Simple circuits of electronic thermostats with your own hands. Simple DIY electronic thermostat Homemade thermostat

Andrey, perhaps the whole problem is in the KU208G triac. 127V is obtained from the fact that the triac skips one of the half-cycles of the mains voltage. Try replacing it with an imported BTA16-600 (16A, 600V), they work more stable. It’s not a problem to buy a BTA16-600 now, and it’s not expensive.

sta9111, to answer this question you will have to remember how our thermostat works. Here is a paragraph from the article: “The voltage at control electrode 1 is set using a divider R1, R2 and R4. A thermistor with negative TCR is used as R4, so when heated its resistance decreases. When the voltage at pin 1 is above 2.5V, the microcircuit is open, the relay is turned on.”

In other words, at the desired temperature, in your case 220 degrees, thermistor R4 should. The voltage drop is 2.5V, let's denote it as U_2.5V. The rating of your thermistor is 1KOhm - this is at a temperature of 25 degrees. This is the temperature indicated in the reference books.

Reference book on thermistors msevm.com/data/trez/index.htm

Here you can see the operating temperature range and TKS: for a temperature of 220 degrees, little is suitable.

The characteristic of semiconductor thermistors is non-linear, as shown in the figure.

Drawing. Volt-ampere characteristics of the thermistor - website/vat.jpg

Unfortunately, the type of your thermistor is unknown, so we will assume that you have an MMT-4 thermistor.

According to the graph, it turns out that at 25 degrees the resistance of the thermistor is exactly 1KOhm. At a temperature of 150 degrees, the resistance drops to approximately 300 Ohms; it is simply impossible to determine more precisely from this graph. Let's denote this resistance as R4_150.

Thus, it turns out that the current through the thermistor will be (Ohm’s law) I= U_2.5V/ R4_150 = 2.5/300 = 0.0083A = 8.3mA. This is at a temperature of 150 degrees, it seems that everything is clear so far, and there seem to be no errors in the reasoning. Let's continue further.

With a supply voltage of 12V, it turns out that the resistance of the circuit R1, R2 and R4 will be 12V/8.3mA=1.445KOhm or 1445Ohm. Subtracting R4_150, it turns out that the sum of the resistances of resistors R1 + R2 will be 1445-300 = 1145 Ohms, or 1.145 KOhms. Thus, you can use a tuning resistor R1 1KOhm, and a limiting resistor R2 470Ohm. This is how the calculation turns out.

This would be all well and good, but few thermistors are designed to operate at temperatures up to 300 degrees. Thermistors ST1-18 and ST1-19 are most suitable for this range. See reference msevm.com/data/trez/index.htm

Thus, it turns out that this thermostat will not provide temperature stabilization at 220 degrees or higher, since it is designed for the use of semiconductor thermistors. You will have to look for a circuit with metal thermal resistances TSM or TSP.

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-acting 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, operating 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 fine tuning is necessary, 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:

• The assembly of such a device is accessible only 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.
• Purchasing the 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.

Required 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 instructions

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 previously 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 set up

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 poor quality parts. 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. Most 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.

Thermostats are widely used in modern appliances, automobiles, heating and air conditioning systems, manufacturing, refrigeration and furnace applications. The operating principle of any thermostat is based on turning on or off various devices after reaching certain temperature values.

Modern digital thermostats are controlled using buttons: touch or regular. Many models also come with a digital panel that displays the set temperature. The group of programmable thermostats is the most expensive. Using the device, you can provide for temperature changes hourly or set the required mode for a week in advance. The device can be controlled remotely: via a smartphone or computer.

For a complex technological process, for example, a steel-smelting furnace, making a thermostat with your own hands is a rather difficult task, which requires serious knowledge. But any home craftsman can assemble a small device for a cooler or incubator.

In order to understand how a temperature controller works, consider a simple device that is used to open and close the damper of a mine boiler and is activated when the air is heated.

To operate the device, 2 aluminum pipes, 2 levers, a return spring, a chain that goes to the boiler, and an adjustment unit in the form of a faucet axle box were used. All components were installed on the boiler.

As is known, the coefficient of linear thermal expansion of aluminum is 22x10-6 0C. When an aluminum pipe with a length of one and a half meters, a width of 0.02 m and a thickness of 0.01 m is heated to 130 degrees Celsius, an elongation of 4.29 mm occurs. When heated, the pipes expand, causing the levers to shift and the damper to close. When cooling, the pipes decrease in length, and the levers open the damper. The main problem when using this scheme is that it is very difficult to accurately determine the response threshold of the thermostat. Today, preference is given to devices based on electronic elements.

