Do-it-yourself temperature regulator up to 300 degrees. Simple circuits of electronic thermostats with your own hands

Maintaining temperature conditions is a very important technological condition not only in production, but also in Everyday life. Having such great importance, this parameter must be regulated and controlled somehow. They produce a huge number of such devices, which have many features and parameters. But making a thermostat with your own hands is sometimes much more profitable than buying a ready-made factory analogue.

Make your own thermostat

General concept of temperature controllers

Devices that record and simultaneously regulate a given temperature value are more common in production. But they also found their place in everyday life. To maintain the necessary microclimate in the house, water thermostats are often used. They make such devices with their own hands for drying vegetables or heating an incubator. Such a system can find its place anywhere.

In this video we will find out what a temperature regulator is:

In reality, most thermostats are only part general scheme, which consists of the following components:

1. A temperature sensor that measures and records, as well as transmits the received information to the controller. This happens due to the conversion of thermal energy into electrical signals recognized by the device. The sensor can be a resistance thermometer or a thermocouple, which have metal in their design that reacts to changes in temperature and changes its resistance under its influence.
2. The analytical unit is the regulator itself. It receives electronic signals and reacts depending on its functions, after which it transmits the signal to the actuator.
3. An actuator is a kind of mechanical or electronic device that, when receiving a signal from the unit, behaves in a certain way. For example, when the set temperature is reached, the valve will shut off the coolant supply. Conversely, as soon as the readings drop below the specified values, the analytical unit will give a command to open the valve.

These are the three main parts of the system for maintaining specified temperature parameters. Although, in addition to them, other parts, such as an intermediate relay, may also participate in the circuit. But they perform only an additional function.

Principle of operation

The principle on which all regulators work is the removal of a physical quantity (temperature), transmission of data to the control unit circuit, which decides what needs to be done in a particular case.

If you are making a thermal relay, the simplest option will be to have a mechanical control circuit. Here, using a resistor, a certain threshold is set, upon reaching which a signal will be given to the actuator.

To get additional functionality and the ability to work with a wider temperature range, you will have to integrate a controller. This will also help increase the service life of the device.

In this video you can see how to make your own thermostat for electric heating:

Homemade temperature controller

There are actually a lot of schemes for making a thermostat yourself. It all depends on the area in which such a product will be used. Of course, it is extremely difficult to create something too complex and multifunctional. But a thermostat that can be used to heat an aquarium or dry vegetables for the winter can be created with a minimum of knowledge.

The simplest scheme

The simplest do-it-yourself thermal relay circuit has a transformerless power supply, which consists of diode bridge with a parallel-connected zener diode, which stabilizes the voltage within 14 volts, and a quenching capacitor. If desired, you can also add a 12-volt stabilizer here.

Creating a thermostat does not require much effort or financial investment.

The entire circuit will be based on a zener diode TL431, which is controlled by a divider consisting of a 47 kOhm resistor, a 10 kOhm resistance and a 10 kOhm thermistor that acts as a temperature sensor. Its resistance decreases with increasing temperature. It is better to select the resistor and resistance to achieve the best operating accuracy.

The process itself is as follows: when a voltage of more than 2.5 volts is generated at the control contact of the microcircuit, it will make an opening, which will turn on the relay, applying a load to the actuator.

You can see how to make a thermostat for an incubator with your own hands in the video presented:

Conversely, when the voltage drops lower, the microcircuit will close and the relay will turn off.

To avoid rattling of the relay contacts, it is necessary to select it with a minimum holding current. And parallel to the inputs you need to solder a 470×25 V capacitor.

When using an NTC thermistor and a microcircuit that have already been used, you should first check their performance and accuracy.

Thus, it turns out to be a simple device regulating temperature. But with the right ingredients, it works excellently in a wide range of applications.

Indoor device

Such do-it-yourself thermostats with an air temperature sensor are optimally suited for maintaining the specified microclimate parameters in rooms and containers. It is fully capable of automating the process and controlling any heat emitter, from hot water to heating elements. At the same time, the thermal switch has excellent performance data. And the sensor can be either built-in or remote.

Here the thermistor, designated R1 in the diagram, acts as a temperature sensor. The voltage divider includes R1, R2, R3 and R6, the signal from which is sent to the fourth pin of the operational amplifier chip. The fifth pin of DA1 receives a signal from the divider R3, R4, R7 and R8.

The resistance of the resistors must be selected in such a way that at the minimum low temperature of the measured medium, when the resistance of the thermistor is maximum, the comparator is positively saturated.

The voltage at the output of the comparator is 11.5 volts. At this time, transistor VT1 is in the open position, and relay K1 turns on the actuator or intermediate mechanism, as a result of which heating begins. As a result, the ambient temperature rises, which reduces the resistance of the sensor. At input 4 of the microcircuit, the voltage begins to increase and, as a result, exceeds the voltage at pin 5. As a result, the comparator enters the negative saturation phase. At the tenth output of the microcircuit, the voltage becomes approximately 0.7 Volts, which is a logical zero. As a result, transistor VT1 closes, and the relay turns off and turns off the actuator.

On the LM 311 chip

This do-it-yourself temperature controller is designed to work with heating elements and is capable of maintaining the specified temperature parameters within the range of 20-100 degrees. This is the safest and most reliable option, since its operation uses galvanic isolation of the temperature sensor and control circuits, and this completely eliminates the possibility of electric shock.

