How to check a varistor: external inspection and testing with a multimeter. Using Thermistors to Limit Current Surges in Power Supplies How to Test Thermal Resistance with a Multimeter

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 built-in contact group. 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 power supply are sufficient. direct current. 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 (-) 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, whereas in bipolar ones 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 keys. 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. Combined performance field effect transistor 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 source will be needed for charging DC voltage at 9-15 V.

The unpretentiousness and relative physical stability of posistors allows them to be used as a sensor for self-stabilizing systems, as well as to implement overload protection. The principle of operation of these elements is that their resistance increases when heated (unlike thermistors, where it decreases). Accordingly, when checking posistors for performance with a tester or multimeter, it is necessary to take into account temperature correlation.

We determine characteristics by marking

The wide range of applications of PTC thermistors implies their wide range, since the characteristics of these devices must correspond to various operating conditions. In this regard, for testing it is very important to determine the series of the element; marking will help us with this.

For example, let's take the radio component C831, its photograph is shown below. Let's see what can be determined from the inscriptions on the body of the part.

Considering the inscription “RTS”, we can state that this element is a posistor “C831”. Having generated a request in a search engine (for example, “RTS C831 datasheet”), we find the specification (datasheet). From it we learn the name (B59831-C135-A70) and series (B598*1) of the part, as well as the main parameters (see Fig. 3) and purpose. The latter indicates that the element can play the role of a self-restoring fuse, protecting the circuit from short-circuit protection and overcurrent.

Decoding the main characteristics

Let's briefly look at the data shown in the table in Figure 3 (for convenience, the lines are numbered).

Figure 3. Table with the main characteristics of the B598 series*1

Short description:

1. a value characterizing the maximum level of operating voltage when the device is heated to 60°C, in this case it corresponds to 265 V. Considering that there is no definition of DC/AC, it can be stated that the element operates with both alternating and direct voltage.
2. The nominal level, that is, the voltage in normal operation, is 230 volts.
3. The estimated number of element operation cycles guaranteed by the manufacturer, in our case there are 100.
4. A value describing the value of the reference temperature, after which a significant increase in the resistance level occurs. For clarity, we present a graph (see Fig. 4) of temperature correlation.

Rice. 4. Dependence of resistance on temperature, the temperature transition point (reference temperature) for C831 is highlighted in red

As can be seen in the graph, R increases sharply in the range from 130°C to 170°C, respectively, the reference temperature will be 130°C.

1. Compliance with the nominal R value (that is, tolerance) is indicated as a percentage, namely 25%.
2. Operating temperature range for minimum (-40°C to 125°C) and maximum (0-60°C) voltage.

Deciphering the specifications of a specific model

These were the main parameters of the series, now let’s look at the specification for C831 (see Fig. 5).

Brief transcript:

1. The current value for normal operation, for our part is almost half an ampere, namely 470 mA (0.47 A).
2. This parameter indicates the current at which the resistance value begins to change significantly upward. That is, when a current of 970 mA flows through C831, the “protection” of the device is triggered. It should be noted that this parameter is associated with the temperature transition point, since the passing current leads to heating of the element.
3. The maximum permissible current value for switching to the “protective” mode, for the C831 is 7 A. Please note that the maximum voltage is indicated in the column, therefore, you can calculate the permissible amount of power dissipation, exceeding which will most likely lead to the destruction of the part.
4. The response time for the C831 at a voltage of 265 volts and a current of 7 amperes will be less than 8 seconds.
5. The amount of residual current required to maintain the protective mode of the radio component in question is 0.02 A. It follows from this that maintaining the triggered state requires a power of 5.3 W (I r x V max).
6. Device resistance at a temperature of 25°C (3.7 Ohms for our model). Note that by measuring this parameter with a multimeter, checking the posistor for serviceability begins.
7. The minimum resistance value for the C831 model is 2.6 Ohms. To complete the picture, we will once again present a graph of the temperature dependence, where the nominal and minimum values ​​of R will be marked (see Fig. 6).

Figure 6. Temperature correlation plot for B59831, RN and Rmin values ​​marked in red

Please note that at the initial stage of heating the radio component, its parameter R decreases slightly, that is, in a certain temperature range, our model begins to exhibit NTS properties. This feature, to one degree or another, is characteristic of all posistors.