Scheme of operation of a simple thermostat

Typically, relay-based circuits are used to maintain a set temperature. The main elements included in this equipment are:

• temperature sensor;
• threshold circuit;
• actuator or indicator device.

Semiconductor elements, thermistors, resistance thermometers, thermocouples and bimetallic thermal relays can be used as sensors.

The thermostat circuit reacts when the parameter exceeds a given level and turns on the actuator. The simplest version of such a device is an element based on bipolar transistors. The thermal relay is based on a Schmidt trigger. A thermistor acts as a temperature sensor - an element whose resistance changes depending on the increase or decrease in degrees.

R1 is a potentiometer that sets the initial offset on thermistor R2 and potentiometer R3. Due to the adjustment, the actuator is activated and relay K1 is switched when the resistance of the thermistor changes. In this case, the operating voltage of the relay must correspond to the operating power supply of the equipment. To protect the output transistor from voltage surges, a semiconductor diode is connected in parallel. The load value of the connected element depends on the maximum current of the electromagnetic relay.

Attention! On the Internet you can see pictures with thermostat drawings for various equipment. But quite often the image and description do not correspond to each other. Sometimes the pictures may simply show other devices. Therefore, production can begin only after carefully studying all the information.

Before starting work, you should decide on the power of the future thermostat and the temperature range in which it will operate. The refrigerator will require some elements, and the heating will require others.

Three element thermostat

One of the elementary devices, using an example of which you can assemble and understand the principle of operation, is a simple do-it-yourself thermostat designed for a fan in a PC. All work is done on a breadboard. If there are problems with the pin, then you can take a solderless board.

The thermostat circuit in this case consists of only three elements:

• power MOSFET transistor (N channel), you can use IRFZ24N MOSFET 12 V and 10 A or IFR510 Power MOSFET;
• potentiometer 10 kOhm;
• NTC thermistor 10 kOhm, which will act as a temperature sensor.

The temperature sensor reacts to an increase in degrees, due to which the entire circuit is activated and the fan turns on.

Now let's move on to the setup. To do this, turn on the computer and adjust the potentiometer, setting the value for the fan turned off. At the moment when the temperature approaches critical, we reduce the resistance as much as possible before the blades rotate very slowly. It is better to do the setup several times to make sure the equipment is working effectively.

The modern electronics industry offers elements and microcircuits that differ significantly in appearance and technical characteristics. Each resistance or relay has several analogues. It is not necessary to use only those elements that are indicated in the diagram; you can take others that match the parameters of the samples.

Thermostats for heating boilers

When adjusting heating systems, it is important to accurately calibrate the device. To do this you will need a voltage and current meter. To create a working system, you can use the following diagram.

Using this scheme, you can create external equipment for monitoring a solid fuel boiler. The role of the zener diode here is performed by the K561LA7 microcircuit. The operation of the device is based on the ability of a thermistor to reduce resistance when heated. The resistor is connected to the electricity voltage divider network. The required temperature can be set using variable resistor R2. The voltage is supplied to the 2I-NOT inverter. The resulting current is supplied to capacitor C1. A capacitor is connected to 2I-NOT, which controls the operation of one trigger. The latter is connected to the second trigger.

Temperature control proceeds according to the following scheme:

• as the degrees drop, the voltage in the relay increases;
• When a certain value is reached, the fan connected to the relay turns off.

It is better to solder on a mole rat. As a battery, you can take any device operating within 3-15 V.

Carefully! Installing homemade devices for any purpose on heating systems can lead to equipment failure. Moreover, the use of such devices may be prohibited at the level of services providing communications in your home.

Digital thermostat

In order to create a fully functioning thermostat with accurate calibration, you cannot do without digital elements. Consider a device for monitoring temperatures in a small storage area for vegetables.

The main element here is the PIC16F628A microcontroller. This chip provides control of various electronic devices. The PIC16F628A microcontroller contains 2 analog comparators, an internal oscillator, 3 timers, CCP comparison modules and USART data transfer exchange modules.