Like most similar schemes, it is based on a bridge direct current, in one arm of which a comparator is connected, and in the other – a temperature sensor. The comparator monitors the mismatch of the circuit and reacts to the state of the bridge when it passes the balance point. At the same time, he tries to balance the bridge using a thermistor, changing its temperature. And thermal stabilization can occur only at a certain value.

Resistor R6 sets the point at which balance should be formed. And depending on the temperature of the environment, the thermistor R8 can be included in this balance, which allows you to regulate the temperature.

In the video you can see an analysis of a simple thermostat circuit:

If the temperature set by R6 is lower than required, then the resistance on R8 is too high, which reduces the current on the comparator. This will cause current to flow and open the seven-stor VS1 which will turn on a heating element. The LED will indicate this.

As the temperature rises, the resistance of R8 will begin to decrease. The bridge will tend to a balance point. On the comparator, the potential of the inverse input gradually decreases, and on the direct input it increases. At some point the situation changes, and the process occurs in the opposite direction. Thus, the temperature controller will turn the actuator on or off depending on the resistance R8.

If LM311 is not available, then it can be replaced with the domestic KR554CA301 microcircuit. It turns out to be a simple do-it-yourself thermostat with minimal costs, high accuracy and reliable operation.

Required materials and tools

The assembly of any electric temperature controller circuit itself does not take much time and effort. But to make a thermostat, you need minimal knowledge in electronics, set of parts according to the diagram and tools:

1. Pulse soldering iron. You can use a regular one, but with a thin tip.
2. Solder and flux.
3. Printed circuit board.
4. Acid to etch the tracks.

Advantages and disadvantages

Even a simple do-it-yourself thermostat has a lot of advantages and positive points. Let's talk about factory ones multifunctional devices and it’s not necessary at all.

Temperature regulators allow:

1. Maintain a comfortable temperature.
2. Save energy resources.
3. Do not involve a person in the process.
4. Observe technological process, improving quality.

Among the shortcomings we can name high cost factory models. Certainly, homemade devices This doesn't apply. But the production ones, which are required when working with liquid, gaseous, alkaline and other similar media, have a high cost. Especially if the device must have many functions and capabilities.

In everyday life and farmsteads, it is often necessary to maintain the temperature regime of a room. Previously, this required a fairly huge circuit made on analog elements; we will consider one of these for general development. Today everything is much simpler; if it is necessary to maintain the temperature in the range from -55 to +125°C, then the programmable thermometer and thermostat DS1821 can perfectly cope with this goal.

Thermostat circuit on a specialized temperature sensor. This DS1821 temperature sensor can be bought cheaply from ALI Express (to order, click on the picture just above)

The temperature threshold for turning on and off the thermostat is set by the TH and TL values ​​in the sensor memory, which must be programmed into the DS1821. If the temperature exceeds the value recorded in the TH cell, a logical one level will appear at the sensor output. To protect against possible interference, the load control circuit is implemented in such a way that the first transistor is locked into that half-wave of the mains voltage when it is equal to zero, thereby applying a bias voltage to the gate of the second field-effect transistor, which turns on the optosimistor, which already opens the VS1 smistor that controls the load . The load can be any device, such as an electric motor or a heater. The locking reliability of the first transistor must be adjusted by selecting the desired value of resistor R5.

The DS1820 temperature sensor is capable of recording temperatures from -55 to 125 degrees and operating in thermostat mode.

Thermostat circuit on the DS1820 sensor

If the temperature exceeds the upper threshold TH, then the output of the DS1820 will be a logical one, the load will be disconnected from the network. If the temperature drops below the lower programmed level TL, a logical zero will appear at the output of the temperature sensor and the load will be turned on. If there are any unclear points, the homemade design was borrowed from No. 2 for 2006.

The signal from the sensor passes to the direct output of the comparator on the CA3130 operational amplifier. The inverting input of the same op-amp receives the reference voltage from the divider. Variable resistance R4 sets the required temperature regime.

Thermostat circuit on sensor LM35

If the potential at the direct input is lower than that set at pin 2, then at the comparator output we will have a level of about 0.65 volts, and if vice versa, then at the comparator output we will have a high level of about 2.2 volts. The signal from the output of the op-amp through transistors controls the operation of the electromagnetic relay. At high level it turns on, and when low it turns off, switching the load with its contacts.

TL431 is a programmable zener diode. Used as a voltage reference and power supply for low power circuits. The required voltage level at the control pin of the TL431 microassembly is set using a divider on resistors Rl, R2 and a thermistor with negative TCS R3.

If the voltage at the TL431 control pin is higher than 2.5V, the microcircuit passes current and turns on the electromagnetic relay. The relay switches the control output of the triac and connects the load. As the temperature increases, the resistance of the thermistor and the potential at the control contact TL431 decreases below 2.5V, the relay releases its front contacts and turns off the heater.

Using resistance R1, we adjust the level of the desired temperature to turn on the heater. This circuit is capable of controlling a heating element up to 1500 W. The relay is suitable for RES55A with an operating voltage of 10...12 V or its equivalent.

The design of an analog thermostat is used to maintain a set temperature inside an incubator, or in a box on the balcony for storing vegetables in winter. Meals are provided from car battery at 12 volts.

The design consists of a relay in the event of a temperature drop and turns off when the preset threshold rises.

The temperature at which the thermostat relay operates is set by the voltage level on pins 5 and 6 of the K561LE5 microcircuit, and the relay off temperature is set by the potential on pins 1 and 21. The temperature difference is controlled by the voltage drop across resistor R3. A thermistor with negative TCR is used as temperature sensor R4, i.e.

