Homemade electronic temperature controller. How to assemble a thermostat at home

It is used in many technological processes, including domestic heating systems. The factor determining the operation of the thermostat is the outside temperature, the value of which is analyzed and when the set limit is reached, the flow rate is reduced or increased.

Temperature controllers come in various designs and today there are a lot of industrial versions on sale that work according to different principles and are intended for use in different areas. Also available are the simplest electronic circuits, which anyone can assemble with the appropriate knowledge of electronics.

Description

The thermostat is a device installed in power supply systems and allows you to optimize the energy consumption for heating. The main elements of the thermostat:

Temperature sensors- control the temperature level by generating electrical impulses of the appropriate size.
Analytical block– processes the electrical signals coming from the sensors and converts the temperature value into a value characterizing the position of the executive body.
Executive agency– regulates the feed by the amount indicated by the analytical unit.

A modern thermostat is a microcircuit based on diodes, triodes or a zener diode that can convert heat energy into electrical energy. Both in industrial and home-made versions, this single block, to which the thermocouple is connected, remote or located here. The thermostat is switched on in series electrical circuit power supply of the executing body, thus reducing or increasing the value of the supply voltage.

Principle of operation

The temperature sensor delivers electrical impulses, the current value of which depends on the temperature level. The inherent ratio of these values allows the device to very accurately determine the temperature threshold and decide, for example, how many degrees the air supply damper to the solid fuel boiler should be opened, or the hot water supply damper should be open. The essence of the operation of the thermostat is to convert one value to another and correlate the result with the current level.

Simple homemade regulators, as a rule, have a mechanical control in the form of a resistor, by moving which, the user sets the required temperature threshold, that is, indicating at what outside temperature it will be necessary to increase the supply. With more advanced functionality, industrial devices can be programmed to wider limits, using a controller, depending on various temperature ranges. They do not have mechanical controls, which contributes to long work.

How to DIY

Self-made regulators are widely used in domestic conditions, especially since the necessary electronic parts and circuits can always be found. Heating the water in the aquarium, turning on the ventilation of the room when the temperature rises, and many other simple technological operations can be completely shifted to such automation.

Schemes of autoregulators

At present, lovers homemade electronics, two automatic control schemes are popular:

Based on an adjustable zener diode type TL431 - the principle of operation is to fix the excess voltage threshold of 2.5 volts. When it is broken on the control electrode, the zener diode comes into the open position and a load current passes through it. In the event that the voltage does not break through the threshold of 2.5 volts, the circuit comes into the closed position and disconnects the load. The advantage of the circuit is its extreme simplicity and high reliability, since the zener diode is equipped with only one input for supplying an adjustable voltage.
A thyristor microcircuit of the K561LA7 type, or its modern foreign counterpart CD4011B - the main element is the thyristor T122 or KU202, which acts as a powerful switching link. The current consumed by the circuit in normal mode does not exceed 5 mA, at a resistor temperature of 60 to 70 degrees. The transistor comes into the open position when pulses are received, which in turn is a signal to open the thyristor. In the absence of a radiator, the latter acquires throughput up to 200 W. To increase this threshold, you will need to install a more powerful thyristor, or equip an existing radiator, which will increase the switching capacity to 1 kW.

Necessary materials and tools

Assembling it yourself will not take much time, but some knowledge in the field of electronics and electrical engineering, as well as experience with a soldering iron, will definitely be required. To work, you need the following:

Soldering iron pulse or conventional with a thin heating element.
Printed circuit board.
Solder and flux.
Acid for etching tracks.
Electronic parts according to the selected scheme.

Thermostat circuit

Walkthrough

Electronic elements must be placed on the board in such a way that they can be easily mounted without touching the neighboring parts with a soldering iron, near the parts that actively generate heat, the distance is made somewhat larger.
The tracks between the elements are etched according to the drawing, if there is none, then a sketch is first made on paper.
It is imperative to check the performance of each element, and only after that the landing on the board is performed, followed by soldering to the tracks.
It is necessary to check the polarity of diodes, triodes and other parts in accordance with the diagram.
It is not recommended to use acid for soldering radio components, since it can short-circuit nearby adjacent tracks, for insulation, rosin is added to the space between them.
After assembly, the device is adjusted by selecting the optimal resistor for the most accurate threshold for opening and closing the thyristor.

