The working principle of a thermistor. What is a thermistor and its use in electronics

And consisting of a semiconductor material, which, with a slight change in temperature, greatly changes its resistance. Typically, thermistors have negative temperature coefficients, meaning their resistance decreases as temperature increases.

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 of 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.

Thermistors are made of metal oxides with semiconductor properties. During production, these devices are given the following form:

disc-shaped;
core;
spherical like a pearl.

The operation of a thermistor is based on the principle of a strong change in resistance with a small change in temperature. At the same time, at a given current strength 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 across the device are measured. According to Ohm's simple law, R=U/I determines the resistance. Next, they look at the resistance versus temperature curve, which can be used to tell 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 accuracy.

Thermistor material

The material of the vast majority of thermistors is semiconductor ceramics. The manufacturing process involves sintering powders of metal nitrides and 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 lower than 250 °C, then the ceramic composition includes magnesium, cobalt and nickel. Ceramics of this composition show 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 semiconductor ceramic.

Instrument manufacturing process

Spherical thermistors are made by coating them on two platinum wires at high temperature (1100 °C). After this, the wire is cut to give the required shape to the thermistor contacts. A glass coating is applied to the spherical device to seal it.

In the case of disk thermistors, the process of making contacts consists of 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 whose working material is platinum. These detectors change their resistance linearly with temperature changes. 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 when temperature changes over the entire operating range.
High level of instrument stability and repeatability of the readings obtained.
Small size that allows you to quickly respond to temperature changes.

Thermistor resistance

This physical quantity decreases in value as the temperature increases, and it is important to take into account 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, devices with a resistance exceeding 10 kOhm are used.

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

Stability and accuracy

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

Basic parameters of thermistors

The following physical parameters are basic for each type of thermistor (the names are explained in English):

R 25 - device resistance in Ohms at room temperature (25 °C). You can simply check this characteristic of the thermistor using a multimeter.
Tolerance of R 25 - the tolerance value for resistance deviation on the device from its set value at a temperature of 25 °C. As a rule, this value does not exceed 20% of R25.
Max. Steady State Current - the maximum value of current in Amperes that can flow through the device for a long time. Exceeding this value threatens a rapid drop in resistance and, as a result, failure of the thermistor.
Approx. R of Max. Current - this value shows the resistance value in Ohms that the device acquires when a maximum current passes through it. This value should be 1-2 orders of magnitude less than the thermistor resistance at room temperature.
Dissip. Coef. - coefficient that shows the temperature sensitivity of the device to the power it absorbs. This coefficient shows the amount of power in mW that must be absorbed by the thermistor in order for its temperature to increase by 1 °C. This value is important because it shows how much power needs to be expended 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 turning off the power in order to be ready when it is turned 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 will decrease 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 capacity in microfarads that can be discharged through a given device without damaging it. This value is indicated for a specific voltage, for example, 220 V.

How to check the thermistor for functionality?

To roughly check the thermistor for its serviceability, you can use a multimeter and a regular 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 terminals of the multimeter. In this case, polarity does not matter. The multimeter will show a certain resistance in Ohms, it should be written down.

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 properly.

If the resistance on the multimeter has not changed or, conversely, has dropped sharply, then the device is unsuitable for use.

Note that this check is rough. To accurately test a device, it is necessary to measure two indicators: its temperature and the corresponding resistance, and then compare these values with 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. These areas include computers, high-precision equipment in industrial plants, and devices for transmitting various data. Thus, a 3D printer thermistor is used as a sensor that monitors the temperature of the heating table or print head.

One common use of 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 can burn the entire microcircuit, so a thermistor is included in the circuit.

When turned on, this device was at room temperature and had enormous resistance. This resistance allows you to effectively reduce the current surge at the time of start-up. Next, the device heats up due to the current passing through it and the release of heat, and its resistance sharply decreases. The calibration of the thermistor is such that the operating temperature of the computer chip leads to virtually zero resistance of the thermistor, and no voltage drop occurs 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 cost-effective and quite simple.

Examples of thermistors

There is currently 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 kOhm with a 10% tolerance. Used as a temperature measurement sensor in consumer and automotive electronics.
The B57153-S disk device has a maximum permissible current of 1.8 A with a resistance of 15 Ohms at room temperature. Used as a starting current limiter.

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

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

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

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

A thermistor is a temperature-dependent resistor, made of a semiconductor material that has a negative temperature coefficient and high sensitivity, a posistor isa temperature-dependent resistor having a positive coefficient.Thus, with an increase in the temperature of the posistor body, its resistance also increases, and with an increase in the temperature of the thermistor, its resistance correspondingly decreases.

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

Thermistors can generally be classified into:

Low temperature class (operating 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 (operating temperature from 900 K to 1300 K).

All these elements, both thermistors and posistors, can operate under a variety of climatic external conditions and under 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 indirectly heated thermistors. The housings of such devices contain the thermistor itself and a galvanically isolated heating element, 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 heating element of the thermistor.

Depending on how the operating point is selected on the current-voltage characteristic of a particular component, the operating mode of the thermistor in the circuit is also determined. And the current-voltage characteristic itself is related to the design features and 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 an operating point set in the linear section of the current-voltage characteristic.

But the operating point is traditionally set on the falling section of the current-voltage characteristic (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 installations for controlling the flow of bulk solids and liquids.

Most Popular Today medium-temperature thermistors and posistors with TKS 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 low TCR from 0.5% to 0.7% per 1 K, made on the basis of silicon. Their resistance changes 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 powerful ones. These components fit easily into circuit diagrams and do not take up much space on boards.