The design is small and consists of only two units - a measuring unit based on a comparator based on the 554CA3 op amp and a load switch up to 1000 W built on the KR1182PM1 power regulator.

The third direct input of the op-amp receives a constant voltage from a voltage divider consisting of resistances R3 and R4. The fourth inverse input is supplied with voltage from another divider across resistance R1 and the MMT-4 thermistor R2.

The temperature sensor is a thermistor located in a glass flask with sand, which is placed in the aquarium. The main unit of the design is the m/s K554SAZ - voltage comparator.

From the voltage divider, which also includes a thermistor, the control voltage goes to the direct input of the comparator. The other input of the comparator is used to adjust the required temperature. A voltage divider is made from resistances R3, R4, R5, which form a bridge sensitive to temperature changes. When the temperature of the water in the aquarium changes, the resistance of the thermistor also changes. This creates a voltage imbalance at the comparator inputs.

Depending on the voltage difference at the inputs, the output state of the comparator will change. The heater is made in such a way that when the water temperature decreases, the aquarium thermostat automatically starts up, and when it increases, on the contrary, it turns off. The comparator has two outputs, collector and emitter. To control the field-effect transistor, a positive voltage is required, therefore, it is the collector output of the comparator that is connected to the positive line of the circuit. The control signal is obtained from the emitter terminal. Resistors R6 and R7 are the output load of the comparator.

To turn the heating element on and off in the thermostat, an IRF840 field-effect transistor is used. To discharge the transistor gate, there is a diode VD1.

The thermostat circuit uses a transformerless power supply. Excess alternating voltage is reduced due to the reactance of capacitance C4.

The basis of the first thermostat design is a PIC16F84A microcontroller with a DS1621 temperature sensor having an l2C interface. When the power is turned on, the microcontroller first initializes the internal registers of the temperature sensor and then configures it. The thermostat on the microcontroller in the second case is already made on PIC16F628 with a DS1820 sensor and controls the connected load using relay contacts.

DIY temperature sensor

Dependence of voltage drop on p-n junction semiconductors on temperature, is perfect for creating our homemade sensor.

The need to adjust the temperature regime arises when using various heating or refrigeration equipment systems. There are many options, and they all require a control device, without which the systems can operate either in maximum power mode or at a complete minimum of capabilities. Control and adjustment are carried out using a thermostat - a device that can influence the system through a temperature sensor and turn it on or off as needed. When using ready-made equipment kits, control units are included in the delivery package, but for homemade systems you have to assemble the thermostat yourself. The task is not the easiest, but quite solvable. Let's take a closer look at it.

The principle of operation of the thermostat

A thermostat is a device that can respond to changes in temperature. Based on the type of action, a distinction is made between trigger-type thermostats, which turn off or turn on heating when a specified limit is reached, or smooth-action devices with the ability to fine-tune and accurately adjust, capable of controlling temperature changes in the range of fractions of a degree.

There are two types of thermostats:

1. Mechanical. It is a device that uses the principle of expansion of gases when temperature changes, or bimetallic plates that change their shape when heated or cooled.
2. Electronic. It consists of a main unit and a temperature sensor that sends signals about an increase or decrease in the set temperature in the system. Used in systems requiring high sensitivity and fine adjustment.

Mechanical devices do not allow for high precision settings. They are both a temperature sensor and an actuator, combined into a single unit. A bimetallic strip used in heating devices is a thermocouple made of two metals with different coefficients of thermal expansion.

The main purpose of the thermostat is to automatically maintain the required temperature

When heated, one of them becomes larger than the other, causing the plate to bend. The contacts installed on it open and stop heating. When cooled, the plate returns to its original shape, the contacts close again and heating resumes.

The chamber with the gas mixture is a sensitive element of the refrigerator thermostat or heating thermostat. When temperature changes, the volume of gas changes, which causes movement of the surface of the membrane connected to the lever of the contact group.

The thermostat for heating uses a chamber with a gas mixture that works according to Gay-Lussac's law - when the temperature changes, the volume of gas changes

Mechanical thermostats are reliable and provide stable operation, but the operating mode is adjusted with a large error, almost “by eye”. If necessary fine tuning, providing adjustment within a few degrees (or even finer), electronic circuits are used. The temperature sensor for them is a thermistor, which is capable of distinguishing the smallest changes in the heating mode in the system. For electronic circuits, the situation is the opposite - the sensitivity of the sensor is too high and it is artificially coarsened, bringing it to the limits of reason. The principle of operation is a change in the resistance of the sensor caused by fluctuations in the temperature of the controlled environment. The circuit reacts to changes in signal parameters and increases/decreases heating in the system until another signal is received. The capabilities of electronic control units are much higher and allow you to obtain temperature settings of any accuracy. The sensitivity of such thermostats is even excessive, since heating and cooling are processes with high inertia, which slow down the reaction time to changing commands.

Scope of homemade device

Making a mechanical thermostat at home is quite difficult and irrational, since the result will operate in too wide a range and will not be able to provide the required adjustment accuracy. Most often, homemade electronic thermostats are assembled, which allow you to maintain the optimal temperature of a heated floor, incubator, provide the desired water temperature in the pool, heat the steam room in the sauna, etc. There can be as many options for using a homemade thermostat as there are systems in the house that need to be configured and adjusted. For rough adjustments using mechanical devices, it is easier to purchase ready-made elements; they are inexpensive and quite accessible.

Advantages and disadvantages

A homemade thermostat has certain advantages and disadvantages. The advantages of the device are:

• High maintainability. A thermostat made by yourself is easy to repair, since its design and operating principle are known to the smallest detail.
• The costs of creating a regulator are much lower than when purchasing a ready-made unit.
• It is possible to change the operating parameters to obtain a more suitable result.