Scope of homemade thermostats

In everyday life, the use of a thermostat is most often found among summer residents who operate home-made incubators, and as practice shows, they are no less effective than factory models. In fact, such a device can be used wherever it is necessary to perform some actions depending on the temperature readings. Similarly, it is possible to equip the lawn spraying or watering system, the extension of light-shielding structures, or simply sound or light alarms that warn of something with automation.

DIY repair

Assembled by hand, these devices last a long time, but there are several standard situations when repairs may be required:

Failure of the adjusting resistor - happens most often, since the copper tracks wear out, inside the element along which the electrode slides, it is solved by replacing the part.
Overheating of the thyristor or triode - the power was incorrectly selected or the device is located in a poorly ventilated area of \u200b\u200bthe room. In order to avoid this in the future, thyristors are equipped with radiators, or the thermostat should be moved to a zone with a neutral microclimate, which is especially important for wet rooms.
Incorrect temperature control - possible damage to the thermistor, corrosion or dirt on the measuring electrodes.

Advantages and disadvantages

Undoubtedly, the use of automatic control is already an advantage in itself, since the energy consumer receives such opportunities:

Saving energy resources.
Constant comfortable room temperature.
No human involvement required.

Automatic control has found particularly great application in the heating systems of apartment buildings. Inlet valves equipped with thermostats automatically control the supply of heat carrier, thanks to which residents receive significantly lower bills.

The disadvantage of such a device can be considered its cost, which, however, does not apply to those that are made by hand. Only industrial devices designed to control the supply of liquid and gaseous media are expensive, since the actuator includes a special motor and other valves.

Although the device itself is quite undemanding to operating conditions, the accuracy of response depends on the quality of the primary signal, and this especially applies to automation operating in conditions of high humidity or in contact with aggressive media. Thermal sensors in such cases should not come into direct contact with the coolant.

The leads are placed in a brass sleeve and hermetically sealed with epoxy glue. You can leave the end of the thermistor on the surface, which will contribute to greater sensitivity.

SCHEMES OF THERMOREGLATORS

There are a large number of electrical circuit diagrams that can maintain the desired set temperature to within 0.0000033°C. These schemes include offset temperature correction, proportional, integral and differential control.
The hotplate regulator (Figure 1.1) uses an Allied Electronics K600A thermistor (Positive Temperature Coefficient Thermistor, or TCR) built into the cooker to maintain the ideal cooking temperature. The potentiometer can be used to regulate the start of the seven-storey controller and, accordingly, turn the heating element on or off. The device is designed to operate in electrical network with a voltage of 115 V. When the device is connected to a 220 V network, it is necessary to use another supply transformer and a seven-storer.

Figure 1.1 Electric stove temperature controller

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 setting the resistor R2 (Fig. 1.2), the temperature regulated by the posistor R1 is set. Thyristor Q2 is selected based on the connected load in terms of power and voltage. Diode D3 is defined for a voltage of 200 V. Resistors R12, R13 and diode D2 control the thyristor when the supply voltage passes through zero.

Figure 1.2 Dosing heater power controller

A simple circuit (Fig. 1.3) with a switch when the supply voltage crosses 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 to control the electric or gas furnace. Thyristor switching occurs at low currents. The measuring resistor NTC SENSOR has a negative temperature coefficient. Resistor Rp sets the desired temperature.

Figure 1.3 Scheme of a temperature controller with load switching when the power goes through zero.

The device (Fig. 1.4) provides proportional control of the temperature of a small low-power furnace with an accuracy of 1 ° C relative to the temperature set using a potentiometer. The circuit uses an 823V voltage regulator, which is powered by the same 28V supply as the oven. A 10-turn wire-wound potentiometer must be used to set the temperature. The powerful Qi transistor operates in or close to saturation, but no heatsink is needed to cool the transistor.

Figure 1.4 Schematic diagram of a thermostat for a low-voltage heater

To control the sevenstor 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), turning on or off the heating element, providing the necessary heating. The control pulse at the moment the mains voltage passes through zero is suppressed or passed under the action of a differential amplifier and a resistance bridge in an integrated circuit (IC). The width of the serial output pulses on pin 10 of the IC is controlled by a potentiometer in the trigger circuit R(trigger)? as shown in the table in Fig. 1.5, and should vary depending on the parameters of the triac used.

