Simple circuits of electronic thermostats with your own hands

We continue our section on electronic homemade products; in this article we will consider devices that support a certain thermal regime, or signal when the desired temperature has been reached. Such devices have a very wide scope of application: they can maintain a given temperature in incubators and aquariums, heated floors, and even be part of a smart home. For you, we have provided instructions on how to make a thermostat with your own hands and at a minimum cost.

Why do you need a thermostat for an incubator?

In order for young birds to be hatched efficiently in an incubator, it is necessary to regularly maintain humidity and temperature at optimal levels. Temperature indicators differ depending on the breed of birds and the stage of their incubation; accordingly, they must be regulated. They range from 35 to 39 degrees. And in order to be able to control the temperature, a microcontroller (thermostat) is required.

Many modern factory incubators are equipped with analog thermostats, which need to be frequently adjusted depending on the temperature readings. Most often, alcohol or mercury thermometers are used to maintain temperature.

However, digital temperature microcontrollers have more advantages over analog devices:

• the required temperature is ensured inside the device;
• it becomes possible to control the operation of heating elements;
• based on current indicators, you can control the temperature;
• the process is automated and does not require regular adjustments;
• Electricity is saved because when the required temperature is reached, the heating elements are turned off.

The editors of the site advise you to familiarize yourself with the labeling of imported and Soviet ceramic capacitors.

Sequence of work

Having mounted the circuit in the chosen way, you should install the sensor in such a way that, during operation, it controls the temperature of the required object.

• The variable resistor should be installed in such a way that it can be easily accessed.
• After that, you need to put a scale of set temperatures that will be maintained by the thermostat.

After all this work has been completed, connect the power cord to the device (if you do this earlier, it will greatly interfere with operation).

After assembling and configuring the device, it is placed in the housing.

Aquarium heater

Less commonly, such a thermostat was used to maintain a set temperature in aquariums with tropical fish. This need arose due to the fact that the majority of thermal heaters produced for these purposes have a mechanical thermostat combined with a heating element in one housing. And therefore, they maintain their own temperature, and not the surrounding temperature, within specified limits. This works well only in rooms with a stable air temperature, within one or two degrees.

Installation features

• due to the inertia of water, the sensor and the heater must be spaced apart, but within direct visibility (without blocking by plants and decorative elements) from each other;
• due to the electrical conductivity of water, the sensor must be insulated either with means of good thermal conductivity or with a thin layer of conventional sealant;
• It is allowed to use both conventional aquarium heaters and adjustable ones with the temperature set to maximum.

You can find other areas of application for this simple-to-manufacture device. For example, for seedling greenhouses, drying cabinets, various thermal baths. What is your imagination enough for? Only if the load is susceptible to short circuiting is it necessary to add a 1 A fuse.

PS As mentioned above, this simple thermostat was used in incubators before, but now it has been replaced by microcontroller-controlled thermostats that can automatically lower the temperature during the incubation cycle. And the incubators themselves have acquired the function of regulating humidity and turning eggs.

Model overview

Capillary thermostats for domestic use must be reliable, durable, and have a simple connection diagram. Next, a description of the 3 most suitable thermostats for domestic use will be given.

AZT-6

Mechanical thermostat. Main purpose: temperature control of water heaters. The device is equipped only with a knob for adjusting the operating temperature. Operates in modes - 15 + 85 degrees C.

Among the advantages:

1. Easy to connect.
2. Fast cut-off.
3. Miniature appearance.
4. Low price.

AZT-6 is an excellent assistant for monitoring air temperature changes in the house.

Ballu BMT-2

Temperature controller for heaters and ventilation systems. Operating range +5–35 °C. The device is used to switch between heating devices and ventilation systems. Has a built-in notification about changing the operating mode. Can be used to connect 4 devices at once into a common system, with their subsequent division according to temperature conditions.

Pros:

1. High reliability.
2. Ease of Management.
3. Multiple connection pairs.
4. Not a high price.

The Ballu BMT-2 regulator can be trusted to control the temperature in the house with an electrical load of more than 10 amperes.

Terneo EG

The device is designed to control the temperature in the incubator. The model is completely electronic. Has an additional socket on the body. Working in incubators is not the only purpose of this model. It can be used for work in warehouses and basements.

Pros:

1. Easy setup of parameters.
2. Easy to read display.
3. Availability of sound notification about operation.
4. Additional socket.