The disadvantages include:

• Assembly of such a device is only available to people who have sufficient training and certain skills in working with electronic circuits and a soldering iron.
• The quality of operation of the device largely depends on the condition of the parts used.
• The assembled circuit requires adjustment and alignment on a control stand or using a reference sample. It is impossible to obtain a ready-made version of the device immediately.

The main problem is the need for training or, at a minimum, the participation of a specialist in the process of creating the device.

How to make a simple thermostat

The manufacture of a thermostat occurs in stages:

• Selecting the type and circuit of the device.
• Acquisition necessary materials, tools and parts.
• Device assembly, configuration, commissioning.

The manufacturing stages of the device have their own characteristics, so they should be considered in more detail.

Necessary materials

Materials required for assembly include:

• Foil getinax or circuit board;
• Soldering iron with solder and rosin, ideally a soldering station;
• Tweezers;
• Pliers;
• Magnifier;
• Wire cutters;
• Insulating tape;
• Copper connecting wire;
• Necessary parts according to the electrical diagram.

Other tools or materials may be needed during the process, so this list should not be considered exhaustive or definitive.

Device diagrams

The choice of scheme is determined by the capabilities and level of training of the master. The more complex the circuit, the more nuances will arise when assembling and configuring the device. At the same time, the simplest schemes make it possible to obtain only the most primitive devices that operate with a high error.

Let's consider one of the simple schemes.

In this circuit, a zener diode is used as a comparator

The figure on the left shows the regulator circuit, and on the right is the relay block that turns on the load. The temperature sensor is resistor R4, and R1 is a variable resistor used to adjust the heating mode. The control element is a zener diode TL431, which is open as long as there is a load on its control electrode above 2.5 V. Heating of the thermistor causes a decrease in resistance, causing the voltage on the control electrode to drop, the zener diode closes, cutting off the load.

The other scheme is somewhat more complicated. It uses a comparator - an element that compares the readings of a temperature sensor and a reference voltage source.

A similar circuit with a comparator is applicable for adjusting the temperature of a heated floor.

Any change in voltage caused by an increase or decrease in the resistance of the thermistor creates a difference between the standard and the operating line of the circuit, as a result of which a signal is generated at the output of the device, causing the heating to turn on or off. Such schemes, in particular, are used to regulate the operating mode of heated floors.

Step-by-step instruction

The assembly procedure for each device has its own characteristics, but some general steps can be identified. Let's look at the build progress:

1. We prepare the device body. This is important because the board cannot be left unprotected.
2. We are preparing the payment. If you use foil getinax, you will have to etch the tracks using electrolytic methods, having first painted them with paint insoluble in the electrolyte. A circuit board with ready-made contacts greatly simplifies and speeds up the assembly process.
3. Using a multimeter, we check the performance of the parts and, if necessary, replace them with serviceable samples.
4. According to the diagram, we assemble and connect all the necessary parts. It is necessary to ensure the accuracy of the connection, correct polarity and direction of installation of diodes or microcircuits. Any mistake can lead to the failure of important parts that will have to be purchased again.
5. After completing assembly, it is recommended to carefully inspect the board again, check the accuracy of the connections, the quality of soldering and other important points.
6. The board is placed in the case, a test run is carried out and the device is configured.

How to setup

To configure the device, you must either have a reference device or know the voltage rating corresponding to a particular temperature of the controlled environment. Individual devices have their own formulas showing the dependence of the voltage on the comparator on temperature. For example, for the LM335 sensor this formula looks like:

V = (273 + T) 0.01,

where T is the required temperature in Celsius.

In other schemes, adjustment is made by selecting the values ​​of adjusting resistors when creating a certain, known temperature. In each specific case, our own methods can be used, optimally suited to the existing conditions or equipment used. The requirements for the accuracy of the device also differ from each other, so in principle there is no single adjustment technology.

Basic faults

The most common malfunction of homemade thermostats is instability of the thermistor readings caused by low quality details. In addition, there are often difficulties with setting modes caused by mismatches in ratings or changes in the composition of parts necessary for the correct operation of the device. Majority possible problems directly depend on the level of training of the technician who assembles and configures the device, since skills and experience in this matter mean a lot. However, experts say that making a thermostat with your own hands is a useful practical task that gives good experience in creating electronic devices.

If you don’t have confidence in your abilities, it’s better to use a ready-made device, of which there are plenty on sale. It must be taken into account that a regulator failure at the most inopportune moment can cause serious troubles, the elimination of which will require effort, time and money. Therefore, when deciding on self-assembly, you should approach the issue as responsibly as possible and carefully weigh your options.

THERMOREGULATOR DIAGRAMS

Exists a large number of electrical circuit diagrams that can maintain the desired set temperature with an accuracy of 0.0000033 °C. These circuits include temperature correction, proportional, integral and differential control.
The electric stove regulator (Figure 1.1) uses a posistor (positive temperature coefficient thermistor, or PTC) type K600A from Allied Electronics, built into the stove to maintain the ideal cooking temperature. The potentiometer can be used to regulate the start of the seven-actor regulator and, accordingly, the heating element on or off. The device is designed to operate in electrical network with a voltage of 115 V. When connecting the device to a 220 V network, you must use a different supply transformer and a seven-stor.