A typical posistor has the shape of a ceramic disk; sometimes several elements are installed in series in one housing, but more often - in a single design with a protective enamel coating. PTC resistors are often used as fuses to protect electrical circuits from voltage and current overloads, as well as temperature sensors and auto-stabilizing elements, due to their unpretentiousness and physical stability.

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

One of the simplest and most popular uses of a thermistor is to effectively limit inrush current. At the moment voltage is applied to the power supply from the network, an extremely sharp surge of 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 restores its original resistance after a few minutes, as soon as it cools down to room temperature.

NTC and PTC thermistors

Currently, the industry produces a huge range of thermistors, posistors and NTC thermistors. Each individual model or series is manufactured for operation in certain conditions, and certain requirements are imposed on them.

Therefore, simply listing the parameters of posistors and NTC thermistors will be of little use. We'll take a slightly different route.

Every time you get your hands on a thermistor with easy-to-read markings, you need to find a reference sheet or datasheet for this thermistor model.

If you don’t know what a datasheet is, I advise you to take a look at this page. In a nutshell, the datasheet contains information on all the main parameters of this component. This document lists everything you need to know to apply a specific electronic component.

I had this thermistor in stock. Take a look at the photo. At first I knew nothing about him. There was minimal information. Judging by the marking, this is a PTC thermistor, that is, a posistor. It says so on it - PTC. The following is the marking C975.

At first it may seem that it is unlikely that it will be possible to find at least some information about this posistor. But, don’t hang your nose! Open the browser, type a phrase like these into Google: “posistor c975”, “ptc c975”, “ptc c975 datasheet”, “ptc c975 datasheet”, “posistor c975 datasheet”. Next, all that remains is to find the datasheet for this posistor. As a rule, datasheets are formatted as a PDF file.

From the found datasheet on PTC C975, I learned the following. It is produced by EPCOS. Full title B59975C0160A070(B599*5 series). This PTC thermistor is used to limit current during short circuits and overloads. Those. This is a kind of fuse.

I will give a table with the main technical characteristics for the B599*5 series, as well as a brief explanation of what all these numbers and letters mean.

Now let's turn our attention to the electrical characteristics of a particular product, in our case it is a PTC C975 posistor (full marking B59975C0160A070). Take a look at the following table.

I R - Rated current (mA). Rated current. This is the current that a given posistor can withstand for a long time. I would also call it working, normal current. For the C975 posistor, the rated current is just over half an ampere, specifically 550 mA (0.55A).

I S - Switching current (mA). Switching current. This is the amount of current flowing through a posistor at which its resistance begins to increase sharply. Thus, if a current of more than 1100 mA (1.1A) begins to flow through the C975 posistor, it will begin to fulfill its protective function, or rather, it will begin to limit the current flowing through itself due to an increase in resistance. Switching current ( I S) and reference temperature ( Tref) are connected, since the switching current causes the posistor to heat up and its temperature reaches the level Tref, at which the resistance of the posistor increases.

I Smax - Maximum switching current (A). Maximum switching current. As we can see from the table, for this value the voltage value on the posistor is also indicated - V=Vmax. This is no accident. The fact is that any posistor can absorb a certain power. If it exceeds the permissible limit, it will fail.

Therefore, the voltage is also specified for the maximum switching current. In this case it is equal to 20 volts. Multiplying 3 amperes by 20 volts, we get a power of 60 watts. This is exactly the power our posistor can absorb when limiting the current.

I r - Residual current (mA). Residual current. This is the residual current that flows through the posistor, after it has triggered, and begins to limit the current (for example, during an overload). The residual current keeps the posistor heated so that it is in a “warm” state and acts as a current limiter until the cause of the overload is eliminated. As you can see, the table shows the value of this current for different voltages on the posistor. One for maximum ( V=Vmax), another for nominal ( V=V R). It is not difficult to guess that by multiplying the limiting current by the voltage, we get the power that is required to maintain the posistor heating in the activated state. For a posistor PTC C975 this power is 1.62~1.7W.

What's happened R R And Rmin The following graph will help us understand.

R min - Minimum resistance (Ohm). Minimal resistance. The smallest resistance value of the posistor. The minimum resistance, which corresponds to the minimum temperature after which the range with positive TCR begins. If you study the graphs for posistors in detail, you will notice that up to the value T Rmin On the contrary, the resistance of the posistor decreases. That is, a posistor at temperatures below T Rmin behaves like a “very bad” NTC thermistor and its resistance decreases (slightly) with increasing temperature.

R R - Rated resistance (Ohm). Nominal resistance. This is the resistance of the posistor at some previously specified temperature. Usually this 25°C(less often 20°С). Simply put, this is the resistance of a posistor at room temperature, which we can easily measure with any multimeter.

Approvals - literally translated, this is approval. That is, it is approved by such and such an organization that deals with quality control, etc. Not particularly interested.

Ordering code - serial number. Here, I think, it’s clear. Full product labeling. In our case it is B59975C0160A070.

From the datasheet for the PTC C975 posistor, I learned that it can be used as a self-resetting fuse. For example, in an electronic device that in operating mode consumes a current of no more than 0.5A at a supply voltage of 12V.

Now let's talk about the parameters of NTC thermistors. Let me remind you that the NTC thermistor has a negative TCS. Unlike posistors, when heated, the resistance of an NTC thermistor drops sharply.

I had several NTC thermistors in stock. They were mainly installed in power supplies and all sorts of power units. Their purpose is to limit the starting current. I settled on this thermistor. Let's find out its parameters.