Terneo EG is very reliable. Through this device, you can connect several devices and set the response temperature for each device.

What parts will you need: DIY thermostat

For a temperature sensor, a thermistor is most often used; this is an element that regulates electrical resistance depending on the temperature reading.

Semiconductor parts are also often used:

• Diodes;
• Transistors.

Temperature should have the same effect on their characteristics. That is, when heated, the transistor current should increase and at the same time it should stop working, despite the incoming signal. It should be taken into account that such parts have a big drawback. It is too difficult to calibrate, or more precisely, it will be difficult to associate these parts with some temperature sensors.

However, at the moment the industry does not stand still, and you can see devices from the 300 series, this is the LM335, which is increasingly recommended by experts and the LM358n. Despite the very low cost, this part occupies the first position in the markings and is oriented towards combination with household appliances. It is worth mentioning that modifications of this part LM 235 and 135 are successfully used in the military and industrial sectors. Including about 16 transistors in its design, the sensor is capable of working as a stabilizer, and its voltage will completely depend on the temperature indicator.

The dependency is as follows:

1. For each degree there will be about 0.01 V, if you focus on Celsius, then at 273 the output result will be 2.73V.
2. The operating range is limited to -40 to +100 degrees. Thanks to such indicators, the user completely gets rid of adjustments by trial and error, and the required temperature will be ensured in any case.

Also, in addition to the temperature sensor, you will need a comparator, it is best to purchase LM 311, which is produced by the same manufacturer, a potentiometer to generate a reference voltage and an output setting to turn on the relay. Don't forget to purchase a power supply and special indicators.

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 (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 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 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 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 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-setting ensures 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 deal with this, a large tracking circuit is connected between points "A" and "B" to set the transistor to a constant 3V without taking into account the voltage required by the heating element. The output of the 741 op amp is compared in the AD301A to a ramp voltage synchronous with the 400 Hz mains voltage. The AD301A microcircuit works as a pulse-width modulator that includes a 2N2219-2N6246 transistor switch. The switch provides controlled power to the 1000 µF capacitor and 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 is turned off. If the temperature drops, transistor Q2 turns off, 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 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 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. 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 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 connected to the time-setting circuit of the '555 monostable 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 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), 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 regulator on the comparator
Website administration address

Communities › Kulibin Club › Blog › Electrics: Temperature sensors, we make it ourselves.

Sometimes there is a need for temperature control of some process, be it a car or a national economy. There are many different thermal control schemes, but the sensors usually have an inconvenient design that does not allow for mounting in a controlled environment. Let's talk about sensors.

As a rule, semiconductor devices - thermistors - serve as sensors for measuring circuits:

The case may be different, but inside there will still be about the same droplet with leads.

The second common temperature sensor is the DS1820:

They are often sold as follows:

Inside is the same DS18B20 microcircuit with three pins, even without thermal paste.

Now let's try to implement these radio components into a car, for example, for digital display of coolant temperature or control of electric fans.

We will need a donor sensor - any suitable thread and cost. In my case, this is the Volga-UAZ sensor TM 106-10

:

We take a drill as a lathe and carefully clamp the sensor into the chuck. Using a metal hacksaw, we cut off the rolling. When the sensor falls apart into its component parts, use a drill to even out the edge of the sensor with a file. We receive a blank housing for introducing our radio component there.

Then you can go in two ways: 1. Pour molten solder into the body, drill a channel in this solder and insert a thermistor there. You can fill the housing cavity with thermal paste and stick the thermistor into it, but tin’s thermal conductivity is several orders of magnitude better than thermal paste, so thermal paste must of course be used, but it’s better to apply a thin layer of it.

The disadvantage of this method is the high inertia of the resulting sensor.

2. Do it the way I do it. Take a telescopic antenna from some old unnecessary device:

If you threw them away before, you did it in vain, because such antennas are a source of wonderful thin-walled brass tubes of different diameters:

We select the tube most suitable for the thermistor - it should be inserted as tightly as possible into the tube. We measure and again use the drill, cut off the piece of tube we need - it’s better to cut with a needle file. We take our blank body and drill its end according to the diameter of the tube. We tin the end of the body with tin, strip the tube down to brass and also tin it. We insert the tube into the case and solder them to each other, an 80W soldering iron is enough for your eyes. It should look something like this (the end is already sealed with a small piece of copper foil 1mm thick):

We check the resulting sensor housing for leaks. I do it not very technologically - using tongue suction

If everything is in order with the tightness, we proceed to the next stage: installing the thermistor and connector.

