How to decipher the symbols of the ntc 10 1412 thermistor. Measuring temperature using an NTC thermistor. NTC thermistors and posistors

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 not a comprehensive list of 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 the temperature over an almost continuous range and send the readings to the serial monitor, so we will not 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 the data from the table in the data sheet, or (if you like to get a little 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. Let's go through them. Reading from the ADC may produce one value on one sample, and then a slightly different value on the next sample. To avoid the influence of noise, we can read the values ​​from the ADC pin multiple times and then average the values ​​to get a more constant value. This constant is used in the readThermistor function. */ const int SAMPLE_NUMBER = 10; /* To use the beta equation, we must know the value of the second resistor in our divider. If you are using a resistor with a large tolerance, such as 5% or even 1%, measure it and place the result in ohms here. */ const double BALANCE_RESISTOR = 9710.0; // This helps to calculate the resistance of the thermistor (see article for details). const double MAX_ADC = 1023.0; /* This constant depends on the thermistor and should be in the datasheet, or see the article on how to calculate it using the beta equation. */ const double BETA = 3974.0; /* Required for the conversion equation as a "typical" room temperature. */ const double ROOM_TEMP = 298.15; // room temperature in Kelvin /* Thermistors have a typical resistance at room temperature, we will indicate it here. Again, necessary for the equation to transform. */ const double RESISTOR_ROOM_TEMP = 10000.0; //================================================================ ============================== // Variables //============== ===================================================== =============== // Here we will store the current temperature double currentTemperature = 0; //================================================================ ============================== // Pin declarations //============= ===================================================== ================ // Inputs: int thermistorPin = 0; // ADC input, voltage divider output //========================================= ====================================== // Initialization //====== ===================================================== ======================= void setup() ( // Set the port speed for sending messages Serial.begin(9600); ) //==== ===================================================== ====================================== // Main loop //===== ===================================================== ========== void loop() ( /* The main loop is quite simple, it prints the temperature to the serial port monitor. The heart of the program is in the readThermistor function. */ currentTemperature = readThermistor(); delay(3000); /* Here we describe what to do if the temperature is too high, too low or just right. */ if (currentTemperature > 21.0 && currentTemperature< 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 the 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 circuit.

And they belong to the category of devices based on semiconductors. These devices are widely used in electrical engineering. They are made from special semiconductor materials with a high negative temperature coefficient. Many devices use a thermistor, the operating principle of which is based on the dependence of electrical resistance on temperature. The quality of any device primarily depends on the physical properties of the semiconductor, as well as on the shape and size of the thermistor itself.

Thermistors: design and principle of operation

A thermistor is a thermistor with a negative temperature coefficient of resistance. These devices are manufactured in the form of semiconductor rods and covered with a protective layer of enamel paint.

Connection to other parts is made using contact caps and terminals, for which only a dry environment is suitable. Some models of thermistors use a metal sealed housing to house them. In this case, they become resistant to any aggressive influences and can be used even with high indoor humidity.

In order for the design of the device to be airtight, glass and tin are used. The performance of thermistors is improved when metal foil is used to wrap the cores. Down conductors are made of nickel wire. The nominal resistance values ​​in various devices are in the range of 1-200 kOhm, and the temperature range is from -100 to +1290C.

Thermistors work based on the properties of individual types of conductors, changing resistance values ​​under the influence of different temperatures. The main conductors used in these devices are pure copper and platinum. It should be noted that the value of the negative temperature coefficient of thermistors significantly exceeds the same parameters characteristic of ordinary metals.

Applications of thermistors

Thermistors used as sensors can operate in two modes. In the first case, the temperature regime depends only on the ambient temperature. The current passing through the thermistor is very small and the device practically does not heat up. The second mode involves heating the thermistor with an electric current passing inside it. In this case, the temperature value will depend on various changing conditions of heat transfer. This may be the density of the gas environment surrounding the device, the intensity of the airflow and other factors.

Each thermistor, whose operating principle is based on a decrease in resistance as the temperature increases, is used in certain areas of electrical engineering. They are used to measure and compensate temperature in large household electrical appliances - refrigerators and freezers, dishwashers and other equipment. These devices are widely used in automotive electronics. With their help, the temperature of the coolant or oil is measured, as well as the temperature indicators of other elements of the car.