1. Full model name (we have B59831-C135-A70), this information may be useful for searching for analogues.

Now, knowing the specification, you can move on to testing for functionality.

Determining serviceability by appearance

Unlike other radio components (for example, such as a transistor or diode), a failed PTC resistor can often be determined by appearance. This is due to the fact that due to exceeding the permissible dissipation power, the integrity of the housing is compromised. Having found a posistor on the board with such a deviation from the norm, you can safely unsolder it and start looking for a replacement, without bothering yourself with the testing procedure with a multimeter.

If the external examination does not produce results, we proceed to testing.

Step-by-step instructions for checking a posistor with a multimeter

For the testing process, in addition to the measuring device, you will need a soldering iron. Having prepared everything you need, we begin to act in the following order:

1. We connect the part under test to the multimeter. It is advisable that the device be equipped with “crocodiles”; otherwise, we solder a wire to the terminals of the element and wind it onto different probe needles.
2. We turn on the measurement mode of the least resistance (200 Ohms). The device will show the nominal value of R, characteristic of the model being tested (usually less than one to two tens of ohms). If the reading differs from the specification (taking into account the error), it can be stated that the radio component is faulty.
3. We carefully heat the body of the tested part using a soldering iron, the R value will begin to increase sharply. If it remains unchanged, the element must be changed.
4. We disconnect the multimeter from the part being tested, let it cool, and then repeat the steps described in steps 1 and 2. If the resistance has returned to the nominal value, then the radio component can most likely be considered serviceable.

Learn about thermistors and how to program an Arduino to measure their data.

Have you ever wondered how some devices, such as thermostats, 3D printer heating pads, car engines and ovens measure temperature? In this article you can find out!

Knowing the temperature can be very useful. Knowing the temperature can help regulate the room temperature to a comfortable level, ensure that the 3D printer's heating pad is hot enough for materials like ABS to stick to its surface, and prevent the motor from overheating or the food you're cooking from burning.

In this article we consider only one type of sensor that can measure temperature. This sensor is called a thermistor.

A thermistor has a resistance that is much more dependent on temperature than other types of resistors.

We will use Arduino to measure and process the thermistor readings, after which we will convert these readings into an easy-to-read temperature unit format.

Below is a photo of the thermistor we are going to use:

Required Components

Accessories

• Arduino (Mega or Uno or any other model);
• several jumpers;
• soldering iron and solder (may be needed if your thermistor will not fit into the connectors on the Arduino board).

Software

• Arduino IDE

Theory

In typical resistor use, you don't want its resistance to change with temperature. This is not realistic in real life, it can only provide a small change in resistance for a large change in temperature. If this were not the case, then resistors would have strange effects on the operation of circuits, for example an LED might glow much brighter or dimmer as the ambient temperature changes.

But what if you actually want LED brightness to be a function of temperature? This is where the thermistor comes in. As you might have guessed, a thermistor's resistance changes greatly with small changes in temperature. To illustrate this, below is the resistance curve of a thermistor:

The figure shows only units of measurement without actual values, since the resistance range depends on the type of specific thermistor. As you can see, as the temperature increases, the resistance of the thermistor decreases. This is the distinguishing property of a Negative Temperature Coefficient resistor, or NTC thermistor for short.

There are also Positive Temperature Coefficient (PTC) thermistors, whose resistance increases as the temperature rises. However, PTC thermistors have a tipping point and change their resistance greatly at a certain temperature. This makes interacting with PTC thermistors a little more complicated. For this reason, NTC thermistors are preferred in most cheap temperature meters.

In the rest of the article, as you can guess, we will talk about NTC thermistors.

Four approaches to finding a formula for plotting a curve

Now that we have a better understanding of the behavior of thermistors, you may be wondering how we can use Arduino to measure temperature. The curve in the graph above is non-linear and therefore a simple linear equation does not work for us (we can actually derive an equation, but more on that later).

So what to do?

Before you continue, think about how you would do this on an Arduino or even in a circuit without microprocessor components.

There are several ways to solve this problem, which are listed below. This is far from full list all techniques, but it will show you some popular approaches.

