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How to distinguish a resistor from a thermistor. Using thermistors to limit surge current in power supplies. Thermistor - characteristics and principle of operation

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°С(less often 20°C). 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.

Today we’ll talk about other most common radio components, such as transistors, thermistors, reed switches and others.

Thermistors

Thermistors are semiconductor devices that change their resistance depending on temperature. Thermistors are divided into two types:

NTC — with negative temperature coefficient) - the thermistor resistance decreases with increasing temperature. They are widely used in various fields of radio electronics, especially where temperature control is important. PTC — positive temperature coefficient) - the resistance of the posistor increases with decreasing temperature. Unlike thermistors, they are much less common at the moment. Perhaps a classic example of the use of posistors is televisions with electro-ray tubes, where they act as stabilizing heating elements in kinescope demagnetization circuits.

The method for testing thermistors and posistors is the same. We will need a multimeter and a heating device, a hair dryer or a soldering iron. On the multimeter, set the ohmmeter mode and connect its probes to the thermistor terminals. Remember the resistance value. After this, we begin to heat the thermistor; the resistance value, depending on the type (PTC or NTC), will increase or decrease in proportion to the heating. This indicates the serviceability of the radio element. If the resistance does not change or is initially close to 0, then the part is faulty.

Reed switches belong to the class of magnetically controlled switching devices; the word “reed switch” itself is an abbreviation for sealed contact. It is a glass flask with a contact group built into it. The contacts are made of ferromagnetic material and are activated under the influence of a magnetic field. An ordinary magnet can act in this capacity. Often found in various sensors and security alarm systems.

It’s easy to check the reed switch; for this you will need a multimeter and a magnet. We set the tester to dial and connect the reed switch to the probes. The value on the display will be 1 - that is, our contact is open. We bring the magnet to the reed switch - the contact closes and the multimeter emits a sound signal. So the reed switch is ok.

Hall Sensor

Hall sensors are similar in purpose to reed switches, that is, they are magnetically controlled devices, but unlike them they are not electromechanical, but electronic. Their main advantage over a reed switch is the absence of mechanical contacts, and therefore durability. They are primarily used as non-contact sensors.

To check the sensor, a conventional multimeter and a DC power source are sufficient. Any Hall sensor has three terminals - positive, common and signal. The positive terminal is usually the first, when viewed from the marking side, the middle one is common, and the third is signal. This means we connect our power supply with a plus to the first pin and a minus to the middle one. Now we take the tester, switch it to DC measurement mode and connect the positive probe to the first terminal, and the negative probe to the third signal terminal. The multimeter should show a voltage close to zero. Now we bring a magnet to our sensor and the voltage should increase to a value close to the voltage of the power source. This indicates that the Hall sensor is working.

Transistors

There are mainly three types of transistors found in electronics −

  • bipolar
  • field

Bipolar The transistor is perhaps the most common among all. In its structure it can be compared to two diodes, since it has two p-n transition, and the diode structure is a regular p-n transition. The common connection point is called base, and the extreme ones - collector And emitter. Depending on the type, the bipolar transistor can be direct conduction p-n-p or reverse n-p-n. Transistor p-n-p structures can be represented as two diodes with cathodes directed towards each other, and n-p-n structures, respectively, the anodes will be connected by the base.

It turns out that a bipolar transistor can be checked for serviceability in the same way as diodes; in the forward direction, the voltage drop across the junction will be equal to a certain value, and in the reverse direction it should tend to infinity. Let's make sure of this.

We take some transistor, find out its pinout, or as they say pinout. In other words, we find out which pins it will have as a base, collector and emitter. Now take the multimeter and set it to diode testing mode. If the transistor is caught n-p-n structure, which means we connect the red (+) probe to the base, and the black (-) probe to the collector. The display should show the value corresponding to the voltage drop across the junction. Next, we leave the positive probe on the base, and connect the black one to the emitter terminal. The display should also show some value. Now we check the base-emitter and base-collector junction in the opposite direction. In both cases, the value on the display should be close to infinity, that is, 1.