In an air conditioner, thermistors are installed in the heat distributor. In addition, they are used as a room temperature monitoring sensor. Thermistors are used to block the doors of heating devices; they are installed in underfloor heating heaters and gas boilers. Thermistors are used when it is necessary to determine the level of non-standard liquids, for example, liquid nitrogen. In general, they are most widely used in industrial electronics.

1. WHAT IS THIS?
A thermistor is a semiconductor resistor that uses the temperature dependence of the semiconductor resistance.
Thermistors are characterized by a large temperature coefficient of resistance (TCR), the value of which exceeds that of metals by tens and even hundreds of times.
Thermistors are designed very simply and are manufactured in various shapes and sizes


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

The thermistor was invented by Samuel Reuben in 1930.

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

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

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

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

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

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

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

Application of NTC thermistors


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

Application of RTS thermistors

Illustrations for the use of thermistors:


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


- in home weather stations, air conditioners, microwave ovens


- in refrigerators, kettles, heated floors


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


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

5. Examples of amateur radio designs using thermistors

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


The main advantages of the circuit include the fact that it eliminates even small surges of current through the filament of the incandescent lamp when turned on. This is ensured thanks to the thermistor installed in the protection device. R3. At the initial moment of connection to the network, the thermistor R3 has a maximum resistance that limits the current flowing through this resistor. When the thermistor is gradually heated R3 its resistance gradually decreases, resulting in current flowing through the incandescent lamp and the resistor R2 also increases smoothly. The device circuit is designed in such a way that when the incandescent lamp reaches a voltage of 180-200 V across the resistor R2 the voltage drops, which causes the electromagnetic relay K1 to operate. In this case, relay contacts KL1 and K1.2 are closed.
Please note that in the incandescent lamp circuit there is another resistor connected in series - R4, which also limits current surges and protects the circuit from overloads. When the contacts of relay KL1 are closed, the control electrode of the thyristor is connected VS1 to its anode, and this in turn leads to the opening of the thyristor, which ultimately bypasses the thermistor R3, turning it off. Relay contacts K1.2 bypass resistor R4, which leads to an increase in voltage on incandescent lamps H2 and NZ, and their threads begin to glow more intensely.
The device is connected to an AC mains voltage of 220 V with a frequency of 50 Hz using an electrical connector X1 "fork" type. Switching the load on and off is provided by a switch S1. A fuse F1 is installed at the input of the device, protecting the input circuits of the device from overloads and short circuits due to improper installation. The inclusion of the device in the alternating current network is controlled by an HI glow discharge indicator lamp, which lights up immediately after switching on. In addition, a filter is assembled at the device input that protects against high-frequency interference that penetrates the device’s power supply.
In the manufacture of incandescent lamp protection devices H2 and NZ The following components are used: thyristor VS1 type KU202K; rectifier diodes VD1-4 type KDYu5B; indicator light H1 type TN-0.2-1; incandescent lamps H2, NC type 60W-220-240V; capacitors S1-2 type MBM-P-400V-0.1 µF, SZ - K50-3-10B-20 µF; resistors R1 type VSA-2-220 kOhm, R2 - VSA-2-10 Ohm, R3 - MMT-9, R4 - homemade wire with a resistance of 200 Ohms or type C5-35-3BT-200 Ohms; electromagnetic relay K1 type RES-42 (passport RS4.569.151); electrical.connector X1 plug type with electrical cable; switch S1 type P1T-1-1.
When assembling and repairing the device, other components may be used. Resistors of type BC can be replaced with resistors of types MLT, MT, S1-4, ULI; MBM type capacitors - for K40U-9, MBGO, K42U-2, K50-3 type capacitor - for K50-6, K50-12, K50-16; electromagnetic relay type RES-42 - on relay types RES-9 (passport RS4.524.200), RVM-2S-110, RPS-20 (passport RS4.521.757); thyristor type KU202K - on KU202L, KU202M, KU201K, KU201L; thermistor of any series.
To adjust and set up an incandescent lamp protection device, you will need an IP and an autotransformer that allows you to increase the AC supply voltage to 260 V. The voltage is supplied to the input of the device X1, and it is measured at points A and B, using an autotransformer to set the voltage on incandescent lamps to 200 V. Instead of a constant resistor R2 install a wirewound variable resistor type PPZ-ZVt-20 Ohm. Smoothly increasing the resistance of the resistor R2 mark the moment when relay K1 operates. Before making this adjustment, the thermistor R3 is bridged with a short-circuited jumper.
After checking the voltage on incandescent lamps with resistors temporarily closed R2 and R3 remove the jumpers, replace the resistor R2 with the appropriate resistance, check the delay time of the electromagnetic relay, which should be within 1.5-2 s. If the relay response time is significantly longer, then the resistor resistance R2 needs to be increased by a few ohms.
It should be noted that this device has a significant drawback: turning it on and off can only be done after the thermistor R3 has completely cooled down after heating and is ready for a new switching cycle. The cooling time of the thermistor is 100-120 s. If the thermistor has not yet cooled down, the device will operate with a delay only due to the resistor included in the circuit R4.

