Diy temperature controller: principle of operation, on and off circuit

Often, for the operation of a device or an entire system, it is necessary to maintain a certain temperature regime. This is important when operating heating or cooling circuits, building incubator-type devices. One of the simplest devices to control the temperature is a thermostat. Such a device can be purchased at specialized retail outlets, but you can make such a temperature controller with your own hands.

Appointments and characteristics

The operation of the thermal relay is based on the device's ability to control the switching on and off of circuit nodes depending on temperature changes. In fact, it is a device located between the controlled elements and temperature sensors. Structurally the device is an electronic circuit or a device made of a special material.

The first type involves the use of remote or built-in sensors, and the second - uses the properties of various materials to change their parameters when the characteristics of the electrical network change. That is, the control takes place in a contact or non-contact way. But despite the fundamental differences, the essence of the work of thermostats is the same. By registering a change in temperature, the device breaks or connects the units of equipment or equipment connected to it in automatic mode.

Thanks to their use, the temperature of air, water, surfaces of various devices and radioelements has a stable value.

Each environment has its own specific device placement. Its response accuracy depends not only on the quality of the regulator itself, but also on the correct placement.

Thermostats are available in various types. They can be classified according to the following criteria:

1. By appointment. They are divided into indoor and outdoor.
2. Installation method. There are independent thermostats, both capable of being located on any surface, and located only inside the equipment.
3. Functionality. Thermostats can register only one signal or several at once. In this case, the second type is called multichannel. They can maintain the temperature value both on several devices at the same time, using independent channels, or on only one.
4. Setting method. The control of operating modes and adjustment of the device can be mechanical, electronic or electromechanical.
5. Hysteresis. In thermostats, it is understood as the temperature value at which the signal changes to the opposite sign, as well as the phenomenon when there is a delay in switching the signal depending on the magnitude of the influence. It is he who makes it possible to reduce the switching frequency, for example, when the temperature in the heater rises. But it should be understood that a large value of the hysteresis leads to a temperature jump.
6. The type of temperature sensors. The sensors connected to thermostats can be of contact and non-contact action. For example, using infrared radiation in their work or the property of a bimetallic plate.

Fixture parameters

As with any equipment, temperature controllers are characterized by a set of parameters. The accuracy of the device's response primarily depends on them. These characteristics depend not only on the quality of the elements used in the construction of the thermostat circuit, but also on the implementation of the system, which allows avoiding the influence of extraneous factors. The main characteristics include:

1. Switching time. Depends on the controller implementation scheme and the sensor installation method, which determines its inertia.
2. Adjustable range. Sets the limit values ​​of the temperature regime in which the device can operate.
3. Supply voltage. This is the value of the operating voltage required for the normal operation of the device.
4. Active load. Shows what maximum power the temperature controller can control.
5. Protection class. It characterizes the safety of the device. Designated according to the international electrical safety classification.
6. Alarm system. The regulator can use an LED signaling device or a liquid crystal display. Focusing on it, the user can immediately see in which mode the control device is operating.
7. Working temperature. Indicates the range within which the correct operation of the thermostat is ensured.
8. Thermal sensor type. As a sensing element, it acts as a temperature indicator, sending data to the controller. Such temperature sensors for turning the device on and off are of different types and designs, and also differ in the method of data transmission.

In addition, the quality characteristics of the device include: ease of use, dimensions, additional features, general appearance.

Therefore, when assembling a thermostat with your own hands, in order to obtain a complete look of the device, it is advisable to think over not only the scheme of the device, but also the case in which it will be located.

Principle of operation

In general, the thermostat can be represented in the form of a block diagram consisting of a temperature sensor, a processing unit and a regulating mechanism. The operation of a mechanical thermal relay is based on the ability of a bimetallic plate to change its shape depending on temperature. For its manufacture, two materials are used, rigidly bonded to each other with different temperature expansion coefficients.

When such a plate is heated, it bends. It is this property that is used in the production of thermal relays. During deformation, the plate closes or opens the contact group, as a result of which the electrical contact is broken or restored. Such a relay can be used in both AC and DC circuits, and the choice of the boundary temperature in them is usually set using a mechanical regulator.

In addition, there are solid state relays (electronic). There are no moving and mechanical parts in their design, but an electronic circuit that calculates temperature changes is used.

The main elements of such devices are a thermistor and a microprocessor. In the first, the electrical parameters change with temperature fluctuations, and the second processes and, depending on the programmed algorithm, switches the contact groups.