If the transistor is caught p-n-p structure, then the testing method is exactly the same, only we connect the negative probe to the base, and alternately connect the positive probe to the collector and emitter.

If the multimeter, when checking in the forward and reverse directions of any transition, shows infinity in both directions, it means the transition is open and such a transistor is faulty. If the value when checking one of the transitions is close to or equal to 0, this clearly indicates a breakdown of the transition and such a transistor is also faulty.

Field transistors differ in their principle of operation from bipolar ones, therefore the method of testing them will be slightly different. The main difference between field-effect transistors and bipolar transistors is that the output current is controlled by changing the applied electric field, that is, voltage, while in bipolar transistors the output current is controlled by the input base current. According to their structure, they are divided into transistors with a control p-n transition ( J-FET) and insulated gate transistors ( MOSFET).

Just like bipolar field-effect transistors, they have three terminals - drain(the area where carriers flock), source(source of current carriers), gate(control electrode). Before checking, first of all, you need to find out the structure of the transistor and which pin is responsible for what.

Well, then we take a multimeter and set it to diode testing mode. We touch the drain with the black negative probe, and touch the source with the red positive probe. The multimeter will show a voltage drop across the junction of 0.5 - 0.8 V. In the opposite direction, the device will show infinity. Next, we leave the black probe on the drain, and touch the red one to the gate and return it to the source again. The multimeter should show a value close to zero, since the transistor has opened. When changing polarity, the value should not change. Now we briefly connect the black probe to the gate and return it to the drain terminal, while leaving the red probe at the source. The field effect transistor should close and the multimeter will again show the voltage drop across the junction. This is the technique for testing an n-channel transistor. For p-channel everything will be exactly the same, we just change the polarity.

And finally IGBT transistors. This is a kind of hybrid of bipolar and field-effect transistors, as evidenced by even its name ( IGBT— insulated gate bipolar transistor). Such transistors are used primarily in power electronics as powerful electronic switches. For example, they can often be found in welding inverters. We can say that in an IGBT transistor, a low-power field-effect transistor is capable of controlling a powerful bipolar one. The combination of field-effect transistor speed and bipolar power is the main advantage of IGBT transistors.

Just as in the case of other types of transistors, before checking the IGBT, it is necessary to find out the purpose of its terminals. The IGBT transistor has a terminal shutter denoted by the letter G–Gate, conclusion emitter E –Emitter and conclusion collector C – Collector. Well, then we start checking with a multimeter. We place the red probe on the gate, the black probe on the emitter. The multimeter should show infinity. When changing the polarity, the result should be the same. Next we put black on the collector, and red on the emitter. The display should show 1, that is, infinity. When the polarity is changed, if there is a shunt diode in the transistor, the multimeter will show the voltage drop across the diode; if there is no diode, the device will show infinity.

In some cases, the multimeter voltage is not enough to open the IGBT transistor, then a constant voltage source of 9-15 V will be needed for charging.

The picture tubes of most TVs use demagnetization systems that have a posistor built into them. Owners who want to carry out repairs themselves need to know how to check such a chain if it fails. The element has physical properties that can be checked with a regular ohmmeter.

Element properties

It is worth studying what a posistor is, how to check it in a circuit - it will become clear later. This element is capable of changing properties depending on temperature. Its physical value resistance is measured. At room temperature, the ohmmeter readings show units or tens of ohms.

When heated in operation, the resistance begins to change upward. The ohmmeter values ​​already show hundreds of kilo-ohms, which indicates the normal state of the element - such a posistor is working. How to check if there is a suspicion of a faulty circuit? Below are ways to resolve this issue.

Due to their properties, posistors are widely used in microelectronics for various purposes:

  • Protection of power circuits. With increased current consumption, the element heats up and increases the resistance to a maximum when a current cutoff is observed.
  • In heating circuits. Thanks to posistors, an automatic heating control system is implemented.
  • In thermal sensor circuits.

Internal structure of the element

A resistor changes its resistance with heating, just like a posistor. How to check the first element? This one is simple. They fluctuate within insignificant limits. A posistor is capable of completely blocking the current passing through it, just like a themistor. Only the latter exhibits an inverse dependence on temperature.