Figure 1.5 Temperature controller on the SN72440 chip

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

Figure 1.6 Temperature controller with diode sensor

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

The RV1 posistor (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 applied 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 potentiometer with a nominal value of 5 kOhm. When the amplitude of this voltage reaches the turn-off voltage of the unijunction transistor, it turns on the thyristor, which supplies voltage to the load. During the negative half-wave of the AC voltage, the thyristor turns off. If the furnace temperature is low, then the thyristor opens a half-wave earlier and produces more heat. If the preset temperature is reached, the thyristor opens later and generates less heat. The circuit is designed for use in devices with an ambient temperature of 100 °F.

Figure 1.7 Thermostat for bread machine

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

Figure 1.8 Scheme of a high-precision thermostat

The device (Fig. 1.9) consists of a triac and a microcircuit, which includes a power source direct current, a 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 resistive load. The control signal is obtained by comparing the voltage obtained from the temperature-sensitive measuring bridge of resistors R4 and R5 and NTC resistor R6 and resistors R9 and R10 in another circuit. All the necessary functions are implemented in the TCA280A chip from Milliard. The values shown are valid for a triac with a control electrode current of 100 mA, for another triac, the values \u200b\u200bof the resistors Rd, Rg and capacitor C1 must be changed. 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.

Presented simple circuit(Fig. 1.10) registers the temperature difference between two objects that require the use of a regulator. For example, to turn on the fans, turn off the heater or to control the valves of the water faucets. Two inexpensive 1N4001 silicon diodes mounted in a resistor bridge are used as sensors. The temperature is proportional to the voltage between the sense and reference diodes, which is applied to pins 2 and 3 of the MC1791 op amp. Since the output of the bridge is only about 2 mV/°C at the temperature difference, a high gain op amp is required. If the load requires more than 10 mA, then a buffer transistor is required.

Figure 1.10 Scheme of a temperature controller with a measuring diode

When the temperature drops below the set value, the voltage difference on the measuring bridge with the thermistor is recorded by a differential operational amplifier, which opens the buffer amplifier on the transistor Q1 (Fig. 1.11) and the power amplifier on the transistor Q2. The power dissipation of transistor Q2 and its load resistor R11 heat the thermostat. R4 thermistor (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 the resistor R7, designed for precise temperature control, are in the thermostat design.

Figure 1.11 Scheme 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 voltage reference on the AD580 chip powers the bridge circuit of the temperature converter, in which the platinum measuring resistor (PLATINUM SENSOR) acts as a sensor. The AD504 op amp amplifies the output of the bridge and drives a 2N2907 transistor, which in turn drives a 60 Hz clocked unijunction transistor oscillator. This generator feeds the control electrode of the thyristor through an isolation transformer. Pre-setting ensures that the thyristor turns on at various points of the AC voltage, which is necessary for accurate adjustment of the heater. A possible disadvantage is the occurrence of high frequency interference, since the thyristor switches in the middle of a sinusoid.

Figure 1.12 Thyristor thermostat

The power transistor switch control assembly (Figure 1.13) for heating 150 W instruments 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 causes transistors Q2 and Q3 to turn on. The collector current of transistor Q2 and the base current of transistor Q3 are determined by resistor R2. The voltage drop across R2 is proportional to the supply voltage, so that the drive current is optimal for Q3 over a wide voltage range.

Figure 1.13 Key for low voltage thermostat

The CA3080A operational amplifier 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 AC voltage load. The triac must be selected Under the adjustable load. The supply voltage for the operational amplifier is non-critical.

Figure 1.14 Temperature controller on a thermocouple

When using triac phase control, the heating current is reduced gradually if the set temperature is approached, which prevents a large deviation from the set value. The resistance of the resistor R2 (Fig. 1.15) is adjusted so that the transistor Q1 is closed at the desired temperature, then the short pulse generator on the transistor Q2 does not function and thus the triac no longer opens. If the temperature drops, then the resistance of the sensor RT increases and the transistor Q1 turns on. Capacitor C1 begins to charge up to the opening voltage of transistor Q2, which opens like an avalanche, forming a powerful short pulse that turns on the triac. The more the transistor Q1 opens, the faster the capacitance C1 is charged 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 controlling a motor with a constant load, such as with a fan. To operate the circuit in cooling mode, resistors R2 and RT must be swapped.