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


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

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


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


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


A more complex cooling fan speed controller circuit has been successfully used in another power supply. Unlike the prototype, it uses “television” transistors. The role of the radiator of the adjustable transistor T2 on it is performed by a free section of foil left on the front side of the board. This circuit allows, in addition to automatically increasing the fan speed when the radiator of the cooled power supply transistors or diode assembly heats up, to set the minimum threshold speed manually, up to the maximum.

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


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

5.4. Multimeter attachment for measuring temperature


A simple attachment containing six resistors allows you to use a digital voltmeter (or multimeter) to measure temperature with a resolution of 0.1 ° C and a thermal inertia of 10...15 s. With such speed, it can also be used to measure body temperature. There is no need to make changes to the measuring device, and the manufacture of the set-top box is also accessible to novice radio amateurs.
A semiconductor thermistor STZ-19 with a nominal resistance of 10 kOhm at t = 20°C was used as a sensor. Together with the additional resistor R3, it forms one half of the measuring bridge. The second half of the bridge is a voltage divider made of resistors R4 and R5. The last thing during calibration is to set the initial value of the output voltage. The multimeter is used in DC voltage measurement mode at 200 or 2000 mV. By appropriately selecting the resistance of resistor R2, the sensitivity of the measuring bridge is changed.
Immediately before measuring the temperature with variable resistor R1, the supply voltage of the measuring circuit is set equal to that at which the initial calibration was performed. The attachment for reading the measured temperature is turned on using push-button switch SB1, and switching from measurement mode to voltage setting mode using switch SB2.
The additional resistor R3 connected in series with the thermistor is calculated using the formula R3 = Rtm(B - 2Tm)/(B + 2Tm), where RTm is the resistance of the thermistor in the middle of the temperature range; B is the thermistor constant; Tm is the absolute temperature in the middle of the measuring range T = t° + 273.
This value of R3 ensures minimal deviation of the characteristic from linear.
The thermistor constant is determined by measuring the resistances RT1 and RT2 of the thermistor at two temperature values ​​T1 and T2 and subsequent calculation using the formula B = ln(RT1/RT2)/(1/T-1/T2).
On the contrary, with known parameters of a thermistor with negative TCR, its resistance for a certain temperature T can be determined by the formula Rt = R-r2oe(B/T"B^J3), where Rt2o is the resistance of the thermistor at a temperature of 20°C.
The attachment is calibrated at two points: Tk- = Tm+0.707(T2-T.)/2 and TK2=Tm-0.707(12-10/2, where Tm = (Tt + T2)/2, Ti and T2 - beginning and the end of the temperature range.
During the initial calibration with a fresh battery, the resistance of the variable resistor R1 is set to the maximum, so that as the capacity is lost and the element voltage decreases, the voltage on the bridge can be kept constant (the set-top box consumes a current of about 8 mA). By adjusting the trimming resistors R2, R5, we achieve compliance in three digits of the readings of the digital multimeter indicator with the temperature values ​​of the thermistor T1 and T2, controlled by an accurate thermometer. If it is not available, use, for example, a medical thermometer to control the temperature within its scale and a stable melting temperature of ice - 0°C.
The author used an M-830 from Mastech as a multimeter. It is better to use multi-turn resistors R2, R5 (SP5-1V, SP5-14). a R1 is single-turn, for example PPB: resistors R3 and R4 are MLT-0.125. To turn on the power and switch the set-top box mode, you can use P2K push-button switches without fixing.
In the manufactured attachment, the boundaries of the measured temperature range were set - T1 = 15°C: T2 = 45°C. In the case of measurements in the range of positive and negative temperatures on the Celsius scale, the sign indication is obtained automatically.