To know how to check the serviceability of a posistor, you should determine its main operating characteristics. These include:

  • nominal resistance at normal ambient temperature (usually 20-25 degrees);
  • The switching resistance is determined at the point of the resistance versus temperature characteristic when the first parameter increases by 2 times compared to the nominal value;
  • the maximum voltage that the element can withstand without failure;
  • current load values: rated, switching, maximum possible and stalling; For testing, these parameters are important only if the posistor will be used in high-precision circuits.

Element in the demagnetization circuit

How to check a posistor on a TV? The answer to the question follows from the principle of its operation. The malfunction of the element is manifested by distortion of the image due to magnetization. To eliminate this defect, the screen design uses a grid connected in series with a posistor. This design is called an external loop, covering the entire surface of the screen from the inside.

The posistor is often soldered into the screen mask circuit, making it difficult to test on site. Before taking measurements, you should unsolder at least one end from the grid. The best option would be to completely remove it from the circuit.

To heat the element, use a regular or hair dryer. To check without external heating, you will need to assemble an electrical circuit and determine the type of posistor by the markings. Based on the device’s passport data, the element’s operating current and the corresponding temperature are determined.

The serviceability of the posistor can be conditionally determined by heating it with a hairdryer. If the resistance increases, then the element is good. However, with this verification option, there remains the possibility of an erroneous result. After all, the resistance of circuit elements changes over the years, which leads to instability of the assembly.

Why do we need a picture tube system?

On TV screens without a demagnetization system, the image would be distorted with a slight influence of the electromagnetic field. It is emitted by all household appliances, the surface of the Earth is penetrated by invisible waves.

Thus, amplifiers, large speakers, and heating elements are often located next to televisions. Without a screen mask, the image would be constantly distorted. During initial operation, a small current flows through the posistor, which does not cause it to heat up. In this case, the mask physically experiences tension from the emerging field.

This applied magnetic field demagnetizes the mask when the TV is turned on. Often this process is accompanied by a sound comparable to hitting a gong. The larger the screen diagonal, the higher the sound pitch. The posistor at this moment passes a high amplitude current through itself, which leads to its heating. The resistance increases and the element closes the circuit.

Options for faults in picture tubes

If, when you first turn it on, the image is distorted or ripples and stripes are observed, then the posistor is most likely to blame. How to check an element in a circuit with a multimeter? It is easier to do this on a cold circuit, because the resistance of the posistor is minimal.

Often the soldered contacts simply fall off due to prolonged use. A posistor refers to circuit elements that constantly operate in a heated state. Using an ohmmeter, check the connection of the screen mask with the output of the second leg of the posistor. If it is minimal, this indicates a reliable connection. Perhaps the element does not trigger the cutoff.

If the posistor is faulty and short-circuited, then when the power supply is turned on for the first time. Provided that this occurs without a visible short circuit in the circuit, you can check the malfunction by completely disconnecting the screen mask and the posistor.

Element in the cooler circuit

If the back of the refrigerator - the radiator - does not heat up, then for self-repair you need to familiarize yourself with how to check the posistor. Two types of starters can be used in the refrigerator: with posistors and with electromagnetic relays. The former spend part of the energy on heat loss in the resistance of the element, the latter are less reliable, but do not heat up.

Most posistors in refrigerators should have a resistance of about 20-30 ohms. When heated, there may be several kiloohms. If the values ​​significantly exceed those given, the element must be replaced. It is important to let the posistor cool to room temperature before taking measurements.

Electronics are sensitive to power quality. When voltage surges occur in the network, components fail. To reduce the likelihood of such an outcome, use . These are components with nonlinear resistance, which in the normal state is very high, and under the influence of a high voltage pulse decreases sharply. As a result, the device absorbs all the pulse energy. In this article we will tell you how to check a varistor for serviceability and distinguish a burnt one from a whole one.