Figure 1.15 Heating thermostat

The proportional thermostat (Fig. 1.16) using the LM3911 chip from National, sets the constant temperature of the quartz thermostat at 75 ° C with an accuracy of ± 0.1 ° C and improves the stability of the crystal oscillator, which is often used in synthesizers and digital counters. Pulse/pause ratio 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 on-time of the microcircuit change the average turn-on current of the thermostat heating element in such a way that the temperature is brought to the set value. The frequency of the rectangular pulse at the output of the IC is determined by the resistor R4 and the capacitor C1. The 4N30 optocoupler opens a powerful composite transistor, which has a heating element in the collector circuit. During the supply of a positive rectangular pulse to the base of the transistor switch, the latter goes into saturation mode and connects the load, and at the end of the pulse 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. Sense bridge error is captured by the AD605 op-amp with high common-mode rejection, low drift, and balanced inputs. A composite transistor with combined collectors (Darlington pair) amplifies the current of the heating element. The PASS TRANSISTOR must accept all power that is not supplied to the heating element. To deal with this, a large servo circuit is connected between points "A" and "B" to set a constant 3V across the transistor, regardless of the voltage required by the heating element. The output of the 741 op amp is compared in the AD301A to a sawtooth voltage, The AD301A acts as a Pulse Width Modulator, including a 2N2219-2N6246 transistor switch that provides controlled power to a 1000 µF capacitor and a thermostatic PASS TRANSISTOR.

Figure 1.17 High Precision Thermostat

circuit diagram thermostat, which operates 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 control the temperature of the electric heater, a proportional on/off sevenstor is used. The circuit, to the right of the dashed line, is a switch that operates when the supply voltage passes through zero, which turns on the triac almost immediately after passing through zero of each half-wave of the mains voltage. The resistance of the resistor R7 is set so that the measuring bridge in the regulator is balanced for the desired temperature. If the temperature is exceeded, then the resistance of the thermistor RT decreases and the transistor Q2 opens, which turns on the control electrode of the thyristor Q3. Thyristor Q3 turns on and short-circuits the gate signal of triac Q4 and the load is turned off. If the temperature drops, transistor Q2 closes, thyristor Q3 turns off, and full power is supplied to the load. Proportional control is achieved by applying a sawtooth voltage generated by transistor Q1 through resistor R3 to the circuit of the measuring bridge, and the period of the sawtooth signal is immediately 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 steps of 8%.

Figure 1.18 Triac thermostat

The scheme of the device (Fig. 1.19) allows the operator to set the upper and lower temperature limits for the regulator, which is necessary during long-term thermal testing of material properties. The design of the switch allows for a choice of control methods: from manual to fully automated cycles. With the help of relay contacts K3, the motor is controlled. 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 condition for switching relay K3 depends on which of the limiting relays was switched 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 comparator thresholds are pre-set by 10-turn potentiometers R3 and R4. Transistor Qi is closed if the input signal is below the reference signal. If the input signal exceeds the reference signal, then the transistor Qi opens and energizes the relay coil K, the upper limit value.

Figure 1.19

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

Figure 1.20 Schematic of temperature detector

The circuit in Figure 1.21 uses an AD261K high-speed isolated amplifier to control the temperature of a laboratory oven with high precision. The multirange bridge contains 10 Ω to 1 mΩ Kelvin-Varley divider sensors that are used to preselect the control point. The choice of the point of control is carried out using a 4-position switch. The bridge can be powered by the AD741J non-inverting stabilizing amplifier, which does not allow common-mode voltage error. A 60Hz passive filter suppresses noise at the input of the AD261K amplifier that 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 taken from the bridge is amplified by the CA3046 microcircuit, which contains 2 paired transistors and one separate output transistor in one package. 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, then the RLA relay turns on. For the opposite function, only the posistor and Rj should be interchanged. The value of the resistor Rj is chosen to approximately reach the desired adjustment point.