The only markings on the body are as follows: 16D-9 F1. After a short search on the Internet, we managed to find a datasheet for the entire series of MF72 NTC thermistors. Specifically, our copy is MF72-16D9. This series of thermistors are used to limit inrush current. The following graph clearly shows how an NTC thermistor works.

At the initial moment, when the device is turned on (for example, a laptop switching power supply, adapter, computer power supply, charger), the resistance of the NTC thermistor is high, and it absorbs the current pulse. Then it warms up, and its resistance decreases several times.

While the device is operating and consuming current, the thermistor is in a heated state and its resistance is low.

In this mode, the thermistor offers virtually no resistance to the current flowing through it. As soon as the electrical appliance is disconnected from the power source, the thermistor will cool down and its resistance will increase again.

Let's turn our attention to the parameters and main characteristics of the NTC thermistor MF72-16D9. Let's take a look at the table.

R 25 - Nominal resistance of the thermistor at 25°C (Ohm). Thermistor resistance at an ambient temperature of 25°C. This resistance can be easily measured with a multimeter. For the thermistor MF72-16D9 this is 16 Ohms. In fact R 25- this is the same as R R(Rated resistance) for a posistor.

Max. Steady State Current - Thermistor maximum current (A). The maximum possible current through the thermistor that it can withstand for a long time. If you exceed the maximum current, an avalanche-like drop in resistance will occur.

Approx. R of Max. Current - Thermistor resistance at maximum current (Ohm). Approximate value of NTC thermistor resistance at maximum current flow. For the MF72-16D9 NTC thermistor, this resistance is 0.802 Ohm. This is almost 20 times less than the resistance of our thermistor at a temperature of 25°C (when the thermistor is “cold” and not loaded with flowing current).

Dissip. Coef. - Energy sensitivity factor (mW/°C). For the thermistor's internal temperature to change by 1°C, it must absorb a certain amount of power. The ratio of absorbed power (in mW) to the change in temperature of the thermistor is what this parameter shows. For our thermistor MF72-16D9 this parameter is 11 milliWatt/1°C.

Let me remind you that when an NTC thermistor heats up, its resistance drops. To heat it up, the current flowing through it is consumed. Therefore, the thermistor will absorb power. The absorbed power leads to heating of the thermistor, and this in turn leads to a decrease in the resistance of the NTC thermistor by 10 - 50 times.

Thermal Time Constant - Cooling time constant (S). The time during which the temperature of an unloaded thermistor will change by 63.2% of the temperature difference between the thermistor itself and the environment. Simply put, this is the time during which the NTC thermistor has time to cool down after current stops flowing through it. For example, when the power supply is disconnected from the mains.

Max. Load Capacitance in μF - Maximum discharge capacity . Test characteristic. Shows the capacitance that can be discharged into an NTC thermistor through a limiting resistor in a test circuit without damaging it. Capacitance is specified in microfarads and for a specific voltage (120 and 220 volts alternating current (VAC)).

Tolerance of R 25 - Tolerance . Permissible deviation of the thermistor resistance at a temperature of 25°C. Otherwise, this is a deviation from the nominal resistance R 25. Typically the tolerance is ±10 - 20%.

That's all the main parameters of thermistors. Of course, there are other parameters that can be found in datasheets, but they, as a rule, are easily calculated from the main parameters.

I hope now, when you come across an electronic component that is unfamiliar to you (not necessarily a thermistor), it will be easy for you to find out its main characteristics, parameters and purpose.

In electronics there is always something to measure or evaluate. For example, temperature. This task is successfully accomplished by thermistors - electronic components based on semiconductors, the resistance of which varies depending on temperature.

Here I will not describe the theory of the physical processes that occur in thermistors, but will move closer to practice - I will introduce the reader to the designation of the thermistor on the diagram, its appearance, some varieties and their features.

On circuit diagrams, the thermistor is designated like this.

Depending on the scope of application and type of thermistor, its designation on the diagram may have slight differences. But you can always identify it by its characteristic inscription t or t° .

The main characteristic of a thermistor is its TKS. TKS is temperature coefficient of resistance. It shows by what amount the resistance of the thermistor changes when the temperature changes by 1°C (1 degree Celsius) or 1 degree Kelvin.

Thermistors have several important parameters. I won’t cite them; this is a separate story.

The photo shows the thermistor MMT-4V (4.7 kOhm). If you connect it to a multimeter and heat it, for example, with a hot air gun or a soldering iron tip, you can make sure that its resistance drops with increasing temperature.

Thermistors are found almost everywhere. Sometimes you are surprised that you didn’t notice them before, didn’t pay attention to them. Let's take a look at the board from the IKAR-506 charger and try to find them.

Here is the first thermistor. Since it is in an SMD case and has a small size, it is soldered onto a small board and installed on an aluminum radiator - it controls the temperature of the key transistors.

Second. This is the so-called NTC thermistor ( JNR10S080L). I'll tell you more about these. It serves to limit the starting current. It's funny. It looks like a thermistor, but serves as a protective element.

For some reason, when we talk about thermistors, they usually think that they are used to measure and control temperature. It turns out that they have found application as security devices.

Thermistors are also installed in car amplifiers. Here is the thermistor in the Supra SBD-A4240 amplifier. Here it is involved in the amplifier overheating protection circuit.

Here's another example. This is a DCB-145 lithium-ion battery from a DeWalt screwdriver. Or rather, his “giblets”. A measuring thermistor is used to control the temperature of the battery cells.