Method 1

Some manufacturers provide information so comprehensive that it contains an entire graph showing specific ranges of integer temperature and resistance values ​​(typical values). One such thermistor can be found in the data sheet from Vishay.

How, with such detailed data, could one implement temperature measurement on Arduino. You would have to hard code all these values ​​into a huge lookup table or very long "switch...case" or "if...else" control structures.

And if the manufacturer did not bother to provide a detailed table, then you will have to measure each point yourself to form such a table. This day will be quite dull for the programmer. But this method is not so bad and has its place in use. If the current project is testing only a few points or even a small range, this may be the preferred method. For example, one such situation arises if you want to measure whether values ​​are within selected temperature ranges and light an LED to indicate this condition.

But in our project we want to measure temperature over an almost continuous range and send the readings to a serial monitor, so we won't use this method.

Method 2

You can try to "linearize" the thermistor's response by adding additional circuitry to it.

One popular way of doing this is to connect a resistor in parallel with the thermistor. Some chips offer to do this for you.

Determining how to select and linearize a portion of the curve, along with choosing the correct resistor value, is a topic for another article. This approach is good if the microprocessor cannot evaluate floating point expressions (such as PICAXE) because it simplifies the response over some temperature range to linear. This also makes it easier to design a circuit that does not have a microprocessor.

But in this article we are using a microprocessor, and we want to measure temperature over the entire range.

Method 3

You can take data from the table into technical description or (if you like to get weird) create your own table by taking your own measurements and recreating the graph in something like Excel. You can then use the curve fitting function to create a formula for that curve. This is not a bad idea, and all the work done will yield a nice formula that you can use in the program. But it will take some time to pre-process the data.

While this is a reasonable approach, we don't want to depend on analyzing all this data. Also, each thermistor is slightly different (but of course this is not a problem if the tolerance is quite low).

Method 4

It turns out there is a general curve fitting formula intended for devices like thermistors. It's called the Steinhart-Hart equation. Below is a version of it (other versions use terms in the second and powers):

\[\frac(1)(T)=A+B\ln(R)+C(\ln(R))^3\]

where R is the resistance of the thermistor at temperature T (in Kelvin).

This is a general curve equation suitable for all types of NTC thermistors. The approximation of the temperature-resistance relationship is "good enough" for most applications.

Note that the equation requires constants A, B and C. These vary between thermistors and must either be specified or calculated. Since we have three unknowns, you need to take three resistance measurements at specific temperatures, which can then be used to create three equations and determine the values ​​of these constants.

Even for those of us who know algebra well, it's still too much work.

Instead, there is an even simpler equation that is less precise but contains only one constant. This constant is denoted as β, and hence the equation is called the β-equation.

\[\frac(1)(T)=\frac(1)(T_o)+(\frac(1)(\beta))\cdot\ln\left(\frac(R)(R_o)\right)\ ]

where R 0 is the resistance at the control temperature T 0 (for example, resistance at room temperature). R is the resistance at temperature T. Temperatures are indicated in Kelvin. β is usually specified in the technical description; and if not, then you only need one measurement (one equation) to calculate this constant. This is the equation I will use to interact with our thermistor since it is the simplest I have come across and does not need to linearize the thermistor response.

Measuring Resistance with Arduino

Now that we've chosen a curve plotting method, we need to figure out how to actually measure resistance using the Arduino before we can pass the resistance information into the β equation. We can do this using a voltage divider:

This will be our circuit for interacting with the thermistor. When the thermistor detects a change in temperature, it will be reflected in the output voltage.

Now, as usual, we use the formula for the voltage divider.

But we are not interested in the output voltage V output, we are interested in the resistance of the thermistor R thermistor. So we'll express it:

This is much better, but we need to measure our output voltage as well as the supply voltage. Since we are using the Arduino's built-in ADC, we can represent the voltage as a numerical value on a specific scale. So the final form of our equation is shown below:

This works because no matter how we represent voltage (in volts or digital units), these units cancel out in the numerator and denominator of the fraction, leaving a dimensionless value. We then multiply it by the resistance to get the result in ohms.

Our Dmax will be equal to 1023, since this is the largest number that our 10-bit ADC can produce. D measured is the measured value by the analog-to-digital converter, which can range from zero to 1023.

All! Now you can start assembling!

Let's put it together

I used a TH10K thermistor.

