Thermostat connection diagram, how to connect a thermostat. Scheme for connecting an infrared heater through a thermostat: possible wiring

With the onset of cold weather, many begin to think about the additional heating of their homes. Because with the beginning heating season usually start repair work in places of gusts of heating mains. Or there are thoughts to go to electric heating as an additional alternative for country house. In this article we will talk about a temperature-controlling device - a thermostat, namely, we will talk about how the thermostat is installed and connected to an infrared heater.

Installation nuances

We will not go into the types and types of regulators, arrange comparisons and tournaments. All of them are good in their own way and will fulfill their purpose, serving faithfully. The first thing you want to pay attention to is the installation location. It does not depend on what type of heaters you have: infrared, panel, convection.

The installation of a thermostat with an air temperature sensor is prohibited in the following places:

• in close proximity to heaters;
• in places where there is a draft;
• in the heating zone of infrared emitters.

All these places are unsuitable for placing a thermostat, because when located near a heater, the air next to it will heat up to the desired temperature earlier, which will lead to false alarms, as a result of which the room will not heat up to a comfortable temperature.

If you install a thermostat in the heating zone of the IR heater, its body will heat up earlier and distort the sensor readings. In places where there is a draft, the sensor will not show the desired temperature and the heaters will overheat the room, consuming excess electricity. The placement of the temperature sensor in height should be done in the comfort zone, at a level of 1.5 meters from the floor.

Wiring diagrams

Always, before installing and connecting the thermostat, read the instructions and passport data for the device. Since the manufacturer indicates the required cable cross-section and gives a connection diagram for their products. In case of deviation from the requirements and savings on wire and thermostats, there is a high probability of equipment failure or a fire hazard.

Scheme of connecting the thermostat to an infrared heater with a power of up to 3.5 kW:

If the space is heated by a group of heaters up to 3.5 kW, then the connection diagram will look like this:

In the event that you are the owner of a three-phase network and heating is carried out by a group of heaters with a total power of more than 3.5 kW, then a magnetic starter is added to the control circuit, which is controlled by a thermostat:

This is how the temperature controller is installed. As you can see, there are some peculiarities in the installation and connection of the thermostat, so it is important to first read the instructions from the manufacturer, and then proceed to the main process.

To create comfort inside a dwelling, there are many devices, among which are various devices that take on the function of adjusting the temperature of water or ambient air. This type of device includes a thermostat, this product is designed, after setting, to independently maintain the temperature of a heater or other heating element by turning the power supply on and off. This article discusses the question of how to connect a thermostat, and also provides a diagram for connecting the controller to a floor heating system.

Types of thermostats

There are two main types of thermostats, which differ depending on the principle of operation:

1. Mechanical devices are thermostats that regulate the temperature of the actuator by opening a contact between two plates of different density. When the sensor is heated, the signal enters the contactor housing and transmits an impulse to open or close the plates;

1. Electronic thermostat. In this case, the information coming from the temperature sensor is analyzed in a digital processor, only after that a command is executed to supply power to the heating element.

In both cases, the control is carried out manually, by setting the required temperature on the controller case. You can also distinguish the classification of thermostats based on visualization and control keys. Thermostats are available with rotary dials with a scale, setting buttons or a touch screen. The principle of operation of all these products is not significantly different from each other.

There is also a classification of thermostats according to the type of placement: external or internal. Depending on the task to be solved, the device can be installed into the wall in a pre-made niche. The construction size of such a device coincides with an ordinary socket, so it is often mounted in a hole cut by a crown.

The outdoor thermostat has a thicker body that is closed on all sides plastic plates. The disadvantage of such a device is its size, due to the impossibility of placing the device inside the wall, it will protrude on a plane, moreover, when connecting a cable to it, it will be necessary to arrange an additional channel from a corrugated pipe or canister.

Areas of application of temperature controllers

Thermostats are widely used in various fields, both in industry and in everyday life. Most often, these devices can be found in underfloor heating systems with a heating element in the form of a heating bundle, which is located in the screed. When power is applied to the electrodes, the wires heat up and give off heat to all surrounding layers, for correct operation the system is equipped with a temperature sensor built into the screed. The controller can be used for electric or water underfloor heating, the principle of its operation does not change from this.

The thermostat is also used in heating or heating boilers for automatic heating level control internal environment. These devices are supplied by many manufacturers. heating appliances already at the manufacturing stage, but even if the boiler design does not provide for this, the controller can be installed on the line yourself.