Causes of malfunction

Varistors are installed in parallel with the protected circuit, and a fuse is placed in series with it. This is necessary so that when the varistor burns out, if the overvoltage pulse is too strong, the fuse will burn out, and not the tracks of the printed circuit board.

The only reason for a varistor to fail is sharp and strong. If the energy of this jump is greater than the varistor can dissipate, it will fail. The maximum energy dissipation depends on the dimensions of the component. They differ in diameter and thickness, that is, the larger they are, the more energy the varistor can dissipate.

Voltage surges can occur during accidents on power lines, during a thunderstorm, or when switching powerful devices, especially inductive loads.

Verification methods

Any repair of electronics and electrical equipment begins with an external inspection, and then proceeds to measurements. This approach allows you to localize most faults. To find a varistor on the board, look at the figure below - this is what varistors look like. Sometimes they can be confused with capacitors, but can be distinguished by their markings.

If the element is burnt out and the markings cannot be read, look at this information on the device diagram. On the board and in the diagram it can be designated by the letters RU. The conventional graphic symbol looks like this.

There are three ways to test a varistor quickly and easily:

  1. Visual inspection.
  2. Call. This can be done with a multimeter or any other device that has a continuity test function.
  3. Resistance measurement. This can be done with a high-range ohmmeter, multimeter or megger.

A varistor fails when a large or prolonged current passes through it. Then the energy is dissipated in the form of heat, and if its amount is greater than that determined by the design, the element burns out. The housing of these components is made of a hard dielectric material, such as ceramic or epoxy coating. Therefore, when it fails, the integrity of the outer coating is most often damaged.

You can visually check the varistor for functionality - there should be no cracks on it, as in the photo:

The next method is to check the varistor with a tester in continuity mode. This cannot be done in the circuit, because the dialing can work through parallel-connected elements. Therefore, you need to unsolder at least one of its legs from the board.

Important: You should not check the elements for serviceability without desoldering them from the board - this may give false readings from the measuring instruments.

Since in the normal state (without voltage applied to the terminals) the resistance of the varistor is high, it should not ring through. The test is performed in both directions, that is, by swapping the multimeter probes twice.

On most multimeters, the continuity mode is combined with the diode testing mode. It can be found by the diode icon on the mode selector scale. If there is a sound indication sign next to it, it probably also has a dial tone.

Another way to test a varistor for breakdown with a multimeter is to measure the resistance. You need to set the device to the maximum measurement limit, in most devices this is 2 MOhms (megaohms, designated as 2M or 2000K). The resistance must be equal to infinity. In practice, it can be lower, within 1-2 MOhm.

Interesting! The same can be done with a megaohmmeter, but not everyone has one. It is worth noting that the voltage at the megohmmeter terminals should not exceed the classification voltage of the component being tested.

This ends the available methods for checking a varistor. This time, the multimeter will help the radio amateur find the faulty element, as in a large number of other cases. Although in practice a multimeter is not always needed in this matter, because the matter rarely goes beyond a visual inspection. Replace the burnt element with a new one, designed for voltage and with a diameter no less than the burnt one, otherwise it will burn out even faster than the previous one.

Materials

The word “thermistor” is self-explanatory: THERMAL RESISTOR is a device whose resistance changes with temperature.

Thermistors are largely nonlinear devices and often have large variations in parameters. This is why many, even experienced engineers and circuit designers, experience inconvenience when working with these devices. However, having taken a closer look at these devices, you can see that thermistors are actually quite simple devices.

First, it must be said that not all devices that change resistance with temperature are called thermistors. For example, resistive thermometers, which are made from small coils of twisted wire or from sputtered metal films. Although their parameters depend on temperature, however, they work differently from thermistors. Typically, the term "thermistor" is applied to temperature-sensitive semiconductor devices.

There are two main classes of thermistors: negative TCR (temperature coefficient of resistance) and positive TCR.

There are two fundamentally different types of manufactured thermistors with positive TCR. Some are made like NTC thermistors, while others are made from silicon. Positive TCR thermistors will be described briefly, with the focus on the more common negative TCR thermistors. Thus, unless there are special instructions, we will be talking about thermistors with negative TCR.

