated. Tirite, vilite, latin, silite are semiconductor materials based on silicon carbide. Zinc oxide is a new material for varistors. As you can see, there are no pure semiconductors here.

The varistor has the property of sharply reducing its resistance from units of GOhm (GigaOhm) to tens of Ohms with an increase in the voltage applied to it above the threshold value. With a further increase in voltage, the resistance decreases even more. Due to the absence of follow currents when the applied voltage changes abruptly, varistors are the main element for the production of surge protection devices.

On this acquaintance with the family of resistors can be considered complete.