When the thermostat is operating, the value of the existing and set temperature is supplied to MT30361 - a three-digit indicator with a common cathode. In order to set the required temperature, use the following buttons: SB1 – to decrease and SB2 – to increase. If you carry out the adjustment while simultaneously pressing the SB3 button, you can set the hysteresis values. The minimum hysteresis value for this circuit is 1 degree. A detailed drawing can be seen on the plan.

When creating any of the devices, it is important not only to correctly solder the circuit itself, but also to think about how best to place the equipment. It is necessary that the board itself is protected from moisture and dust, otherwise short circuits and failure of individual elements cannot be avoided. You should also take care to insulate all contacts.

Video

THERMOREGULATOR DIAGRAMS

There are 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 (Fig. 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 an electrical network with a voltage of 115 V. When connecting the device to a network with a voltage of 220 V, it is necessary to use another supply transformer and a semistor.

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 regulate 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 an integrated circuit (IC). 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 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 approximately 2 mV/°C is supplied 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 converter 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-setting ensures that the thyristor is turned on at various points in 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 given 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, disconnects it.

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 2N2219-2N6246 transistor switch. The switch provides controlled power to a 1000 μF capacitor and a transistor switch (PASS TRANSISTOR) of the thermostat.

Figure 1.17 High altitude thermostat

The schematic diagram of 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 network 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 steps of 8%.

Figure 1.18 Triac thermostat

The device diagram (Fig. 1.19) allows the operator to set 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 5V 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. The DC voltage gain of the LM216 op amp will be set to 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 lights 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 2 W resistors 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 posistor 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

A simple DIY electronic thermostat. I propose a method for making a homemade thermostat to maintain a comfortable room temperature in cold weather. The thermostat allows you to switch power up to 3.6 kW. The most important part of any amateur radio design is the housing. A beautiful and reliable case will ensure a long life for any homemade device. The version of the thermostat shown below uses a convenient, small-sized case and all the power electronics from an electronic timer sold in stores. The homemade electronic part is built on the LM311 comparator microcircuit.

Description of the circuit operation

The temperature sensor is a thermistor R1 with a nominal value of 150k, type MMT-1. Sensor R1 together with resistors R2, R3, R4 and R5 form a measuring bridge. Capacitors C1-C3 are installed to suppress interference. Variable resistor R3 balances the bridge, that is, it sets the temperature.

If the temperature of temperature sensor R1 drops below the set value, its resistance will increase. The voltage at input 2 of the LM311 microcircuit will become greater than at input 3. The comparator will work and its output 4 will set to a high level, the voltage applied to the electronic timer circuit through the HL1 LED will cause the relay to operate and turn on the heating device. At the same time, the HL1 LED will light up, indicating that the heating is turned on. Resistance R6 creates negative feedback between output 7 and input 2. This allows you to set hysteresis, that is, the heating turns on at a temperature lower than it turns off. Power is supplied to the board from the electronic timer circuit. Resistor R1 placed outside requires careful insulation, since the thermostat’s power supply is transformerless and has no galvanic isolation from the network, that is dangerous mains voltage is present on the device elements. The procedure for manufacturing the thermostat and how the thermistor is insulated is shown below.

How to make a thermostat with your own hands

1. The donor of the housing and power circuit is opened - the CDT-1G electronic timer. A timer microcontroller is installed on a gray three-wire cable. Unsolder the cable from the board. The holes for the cable wires are marked (+) - +5 Volt power supply, (O) - control signal supply, (-) - minus power supply. An electromagnetic relay will switch the load.

2. Since the power supply to the circuit from the power unit is not galvanically isolated from the network, all work on checking and setting up the circuit is carried out from a safe 5 volt power source. First, we check the functionality of the circuit elements at the stand.

3. After checking the circuit elements, the design is assembled on the board. The board for the device was not developed and was assembled on a piece of a breadboard. After assembly, a performance check is also carried out on the stand.

4. Thermal sensor R1 is installed externally on the side surface of the socket housing; the conductors are insulated with heat-shrinkable tubing. To prevent contact with the sensor, but also to maintain access of outside air to the sensor, a protective tube is installed on top. The tube is made from the middle part of a ballpoint pen. A hole is cut in the tube for installation on the sensor. The tube is glued to the body.

5. Variable resistor R3 is installed on the top cover of the case, and there is also a hole for the LED. It is useful to cover the resistor body with a layer of electrical tape for safety.

6. The adjustment knob for resistor R3 is homemade and made with your own hands from an old toothbrush of a suitable shape :).

Resistor R3