NTC thermistors are highly sensitive, narrow range, nonlinear devices whose resistance decreases as temperature increases. Figure 1 shows a curve showing the change in resistance depending on temperature and is a typical temperature dependence of resistance. Sensitivity is approximately 4-5%/o C. There is a wide range of resistance values, and the change in resistance can reach many ohms and even kilo-ohms per degree.

R Ro

Fig.1 Negative TCR thermistors are very sensitive and significantly

The degrees are non-linear. Rо can be in ohms, kilo-ohms or mego-ohms:

1-resistance ratio R/Ro; 2- temperature in o C

Thermistors are essentially semiconductor ceramics. They are made from metal oxide powders (usually nickel and manganese oxides), sometimes with the addition of small amounts of other oxides. Powdered oxides are mixed with water and various binders to obtain a liquid dough, which is given the required shape and fired at temperatures above 1000 o C.

A conductive metal covering (usually silver) is welded on and the leads are connected. The completed thermistor is usually coated with epoxy resin or glass, or enclosed in some other housing.

From Fig. 2 you can see that there are many types of thermistors.

Thermistors have the form of disks and washers with a diameter of 2.5 to approximately 25.5 mm, and the shape of rods of various sizes.

Some thermistors are first made as large plates and then cut into squares. Very small bead thermistors are made by directly burning a drop of dough onto two refractory titanium alloy terminals and then dipping the thermistor into glass to create a coating.

Typical parameters

Saying “typical parameters” is not entirely correct, since there are only a few typical parameters for thermistors. There are an equally large number of specifications available for a variety of thermistor types, sizes, shapes, ratings, and tolerances. Moreover, often thermistors produced by different manufacturers are not interchangeable.

You can purchase thermistors with resistances (at 25 o C - the temperature at which the thermistor resistance is usually determined) from one ohm to ten megohms or more. Resistance depends on the size and shape of the thermistor, however, for each specific type, resistance ratings can differ by 5-6 orders of magnitude, which is achieved by simply changing the oxide mixture. When replacing the mixture, the type of temperature dependence of the resistance (R-T curve) also changes and the stability at high temperatures changes. Fortunately, thermistors with high resistance enough to be used at high temperatures also tend to be more stable.

Inexpensive thermistors usually have fairly large parameter tolerances. For example, permissible resistance values ​​at 25 o C vary in the range from ± 20% to ± 5%. At higher or lower temperatures, the spread of parameters increases even more. For a typical thermistor having a sensitivity of 4% per degree Celsius, the corresponding measured temperature tolerances range from approximately ±5°C to ±1.25°C at 25°C. High precision thermistors will be discussed later in this article.

It was previously said that thermistors are narrow range devices. This needs to be explained: most thermistors operate in the range from -80°C to 150°C, and there are devices (usually glass-coated) that operate at 400°C and higher temperatures. However, for practical purposes, the greater sensitivity of thermistors limits their useful temperature range. The resistance of a typical thermistor can vary by a factor of 10,000 or 20,000 at temperatures ranging from -80°C to +150°C. One can imagine the difficulty in designing a circuit that provides accurate measurements at both ends of this range (unless range switching is used). Thermistor resistance, rated at zero degrees, will not exceed several ohms at

Most thermistors use soldering to connect the leads internally. Obviously, such a thermistor cannot be used to measure temperatures above the melting point of solder. Even without soldering, the epoxy coating of thermistors only lasts at a temperature of no more than 200 ° C. For higher temperatures, it is necessary to use glass-coated thermistors with welded or fused leads.

Stability requirements also limit the use of thermistors at high temperatures. The structure of thermistors begins to change when exposed to high temperatures, and the rate and nature of the change is largely determined by the oxide mixture and the method of manufacturing the thermistor. Some drift in epoxy coated thermistors begins at temperatures above 100°C or so. If such a thermistor operates continuously at 150 o C, then the drift can be measured by several degrees per year. Low-resistance thermistors (for example, no more than 1000 ohms at 25 o C) are often even worse - their drift can be noticed when operating at approximately 70 o C. And at 100 o C they become unreliable.