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


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

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


The thermostat, the diagram of which is shown below, is designed to maintain a constant temperature in the air in the room, water in vessels, in thermostats, as well as solutions in color photography. You can connect a heater with a power of up to 500 W to it. The thermostat consists of a threshold device (on transistors T1 and T2), an electronic relay (on transistor T3 and thyristor D10) and a power supply. Temperature sensor The thermistor R5 is used, connected to the voltage supply circuit to the base of the transistor T1 of the threshold device.
If the environment has the required temperature, the threshold device transistor T1 is closed and T2 is open. Transistor TZ and thyristor D10 of the electronic relay are closed in this case, and the mains voltage is not supplied to the heater. As the temperature of the environment decreases, the resistance of the thermistor increases, as a result of which the voltage at the base of transistor T1 increases. When it reaches the device's operating threshold, transistor T1 will open and T2 will close. This will lead to the opening of the T3 transistor. The voltage that appears across resistor R9 is applied between the cathode and the control electrode of thyristor D10 and will be sufficient to open it. The mains voltage is supplied to the heater through the thyristor and diodes D6 - D9.
When the ambient temperature reaches the required value, the thermostat will turn off the voltage from the heater. Variable resistor R11 is used to set the limits of the maintained temperature.
The thermostat uses an MMT-4 thermistor. The Tr transformer is made on a Ш12Х25 core. Winding I contains 8000 turns of PEV-1 0.1 wire, winding II contains 170 turns of PEV-1 0.4 wire.

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


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

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

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


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

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


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

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


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


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

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


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


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

BACK to the RADIO components page

Thermistors are made of special semiconductor alloys or pure metals, whose resistance varies significantly with temperature. Thermistors are also called thermal resistances or thermistors for short (therm^P^^istor).

The main parameter of thermistors is the temperature coefficient of resistance (TCR). The larger it is, the easier it is to record temperature deviations. The more stable it is over time, the more reliable the readings will be.

Based on the TCS sign, NTC and PTC thermistors are distinguished.

In NTC-type thermistors (NTC - Negative Temperature Coefficient), the resistance decreases with increasing ambient temperature. A typical NTC thermistor at 0°C has a resistance of 7...16 kOhm, and at +100°C - 152...339 Ohm (Table 3.11).

Table 3JL Parameters of NTC thermistors (NTC-Thermistor)

NTC thermistors

Material

Resistance range [kOhm]

Tolerance [%]

Power [W]

TCS [%/°С]

Temperature range [°C]

Si, Co, MP

"NTC" (EPCOS company)

Cu, Co, Mn, Ni, Fe

In RTS-type thermistors or, in other words, posistors (English RTS - Positive Temperature Coefficient), the resistance increases with increasing ambient temperature (Table 3.12).

Table 3.12, Parameters of posistors (Posistor)

TCR in percentage terms is higher for posistors than for NTC thermistors. On the other hand, posistors are not high-resistance. This leads to a division of the areas of their application. NTC-type thermistors are most often used to measure temperature, and posistors are used for thermal protection systems and inrush current limitation in power circuits.

The main advantages of thermistors over other temperature sensors are their low cost and high sensitivity, which allows them to record rapid temperature fluctuations. Disadvantages: relatively narrow operating temperature range, “fragile” design and nonlinear characteristics. If the temperature is measured by an MK, then the nonlinearity is easily taken into account programmatically.

In Fig. 3.64, a...t shows diagrams for connecting NTC thermistors to the MK.