Again, we try everything on and cut off the leads of the thermistor so that when installed in the housing, the thermistor is at the end of the tube, or better yet, rests against the end:

The thermistor is now ready for installation. We put a little thermal paste inside the tube, coat the thermistor itself with a little thermal paste and insert it into the tube. After the thermistor has entered the tube under the connector, we place a little pre-prepared poxypol or epoxy plasticine. We press the connector into the polyester and remove the excess. When the Poxypol has completely hardened, you get this nice sensor ready for installation:

And this is how the sensor will stand at its workplace - the measuring part will be completely washed by the working environment:

Well, here’s a picture of a general check of the functionality of the electrical part:

Homemade thermostat: step-by-step instructions

If you have purchased all the necessary components for assembly, all that remains is to review the detailed instructions. We will consider the example of a temperature sensor designed for 12V.

A homemade temperature controller is assembled according to the following principle:

1. We prepare the body. You can use old shells from the meter, for example from the Granit-1 installation.
2. You select the circuit that you like best, but you can also focus on the board from the meter. The forward stroke marked “+” is necessary to connect the potentiometer. The inversion input with o will be used to connect the temperature sensor. If it so happens that the voltage at the direct input is higher than required, the output will be set to a high level and the transistor will begin to supply power to the relay, and it, in turn, to the heating element. As soon as the output voltage exceeds the permissible level, the relay will turn off.
3. In order for the thermostat to operate on time and temperature differences to be ensured, you will need to make a negative connection using a resistor, which is formed between the direct input and output of the comparator.
4. As for the transformer and its power supply, you may need an induction coil from an old electric meter. In order for the voltage to correspond to 12 volts, you will need to make 540 turns. It will only be possible to fit them if the diameter of the wire is no more than 0.4 mm.

That's all. These small steps are where all the work of creating a thermostat with your own hands lies. It may not be possible to do it yourself without certain skills right away, but with the help of photo and video instructions you will be able to test all your skills.

Thanks to its simple design, a self-created thermal controller can be used anywhere.

For example:

• For heated floors;
• For the cellar;
• Heating boiler;
• Can adjust air temperature;
• For the oven;
• For an aquarium where the water temperature will be controlled;
• In order to control the temperature value of the electric boiler pump (its switching on and off);
• And even for a car.

It is not necessary to use a digital, electronic or mechanical commercial thermal switch. Having bought an inexpensive thermal relay, adjust the power on the triac and thermocouple and your homemade device will work no worse than the purchased one.

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.

This is useful: distribution manifold in a heating system.

The simplest scheme

The simplest do-it-yourself thermal relay circuit has a transformerless power supply, which consists of a diode bridge with a parallel-connected zener diode that 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, we get the simplest device that regulates the 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 circuits, it is based on a direct current bridge, 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 semistor VS1 , which will turn on the 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.

Thermostat circuit - second option

After some thought, I came to the conclusion that it is possible to connect here the same controller as on the soldering station, but with a little modification. During the operation of the soldering station, minor inconveniences were identified: the need to set the timers to 0, and sometimes an interference occurs that puts the station into SLEEP mode

. Considering that women do not need to remember the algorithm for switching the timer to mode 0 or 1, the circuit of the same station was repeated, but only the hair dryer channel. And minor improvements led to stable and “interference-free” operation of the thermostat in terms of control

When flashing AtMega8 firmware, you should pay attention to the new fuses. The following photo shows a K-type thermocouple, which is convenient to mount in the oven

I liked the work of the temperature controller on the breadboard and started final assembly on the printed circuit board.

I finished the assembly, the operation is also stable, the readings in comparison with the laboratory thermometer differ by about 1.5°C, which is basically excellent. When setting up, there is an output resistor on the printed circuit board; I have not yet found an SMD of this value in stock.

The LED models the heating elements of the oven. The only note: the need to create a reliable common ground, which in turn affects the final measurement result

The circuit requires a multi-turn tuning resistor, and secondly, pay attention to R16, it may also need to be selected, in my case it is 18 kOhm. So, here's what we have:

In the process of experimenting with the latest thermostat, more minor improvements appeared that qualitatively affected the final result, look at the photo with the inscription 543

- this means the sensor is disconnected or broken.

And finally we move from experiments to the finished design of the thermostat. I implemented the circuit into the electric stove and invited an authoritative commission to take over the work :) The only thing that my wife rejected were the small buttons on the convection control, general power supply and airflow, but this can be solved over time, but for now it looks like this.