Inexpensive devices with larger tolerances are manufactured with less attention to detail and can produce even worse results. On the other hand, some properly designed glass-coated thermistors have excellent stability even at higher temperatures. Glass-coated bead thermistors have very good stability, as do the more recently introduced glass-coated disk thermistors. It should be remembered that drift depends on both temperature and time. For example, it is usually possible to use an epoxy coated thermistor when briefly heated to 150°C without significant drift.

When using thermistors, the nominal value must be taken into account constant power dissipation. For example, a small epoxy-coated thermistor has a dissipation constant of one milliwatt per degree Celsius in still air. In other words, one milliwatt of power in a thermistor increases its internal temperature by one degree Celsius, and two milliwatts increases its internal temperature by two degrees, and so on. If you apply a voltage of one volt to a one-kilo-ohm thermistor that has a dissipation constant of one milliwatt per degree Celsius, you will get a measurement error of one degree Celsius. Thermistors dissipate more power if they are lowered into liquid. The same small epoxy coated thermistor mentioned above dissipates 8 mW/°C when placed in well-mixed oil. Larger thermistors have better consistent dissipation than smaller devices. For example, a thermistor in the form of a disk or washer can dissipate a power of 20 or 30 mW/o C in air; it should be remembered that, just as the resistance of a thermistor changes depending on temperature, its dissipated power also changes.

Equations for thermistors

There is no exact equation to describe the behavior of a thermistor; there are only approximate ones. Let's consider two widely used approximate equations.

The first approximate equation, exponential, is quite satisfactory for limited temperature ranges, especially when using thermistors with low accuracy.

Thermistors

Designation on the diagram, varieties, application

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 t0.

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 10C (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.

Thermistor - characteristics and principle of operation

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:

    NTC thermistors;

    PTC thermistors (aka posistors).

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

NTC thermistors.

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 ICAR 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 250C (for a given thermistor is 8 Ohms) and the maximum operating current are indicated. 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.

Symbol for a posistor in the diagram.

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.

I have already talked in more detail about the use of posistors in the demagnetization circuit of picture tubes here.

Just like NTC thermistors, posistors are used as protection devices. One type of posistor is a self-resetting fuse.

SMD thermistors.

With the active introduction of SMT mounting, manufacturers began to produce thermistors for surface mounting. In appearance, such thermistors differ little from ceramic SMD capacitors. The sizes correspond to the standard series: 0402, 0603, 0805, 1206. It is almost impossible to visually distinguish them on the printed circuit board from nearby 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 have become familiar with thermistors, it’s time to learn about their parameters.

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T You might also be interested to know:

A thermistor is a temperature-sensitive element made of semiconductor material. It behaves like a resistor sensitive to temperature changes. The term "thermistor" is short for temperature-sensitive resistor. A semiconductor material is a material that conducts electrical current better than a dielectric, but not as well as a conductor.

Thermistor operating principle

Like resistance thermometers, thermistors use changes in resistance value as the basis of measurement. However, thermistor resistance is inversely proportional to changes in temperature, rather than directly proportional.

As the temperature around the thermistor increases, its resistance decreases, and as the temperature decreases, its resistance increases.

Although thermistors provide readings as accurate as resistance thermometers, thermistors are often designed to measure over a narrower range. For example, a resistance thermometer's measurement range might be -32°F to 600°F, while a thermistor would measure -10°F to 200°F.

Thermistor operating principle

The measurement range for a particular thermistor depends on the size and type of semiconductor material it uses.

Like thermometers, thermistors respond to changes in temperature by proportionally changing resistance, and both are often used in bridge circuits.

In this circuit, the change in temperature and the inverse relationship between temperature and the thermistor resistance will determine the direction of current flow. Otherwise the circuit will function in the same way as in the case of a resistance thermometer. As the temperature of the thermistor changes, its resistance changes and the bridge becomes unbalanced. Now a current will flow through the device, which can be measured. The measured current can be converted to temperature units using a conversion table, or by calibrating the scale accordingly.