Rice. 3.64. Diagrams for connecting NTC thermistors to the MK (beginning)’.

a) basic circuit for measuring temperature through the ADC of the MK. The dependence of the thermistor resistance on temperature in the general case is nonlinear, so a tabular method with pre-selected coefficients is used. The ADC voltage-to-temperature conversion table is pre-entered into the MK ROM;

b) if the thermistor R) is connected to the power circuit and not to the common wire, then the slope of the dependence of the ADC voltage on temperature changes in the opposite direction;

c) temperature measurement is carried out only at a HIGH level at the output of the MK, which saves current through the divider /?/, in standby mode. The resistor /?/ must be precise;

d) a direct current amplifier on transistor VT1 increases sensitivity, but narrows the temperature range. The base current VT1 may go beyond the norm if the resistance RL is low. The resistor scale (rotation characteristic “B”) is marked in degrees of temperature. The MK monitors the input level and at the moment of the “jump” turns on an external indicator;

e) MK measures the voltage difference across two dividers: R1, R2w R3, R4. Two ADC channels are used in differential mode. Thermistors R1 and are physically installed in different places with different ambient temperatures;

Rice. 3.64. Schemes for connecting NTC thermistors to MK (continued)’.

e) first, capacitor C1 is discharged through resistor R1 at a LOW level from the “O/Z” output of the MK. Then the “O/Z” and “1/Z” lines are configured into input mode, and the “Z/1” line into output mode with a HIGH level. The MK uses a timer to measure the charging time of capacitor C/ through resistor R2JX0 of a certain threshold (the “O/Z” line serves as the input). The capacitor is again discharged through the MK line, after which the charging time of the capacitor is measured in the same way through the thermistor R3. The difference between the two time counts is proportional to the difference in heating temperatures of resistors R2w R3, which must be physically located in different places. Resistor /?/ can be replaced with a jumper if the capacitance of capacitor C/ is small;

g) charge balancing method. The MK can use an ADC or a regular port line with a fixed response threshold at the input. If the voltage on capacitor C/ is greater than the threshold, then a LOW level is set on the “Z/O” line and the capacitance is discharged through resistor R2. If the voltage is less than the threshold, then the “Z/O” line is switched to input mode without a “pull-up” resistor. The capacitor C/ is charged through the thermistor RI. The average number of charge-discharge cycles per unit time is proportional to temperature. The advantage of the method is compensation of interference with the frequency of the supply network and its harmonics;

h) dual-range temperature measurement through the ADC of the MK. At low temperatures, the divider RI, R3 is used, at high temperatures - R2, R3. The number of ranges can be increased by using other output lines of the MK ports. Advantage - compensation for the natural nonlinearity of thermistor R3, increased measurement accuracy;

i) thermistor /?/ is automatically connected to the gap between resistor R2 and the common wire when connected to the XSI socket. Resistor R3 sets the operating voltage at the MK input, close to half the power. In addition, this resistor can simulate the process of rapid temperature changes during test checks;

j) The DAI op-amp is connected according to a voltage follower circuit. NTC thermistor R2 (BCcomponents, catalog number 2322-633-83033) changes its resistance from 941 kOhm to 191 Ohm at temperatures from -40 to +200°C. Intermediate values ​​are indicated in the datasheet;

Rice. 3.64. Schemes for connecting NTC thermistors to the MK (continued): k) accurate temperature measurement through a 22-bit ADC DA1. Platinum thermistor R2 W2102 (Omega Engineering) provides high stability and linearity;

l) original inclusion of two halves of the DAI chip. Resistor RI sets the temperature threshold until the thermistor is completely turned off

n) temperature measurement using the thermistor Y2 and analog comparator MK; o) similar to Fig . 3.64, n, but with the RI thermistor connected to the power circuit (and not to the common wire) and with the ability to calibrate the temperature using a trimmer resistor R2\

n) increasing the accuracy of temperature measurement using an integrated voltage stabilizer DA /. The internal ADC M K is switched to measurement mode from an external reference. Resistor /?/ linearizes the temperature characteristic of thermistor R3 in a narrow range; p) similar to Fig .3.64, p, but with a zener diode VD1 and without linearization of the characteristics;

Rice. 3.64. Connection diagrams for YTS thermistors to MK (end) -. c) resistor R4 balances the bridge containing thermistor R2. Resistors /?/, R3, it is recommended to use high-precision ones, for example, ± 1%. Thermistor R2-pro

fiber TSM-YuOM (copper, -50...+200°C), TSM-YuOP (platinum, -200...+750°C) or homemade, consisting of 11 m of copper wire PEV-0.05. When selecting a replacement, you should know the standard range of ratings of wire-wound measuring thermistors: 100; 500; 1000 Ohm;

r) thermistor R1 is part of the divider, the voltage on which is measured through the ADC of the MK. Capacitor C/ reduces interference at a significant distance /?/ from the MK and at a high level of interference. Thermistor R1 is a home-made wire with a TCR of approximately 10 Ohm/°C. It contains 1300 turns of PEL-0.05 copper wire, wound on a frame with a diameter of 7 mm. The advantage of a wire sensor is a stable and predictable TCS, a wide range of measured temperatures -100...+500°C. If you need to expand the range to -200...+850°C, then you should use an industrial platinum thermistor.