He is almost invisible. It is filled with silicone sealant. When the battery is assembled, this thermistor fits tightly to one of the Li-ion battery cells.

Direct and indirect heating.

According to the heating method, thermistors are divided into two groups:

Direct heating. This is when the thermistor is heated by external ambient air or current that flows directly through the thermistor itself. Directly heated thermistors are typically used for either temperature measurement or temperature compensation. Such thermistors can be found in thermometers, thermostats, chargers (for example, for Li-ion batteries in screwdrivers).

Indirect heating. This is when the thermistor is heated by a nearby heating element. At the same time, it itself and the heating element are not electrically connected to each other. In this case, the resistance of the thermistor is determined by a function of the current flowing through the heating element, not through the thermistor. Thermistors with indirect heating are combined devices.

NTC thermistors and posistors.

Based on the dependence of the change in resistance on temperature, thermistors are divided into two types:

PTC thermistors (aka posistors).

Let's figure out what the difference is between them.

NTC thermistors get their name from the abbreviation NTC - Negative Temperature Coefficient , or "Negative Resistance Coefficient". The peculiarity of these thermistors is that When heated, their resistance decreases. By the way, this is how the NTC thermistor is indicated in the diagram.

Thermistor designation on the diagram

As you can see, the arrows on the designation are in different directions, which indicates the main property of the NTC thermistor: the temperature increases (up arrow), the resistance drops (down arrow). And vice versa.

In practice, you can find an NTC thermistor in any switching power supply. For example, such a thermistor can be found in a computer power supply. We have already seen the NTC thermistor on the IKAR board, only there it was gray-green.

This photo shows an NTC thermistor from EPCOS. Used to limit starting current.

For NTC thermistors, as a rule, its resistance at 25°C is indicated (for this thermistor this is 8 Ohms) and the maximum operating current. This is usually a few amps.

This NTC thermistor is installed in series at the 220V mains voltage input. Take a look at the diagram.

Since it is connected in series with the load, all current consumed flows through it. The NTC thermistor limits the inrush current, which occurs due to the charging of electrolytic capacitors (in diagram C1). An inrush of charging current can lead to breakdown of the diodes in the rectifier (diode bridge on VD1 - VD4).

Each time the power supply is turned on, the capacitor begins to charge, and current begins to flow through the NTC thermistor. The resistance of the NTC thermistor is high, since it has not yet had time to heat up. Flowing through the NTC thermistor, the current heats it up. After this, the resistance of the thermistor decreases, and it practically does not interfere with the flow of current consumed by the device. Thus, due to the NTC thermistor, it is possible to ensure a “smooth start” of the electrical device and protect the rectifier diodes from breakdown.

It is clear that while the switching power supply is turned on, the NTC thermistor is in a “heated” state.

If any elements in the circuit fail, then the current consumption usually increases sharply. At the same time, there are often cases when an NTC thermistor serves as a kind of additional fuse and also fails due to exceeding the maximum operating current.

The failure of the key transistors in the charger's power supply led to the maximum operating current of this thermistor being exceeded (max 4A) and it burned out.

PTC resistors. PTC thermistors.

Thermistors, whose resistance increases when heated, are called posistors. They are also PTC thermistors (PTC - Positive Temperature Coefficient , "Positive Resistance Coefficient").

It is worth noting that posistors are less widespread than NTC thermistors.

PTC resistors are easy to detect on the board of any color CRT TV (with a picture tube). There it is installed in the demagnetization circuit. In nature, there are both two-terminal posistors and three-terminal ones.

The photo shows a representative of a two-terminal posistor, which is used in the demagnetization circuit of a kinescope.

The working fluid of the posistor is installed inside the housing between the spring terminals. In fact, this is the posistor itself. Outwardly it looks like a tablet with a contact layer sprayed on the sides.

As I already said, posistors are used to demagnetize the picture tube, or rather its mask. Due to the Earth's magnetic field or the influence of external magnets, the mask becomes magnetized, and the color image on the kinescope screen is distorted and spots appear.

Probably everyone remembers the characteristic “clang” sound when the TV turns on - this is the moment when the demagnetization loop works.

In addition to two-terminal posistors, three-terminal posistors are widely used. Like these ones.

Their difference from two-terminal ones is that they consist of two “pill” posistors, which are installed in one housing. These “tablets” look exactly the same. But that's not true. In addition to the fact that one tablet is slightly smaller than the other, their resistance when cold (at room temperature) is different. One tablet has a resistance of about 1.3 ~ 3.6 kOhm, while the other has only 18 ~ 24 Ohm.

Three-terminal posistors are also used in the kinescope demagnetization circuit, like two-terminal ones, but their connection circuit is slightly different. If the posistor suddenly fails, and this happens quite often, then spots with an unnatural color display appear on the TV screen.

And capacitors. They are not marked, which makes their identification difficult. In appearance, SMD thermistors are very similar to ceramic SMD capacitors.

Built-in thermistors.

Built-in thermistors are also actively used in electronics. If you have a soldering station with tip temperature control, then a thin-film thermistor is built into the heating element. Thermistors are also built into the hair dryer of hot-air soldering stations, but there it is a separate element.

It is worth noting that in electronics, along with thermistors, thermal fuses and thermal relays (for example, KSD type) are actively used, which are also easy to find in electronic devices.

Now that we are familiar with thermistors, it’s time.