Figure 1.22 Temperature controller with PTC

The controller circuit (Figure 1.23) adds many stages of leading signal to the normally amplified output of National's LX5700 temperature sensor to at least partially compensate for measurement delays. Gain by constant voltage The LM216 op amp will be set to 10 with 10 and 100 mΩ resistors, resulting in 1 V/°C at the op amp output. The output of the op-amp activates an optocoupler that drives a conventional thermostat.

Figure 1.23 Temperature controller with optocoupler

The circuit (Fig. 1.24) is used to control the temperature in a gas-fired industrial heating installation with a high heat output. When the AD3H op-amp-comparator switches at the required temperature, the 555 single vibrator is started, the output of which opens the transistor switch, and therefore turns on the gas valve and ignites the heating system burner. After single impulse the burner turns off regardless of the state of the output of the op amp. The 555 timer time constant compensates for delays in the system where the heat is turned off before the AD590 reaches the switch point. The 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 start-up process, the signal generated by the AD741 operational amplifier 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), so the maximum power dissipation of 2 W resistors is used to maintain the temperature in the quartz. The posistor has a resistance of about 1 kOhm at room temperature. Transistor types are not critical, but should have low leakage currents. The thermistor current from about 1 mA should be much greater than the base current of 0.1 mA 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 taken by the AD301 operational amplifier, which is included as a differential comparator amplifier. In a cold state, the sensor resistance is less than 500 ohms, while the output of the operational amplifier saturates and gives a positive signal at the output, which opens a powerful transistor and the heating element starts to heat up. As the element heats up, the resistance of the sensor also increases, which returns the bridge to the balancing state, and the heating is turned off. The accuracy reaches 0.01 °C.

Figure 1.26 Temperature controller on the comparator

Andrew, perhaps the whole problem is in the triac KU208G. 127V is obtained from the fact that the triac passes one of the half-cycles of the mains voltage. Try to replace it with imported BTA16-600 (16A, 600V), they work more stable. BTA16-600 is not a problem to buy now, and it is 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 the control electrode 1 is set using the divider R1, R2 and R4. As R4, a thermistor with a negative TCR is used, therefore, when heated, its resistance decreases. When the voltage at pin 1 is higher than 2.5V, the microcircuit is open, the relay is turned on.

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

Thermistor Handbook msevm.com/data/trez/index.htm

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

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

Picture. Volt-ampere characteristic of the thermistor - site/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 just 1KΩ. At a temperature of 150 degrees, the resistance drops to about 300 ohms, it is simply impossible to determine more precisely from this graph. Let's designate this resistance as R4_150.

Thus, it turns out that the current through the thermistor will be (Ohm's law) I \u003d U_2.5V / R4_150 \u003d 2.5 / 300 \u003d 0.0083A \u003d 8.3mA. This is at a temperature of 150 degrees, it seems, so far everything is clear, and there seem to be no errors in 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 = 1.445KΩ or 1445Ω. Minus R4_150, it turns out that the sum of the resistances of the resistors R1 + R2 will be 1445-300 = 1145 Ohm, or 1.145 KOhm. Thus, it is possible to apply a tuning resistor R1 1KΩ, and a limiting resistor R2 470Ω. Here is the calculation.

All this is good, only a few thermistors are designed to operate at temperatures up to 300 degrees. Most of all, thermistors ST1-18 and ST1-19 are suitable for this range. See reference msevm.com/data/trez/index.htm

Thus, it turns out that this thermostat will not provide temperature stabilization of 220 degrees and above, since it is designed for the use of semiconductor thermistors. You will have to look for a circuit with metal RTDs TCM or TSP.

hello to all lovers electronic homemade products. Recently, I quickly made an electronic thermostat with my own hands, the device diagram is very simple. As an actuator, an electromagnetic relay with powerful contacts is used that can withstand currents up to 30 amperes. Therefore, the considered homemade product can be used for various household needs.

According to the scheme below, the thermostat can be used, for example, for an aquarium or for storing vegetables. For someone it can be useful when used in conjunction with an electric boiler, and someone can adapt it for a refrigerator.

