Temperature controller. Scheme, description

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Temperature controller. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers

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To maintain the temperature of the air in the room, in the balcony box for storing food, water in the tank, a thermostat is suitable, the description of which is given below. It has high precision, long-term stability, and can drive a comparatively powerful heater.

The scheme of the device is shown in fig. one.

(click to enlarge)

The B1 temperature sensor is a specialized AD22100KT microcircuit, the output voltage of which depends almost linearly on the ambient temperature. You can calculate the voltage value U in volts at a temperature T, given in degrees Celsius, using the formula

where Up is the sensor supply voltage, V.

A voltage comparator is assembled on the DA2 chip. The sensor signal is fed to its input through the R1C3 filter, which suppresses interference and interference. A photothyristor U1 is used as an actuating element, switching the heater through a diode bridge VD3-VD6. LED HL2 signals about the given command to turn on the heater.

On the step-down transformer T1 and diodes VD1, VD2, a rectifier is assembled, and on the DA1 chip, a sensor supply voltage regulator. LED HL1, showing the presence of voltage at the output of the rectifier, signals the connection of the device to the network.

The regulator works as follows. The comparator inputs are supplied with voltage from the output of sensor B1 and exemplary from the engine of the variable resistor R3. If the temperature is higher than the set one, the current in the photothyristor control circuit does not flow and the latter is closed. The heater is de-energized.

As the temperature drops, the voltage at the output of the sensor will become less than the exemplary one, which will lead to the switching of the comparator. The photothyristor will open, closing the heater power circuit. The temperature of the object will rise and the instrument will return to its original state in which the heater is turned off.

Structurally, the thermostat consists of three units connected by wire harnesses with connectors. The first node is temperature sensor B1. The AD22100KT device indicated in the diagram operates in the temperature range 0...+100°С. To work in the range -40...+85 °C, you need AD22100AT, and in the range -50...+150 °C - AD22100ST. For use in a liquid medium, the sensor must be protected from direct contact with it, and its connecting wires must be reliably insulated.

The second node is the printed circuit board on which the microcircuits are installed, most of the other parts and the plugs of the X1 and X2 connectors. The sketch of the board is shown in fig. 2. It is designed for oxide capacitors of the K50-35 series or similar imported ones and for fixed resistors of the power indicated on the diagram. Wire multi-turn tuning resistor SP5-2VB (R3) is installed outside the board. Forks X1, X2 series PLS. Similar ones can be found in faulty computer modules. Diodes VD1, VD2, if necessary, are replaced by other rectifiers, for example, KD105B, KD106A.

Temperature controller

The third node is the power elements of the regulator and LEDs. All of them, together with the printed circuit board of the second node, are placed in a housing made of insulating material. A bridge of VD3-VD6 diodes can be replaced with a similar single-case assembly, for example, KVRS1006 or KVRS1004. With a load current of more than 1.. .2 A, the diode bridge and the photothyristor require heat sinks, and if the heater power exceeds 2,2 kW, these elements should be replaced with others of the appropriate power. Transformer T1 must provide a rectified voltage of 12 ... 15 V at a load current of 100 mA. Any LED will do.

The adjustment begins with setting the boundaries of the temperature interval in which the controller is planned to operate. To do this, select the values of resistors R2 and R4. The voltage drop across the resistor R4 should be equal to that calculated by the above formula for the minimum, and on the series-connected R3 and R4 - for the maximum temperature of the interval.

The value of the resistor R6 is chosen so that a current of approximately 1 mA flows in the control circuit of the photothyristor U100. If the heater is turned on and off with a "bounce", it can be eliminated by setting the resistor R9 shown in the diagram with dashed lines. It will create a small comparator switching hysteresis.

Author: I. Nechaev, Kursk

See other articles Section Power regulators, thermometers, heat stabilizers.

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