1. WHAT IS THIS?
A thermistor is a semiconductor resistor that uses the temperature dependence of the semiconductor resistance.
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 designed very simply and are manufactured in various shapes and sizes

In order to more or less imagine the physical basis of the operation of this radio component, you should first become familiar with the structure and properties of semiconductors (see my article “Semiconductor Diode”).
A quick reminder. Semiconductors contain two types of free electric charge carriers: “-” 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 equilibrium. When the temperature changes, this equilibrium is disrupted: if the temperature increases, then the concentration of carriers increases (conductivity increases, resistance decreases), and if it decreases, then the concentration of free carriers also decreases (conductivity decreases, resistance increases).
The dependence of the resistivity of a semiconductor on temperature is shown in the graph.
As you can see, if the temperature tends to absolute zero (-273.2C), then the semiconductor becomes an almost ideal dielectric. If the temperature increases greatly, then, on the contrary, it becomes an almost ideal conductor. But the most important thing is that the R(T) dependence of a semiconductor is strongly expressed in the range of ordinary temperatures, say, from -50C to +100C (you can take it a little wider).

The thermistor was invented by Samuel Reuben in 1930.

2. MAIN PARAMETERS
2.1. Nominal resistance - resistance of the thermistor 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 resistivity of a substance when the temperature changes by 1°C (1K).
There are thermistors with negative ( thermistors) and positive ( posistors) TKS. They are also called NTC thermistors (Negative temperature coefficient) and PTC thermistors (Positive temperature coefficient), respectively. For posistors, as the temperature increases, the resistance also increases, but for thermistors, the opposite is true: as the temperature increases, the resistance decreases.
The TCS 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 to operate at 4.2 K and below and at 900–1300 K. The most widely used are medium temperature thermistors with a TCR of -2.4 to -8.4%/K and a nominal resistance of 1–106 ohms .

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 a “+” sign. There is no negative absolute temperature. Designation: T, unit of measure 1K (Kelvin). 1K=1°C, therefore the formula for converting temperature from the Celsius scale to the thermodynamic temperature scale is very simple: T=t+273 (approximately) or, accordingly, vice versa: t=T-273. Here t is the temperature on the Celsius scale.
The relationship between the Celsius and Kelvin scales is shown in

2.4. Rated power dissipation is the power at which the thermistor maintains its parameters within the limits specified by the technical specifications during operation.

3. OPERATING MODE
The operating mode of thermistors depends on which part of the static current-voltage characteristic (volt-ampere characteristic) the operating point is selected. In turn, the current-voltage characteristic depends both on the design, dimensions and main parameters of the thermistor, and on temperature, thermal conductivity of the environment, and the thermal connection between the thermistor and the environment. Thermistors with an operating point at the initial (linear) section of the current-voltage characteristic are used to measure and control temperature and compensate for temperature changes in the parameters of electrical circuits and electronic devices. Thermistors with an operating point in the descending section of the current-voltage characteristic (with negative resistance) are used as starting relays, time relays, power meters of electromagnetic radiation in the microwave, temperature and voltage stabilizers. The operating mode of the thermistor, in which the operating point is also on the descending section of the current-voltage characteristic (this uses the dependence of the thermistor resistance on the temperature and thermal conductivity of the environment), is typical for thermistors used in thermal systems. control and fire alarm, regulation of the level of liquid and granular media; the action of such thermistors is based on the occurrence of a relay effect in the circuit with the thermistor when the ambient temperature changes or the conditions of heat exchange between the thermistor and the medium.
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, 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 thermistors made from BaTiO-based solid solutions. They are called posistors. There are known thermistors with a small positive TCR (0.5–0.7%/K), made on the basis of silicon with electronic conductivity; their resistance changes with temperature approximately linearly. Such thermistors are used, for example, for temperature stabilization of electronic devices using transistors.
In Fig. shows the dependence of the thermistor resistance on temperature. Line 1 - for TKS< 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 conditions of heat transfer, for example, the intensity of the blowing, the density of the surrounding gaseous medium, etc.
Since thermistors have a negative coefficient (NTC), and posistors have a positive coefficient (RTS), they will be designated accordingly in the diagrams.

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 increases when the temperature exceeds an acceptable limit. This feature makes them ideal for a variety of applications in modern electronic equipment.

Application of RTS thermistors

Illustrations for the use of thermistors:

- temperature sensors for cars, 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 consumption sensors, water flow sensors