The regulator maintains the set temperature with an accuracy of 2 degrees. This happens at the moment of heating, due to the inertia of the entire structure (the heating elements cool down, the internal frame is temperature equalized), in general, I really liked the scheme in the work, and therefore it is recommended for independent repetition. Author - GOVERNOR

.

Discuss the article THERMOREGULATOR DIAGRAM

Autonomous heating of a private house allows you to choose individual temperature conditions, which is very comfortable and economical for residents. In order not to set a different mode indoors every time the weather changes outside, you can use a thermostat or thermostat for heating, which can be installed on both radiators and the boiler.

Thermostat - what is it?

Before you understand what a thermostat is, it would be useful to find out what the water heating tank actually consists of.

Elements of the water heating apparatus:

• The tank body, regardless of volume, can be metal or plastic. One option is a combination of both materials
• The internal tank is a container made of stainless steel. The inside is covered with a layer of enamel and porcelain glass
• The magnesium anode for the water heater serves as reliable protection for the inner surface of the tank from corrosion.
• Heating element, or heating element, responsible for heating water
• A safety valve is designed to relieve excess pressure inside the tank.

Types of thermostats:

• A bimetallic or rod thermostat is a metal rod.
  It is inserted into the heating element tube, the length of which is about 25-45 cm, located on the boiler body. Most often you can find a bimetallic thermostat for a Termex water heater.
• A capillary thermostat is a polyester tube filled with water, inside which there is a built-in switching device (regulator), a temperature sensor, and a capillary tube.
  The principle of operation of the mechanism is as follows. When heated, a process of volumetric expansion of the liquid occurs in the sensor and capillary tube. It begins to act on the thermostat membrane, switching the electrical contact
• The thermostat for an electronic type boiler comes in 2 types. They are usually installed in pairs, 1 of them is a control thermostat, 2 is a safety thermostat, which turns off the device in the event of an accident.

How to choose a thermostat:

• The geometric dimensions of the heating element are taken as a basis
• Tank volume, power, technical characteristics are determined
• A piece of advice from an experienced professional won't hurt either. He can suggest an alternative to the broken part.

Common causes of failure:

• The capillary copper tube, which is very sensitive to mechanical damage, often fails. It cannot be repaired and must be replaced
• The electrical connectors of the heating element and thermostat have poor adhesion
• Constant voltage drops in the power supply network can cause a breakdown. To avoid problems when purchasing a boiler, a voltage stabilizer is additionally installed
• If there is a large amount of scale, the thermostat may constantly turn on and off
• The anode for the water heater is an important element of the water heater, but it must be periodically replaced with a new one. When its mechanical strength decreases due to corrosion, all other components also fail.

Operating principle of the thermostat:

• Using a lever, button, or switch, the optimal temperature level is set
• The thermostat automatically measures the water temperature and turns on the heating element if necessary. The water starts to heat up
• When the required heating level is reached, the thermostat for the boiler automatically breaks the circuit and the heating element turns off
• Over time, the water cools down, the thermostat relay is activated again, the circuit is closed, and the water begins to heat up again

Types of thermostats:

1. In some expensive models of boilers with a more complex device, as many as 3 electronic thermostats are installed that control the heating element.
2. The thermostat for Ariston water heater comes in 2 types, electronic and electromechanical. The first one works using a high-precision sensor, the operating principle of the second one is based on the properties of bimetals.
3. Depending on the installation method, thermostats can be surface-mounted or mortise-mounted. According to the range of capabilities, they can be simple, two-zone, programmable.
4. Models of Italian thermostats are distinguished by first-class quality, high measurement accuracy, low inertia, and reliability.
5. Swedish companies produce mainly electronic devices. One example is a thermostat for Electrolux water heaters, equipped with a touch screen, programmable device. It allows you to set the required temperature regime for a day, all week.

Tags: temperature sensor, sensor manufacturing

Comments 153

I installed exactly the same one on a double-circuit boiler. The third season is already plowing. The power supply is from an antenna amplifier. Did you have a controller with a sensor?

no, as far as I remember, I bought the sensor separately at Chip and Deep

But in general, then I assembled everything I had planned on the DS18B20

And this option: a thermocouple attached to the pipe and a simple multimeter in temperature measurement mode

Tell me, what kind of display board is it? Homemade?

no, not homemade - a friend bought a “bunch for a heel” on Aliexpress and gave one to me:

Have you ever thought of stirring up such a thing yourself?