I also used a 10k resistor as R balance in our voltage divider. I didn't have the β constant, so I calculated it myself.

Below is a complete diagram of the device. It's pretty simple.

And this is what the final layout looks like:

Program code for Arduino

The code is provided with a lot of comments to help you understand the logic of the program.

Basically it measures the voltage across the divider, calculates the temperature and then displays it on the serial port terminal.

For fun, there are also some "if...else" statements added to show how you can act depending on the temperature range.

//================================================================ ============================== // Constants //============== ===================================================== =============== // Thermistor related: /* Here we have several constants that make editing the code easier. The heart of the program is in the readThermistor function. */ currentTemperature = readThermistor();< 24.0) { Serial.print("It is "); Serial.print(currentTemperature); Serial.println("C. Ahhh, very nice temperature."); } else if (currentTemperature >= 24.0) ( Serial.print("It is "); Serial.print(currentTemperature); Serial.println("C. I feel like a hot tamale!"); ) else ( Serial.print("It is ") ; Serial.print(currentTemperature); Serial.println("C. Brrrrrr, it"s COLD!"); ) ) //==================== ===================================================== ======== // Functions //====================================== ========================================================= //////// //////////////////// ////// readThermistor /////// /////////////// ///////////// /* This function reads the values ​​from the analog pin as shown below. Converts the input voltage to a digital representation using A/D conversion. However, this is done several times so that we. could average the value to avoid measurement errors. This averaged value is then used to calculate the resistance of the thermistor. The resistance is then used to calculate the temperature of the thermistor. Finally, the temperature is converted to degrees Celsius. */ double readThermistor() ( // variables double rThermistor = 0. ; // Stores the thermistor resistance value double tKelvin = 0; // Stores the calculated temperature double tCelsius = 0; // Stores temperature in degrees Celsius double adcAverage = 0; // Stores the average voltage value int adcSamples; // An array to store individual // voltage measurements /* Calculate the average thermistor resistance: As mentioned above, we will read the ADC values ​​multiple times to get an array of samples. A small delay is used for the analogRead function to work correctly. */ for (int i = 0; i< SAMPLE_NUMBER; i++) { adcSamples[i] = analogRead(thermistorPin); // прочитать значение на выводе и сохранить delay(10); // ждем 10 миллисекунд } /* Затем мы просто усредняем все эти выборки для "сглаживания" измерений. */ for (int i = 0; i < SAMPLE_NUMBER; i++) { adcAverage += adcSamples[i]; // складываем все выборки. . . } adcAverage /= SAMPLE_NUMBER; // . . . усредняем их с помощью деления /* Здесь мы рассчитываем сопротивление термистора, используя уравнение, описываемое в статье. */ rThermistor = BALANCE_RESISTOR * ((MAX_ADC / adcAverage) - 1); /* Здесь используется бета-уравнение, но оно отличается от того, что описывалось в статье. Не беспокойтесь! Оно было перестроено, чтобы получить более "красивую" формулу. Попробуйте сами упростить уравнение, чтобы поупражняться в алгебре. Или просто используйте показанное здесь или то, что приведено в статье. В любом случае всё будет работать! */ tKelvin = (BETA * ROOM_TEMP) / (BETA + (ROOM_TEMP * log(rThermistor / RESISTOR_ROOM_TEMP))); /* Я буду использовать градусы Цельсия для отображения температуры. Я сделал это, чтобы увидеть типовую комнатную температуру, которая составляет 25 градусов Цельсия. */ tCelsius = tKelvin - 273.15; // преобразовать кельвины в цельсии return tCelsius; // вернуть температуру в градусах Цельсия }

Possible next steps

Everything in this article shows a fairly simple way to measure temperature using a cheap thermistor. There are a couple more ways to improve the scheme:

• add a small capacitor in parallel with the divider output. This will stabilize the voltage and may even eliminate the need to average a large number of samples (as was done in the code) - or at least we will be able to average fewer samples;
• Use precision resistors (less than 1% tolerance) to get more predictable measurements. If measurement accuracy is critical to you, keep in mind that self-heating of the thermistor may affect the measurements; Self-heating is not compensated for in this article.