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

5. Examples of amateur radio designs using thermistors

5.1. Thermistor-based incandescent lamp protection device
To limit the initial current, sometimes it is enough to connect a constant resistor in series with the incandescent lamp. In this case, the correct choice of resistor resistance depends on the power of the incandescent lamps and the current consumed by the lamp. In the technical literature there is information about the results of measuring current surges through the lamp in its cold and warm states when connected in series with the lamp with a limiting resistor. 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 rated resistance of the incandescent lamp in operating 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 surges of current through the filament of the incandescent lamp when turned on. This is ensured thanks to the thermistor installed in the protection device. R3. At the initial moment of connection to the network, the thermistor R3 has a maximum resistance that limits the current flowing through this resistor. When the thermistor is gradually heated R3 its resistance gradually decreases, causing current through the incandescent lamp and resistor R2 also increases smoothly. The device circuit is designed in such a way that when the incandescent lamp reaches a voltage of 180-200 V across the resistor R2 the voltage drops, which causes the electromagnetic relay K1 to operate. In this case, relay contacts KL1 and K1.2 are closed.
Please note that in the incandescent lamp circuit there is another resistor connected in series - R4, which also limits current surges and protects the circuit from overloads. When the contacts of 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 bypasses the thermistor R3, turning it off. Relay contacts K1.2 bypass resistor R4, which leads to an increase in voltage on incandescent lamps H2 and NZ, and their threads begin to glow more intensely.
The device is connected to an AC mains voltage of 220 V with a frequency of 50 Hz using an electrical connector X1 "fork" type. Switching the load on and off is provided by a switch S1. A fuse F1 is installed at the input of the device, protecting the input circuits of the device from overloads and short circuits due to improper installation. The inclusion of the device in the alternating current network is controlled by an HI glow discharge indicator lamp, which lights up immediately after switching on. In addition, a filter is assembled at the device input that protects against high-frequency interference that penetrates the device’s power supply.
In the manufacture of incandescent lamp protection devices H2 and NZ The following components are used: thyristor VS1 type KU202K; rectifier diodes VD1-4 type KDYu5B; indicator light H1 type TN-0.2-1; incandescent lamps H2, NC type 60W-220-240V; capacitors S1-2 type MBM-P-400V-0.1 µF, SZ - K50-3-10B-20 µF; resistors R1 type VSA-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 electrical cable; switch S1 type P1T-1-1.
When assembling and repairing the device, other components may be used. Resistors of type BC can be replaced with resistors of types MLT, MT, S1-4, ULI; MBM type capacitors - for K40U-9, MBGO, K42U-2, K50-3 type capacitor - for K50-6, K50-12, K50-16; electromagnetic relay type RES-42 - on 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 set up an incandescent lamp protection device, you will need an IP and an autotransformer that allows you to increase the AC supply voltage to 260 V. The voltage is supplied to the input of the device X1, and it is measured at points A and B, using an autotransformer to set the voltage on incandescent lamps to 200 V. Instead of a constant resistor R2 install a wirewound variable resistor type PPZ-ZVt-20 Ohm. Smoothly increasing the resistance of the resistor R2 mark the moment when relay K1 operates. Before making this adjustment, the thermistor R3 is bridged with a short-circuited jumper.
After checking the voltage on incandescent lamps with resistors temporarily closed R2 and R3 remove the jumpers, replace the resistor R2 with the appropriate resistance, check the delay time of the electromagnetic relay, which should be within 1.5-2 s. If the relay response time is significantly longer, then the resistor resistance R2 needs to be increased by a few ohms.
It should be noted that this device has a significant drawback: turning it on and off can only be done after the thermistor R3 has completely cooled down after heating and is ready for a new switching cycle. The cooling time of the thermistor is 100-120 s. If the thermistor has not yet cooled down, 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, availability of elements (radio components) necessary for assembly, especially those used as temperature sensors, manufacturability of assembly and installation in the power supply housing were taken into account.
According to these criteria, V. Portunov’s scheme turned out to be the most successful. It allows you to reduce wear on the fan and reduce the noise level created 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 determined 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 housing of these diodes allows you to do without any dielectric spacers when installing power supply transistors on the heat sink. The prevalence of diodes and their accessibility to radio amateurs played an important role.

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 selected based on the maximum permissible value of the base current VT1. Resistor R2 determines the response threshold of the regulator.
It should be noted that the number of diodes of the temperature sensor depends on the static current transfer coefficient of the composite transistor VT1,VT2. If, with the resistance of resistor R2 indicated in the diagram, room temperature and the power on, the fan impeller is motionless, the number of diodes should be increased. It is necessary to ensure that after the supply voltage is applied, it confidently begins to rotate at a low frequency. Naturally, if the rotation 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 diodes VD1-VD4 of the same name 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 of high-voltage transistors on the reverse side. Transistor VT2 with resistors R1, R2 and transistor VT1 soldered to its terminals (Fig. 2) is installed with the emitter output in the “+12 V fan” hole of the power supply board (previously the red wire from the fan was connected there). Setting up the device comes down to selecting resistor R2 2.. 3 minutes after turning on the PC and warming up the power supply transistors. Temporarily replacing R2 with a variable (100-150 kOhm), select such a resistance so that at rated load the heat sinks of the power supply transistors heat up no more than 40ºC.
To avoid electric shock (heat sinks are under high voltage!), you can only “measure” the temperature by touch after 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 (the 10 kOhm rating is not critical). The transistor in the circuit is of the KT503 type. As determined experimentally, its operation is more stable than other types of transistors. It is advisable to use a multi-turn trimmer, 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 it is missing, it can be replaced with two diodes. More powerful fans with a current consumption of more than 100 mA should be connected through a compound transistor circuit (the second KT815 transistor).

Diagrams of the other two, relatively simple and inexpensive power supply cooling fan speed controllers, are often provided on the Internet (CQHAM.ru). Their peculiarity is that the TL431 integral stabilizer is used as a threshold element. You can quite simply “get” this chip by disassembling old ATX PC power supplies.
The author of the first scheme is Ivan Shor. Upon repetition, it became clear that it was advisable to use a multi-turn resistor of the same value as a tuning resistor R1. The thermistor is attached to the radiator of the cooled diode assembly (or to its body) using KPT-80 thermal paste.

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

A more complex cooling fan speed controller circuit has been successfully used in another power supply. Unlike the prototype, it uses “television” transistors. The role of the radiator of the adjustable transistor T2 on it is performed by a free section of foil left on the front side of the board. This circuit allows, in addition to automatically increasing the fan speed when the radiator of the cooled power supply transistors or diode assembly heats up, 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 one is much safer; in addition, if you use a non-inertial thermistor of the STZ-19 type, 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 imbalance 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 imbalance of the bridge, and therefore to the measured temperature. Transistors VT1 and VT2 are used as low-voltage zener diodes. They can be replaced with KT3102 with any letter index. Setting up the device begins by 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 this, potentiometers R2 and R3 are set to the 1st middle position. To calibrate the thermometer, you can use the following method. 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 with a reference thermometer.
After turning on the power, perform the following operations:
a) move switch S2 to the “CALIBRATION” position and use resistor R8 to set the arrow to the zero scale mark;
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 use resistor R3 to set the instrument needle 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 setup can be considered complete.