Figure 1.1 Electric stove temperature regulator

The LM122 timer manufactured by National is used as a dosing thermostat with optical isolation and synchronization when the supply voltage passes through zero. By installing resistor R2 (Fig. 1.2), the temperature controlled by posistor R1 is set. Thyristor Q2 is selected based on the connected load in terms of power and voltage. Diode D3 is specified for a voltage of 200 V. Resistors R12, R13 and diode D2 implement control of the thyristor when the supply voltage passes through zero.

Figure 1.2 Dosing heater power regulator

A simple circuit (Fig. 1.3) with a switch when the supply voltage passes through zero on the CA3059 microcircuit allows you to control the on and off of the thyristor, which controls the coil of the heating element or relay for controlling an electric or gas oven. The thyristor switches at low currents. The NTC SENSOR measuring resistance has a negative temperature coefficient. Resistor Rp sets the desired temperature.

Figure 1.3 Diagram of a thermostat with load switching when the power passes through zero.

The device (Fig. 1.4) provides proportional control of the temperature of a small, low-power oven with an accuracy of 1 °C relative to the temperature set using a potentiometer. The circuit uses an 823V voltage regulator, which, like the furnace, is powered by the same 28V source. A 10-turn wirewound potentiometer must be used to set the temperature. The Qi power transistor operates at or near saturation, but does not require a heatsink to cool the transistor.

Figure 1.4 Thermostat circuit for a low-voltage heater

To control the semistor when the supply voltage passes through zero, a switch on the SN72440 chip from Texas Instruments is used. This microcircuit switches the TRIAC triac (Fig. 1.5), which turns the heating element on or off, providing the necessary heating. The control pulse at the moment the network voltage passes through zero is suppressed or passed under the action of a differential amplifier and a resistance bridge in integrated circuit(IS). Is the width of the serial output pulses at pin 10 of the IC controlled by the potentiometer in the R(trigger) circuit? as shown in the table in Fig. 1.5, and should vary depending on the parameters of the triac used.

Figure 1.5 Thermoregulator on the SN72440 chip

A typical silicon diode with a temperature coefficient of 2 mV/°C can maintain temperature differences of up to ±10°F] with an accuracy of approximately 0.3°F over a wide temperature range. Two diodes connected to the resistance bridge (Fig. 1.6)^ produce a voltage at terminals A and B, which is proportional to the temperature difference. The potentiometer adjusts the bias current, which corresponds to a preset temperature bias region. The low output voltage of the bridge is amplified by the MCI741 operational amplifier from Motorola to 30 V when the input voltage changes by 0.3 mV. A buffer transistor is added to connect the load using a relay.

Figure 1.6 Temperature controller with diode sensor

Temperature on the Fahrenheit scale. To convert temperature from Fahrenheit to Celsius, subtract 32 from the original number and multiply the result by 5/9/

The posistor RV1 (Fig. 1.7) and a combination of variable and constant resistors form a voltage divider coming from a 10-volt Zener diode (zener diode). The voltage from the divider is supplied to the unijunction transistor. During the positive half-wave of the mains voltage, a sawtooth voltage appears on the capacitor, the amplitude of which depends on the temperature and the resistance setting on the 5 kOhm potentiometer. When the amplitude of this voltage reaches the gate voltage of the unijunction transistor, it turns on the thyristor, which supplies voltage to the load. During the negative half-wave of the alternating voltage, the thyristor turns off. If the oven temperature is low, the thyristor opens earlier in the half-wave and produces more heat. If the preset temperature is reached, the thyristor opens later and produces less heat. The circuit is designed for use in applications with an ambient temperature of 100°F.

Figure 1.7 Temperature regulator for bread machine

A simple controller (Fig. 1.8), containing a thermistor bridge and two operational amplifiers, regulates temperature with very high accuracy (up to 0.001 ° C) and a large dynamic range, which is necessary when environmental conditions change rapidly.

Figure 1.8 High accuracy thermostat circuit

The device (Fig. 1.9) consists of a triac and a microcircuit, which includes a DC power supply, a supply voltage zero crossing detector, a differential amplifier, a sawtooth voltage generator and an output amplifier. The device provides synchronous switching on and off of the ohmic load. The control signal is obtained by comparing the voltage received from the temperature-sensitive measuring bridge of resistors R4 and R5 and the NTC resistor R6, as well as resistors R9 and R10 in another circuit. All necessary functions are implemented in the TCA280A microcircuit from Milliard. The values ​​shown are valid for a triac with a control electrode current of 100 mA; for another triac, the values ​​of resistors Rd, Rg and capacitor C1 must change. Proportional control limits can be set by changing the value of resistor R12. When the mains voltage passes through zero, the triac will switch. The sawtooth oscillation period is approximately 30 seconds and can be set by changing the capacitance of capacitor C2.

The simple diagram presented (Fig. 1.10) registers the temperature difference between two objects that require the use of a regulator. For example, to turn on fans, turn off the heater or control water mixer valves. Two inexpensive 1N4001 silicon diodes installed in a resistor bridge are used as sensors. The temperature is proportional to the voltage between the measuring and reference diode, which is supplied to pins 2 and 3 of the MC1791 operational amplifier. Since only about 2 mV/°C comes from the bridge output when the temperature difference occurs, a high-gain operational amplifier is required. If the load requires more than 10 mA, then a buffer transistor is needed.