Thought. On the DS1820 sensor. But it so happened that I went to visit a friend, we started talking over a glass of tea, I told him what I wanted to do, and he took this device from the shelf and gave it to me. Now the need for independent production seems to have disappeared. But I’ve already done it according to this scheme before and I have everything for it:

Dallas works better compared to thermistors, and digitally. However, the range is small.

why is it too small? More than enough for use in cars.

Dallas, in principle, has a better measurement range. But the upper limit is critical. The thermistor, as far as I remember, is not reliable. Although if the potential sits at 12 volts, it works. But Dallas needs stable power.

Can you tell me more about what the top bar is critical? I heated the sensor to more than 120 degrees with a hairdryer, it seemed to work after that.

The upper temperature range seems to be 125 degrees in Dallas. That is, -50 and +125. And if you need a controlled temperature above 125, then Dallas will not cope. In general, its accuracy is normal, but there is a delay of 0.5-1 seconds. There is a 3-wire connection, it is possible to connect via 2 wires. There will be a delay and the range will be less.

I know about these connections, just for fun I tried to heat the sensor with a hairdryer, I was able to see 127.9 as much as possible on it, beyond zero, when it cools down it returns to normal)

no, this is already a proven technology. I only got confused by taking pictures of everything, formulating it and posting it here)

I understand that it’s used, but is it really worth the effort given the relatively low cost of the sensor? Although of course there are rare and expensive sensors...

no, it’s not a matter of cost, it’s a matter of fitting a Chinese sensor into the required structure. You need to regulate the temperature of the water, for example, with a boiler - you just can’t put the sensor in the water; you have to screw it in somehow, so you need a housing.

By the way, what a cool idea...listen, how much would the cheapest sensor like this cost? If only it would open the circuit like a thermostat and then there would be no price...

I understand that it’s used, but is it really worth the effort given the relatively low cost of the sensor? Although of course there are rare and expensive sensors...

Well, look - I, for example, have a Bosch Mono-Jetronic, according to all the tables, DTVV and DTOZH should (at the same air temperature and coolant) give the “brains” the same resistance. At the same time, the DTVV is quite adequate, but cannot be replaced (due to design features). And the DTOZH is “buggy”; when the readings differ, the ECU begins to “adjust”, because can’t figure out who to believe (DTOZH or DTVV)! I bought 4 (FOUR) different sensors - they all give different resistance at the same temperature! And with the technology described above, it is possible to select a cheap thermistor for almost any resistance value at a given temperature! Yes, whatever, you can replace BOTH thermistors (by selecting the required resistance) with both DTOZH and DTVV! And this will help solve several problems with “glitches” of the electronic power system at once! Moreover, the price of Chinese thermistors, consumables, etc. There is no comparison with “branded” sensors (which sometimes cannot be replaced, or they cost like an airplane wing)! Am I explaining this clearly? )))

Thermostat cost, how to choose

From time to time, any device needs to be replaced, thermostats are no exception. At the same time, it is important to buy a model that meets all the requirements of functionality and safe operation.

What to look for when purchasing:

• It is advisable to install a thermostat similar to the one that failed
• If there is no alternative, then choose a model that has the same type, mounting method, installation dimensions
• An alternative device must perform the same functions, be it regulation, protection, automatic shutdown, etc.
• Be sure to pay attention to what current the analogue of the previous device is designed for.

The cost of thermostats largely depends on the boiler model. If this is an economy class sample, then the replacement procedure will not cost much. You will be able to save on the cost of parts and repair work. For more expensive models of water heaters, the approximate price range looks like this:

• For owners of the Electrolux brand, the spare part will cost 560 – 3,000 rubles
• The cost of a part for Termex starts from 360 rubles. up to 1,400 rub.
• For Ariston, the average cost varies from 550 rubles. up to 9,500 rub.

Description of the operation of the aquarium thermostat

The circuit diagram of the regulator is shown in Figure 5.1.1. The circuit is implemented on an integrated comparator DA1 brand 554CA3. The role of a power radio element is played by thyristor VS1 of the KU202M brand. The DA1 chip is powered by a stabilizer based on radio elements VD1, R11.

The reference voltage from the divider R1, R2 is suitable for the direct input of the comparator. Thanks to potentiometer R2, it is possible to change the size of the reference voltage, and therefore the temperature at which the comparator switches.