Do-it-yourself electronic thermostat, device diagram

As I said, the circuit is very simple, contains a minimum of inexpensive and common radio components. Typically, thermostats are built on a comparator chip. Because of this, the device becomes more complicated. This homemade product is built on an adjustable zener diode TL431:

Now let's talk more about the details that I used.

Device details:

12 volt step down transformer
Diodes; IN4007, or others with similar specifications 6 pcs.
electrolytic capacitors; 1000 microns, 2000 microns, 47 microns
Chip stabilizer; 7805 or other 5 volt
Transistor; KT 814A, or another p-n-p with a collector current of at least 0.3 A
Adjustable zener diode; TL431 or Soviet KR142EN19A
Resistors; 4.7 kΩ, 160 kΩ, 150 ohm, 910 ohm
Variable resistor; 150 Kom
Thermistor as a sensor; about 50 kw with negative tks
Light-emitting diode; any with the lowest current consumption
Relay electromagnetic; any 12 volt with a current consumption of 100 mA or less
Button or toggle switch; for manual control

How to make a thermostat with your own hands

The burned-out electronic counter Granit-1 was used as a case. The board on which all the main radio components are located is also from the meter. Inside the case fit a power supply transformer and an electromagnetic relay:

As a relay, I decided to use an automotive one, which can be purchased at any car dealer. Coil operating current approximately 100 milliamps:

Since the adjustable zener diode is low-power, its maximum current does not exceed 100 milliamps, it will not work to directly connect the relay to the zener diode circuit. Therefore, I had to use a more powerful transistor KT814. Of course, the circuit can be simplified if a relay is used, in which the current through the coil will be less than 100 milliamps, for example, or SRA-12VDC-AL. Such relays can be connected directly to the zener diode cathode circuit.

I'll talk a little about the transformer. As which I decided to use non-standard. I have a voltage coil from an old induction electric energy meter lying around:

As you can see in the photo, there is free place for the secondary winding, I decided to try winding it and see what happens. Of course, the cross-sectional area of \u200b\u200bthe core is small, and, accordingly, the power is small. But for this temperature controller, this transformer is enough. According to my calculations, I got 45 turns per 1 volt. To get 12 volts at the output, you need to wind 540 turns. To fit them, I used a wire with a diameter of 0.4 mm. Of course, you can use a ready-made one with an output voltage of 12 volts or an adapter.

As you noticed, the circuit has a 7805 stabilizer with a stabilized output voltage of 5 volts, which feeds the control output of the zener diode. Thanks to this, the temperature controller turned out with stable characteristics that will not change from changes in the supply voltage.

As a sensor, I used a thermistor, which at room temperature has a resistance of 50 kΩ. When heated, the resistance of this resistor decreases:

To protect it from mechanical influences, I used heat-shrinkable tubes:

The place for the variable resistor R1 was found on the right side of the thermostat. Since the axis of the resistor is very short, I had to solder a flag on it, for which it is convenient to turn. On the left side, I placed the manual control toggle switch. With it, it is easy to control the operating state of the device, while not changing the set temperature:

Despite the fact that the terminal block of the former electric meter is very bulky, I did not remove it from the case. It clearly includes a plug from any device, such as an electric heater. By removing the jumper (yellow on the right in the photo) and turning on the ammeter instead of the jumper, you can measure the current supplied to the load:

Now it remains to calibrate the thermostat. For this we need . It is necessary to connect both sensors of the device together with electrical tape:

Use a thermometer to measure the temperature of various hot and cold objects. Using a marker, apply a scale and markings on the thermostat, the moment the relay is turned on. I got from 8 to 60 degrees Celsius. If someone needs to shift the operating temperature in one direction or another, this is easy to do by changing the values of the resistors R1, R2, R3:

So we made an electronic thermostat with our own hands. Outwardly it looks like this:

In order not to see the inside of the device, through the transparent cover, I covered it with adhesive tape, leaving a hole for the HL1 LED. Some radio amateurs who decide to repeat this scheme complain that the relay turns on, not very clearly, as if rattling. I did not notice anything, the relay turns on and off very clearly. Even with a slight change in temperature, no chatter occurs. If, nevertheless, it arises, it is necessary to select more accurately the capacitor C3 and the resistor R5 in the base circuit of the KT814 transistor.