5.4. Multimeter attachment 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. There is no need to make changes to the measuring device, and the manufacture of the set-top box is also accessible 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 the additional resistor R3, it forms one half of the measuring bridge. The second half of the bridge is a voltage divider made of resistors R4 and R5. The last thing during calibration is to set the initial value of the output voltage. The multimeter is used in DC voltage measurement mode at 200 or 2000 mV. By appropriately selecting the resistance of resistor R2, the sensitivity of the measuring bridge is changed.
Immediately before measuring the temperature with variable resistor R1, set the supply voltage of the measuring circuit equal to that at which the initial calibration was performed. The attachment for reading the measured temperature is turned on using push-button switch SB1, and switching from measurement mode to voltage setting mode using switch SB2.
The additional resistor R3 connected in series with the thermistor is calculated using 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 is the absolute temperature in the middle of the measuring range T = t° + 273.
This value of R3 ensures minimal deviation of the characteristic from linear.
The thermistor constant is determined by measuring the resistances RT1 and RT2 of the thermistor at two temperature values T1 and T2 and subsequent calculation using the formula B = ln(RT1/RT2)/(1/T-1/T2).
On the contrary, with known parameters of a thermistor with negative TCR, its resistance for a certain temperature T can be determined by the formula Rt = R-r2oe(B/T"B^J3), where Rt2o is the resistance of the thermistor at a temperature of 20°C.
The attachment is calibrated at two points: Tk- = Tm+0.707(T2-T.)/2 and TK2=Tm-0.707(12-10/2, where Tm = (Tt + T2)/2, Ti and T2 - 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 capacity is lost and the element voltage decreases, the voltage on the bridge can be kept constant (the set-top box consumes a current of about 8 mA). By adjusting the trimming resistors R2, R5, we achieve compliance in three digits of the readings of the digital multimeter indicator with the temperature values of the thermistor T1 and T2, 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 an M-830 from Mastech as a multimeter. It is better to use multi-turn resistors R2, R5 (SP5-1V, SP5-14). a R1 is single-turn, for example PPB: resistors R3 and R4 are MLT-0.125. To turn on the power and switch the set-top box mode, you can use P2K push-button switches without fixing.
In the manufactured attachment, the boundaries of the measured temperature range were set - T1 = 15°C: T2 = 45°C. In the case of measurements in the range of positive and negative temperatures 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 as the temperature drops. 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 low, and a constant bias is applied to the base of the transistor VT1, which keeps it in the on 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 applies bias to the base circuit of transistor T4, in the emitter circuit of which an electromagnetic relay is connected. Transistor VT4 opens and turns on relay K1. By adjusting resistor R3, you can select the trigger thresholds and, therefore, the temperature that the device will automatically maintain. Diode VD2, connected in the opposite direction, bypasses the relay winding and protects the transistor from breakdown when the relay is turned on when a self-inductive emf occurs in its winding. Simultaneously with the relay activation, the HL1 LED begins to light, which is used as an indicator of the operation of the entire device. Zener diode VD1 and resistor R9 form the simplest parametric voltage stabilizer to power the electronic circuit of the device, 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 at a radio store. MLT type resistors, transistor VT1 -MP41; VT2, VT3 and VT4 - MP26. Instead, you can use any p-n-p transistors designed for a voltage of at least 20 V. Relay K1 - type RES-10 or similar, triggered at a current of 10-15 mA with switching or breaking contacts. If you can’t find the relay you need, don’t 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 a printed circuit board, which is located in the room along with an 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 triac VS1, which unlocks it. The circuit is closed.
Now about setting up the electronic circuit. Before connecting the contacts of relay 4 to thyristor VS1, the thermostat must be tested and adjusted. You can do it like this.
Take a thermistor, solder a long wire with double-layer insulation to it and place it in a thin glass tube, sealing it with epoxy resin on both ends to seal it. Then turn on the power to the electronic regulator, lower the tube with the thermistor into a glass of ice and, by rotating the trimmer resistor slider, get the relay to operate.

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 air in the room, water in vessels, thermostats, as well as solutions in color photography. You can connect a heater with a power of up to 500 W to it. The thermostat consists of a threshold device (on transistors T1 and T2), an electronic relay (on transistor T3 and thyristor D10) and a power supply. Temperature sensor The thermistor R5 is used, connected to the voltage supply circuit to the base of the transistor T1 of the threshold device.
If the environment has 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. As the temperature of the environment decreases, the resistance of the thermistor increases, as a result of which the voltage at the base of transistor T1 increases. When it reaches the device's operating threshold, transistor T1 will open and T2 will close. This will lead to the opening of the T3 transistor. The voltage that appears across resistor R9 is applied between the cathode and the control electrode of thyristor D10 and will be sufficient to open it. The mains voltage is supplied to the heater through the thyristor and diodes D6 - D9.
When the ambient temperature reaches the required value, the thermostat will turn off the voltage from the heater. Variable resistor R11 is used to set the limits of the maintained temperature.
The thermostat uses an MMT-4 thermistor. The Tr transformer is made on a Ш12Х25 core. Winding I contains 8000 turns of PEV-1 0.1 wire, winding II contains 170 turns of PEV-1 0.4 wire.