The opposite input of the comparator (pin 4 of DA1) receives a measured voltage, the size of which is set by the resistance of thermistor R4 (water temperature sensor in the aquarium). If the voltage at pin 4 of DA1 is greater than the voltage at its pin 3, a log appears at the output of the microcircuit. 0. A current flows through the +12 V electrical circuit - R8 - pin 9, 2 IC1 - R9, which unlocks the thyristor VS1 and turns on the LED HL1.

When thyristor VS1 is unlocked, the power circuit of the water heater in the aquarium is closed and it heats the water. If the water temperature increases, the resistance of thermistor R4 becomes less. When the voltage at pin 4 of DA1 is lower than the voltage at pin 3, the comparator changes its output state and the thyristor turns off. As a result, the heater is turned off from the network.

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.

How to install correctly

To extend the life of the thermostat, use the following recommendations:

• do not install electronics without additional protection outdoors or in rooms with high humidity levels;
• if necessary, remove the control sensor into an unfavorable environment;
• exclude the placement of the regulator opposite heat guns or other “generators” of cold or heat;
• To increase accuracy, choose a location without active convection currents.

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.

Diagram of a homemade thermostat part

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 does not have galvanic isolation from the network, that is, dangerous mains voltage is present on the elements of the device. The procedure for manufacturing the thermostat and how the thermistor is insulated is shown below.

Thermostat microcircuit

The modern level of integration of electronic devices makes it possible to design this device in a single microcircuit; such microcircuits can often be found in a wide variety of household and industrial devices.

However, when such a microcircuit fails, there is often simply nothing to replace it with. Therefore, in order to repair a thermostat, often, instead of such a microcircuit, a homemade thermostat assembled on separate elements is used.

Of course, such a device is much larger than a microcircuit, however, if the dimensions of the device allow, then the use of such a device can be completely justified.

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:

https://youtube.com/watch?v=bXNiBuC6LSM

In reality, most thermostats are only part of an overall circuit, 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.

Advantages and disadvantages

Even a simple do-it-yourself thermostat has a lot of advantages and positive aspects. There is no need to talk about factory multifunctional devices at all.

Temperature regulators allow:

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

The disadvantages include the high cost of factory models. Of course, this does not apply to homemade devices. 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.

How does a digital thermostat work?

Accurate temperature control is best achieved through the use of digital thermostats. They differ from simple designs in the method of signal processing. The voltage is removed from the sensor, passes through the analog-to-digital converter and enters the comparative side. The digitally obtained initial temperature value is then compared with that received from the sensor, after which the control device receives the appropriate command.

Thanks to this method, the measurement accuracy is increased and is almost independent of ambient temperature or interference. Sensitivity and stability are most often limited by the system capacity and sensor capabilities. The digital signal easily allows you to display the temperature on a special display.

Review of digital thermostat models

The Ringder THC-220 thermostat is an inexpensive model that is perfect for a small DIY home incubator.
Thanks to the external block of sockets and temperature control from 16 to 42 degrees, it can be used in the off-season, and not just in summer. Technical characteristics of the device:

• humidity and temperature in the sensor area are displayed on a special display;
• the displayed temperature varies from -40 to 100 degrees, and humidity – up to 99 percent;
• one or another mode is displayed as a specific symbol;
• the temperature setting step is 0.7 degrees;
• The timer has a 24-hour format and is divided into night and day;
• one channel has a load capacity of 1200 W;
• the temperature in a large room can vary by up to one degree.

Another factory model of the digital controller is XM-18. In Russia you can buy it with an English or Chinese interface. It is more complex and costs more than the previous device.

It's not difficult to deal with him. Depending on the required temperature inside the incubator, special keys can control the factory program. The front panel has screens that display temperature, humidity and additional parameters. Active modes are indicated by LEDs; in case of dangerous deviations, light and sound alarms are triggered.

Characteristics of XM-18:

• temperature operating range - from 0 to 40.5 degrees, probability of deviation - 0.1 degrees;
• the permissible load along the heater channel is 1760 W;
• permissible load on the humidity, alarm and motor channels – 220 W;
• There is an interval of up to 999 minutes between turning eggs;
• cooling fan runs 999 seconds between permissible periods between inversions;
• indoor temperatures are allowed from -10 to 60 degrees, and relative humidity is up to 85 percent.