Figure 1.10 Circuit diagram of a thermostat with a measuring diode

When the temperature drops below the set value, the voltage difference across the measuring bridge with the thermistor is recorded by a differential operational amplifier, which opens the buffer amplifier on transistor Q1 (Fig. 1.11) and the power amplifier on transistor Q2. The power dissipation of transistor Q2 and its load resistor R11 heats the thermostat. Thermistor R4 (1D53 or 1D053 from National Lead) has a nominal resistance of 3600 Ohms at 50 °C. The voltage divider Rl-R2 reduces the input voltage level to the required value and ensures that the thermistor operates at low currents, providing low heating. All bridge circuits, with the exception of resistor R7, designed for precise temperature control, are located in the thermostat design.

Figure 1.11 Diagram of a thermostat with a measuring bridge

The circuit (Fig. 1.12) provides linear temperature control with an accuracy of 0.001 °C, with high power and high efficiency. The AD580's voltage reference powers the temperature transducer bridge circuit, which uses a platinum sense resistor (PLATINUM SENSOR) as a sensor. The AD504 op amp amplifies the bridge output and drives a 2N2907 transistor, which in turn drives a 60 Hz synchronized unijunction transistor oscillator. This generator powers the control electrode of the thyristor through an isolation transformer. Pre-installation contributes to the fact that the thyristor is turned on at various points of the alternating voltage, which is necessary for precise adjustment of the heater. A possible disadvantage is the occurrence of high-frequency interference, since the thyristor switches in the middle of a sine wave.

Figure 1.12 Thyristor thermostat

The power transistor switch control assembly (Figure 1.13) for heating 150-W tools uses a tap on the heating element to force the switch on transistor Q3 and the amplifier on transistor Q2 to saturate and set low power dissipation. When a positive voltage is applied to the input of transistor Qi, transistor Qi turns on and drives transistors Q2 and Q3 into the on state. The collector current of transistor Q2 and the base current of transistor Q3 are determined by resistor R2. The voltage drop across resistor R2 is proportional to the supply voltage, so that the control current is at the optimal level for transistor Q3 over a wide voltage range.

Figure 1.13 Key for low-voltage thermostat

The operational amplifier CA3080A manufactured by RCA (Fig. 1.14) includes together a thermocouple with a switch that is triggered when the supply voltage passes through zero and is made on the CA3079 microcircuit, which serves as a trigger for a triac with an alternating voltage load. The triac must be selected for the regulated load. The supply voltage for the operational amplifier is not critical.

Figure 1.14 Thermocouple thermostat

When using phase control of a triac, the heating current is reduced gradually as the set temperature is approached, which prevents large deviations from the set value. The resistance of resistor R2 (Fig. 1.15) is adjusted so that transistor Q1 is closed at the desired temperature, then the short pulse generator on transistor Q2 does not function and thus the triac no longer opens. If the temperature decreases, the resistance of the RT sensor increases and transistor Q1 opens. Capacitor C1 begins to charge to the opening voltage of transistor Q2, which opens like an avalanche, forming a powerful short pulse that turns on the triac. The more transistor Q1 opens, the faster capacitance C1 charges and the triac switches earlier in each half-wave and, at the same time, more power appears in the load. The dotted line represents an alternative circuit for regulating a motor with a constant load, such as a fan. To operate the circuit in cooling mode, resistors R2 and RT must be swapped.

Figure 1.15 Thermostat for heating

The proportional thermostat (Fig. 1.16) using the LM3911 chip from National sets a constant temperature of the quartz thermostat at 75 ° C with an accuracy of ±0.1 ° C and improves the stability of the quartz oscillator, which is often used in synthesizers and digital meters. The pulse/pause ratio of the rectangular pulse at the output (on/off time ratio) varies depending on the temperature sensor in the IC and the voltage at the inverse input of the microcircuit. Changes in the duration of switching on the microcircuit change the average switching current of the thermostat heating element in such a way that the temperature is brought to a predetermined value. The frequency of the rectangular pulse at the output of the IC is determined by resistor R4 and capacitor C1. The 4N30 optocoupler opens a powerful compound transistor, which has a heating element in the collector circuit. When a positive rectangular pulse is applied to the base of the transistor switch, the latter goes into saturation mode and connects the load, and when the pulse ends, turns it off.

Figure 1.16 Proportional thermostat

The regulator (Fig. 1.17) maintains the temperature of the furnace or bath with high stability at 37.5 °C. The bridge mismatch is captured by the AD605 high common mode rejection, low drift, and balanced input op amp. A composite transistor with combined collectors (Darlington pair) amplify the current of the heating element. The transistor switch (PASS TRANSISTOR) must accept all the power that is not supplied to the heating element. To cope with this, a large tracking circuit is connected between points "A" and "B" to set the transistor to a constant 3V without regard to the voltage required by the heating element. The output of the 741 op amp is compared in the AD301A to the sawtooth voltage, synchronous with the mains voltage with a frequency of 400 Hz. The AD301A chip operates as a pulse-width modulator, including a transistor switch 2N2219-2N6246. The key provides controlled power to a 1000 μF capacitor and a transistor switch (PASS TRANSISTOR) of the thermostat.

Figure 1.17 High altitude thermostat

Schematic diagram A thermostat that is triggered when the mains voltage passes through zero (ZERO-POINT SWITCH) (Fig. 1.18) eliminates electromagnetic interference that occurs during phase control of the load. To accurately regulate the temperature of the electric heating device, proportional switching on/off of the semistor is used. The circuit to the right of the dashed line is a zero-crossing switch that turns on the triac almost immediately after the zero-crossing of each half-wave of the mains voltage. The resistance of resistor R7 is set so that the measuring bridge in the regulator is balanced for the desired temperature. If the temperature is exceeded, the resistance of the posistor RT decreases and transistor Q2 opens, which turns on the control electrode of thyristor Q3. Thyristor Q3 turns on and short-circuits the control electrode signal of triac Q4 and the load turns off. If the temperature drops, transistor Q2 turns off, thyristor Q3 turns off, and full power is applied to the load. Proportional control is achieved by applying a ramp voltage generated by transistor Q1 through resistor R3 on the measuring bridge circuit, and the period of the sawtooth signal is 12 cycles of the mains frequency. From 1 to 12 of these cycles can be inserted into the load and, thus, the power can be modulated from 0-100% in 8% steps.