A thermistor (or thermistor) is a resistor that changes its electrical resistance depending on temperature.

There are two types of thermistors: PTC - with a positive temperature coefficient, and NTC - with a negative temperature coefficient. A positive coefficient means that as the temperature rises, the thermistor's resistance increases. An NTC thermistor behaves in the opposite way.

Thermistors also differ in their nominal resistance, which corresponds to room temperature - 25 C°. For example, thermistors with a rating of 100 kOhm and 10 kOhm are popular. Such thermistors are often used in 3D printers.

In this tutorial we will use a NTC 100K thermistor in a glass case. Like this:

Connecting a thermistor to Arduino

To measure the resistance of the thermistor, we connect it as the lower arm of a voltage divider. We will connect the middle point of the divider to the Arduino analog input - A0. A similar method was used in .

We talked in detail about the analog inputs of Arduino in the lesson:

Schematic diagram

Layout appearance

What resistance should the resistor have in the upper arm of the divider? As a rule, a resistor is used with a resistance that matches the value of the thermistor. In our lesson we use a resistor with R1 = 102 kOhm, it can be easily obtained by connecting two 51 kOhm resistors in series.

Program for calculating thermistor resistance

The first program we will write will calculate the resistance of the thermistor in ohms.

#define SERIAL_R 102000 // resistance of the serial resistor, 102 kOhm const byte tempPin = A0; void setup() ( Serial.begin(9600); pinMode(tempPin, INPUT); ) void loop() ( int t = analogRead(tempPin); float tr = 1023.0 / t - 1; tr = SERIAL_R / tr; Serial. println(tr); delay(100);

Result of the program:

You can notice that the measured resistance of the thermistor is less than 100 kOhm, which means the ambient temperature is below 25 C°. The next step is to calculate the temperature in degrees Celsius.

A program for calculating the temperature on a thermistor

To calculate the temperature value, use the Steinhart-Hart formula:

The equation has parameters A, B and C, which must be taken from the specification for the sensor. Since we do not need great accuracy, we can use a modified equation (B-equation):

In this equation, only parameter B remains unknown, which for an NTC thermistor is equal to 3950. The remaining parameters are already known to us:

  • T0 is the room temperature in Kelvin, for which the thermistor rating is indicated; T0 = ​​25 + 273.15;
  • T is the desired temperature, in Kelvin;
  • R is the measured resistance of the thermistor in Ohms;
  • R0 is the nominal resistance of the thermistor in Ohms.

Let's modify the program for Arduino by adding temperature calculation:

#define B 3950 // B-coefficient #define SERIAL_R 102000 // series resistor resistance, 102 kOhm #define THERMISTOR_R 100000 // nominal thermistor resistance, 100 kOhm #define NOMINAL_T 25 // nominal temperature (at which TR = 100 kOhm) const byte tempPin = A0; void setup() ( Serial.begin(9600); pinMode(tempPin, INPUT); ) void loop() ( int t = analogRead(tempPin); float tr = 1023.0 / t - 1; tr = SERIAL_R / tr; Serial. print("R="); Serial.print(", t="); float steinhart = tr / THERMISTOR_R; // (R/Ro) steinhart = log(steinhart); / ln(R/Ro) steinhart /= B; // 1/B * ln(R/Ro) steinhart += 1.0 / (NOMINAL_T + 273.15); // + (1/To) steinhart = 1.0 / steinhart; / Invert steinhart -= 273.15; Serial.println(steinhart);

Result:

Already better! The program shows us the temperature in degrees Celsius. As expected, it is slightly below 25 C°.

Quests

  1. Thermometer with display. Let's connect a symbolic LCD display to the circuit and write a program that will display the temperature on it every 100 milliseconds.
  2. Overheat alarm. Let's add a buzzer to the circuit and write a program that will continuously calculate the temperature. The program should also have a condition: if the temperature exceeds 70 C°, then we turn on the buzzer.