Connecting a thermostat

Since temperature controllers can be used both to control heating elements and to control a cooler, there are two types of contacts and terminals in the design of the device. During the independent connection of the device to the system, it is necessary to strictly observe the polarity of the contacts and avoid contradictions in the circuit.

No electrical connection is required to connect a mechanical thermostat, since all control and opening of the switch is done by physically changing the characteristics of the heating plate. To connect this appliance you need to follow the algorithm below:

1. In the documentation for the devices, there is a designation of the terminals by numbers; in accordance with these indicators, it is necessary to assemble the system. First of all, you need to connect the zero cable to the box electrodes and lead it immediately to the consumed heating elements, for example, a warm floor;
2. The phase is brought into the controller directly, without connection to household appliances. The box itself will distribute electricity at the moment the contacts are turned on. In some devices, it is necessary to lay a jumper inside the thermostat from the positive wire to the operation indicator, which shows a signal at the moment the heater is turned on and throughout the entire period of operation;
3. The control unit contains terminals for connecting a cooling heating element, as well as for an external temperature sensor. All devices must be connected in series, the current must be completely disconnected. This is a typical thermostat connection scheme, which is most common in underfloor heating or infrared space heating systems;
4. The temperature sensor is connected last, after which a test run of the system and a voltage check on all elements are performed.

There is also a thermostat connection scheme using a magnetic circuit breaker, most often this scheme is used in the presence of several controlled devices that require high voltage current for operation. In this case, the machine is connected to an open network of a positive cable in parallel with a thermostat, in addition there is a connecting cable with a control device. Current is supplied to consumer devices through a circuit breaker, but it is controlled by a thermostat. The heating elements are connected to the controller only on a parallel line and through the machine, this allows the system to be operated with high voltage without interruption and in a safe mode. In the event of an emergency, the switch will trip and completely de-energize all devices.

Thus, it can be seen from the diagram that the thermostat is connected to heating or cooling devices immediately before applying voltage to them, that is, the controller will be the first element in the system. Many thermostats are equipped with an electronic microcircuit and a processor that, in addition to temperature readings, provide additional data on various indicators, such as the state of humidity in the room, pressure and the time required to reach the set parameters. Such devices have a cost much higher than mechanical household thermostats.

Connecting the thermostat to the underfloor heating system

Depending on the type heating cable in the underfloor heating system, the connection scheme will be different. There are two types of floor: with a single-core and two-core bundle, the principle of operation between them is similar, but a multi-core cable has a working life, as well as technical indicators in terms of speed and height of heating is much higher.

Connecting a thermostat to a single-core system is easier - just connect two neutral cables to one terminal, and a phase to the appropriate socket. In this case, the current will pass through the entire length in series along the ring of the bundle.

In a two-core cable, all wires exit from one side, so the connection is made in series - one wire to one terminal. The current in this circuit passes along the entire length of the heating element and returns along the same path in one direction.

Thus, subject to all the rules and the algorithm for connecting a thermostat to any circuit, all that remains is to set the device to the desired parameters by rotating the wheel on the temperature scale.

Video

SCHEMES OF THERMOREGLATORS

Exists a large number of electrical circuit diagrams that can maintain the desired set temperature with an accuracy of 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 temperature controller 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 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. The low output voltage of the bridge is amplified by Motorola's MCI741 op amp to 30V with a 0.3mV change in input voltage. 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 thermistor bridge and two operational amplifiers, regulates the temperature with a very high precision(up to 0.001 °C) and a large dynamic range, which is necessary for rapid changes in 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 controller. 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 a 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 (fig. 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. 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 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, proportional switching on / off of the 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 turned on last, K\ or K2. The control circuit checks the output of the temperature programmer. This DC input signal will be reduced by resistors and R2 by a maximum of 5V and amplified by voltage follower A3. The signal is compared in voltage comparators Aj and A2 with a continuously varying reference voltage from 0 to 5 V. The 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 multi-range 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 a non-inverting AD741J stabilizing amplifier that 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 (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. The DC gain of 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 burner heating system. After the expiration of a single pulse, the burner switches off, regardless of the state of the output of the operational amplifier. 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

The proposed proven and well-proven thermostat operates in the range of 0 - 100 ° C. It provides electronic temperature control by switching the load through a relay. The circuit is assembled using available chips LM35 (temperature sensor), LM358 and TL431.