Figure 1.18 Triac thermostat

The device diagram (Fig. 1.19) allows the operator to set the upper and lower temperature limits for the regulator, which is necessary during long-term thermal tests of material properties. The design of the switch allows for a choice of control methods: from manual to fully automated cycles. Relay K3 contacts control the engine. When the relay is turned on, the motor rotates in the forward direction to increase the temperature. To lower the temperature, the direction of rotation of the motor is reversed. The switching condition of relay K3 depends on which of the limiting relays was turned on last, K\ or K2. The control circuit checks the output of the temperature programmer. This DC input signal will be reduced by resistors and R2 by a maximum of 5 V and amplified by voltage follower A3. The signal is compared in voltage comparators Aj and A2 with a continuously varying reference voltage from 0 to 5 V. The thresholds of the comparators are preset by 10-turn potentiometers R3 and R4. The Qi transistor is turned off if the input signal is lower than the reference signal. If the input signal exceeds the reference signal, then the transistor Qi is cut off and energizes the coil of the relay K, the upper limit value.

Figure 1.19

A pair of National LX5700 temperature transducers (Figure 1.20) provide an output voltage that is proportional to the temperature difference between the two transducers and is used to measure temperature gradients in processes such as cooling fan failure detection, cooling oil movement detection, and observations of other phenomena in cooling systems. With the transmitter in a hot environment (out of coolant or in static air for more than 2 minutes), the 50 ohm potentiometer must be installed so that the output is turned off. Whereas with the converter in a cool environment (in liquid or in moving air for 30 seconds), there should be a position at which the output turns on. These settings overlap, but the final setting ultimately results in a fairly stable regime.

Figure 1.20 Temperature detector circuit

The circuit (Figure 1.21) uses an AD261K high-speed isolated amplifier to precisely control the temperature of a laboratory oven. The multi-band bridge contains 10 ohm to 1 mohm sensors with Kelvin-Varley dividers that are used to preselect the control point. The control point is selected using a 4-position switch. To power the bridge, it is possible to use a non-inverting stabilized amplifier AD741J, which does not allow common-mode voltage error. A 60 Hz passive filter suppresses noise at the input of the AD261K amplifier, which powers the 2N2222A transistor. Next, power is supplied to the Darlington pair and 30 V is supplied to the heating element.

The measuring bridge (Fig. 1.22) is formed by a posistor (a resistor with a positive temperature coefficient) and resistors Rx R4, R5, Re. The signal removed from the bridge is amplified by the CA3046 microcircuit, which in one package contains 2 paired transistors and one separate output transistor. Positive Feedback via resistor R7 prevents ripple if the switching point is reached. Resistor R5 sets the exact switching temperature. If the temperature drops below the set value, the RLA relay turns on. For the opposite function, only the posistor and Rj must be swapped. The value of resistor Rj is selected to approximately achieve the desired adjustment point.

Figure 1.22 Temperature controller with posistor

The regulator circuit (Figure 1.23) adds multiple lead stages to the normally amplified output of National's LX5700 temperature sensor to at least partially compensate for measurement delays. Gain by constant voltage The op amp LM216 will be set to a value of 10 using 10 and 100 mΩ resistors, resulting in a total of 1 V/°C at the op amp output. The output of the op-amp activates an optocoupler, which controls a conventional thermostat.

Figure 1.23 Thermoregulator with optocoupler

The circuit (Fig. 1.24) is used to regulate the temperature in an industrial heating installation that runs on gas and has high thermal power. When the operational amplifier-comparator AD3H switches at the required temperature, the single-vibrator 555 is started, the output signal of which opens the transistor switch, and therefore turns on the gas valve and ignites the burner of the heating system. After a single pulse, the burner turns off, regardless of the state of the op-amp output. The 555 timer's time constant compensates for system delays in which the heat is turned off before the AD590 reaches the switching point. A posistor included in the time-setting circuit of the one-shot 555 compensates for changes in the timer time constant due to changes in ambient temperature. When the power is turned on during the system startup process, the signal generated by the operational amplifier AD741 bypasses the timer and turns on the heating of the heating system, while the circuit has one stable state.

Figure 1.24 Overload Correction

All components of the thermostat are located on the body of the quartz resonator (Fig. 1.25), thus, the maximum power dissipation of the resistors of 2 W serves to maintain the temperature in the quartz. A posistor has a resistance of about 1 kOhm at room temperature. Transistor types are not critical, but should have low leakage currents. The PTC current of approximately 1 mA should be much greater than the 0.1 mA base current of transistor Q1. If you choose a silicon transistor as Q2, then you need to increase the 150-ohm resistance to 680 ohms.

Figure 1.25

The bridge circuit of the regulator (Fig. 1.26) uses a platinum sensor. The signal from the bridge is removed by the operational amplifier AD301, which is included as a differential amplifier-comparator. In a cold state, the resistance of the sensor is less than 500 Ohms, while the output of the operational amplifier comes into saturation and gives a positive signal at the output, which opens a powerful transistor and the heating element begins to heat up. As the element heats up, the resistance of the sensor also increases, which returns the bridge to a state of equilibrium and the heating is turned off. The accuracy reaches 0.01 °C.