Of course, thermistors are only one of the sensors used to measure temperature. Another popular choice is sensor chips (an example of working with one of them is described). This way you won't have to deal with linearization and complex equations. The other two options are thermocouple and infrared sensor type; the latter can measure temperature without physical contact, but it is no longer as cheap.

I hope the article was useful. Leave comments!

I often noticed “popping” noises in switches when turning on light bulbs (especially LEDs). If they have capacitors as a driver, then the “pops” are simply frightening. These thermistors helped solve the problem.
Everyone knows from school that alternating current flows in our network. And alternating current is an electric current that changes in magnitude and direction over time (changes according to a sinusoidal law). That is why the “claps” happen every time. Depends on what moment you are in. At the moment of crossing zero there will be no cotton at all. But I don't know how to turn it on :)
To smooth out starting current, but without affecting the operation of the circuit, I ordered NTC thermistors. They have a very good property: with increasing temperature, their resistance decreases. That is, at the initial moment they behave like ordinary resistance, decreasing their value as they warm up.

A thermistor (thermistor) is a semiconductor device whose electrical resistance varies depending on its temperature.
Based on the type of dependence of resistance on temperature, thermistors are distinguished with negative (NTC thermistors, from the words “Negative Temperature Coefficient”) and positive (PTC thermistors, from the words “Positive Temperature Coefficient” or posistors.)

My task was to increase the service life of light bulbs (not only LEDs), but also to protect switches from damage (burning).
Not long ago I did a review about multi-turn resistance. When I ordered it, I noticed the seller’s product. There I saw these resistances. I immediately ordered everything from the seller.


I ordered at the end of May. The parcel arrived in 5 weeks. I got there with this track.



You can’t immediately tell that there are 50 pieces here.

I counted it, exactly fifty.
When I was selecting thermistors for my tasks, I found this sign from one seller. I think it will be useful to many. 10D-9 is simply deciphered: resistance (at zero) 10 Ohm, diameter 9mm.


Well, I compiled my table based on the experiments that I conducted. It's simple. From the P321 installation, with which I calibrate multimeters, I supplied a calibrated current.
The voltage drop across the thermistor was measured with a conventional multimeter.
There are features:
1. At a current of 1.8A, a smell appears paint coating thermistor.
2. The thermistor can easily withstand 3A.
3. The voltage is not established immediately, but gradually approaches the table value as it warms up or cools down.
4. The resistance of thermistors at a temperature of 24˚C is within 10-11 Ohms.

I have highlighted in red the range that is most applicable in my apartment.
I transferred the table to the chart.


The most effective work is on a steep descent.
Initially, I intended to implant each thermistor into a light bulb. But after testing the received product and taking characteristics, I realized that they (thermistors) needed a more serious load. That is why I decided to install it in switches so that they would work for several light bulbs at once. The resistor leads are a little thin, so I had to get out of the situation this way.

I don’t have a special crimp, so I worked with pliers.


For a single switch I prepared a single terminal block.

For the double I prepared another set. It will be more convenient to install with a terminal block.


The main thing is done. It stood up without any problems.


They've been working for six months now. After installing it in place, I no longer heard the terrible “pops”.
Enough time has passed to conclude that they are suitable. And they are suitable not only for LED bulbs.
But I found such a thermistor directly in the LED driver circuit (ITead Sonoff LED- WiFi Dimming LED)
The Chinese do not install large resistances so as not to interfere with the correct operation of the circuit.


What else did I want to say at the end? Everyone must choose the resistance value themselves in accordance with the tasks being solved. This is not at all difficult for a technically literate person. When I ordered thermistors, there was no information about them at all. You have it now. Look at the dependence graph and order what you think is more suitable for your tasks.
That's all!
Good luck!

I'm planning to buy +80 Add to favorites I liked the review +80 +153

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. For multiple thermistors various types, sizes, shapes, denominations and tolerances, there is an equally large number technical specifications. 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, valid values resistances 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. Thermistors with large sizes have a constant dispersion better than small 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 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. Let's take a look at the board from charger IKAR-506 and let's 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 lithium ion battery DCB-145 from 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 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 multidirectional, 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 in color.

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). Throw charging current can lead to breakdown of 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 for now pulse block The 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 can be easily found 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.

Installed inside the housing between the spring terminals working fluid posistor. 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.