The assembled thermostat according to this scheme turns on the load when the temperature drops. If someone, on the contrary, needs to turn on the load when the temperature rises, then you need to swap the sensor R2 with resistors R1, R3.

A thermostat on the farm is sometimes an indispensable thing that helps control the thermal regime on a home incubator or vegetable dryer. Built-in mechanisms for this purpose often deteriorate quickly or do not differ in decent quality, which forces you to invent a simple thermostat with your own hands.

If you are among those who urgently needed homemade device with the function of thermoregulation, stay here, because all suitable and tested schemes, combined with theory and useful tips are listed below.

What is applicable for?

A temperature controller or thermostat is a device capable of restarting and stopping the operation of heating or cooling units. For example, it allows you to maintain the optimal mode in the incubator, and is also able to turn on the heating in the basement, fixing the low temperature.

How it works?

Before you make a thermostat with your own hands, you need to understand the accompanying theory. Principle this device is identical to the operation of simple measurement sensors capable of changing resistance depending on ambient temperature conditions. A special element is responsible for changing the indicator, and the so-called reference resistance remains unchanged.

In the thermostat device, an integrated amplifier (comparator) reacts to a change in the resistance value, switching microcircuits when a certain temperature is reached.

What should be the schema?

On the Internet and in regulatory documentation, it is easy to find diagrams of thermostats for various purposes, which you can assemble with your own hands. In most cases, the basis of a schematic drawing is the following elements:

Control zener diode, designated TL431;
Integrated amplifier (K140UD7);
Resistors (R4, R5, R6);
Quenching capacitor (C1);
Transistor (KT814);
Diode bridge (D1).

The circuit is powered by a transformerless power supply, and an automotive relay designed for a voltage of 12 volts is perfect as an actuator, provided that the current entering the coil is at least 100 mA.

How to do?

Instructions for making a thermostat with your own hands are based on strict adherence to the chosen scheme, according to which it is necessary to connect all the components into a single whole. For example, an electronic circuit for an incubator is assembled according to the following algorithm:

Examine the image (it is better to print and put in front of you).
Find the necessary parts, including the case and the board (the old ones from the meter will do).
Start with the "heart" - the integrated amplifier K140UD7 / 8, connecting it with a positively charged reverse action, which will give it the functions of a comparator.
Connect the negative resistor MMT-4 in place of “R5”.
Connect the remote sensor using shielded wiring, and the length of the cord can be no more than a meter.
To control the load, include a VS1 thyristor in the circuit, installing it on a small radiator to ensure adequate heat transfer.
Set up the rest of the chain.
Connect to power supply.
Check functionality.

By the way, adding a temperature sensor, assembled device can be safely used not only for incubators, dryers, but also for maintaining the thermal regime in an aquarium or terrarium.

How to properly install?

In addition to high-quality assembly, it is necessary to pay attention to the conditions of its operation, which should include:

Placement - the lower part of the room;
Dryness of the room;
The absence of a number of “knocking down” units: radiating heat or cold (electrical equipment, air conditioning, an open door with a draft).

Having figured out how to connect the thermostat with your own hands, you can proceed to it regular use. The main thing is that the power of the manufactured device is designed for the relay contacts. For example, with a maximum load of 30 Amps, the power should not exceed 6.6 kW.

How to repair?

A factory or homemade thermostat can be repaired so as not to buy a new one and not waste time searching for and assembling the necessary parts. First of all, the device must be found (if you did not install it), because the photo of the thermostat shows that its dimensions are small, which makes the search somewhat difficult.

Advice will help: the thermostat is located next to the temperature mode button.

Signs of a device failure may include the following:

The device has ceased to perform the main function: the temperature has dropped significantly or increased without the reaction of the mechanism;
The connected machine works without going into standby or saving mode;
The unit turned off spontaneously.

Depending on the cause of the malfunction, the following steps must be taken to repair the thermostat with your own hands:

Disconnect the repaired device from the network.
Remove the protective case from the device.
Check the quality of contacts and connections.
Disconnect and pull out the capillary tube.
Get the relay.
Change bellows tube, fix.
If necessary, replace other parts.
Connect wiring back.
Put the relay in place.

Many household and household appliances are equipped with thermostats, and knowing how to fix them, reassemble them with your own hands and install them will significantly save your money, time and effort.