Figure 1.26 Temperature controller on the comparator

The Russian winter is distinguished by its severity and extreme cold, as everyone knows. Therefore, the rooms in which people are located must be heated. Central heating is the most common option, and if this is not available, you can use an individual gas boiler. However, it often happens that neither one nor the other is accessible, for example, in an open field there is a small room of a water pumping station, in which drivers are on duty around the clock. This could be a room in some large uninhabited building or a guard tower. There are plenty of examples.

Way out

All these cases force the installation of electric heating. For small rooms, it is quite possible to get by with a conventional electric oil radiator, but in large rooms, water heating using a radiator is most often installed. If you do not monitor the temperature of the water, sooner or later it may boil, causing the entire boiler to fail. To protect against such cases, thermostats are used.

Device Features

In functional terms, the device can be divided into several separate units: a comparator, as well as load control devices. All these parts will be described below. This information is necessary in order to make a thermostat with your own hands. In this case, a design is proposed in which a conventional bipolar transistor serves as a temperature sensor, so that the use of thermistors can be eliminated. This sensor operates on the basis that the parameters of the transistors of all semiconductor devices depend to a greater extent on the temperature of the medium.

Important nuances

Creating a thermostat with your own hands must take into account two points. Firstly, we are talking about the tendency automatic devices to autogeneration. In the case when the thermal relay is installed between the actuator and the sensor too strong connection, after triggering, the relay immediately turns off and then turns on again. This will occur in cases where the sensor is in close proximity to a cooler or heater. Secondly, all sensors and electronic devices have a certain accuracy. For example, you can track a temperature of 1 degree, but smaller values ​​are much more difficult to track. In this case, simple electronics often begin to make mistakes and make mutually exclusive decisions, especially when the temperature is almost equal to the one set for operation.

Process of creation

If we talk about how to make a thermostat with your own hands, then it is worth saying that the sensor here is a thermistor that reduces its resistance during the heating process. It is connected to the voltage divider circuit. R2 is also included in the circuit, through which the response temperature is set. From the divider, the voltage is supplied to the 2I-NOT element, which is switched on in inverter mode, and then to the base of the transistor, which serves as a discharge gap for capacitor C1. It, in turn, is connected to the input (S) of the RS flip-flop, which is assembled on a pair of elements, as well as to the input of another 2I-NOT. From the divider, the voltage is supplied to the 2I-NOT input, which controls the second input (R) of the RS flip-flop.

How it works

So, we're looking at how to create a simple DIY thermostat, so it's important to understand how it works in different situations. At high temperatures, thermistors are characterized by low voltage, so there is a voltage perceived on the divider logic circuits like zero. In this case, the transistor is open, a logical zero is perceived at the input of the S-trigger, and capacitor C1 is discharged. The output of the trigger is set to a logical one. The relay is in the on mode, and transistor VT2 is open. To understand exactly how to make a thermostat, it is worth noting that this particular relay implementation is focused on cooling the object, that is, it turns on the fan when the temperature is high.

Temperature drop

When the temperature decreases, the resistance of the thermistor increases, which leads to an increase in the voltage across the divider. At a certain moment, transistor VT1 closes, after which capacitor C1 begins charging through R5. Eventually the moment comes to reach the logical one level. It is this that is supplied to one of the inputs of D4, and the voltage from the divider is supplied to the second input of this element. When both inputs are set to logical ones, and a zero appears at the output of the element, the trigger switches to the opposite state. In this case, the relay will be turned off, which will allow you to turn off the fan, if necessary, or turn on the heating. This way you can make a thermostat so that it turns the fan on and off when necessary.

Temperature rise

So, the temperature began to increase again. A zero on the divider will first appear at one of the inputs of D4, and it will remove the zero at the trigger input, changing it to one. Further, as the temperature increases, zero will appear on the inverter. After changing it to one, the transistor will open, which will lead to the discharge of element C1 and the setting of zero at the input of the trigger, which turns off the heating of the coolant in the water heating system or turns on the fan. These handmade ones work quite effectively.

Blocks C1, R5 and VT1 are designed to eliminate self-generation, due to the fact that a turn-off delay time is set on them. It can range from a few seconds to several minutes. We are considering a fairly simple thermostat, created with our own hands, so the above unit also allows us to eliminate the bounce of the temperature sensor. Even with a very small very first pulse, the transistor opens and the capacitor instantly discharges. The chatter will then be ignored. When the transistor closes, the situation repeats. Charging of the capacitor begins only after the completion of the last bounce pulse. Thanks to the introduction of a trigger into the circuit, it is possible to ensure maximum clarity of relay operation. As you know, a trigger can only have two positions.

Assembly

To make a thermostat with your own hands, you can use a special circuit board on which the entire circuit will be assembled using a hinged method. Can also be used printed circuit board. You can use any power within the range of 3-15 volts. Relays should be selected according to this.

Using a similar scheme, you can make a thermostat for an aquarium with your own hands, but please note that it must be attached to the glass from the outside, then there will be no problems with its use.

The relay described above demonstrated very high reliability during operation. The temperature is maintained to within a fraction of a degree. However, it is directly dependent on the time delay determined by the R5C1 circuit, as well as the response to operation, that is, the power of the cooler or heater. The temperature range and the accuracy of its setting are determined by the selection of divider resistors. If you made such a thermostat with your own hands, then it does not need adjustment, but starts working immediately.