Regulator circuitry

Due to the complexity of setting up a mechanical relay, its independent manufacture is almost impossible, therefore radio amateurs make solid-state regulators. To date, a large number of thermal relay circuits of different classes are known. So choosing the right one for a possible repetition will not be difficult.

But before you start making your own thermostat, you need to prepare a number of tools and materials. For this, in addition to the electrical circuit and the radioelements necessary according to it, you will need:

1. Soldering iron or, in the case of using complex microcontrollers, a soldering station.
2. One-sided foil fiberglass. If the electrical circuit contains a large number of radioelements and belongs to the medium or high complexity group, then it is not possible to make it by hanging installation. Therefore, glass fiber laminate is used, on which a printed circuit of the future thermal relay is applied by a convenient method, for example, azure-iron or photolithography.
3. Multimeter. It is necessary to configure the operation of the device and check the correct installation of radioelements.
4. Mini drill. With the help of it, holes are made into which radio elements are installed.
5. Working materials. These include: flux, solder, alcohol solution, electrical tape, or heat shrink tubing.

The manufacturing sequence is as follows. At the first stage, a circuit is selected and its description, the availability of radioelements are studied. At the same time, do not forget that there is an analog for almost every radio component. Then, a printed circuit is made, and a board is already on it. Radioelements, commutation sockets and wires are soldered to the board. As soon as everything is ready, a test run is carried out and, if necessary, the work is adjusted.

Simple devices

The simplest device that reacts to temperature changes can be assembled from several resistances and an integrated amplifier. The resistors used are two half arms that form the measuring and reference parts of the circuit. A thermistor is used as R2, that is, a resistor whose resistance changes depending on the temperature acting on it.

The LM393 integrated amplifier operates in comparator mode, that is, it compares the two signals taken from R1-R2 and R3-R4. As soon as the signal level at the two inputs of the microcircuit is equal, the LM393 switches the load to the mains. A fan can be used as a load. As soon as the fan cools the controlled device, the signal level at the second and third inputs of the comparator will start to differ again. The device will switch its outputs again and power will be removed from the load.

A simple circuit can also be assembled on a thyristor. As its load, you can use a heater, the temperature of which will be regulated by a homemade thermostat.

This circuit can be used in an incubator or aquarium.

The circuit is also based on the comparator's ability to compare voltage levels at its inputs and, depending on this, open its outputs. With the same signal, the current does not flow through the transistor VT1, which means that the control output of the thyristor VS1 is at a low level, and it is closed. The appeared voltage across the resistance R8 leads to its opening. The circuit is powered through the diode VD2 and R10. A Zener diode VD1 is used to stabilize the power supply. The list and values ​​of the elements are given in the table:

Designation	Name	Analogue
R1	10 kΩ
R2	22 k Ohm
R3	100 kΩ
R4 = R6	6.8 kΩ
R5	1 kΩ
R8	470 Ohm
R9	5.1 kΩ
R10	27 kΩ
C1	0.33 μF
VT1	KT117	2N6027
VD1	KS212ZH	BZX30C12
VD2	KD105	1N4004
VS1	KU208G	TAG307-800

Thermal relay on a microcontroller

Having assembled such a thermostat, it can be used in conjunction with a heating system, for example, in conjunction with a boiler. The design is based on the DS1621 microcircuit, which combines a thermometer and a thermostat. Its digital I / O provides an accuracy of ± 0.5 ° C.

When the DS1621 is used as a thermostat, its internal non-volatile memory (EEPROM) stores data on the temperature that must be maintained. And also the control points, upon reaching which the temperature rises or falls. The difference between them forms a hysteresis, while a logical one or zero is formed on the third pin of the microcircuit.

Data is entered into the microcircuit using a microcontroller based on ATTINY2313. The device can maintain temperatures between 10 and 40 degrees. The boiler thermoelement is controlled through a thyristor. The S1 button switches the thermometer on and off. And the buttons S2 and S3 set the temperature. The HL1 LED signals the device's operability. When the boiler thermocouple is heating up, it flashes. TAIWAN 110-230V 6-0-6V 150TA is used as a transformer.

When programming in Features, select: int. RC Osc. 4 MHz; Start-up time: 14 CK + 0 ms; [CKSEL = 0010 SUT = 00] and Brown-out detection disabled; [B0DLEVEL = 111] check the box for Serial program downloading (SPI) enabled; [SPIEN = 0]. And also note the fuses: SUT1, SPIEN, SUTO, CKSEL3, CKSEL2, CKSELO. A properly assembled device works immediately and does not need adjustment.