When choosing a factory thermostat with a temperature sensor for an incubator, it is very important to consider its capabilities. If it is small and made by yourself, then a device that controls only humidity and temperature will be enough for you, and additional capabilities are needed for more complex models for industrial needs

Types of household water heaters

Features of connecting household heaters are directly related to the types of devices, their technical parameters, and overall dimensions.

Traditionally, two types of heaters are used in household practice:

Both types of boiler systems differ from each other in heating technology.

With storage heaters, cold water is collected in a container, heated and then discharged for tapping.

With flow-through units, heating is carried out directly during the flow of cold water in contact with the heater, without collecting liquid in a storage tank.

Household consumers mainly use storage boiler systems. A comparative review of both types of water heaters is given in this article.

Technical device of the storage boiler

A storage-type water heating system, boilers, in a simplified schematic form is a container equipped with electric heating elements or liquid heat exchangers. The storage vessel has pipe lines for cold water supply and hot water outlet.

The designs of indirect heating boilers are additionally equipped with a working coolant area and heating connection lines.

Any modern system, regardless of its design features, is equipped with automation, thanks to which the water heating temperature and the operation of the system as a whole are regulated.

Structural design of heating devices

There are designs of storage boilers designed for installation vertically (wall-mounted) and horizontally (floor-mounted). Of course, each individual case of using certain boilers has its own installation features.

So, if it is planned to install a water heating device on a wall, a preliminary calculation of the load and comparison of the results obtained with the design parameters of the wall of the room on which the device is to be mounted is necessary.

Installing equipment without load calculations threatens to result in a fatal installation error, when a filled boiler can simply collapse along with the flimsy partition on which it was mounted.

According to the equipment instructions, the load must be calculated taking into account four times the weight of the boiler system.

Therefore, if the structure of the supporting wall is frankly weak, the water heater circuit must be supplemented not only with connection lines to the water supply and coolant, but also with reinforced racks - through fasteners.

In classic connection diagrams for wall-mounted boilers, the water supply/discharge pipes of heating devices are marked with the appropriate color – blue/red.

A little theory

Any thermostat structurally includes three main blocks:

Theoretically, a temperature sensor can be represented by a set of four resistances, among which three resistors will be represented by elements with constant electrical parameters, and the fourth by variable ones. They are assembled into a measuring half-arm circuit shown in Figure 1 below:

The diagram shows the principle of connecting resistors to obtain a temperature sensor. As you can see, resistance R2 is variable and changes its physical value in accordance with changes in ambient temperature. When the same supply voltage is supplied to the thermostat, when the resistance in the arm changes, the current in the circuit will increase.

Based on the changes, temperature fluctuations are analyzed, as a result of which the working element causes the thermostat to operate and subsequently turn off or turn on the equipment.

To measure the resistance of resistors, a microcircuit operating in comparator mode is installed as a logical element. Its task is to compare the electrical signals in the two arms. An example of a temperature controller circuit is shown in the figure:

Here, the U1A microcircuit block receives signals from the temperature meter at inputs 2 and 3. When the response temperature is reached, different currents will begin to flow in the arms, and the comparator will send a signal to turn on to the control element of the electronic thermostat.

When the thermometer sensor cools down, the current in the arms of the thermostat will equalize, and the electronic unit will issue a control signal to turn off. The above electronic circuit operates in two stable states - off and on, alternating operating modes occurs in accordance with a given logic.

This thermostat circuit is used in the operation of a personal computer cooler; receiving power from the power supply, the current in the arms is compared. When the power supply overheats, the thermostat will switch the transistor to the opposite state and the fan will start.

This principle can be used not only in fans, but also in a number of other devices:

• to control the operation of electric heating based on temperature readings in the room;
• to set the temperature level in a homemade incubator;
• when connecting a heated floor to control its operation;
• to set the temperature range of engine operation, with forced cooling or shutting down the system when the temperature limit is reached;
• for soldering stations or hand soldering irons;
• in cooling systems and refrigeration equipment with the logic of reducing temperature within certain limits;
• in ovens and stoves for both household and industrial purposes.

The scope of application of the thermostat is not limited in any way; wherever you want to control the temperature level in automatic mode with power management, such a device will be an excellent assistant.

Overview of circuits

Depending on the type of elements that make up the thermostat, there are mechanical and digital thermostats. The operation of the former is based on the operation of a relay, the latter have an electronic unit that controls the processes. We will consider examples of the operation of several schemes below.