Electrical thermostat circuit

Device details

• IC1: LM35DZ temperature sensor
• IC2: TL431 precision voltage reference
• IC3: LM358 double unipolar op amp.
• LED1: 5mm LED
• B1: PNP transistor A1015
• D1 - D4: 1n4148 and 1N400x silicon diodes
• ZD1: 13V zener diode, 400mW
• Trimmer resistor 2.2 k
• R1 - 10k
• R2 - 4.7 M
• P3 - 1.2 K
• R4 - 1k
• R5 - 1k
• P6 - 33 Ohm
• C1 - 0.1 microfarad ceramic
• C2 - 470 uF electrolytic
• Relay 12 V DC single pole double throw 400 Ω or higher

The device performs a simple but very accurate thermal current control which can be used where automatic temperature control is required. The circuit switches the relay depending on the temperature detected by the LM35DZ single-chip sensor. When the LM35DZ detects a temperature higher than the set level (set by the controller), the relay is activated. When the temperature falls below the set temperature, the relay is de-energized. Thus, the desired value of the incubator, thermostat, home heating system, and so on is maintained. The circuit can be powered by any source of AC or DC 12 V, or from a battery. There are several versions of the LM35 temperature sensor:

• LM35CZ and LM35CAZ (in to-92 package) - 40 - +110C
• LM35DZ (in to-92 case) 0 - 100s.
• LM35H and LM35AH (in-46 case) - 55 - +150C

Principle of operation

How does a thermostat work. The basis of the circuit is a temperature sensor, which is a degree-volt converter. The output voltage (at pin 2) changes linearly with temperature from 0 V (at zero) to 1000 mV (at 100 degrees). This greatly simplifies the design of the circuit, as we only need to provide a precision voltage reference (TL431) and an accurate comparator (A1 LM358) in order to build complete thermal controllability of the switch. The regulator and resistor sets the reference voltage (vref) 0 - 1.62 V. The comparator (A1) compares the reference voltage vref from (set by the regulator) with the output voltage of the LM35DZ and decides whether to turn on or turn off the power to the relay. The purpose of R2 is to create hysteresis, which helps prevent the relay from bouncing. The hysteresis is inversely proportional to the value of R2.

Setting

No special instrumentation required. For example, to set the trip to 70C, connect a digital voltmeter or multimeter through test points "TP1" and "ground". Adjust vr1 until you get an accurate reading of 0.7V on the voltmeter. Another version of the circuit, using a microcontroller, see.

Household mechanical temperature controllers have found their application in various heating and cooling systems of apartments, houses and garages. The principle of operation of the thermostat is simple: when the set temperature is reached, the controlled device is turned on or off ( electric heater boiler, air conditioner). Universal thermostats allow you to control both heating appliances and cooling systems. To do this, they have two terminal groups.

A feature of mechanical thermostats is that there is no need to connect to the mains or use batteries. The mechanical thermostat allows only switching (connection or disconnection) electrical circuits, and the control algorithm is determined by the set temperature value. Temperature control by a temperature controller occurs due to a change in the mechanical properties of the materials used as a sensor element of the temperature sensor.

Consider one of the Zilon mechanical room thermostats, type za-1. Having opened the package, the buyer may be surprised not to find the sensor connection diagram. The manufacturer decided to save on paper and made a connection diagram on a sticker by gluing it to the back of the front panel of the thermostat.

The absence of any connection description will add even more headaches, so below we give typical scheme connection of a mechanical thermostat.

Consider the assignment of the thermostat terminals Zilon za-1:
- terminals "1" and "2" are connected to an indicator lamp, which can be used to monitor the activation of the thermostat. The neutral conductor of the power source is connected to terminal "1", and the wire coming from terminal "4" or "5" is connected in series to terminal "2".
- terminals "4", "5" and "6" are intended for connection household appliances. The phase conductor of the power source is connected to terminal "6". When the set temperature is reached, the thermostat switches between terminals "4" and "5".

An alternative option for connecting a thermostat involves using terminal "1" as a terminal for connecting a neutral conductor. Such a connection scheme allows you to make all the necessary connections of the supply conductors inside the thermostat, excluding additional junction boxes from the scheme.

When choosing household mechanical thermostats, you should pay attention to the parameters of the connected load, more precisely, to the operating current of the heater or air conditioner. In our case, the thermostat is designed to switch circuits with a load of not more than 16A.

Large rooms require the installation of sufficiently powerful heaters, so the connection of a thermostat in such systems is best done through an intermediate magnetic starter.

The magnetic starter in the thermostat connection circuit provides control of high load currents with a small value of the control signal (the presence of voltage on the coil). In the above connection diagram, when the thermostat is triggered, voltage is applied to the coil of the magnetic starter, the contacts of which close or open the heater circuit.