5.7. THERMOREGULATOR FOR INCUBATOR
A circuit of a simple and reliable thermal relay for an incubator is proposed. It features low power consumption, heat generation on the power elements and ballast resistor is insignificant.
I propose a circuit for a simple and reliable thermal relay for an incubator. The circuit was manufactured, tested, and verified in continuous operation over 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 °C
Temperature control range from + 24 to 45°C.
Schematic diagram of the device

A comparator is assembled on the DA1 chip. The set temperature is adjusted using variable resistor R4. Thermal sensor R5 is connected to the circuit with a shielded wire in vinyl chloride 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 negative AC feedback. The circuit is powered through a parametric stabilizer made on a VD1 zener diode of type D814A-D. Capacitor C3 is a power filter. To reduce power dissipation, ballast resistor R9 is made up of two 22 kOhm 2 W resistors connected in series. For the same purpose, the transistor switch 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 diodes VD2-VD5 type KD202K,M,R, installed on small U-shaped radiators made of aluminum 1-2 mm thick with an area of 2-2.5 cm2. Thyristor VS1 is also installed on a similar radiator with an area of 10-2.5 cm2. 12 cm2
Lighting lamps HL1...HL4 are used as a heater, connected in series-parallel to increase service life and eliminate emergency situations in the event of the filament of one of the lamps burning out.
Operation of the circuit. When the temperature of the temperature sensor is less than the specified level set by potentiometer R4, the voltage at pin 6 of the DA1 chip is close to the supply voltage. The key on transistor VT1 and thyristor VS1 is open, the heater on HL1...HL4 is connected to the network. As soon as the temperature reaches the set level, the DA1 chip will switch, the voltage at its output will become close to zero, the thyristor switch will close, and the heater will be disconnected from the network. When the heater is turned off, the temperature will begin to decrease, and when it drops 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 appropriate operating voltage. Thermistor R5 type MMT-4 (or another with negative TKS). Its rating 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 to 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. Despite 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 key mode. Thus, it was possible to avoid impulse noise that occurs when switching a load with a significant current consumption.

The heating elements are wirewound resistors (10 Ohm, 10 W) and a P217V control transistor (can be replaced by any modern silicon transistor of the pnp structure). Refrigerator - radiator. The thermistor (MMT-4 3.3 Kom) is soldered to a copper cup into which a thermostatically controlled jar is inserted. You need to wrap several layers of thermal insulation around the cup and make a thermally insulating lid over the jar.
The circuit is powered from a stabilized laboratory power supply. When the circuit is turned on, heating begins, as indicated by the red LED. When the set temperature is reached, the brightness of the red LED decreases and the green LED begins to glow. After the process of “running out” of the temperature is completed, both LEDs glow at full intensity - the temperature has stabilized.
The entire circuit is located inside a U-shaped aluminum radiator. Thus, all elements of the circuit are also thermostatically controlled, which increases the accuracy of the device.

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

The regulator is not critical to the ratings of the elements used. 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 µF, and C2 - 0.5...1 µF. The latter must be designed for an operating voltage of at least 400 V.
A correctly assembled device does not require adjustment. In order for it to function as a dimmer, thermistor R4 must be replaced with a photoresistor or photodiode connected in series with a resistor, the value of which is selected experimentally.
The author's version of the design described here is used to regulate the temperature in a home incubator, therefore, to increase reliability, when the SCR VS1 is open, the lighting lamps connected to the load (four parallel-connected lamps with a power of 60 W at a voltage of 220 V) burn at full intensity. When operating the device in 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 resistor R5 must be made of insulating material and good electrical insulation of thermistor R4 must be ensured.

5.10. DC fluorescent lamp power supply
In these devices, pairs of contacts of the connector of each filament can be connected together and connected to “their” circuit - then even a lamp with burnt-out filaments will work in the lamp.

A diagram of a device version designed to power a fluorescent lamp with a power of 40 W or more is shown in Fig. . Here the bridge rectifier is made using 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, capacitor C2 is charged (through thermistor R1 and diode VD3), and in the other - SZ (through thermistor R2 and diode VD4). Thermistors limit the charging current of the capacitors. Since the capacitors are connected in series, the voltage across lamp EL1 is sufficient to ignite it.
If the thermistors are in thermal contact with the bridge diodes, their resistance will increase when the diodes heat up, which will reduce the charging current.

The inductor, which serves as a ballast resistance, is not necessary in the power devices under consideration and can be replaced with an incandescent lamp, as shown in Fig. . When the device is connected to the network, the lamp EL1 and 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 across them reaches the ignition voltage of lamp EL2, the capacitors will quickly discharge - this is facilitated by diodes VD1, VD2.
By supplementing a conventional incandescent lamp with this device with a fluorescent lamp, you can improve general or local lighting. For a EL2 lamp with a power of 20 W, EL1 should be 75 or 100 W, but if EL2 is used with a power of 80 W, EL1 should be 200 or 250 W. In the latter option, it is permissible to remove the charge-discharge circuits from resistors R2, R3 and diodes VD1, VD2 from the device.

This concludes my review of THERMORESISTORS.
A few more words about another radio component - varistor.
I don’t plan to make a separate article about it, so in short:
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” from the shells of the atom, the more holes are formed - the number of free charge carriers increases, so does the conductivity, and the resistance decreases. This is the case if the semiconductor is pure. In practice, everything is much more complicated. Tirite, vilit, latin, silit 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.

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

At this point, our acquaintance with the family of resistors can be considered complete.