Rice. 3. Thermostat circuit No. 1

In the diagram shown, the measurement occurs due to resistors R1 and R2; with temperature fluctuations, the variable resistor R2 will change the magnitude of the voltage drop. After which, through the thermostat amplifier, represented by a pair of transistors, electric current will begin to flow through relay coil K1.

When the amount of current in the solenoid creates a magnetic flux of sufficient strength, the core will attract and switch the contacts to another position. The disadvantage of such a thermostat is the presence of magnetically conductive parts, which, due to hysteresis, make an additional correction for temperature in addition to the measuring element.

Rice. 4. Thermostat circuit No. 2

This thermostat, unlike a mechanical thermostat, does not use a relay connection, therefore it is more accurate. Its use is justified in situations where a few degrees can make a significant difference, for example, when controlling the heating temperature of an engine or in an incubator.

Here, the change in temperature conditions is recorded by resistor R5, thanks to which the thermostat changes the electrical operating parameters. To compare and enhance the difference in the electrical parameter coming from the half-arms, the K140UD7 microcircuit is used.

To control the load, a thyristor VS1 is installed in the circuit; in this example of a thermostat, the limit is 150 W, but if desired, another parameter can be selected. But it should be taken into account that using a thyristor as a switch leads to its heating, therefore, with an increase in power, it is necessary to install a radiator for better heat transfer.

Assembly and adjustment

When assembling the thermostat, it is necessary to ensure a high-quality connection of all electrical contacts, especially in the power section.

When using a temperature sensor LM-335 or a similar (calibrated) one, there is no need to configure the device, as already noted.

If a thermistor or any semiconductor element is used as a temperature sensor, then adjustments cannot be avoided. It is most convenient to carry it out using a digital thermometer, for example, brand TM-902C.

The sensors of the thermometer and thermostat must be connected using adhesive tape or electrical tape and placed in environments with different temperatures. In this case, each time you need to gradually change the resistance of the variable resistor until the device works. At this moment, you need to record the readings of the digital thermometer and make a corresponding mark opposite the current position of the variable resistor knob.

Assembly

Having prepared the above materials and tools, we proceed to soldering a simple circuit.

1. The positive terminal of the power supply is connected by a wire to the input contact (+) of the cooler;
2. The three terminals of the field-effect transistor are soldered with wires like this: “source” with a cooler, “gate” with a thermistor, “drain” with a variable resistor.
3. Wires connect the free contacts of the thermistor to the “+” of the power supply, and the variable resistor to the “−” of the same block.

Principle of operation

The temperature sensor delivers electrical pulses, the current value of which depends on the temperature level. The built-in ratio of these values ​​allows the device to very accurately determine the temperature threshold and make a decision, for example, how many degrees should the air supply damper to the solid fuel boiler be opened, or the hot water supply valve should be opened. The essence of the thermostat's operation is to convert one value into 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 response threshold, that is, indicating at what outside temperature it will be necessary to increase the flow. Having more advanced functionality, industrial devices can be programmed to wider limits using a controller, depending on different temperature ranges. They do not have mechanical controls, which contributes to long-term operation.

Mechanical thermostat

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.

Mechanical thermostat

DIY temperature controller: power and load

As for the connection of LM 335, it must be serial. All resistances must be selected so that the total current that passes through the temperature sensor corresponds to values ​​from 0.45 mA to 5 mA. The mark should not be exceeded, as the sensor will overheat and show distorted data.

The thermostat can be powered in several ways:

• Using a power supply oriented at 12 V;
• Using any other device whose power supply does not exceed the above figure, but the current flowing through the coil should not exceed 100 mA.

Let us remind you once again that the current in the sensor circuit should not exceed 5 mA; for this reason, you will have to use a high-power transistor. KT 814 is best. Of course, if you want to avoid using a transistor, you can use a relay with a lower current level. It can operate on a voltage of 220 V.

Typical thermal relay circuit

The basis of the design is the LM335 temperature sensor or its analogues, as well as the LM311 compressor. The thermal relay circuit is supplemented by an output device, to which a heater with the installed power is connected. A power supply must be present; indicators can be used if necessary.

A more complex circuit includes transistors, a relay, a zener diode and a capacitor C1, which smoothes out voltage ripples. Current equalization is carried out using a parametric stabilizer. In this case, the device can be powered from any source whose parameters match the relay coil voltage in the range from 12 to 24 volts. The power supply can be stabilized using a conventional diode bridge with a capacitor.