Automatic temperature maintenance in the volume. Encyclopedia of radio electronics and electrical engineering

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

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Automatic temperature maintenance is necessary when keeping individual vegetable stores on balconies during the cold season, as well as to maintain the temperature of aquariums, greenhouses, residential premises. Electric heating can also be used as an additional, corrective one along with other types of heating, for example, in a greenhouse.

In automatic temperature maintenance devices, either contact devices (relays) or non-contact devices (thyristors) are used in the heater power control circuit. It is preferable to use trinistor keys as more reliable.

To control thyristors, they are widely used as the most accessible thyristor control circuits based on an analog of a unijunction transistor.

This circuit (Fig. 1a) is assembled on two bipolar transistors npn and pnp conductivities (VT2, VT3). Such a circuit performs phase-pulse control of the thyristor and ensures that the switching moment of the thyristor is moved to any point of the half-cycle of the mains voltage (Fig. 1, b). The thyristor turn-on control current is provided by the storage capacitor C1, connected between the emitter of the transistor VT2 and the common wire. The energy stored by the capacitance of the capacitor is close to zero at the beginning of the half-cycle and increases during the half-cycle.

The moment of the beginning of the discharge of the capacitor through the control electrode of the thyristor determines the voltage on the base of this transistor, supplied from the control circuit. A decrease in this voltage brings the moment of thyristor opening closer to the beginning of the half-cycle. And at some low control voltage, the thyristor does not open, since the storage capacitor has not yet stored energy sufficient to unlock the thyristor during the time from the beginning of the half-cycle.

Such a scheme well provides automatic temperature control in the volume by its continuous heating. However, for the initial heating of the volume, when the temperature is very low, the control circuit according to the state of the temperature sensor gives a very low control voltage, the thyristor does not open, and the volume is not heated.

Thus, a simple thyristor control circuit based on an analogue of a single-junction transistor does not provide automatic heating of the volume from a temperature that is significantly reduced relative to the required one. This situation is not acceptable to us when the electricity is temporarily cut off.

A simple scheme for automatic temperature control in the volume, free from this shortcoming, is shown in Fig. 2. The circuit provides amplitude control of the thyristor activation and turns on the heating element in the volume from any low temperature for a time until the temperature rises to the temperature set on the R2 temperature controller.

The duration of the heating cycle is controlled by the temperature sensor in the volume R1. With the initial heating of the volume or with a long absence of heating, the resistance of the sensor increases greatly, and when the regulator is connected to the network, the voltage at the base of the transistor VT1 keeps it open. Transistor VT2 opens, and the thyristor turn-on current flows through the thyristor control electrode circuit. The thyristor turns on at the beginning of each half cycle. As the volume heats up, the resistance of the sensor decreases. When the temperature in the volume equals the set one, the transistors VT1 and VT2 close. The thyristor is closed. There is no heating until the temperature in the volume drops to a value no more than 1°C below the set value. Then the heating is switched on again. The included thyristor shunts the control circuit, and it does not consume energy, which makes it possible to reduce the power of the limiting resistor R8.

The glow of the HL2 LED indicates that the device is connected to the network and that the heater circuit is working, while HL1 is not lit. The glow of HL1 indicates heating, while HL2 goes out.

The accuracy of maintaining the temperature of about 1°C is quite acceptable.

When setting up the circuit, you need to select the resistance of the resistor R6 and apply the scale of the temperature controller R2. To select R6, you need to turn on the lighting lamp as a load, break the temperature sensor circuit and, by reducing the resistance of the resistor R6 with 2 kOhm, get the lamp glowing at full heat. In the circuit, set R6 of the received face value. For different instances of thyristors, R6 may vary.

To apply the setpoint scale, turn on the resistor R2 so that in the extreme left position of the slider the resistance of the circuit is the greatest. Place the temperature sensor together with a mercury thermometer in a vessel with water and bring the water temperature (by heating it or adding ice) to the desired temperature at the beginning of the setpoint scale. Then, reducing the resistance of the resistor R3 from 47 kOhm, light the lamp. Record the resistance value R3. Move the R2 engine to the extreme right position.

Increasing the temperature of the water, fix the temperature at which the lamp goes out. This is the upper temperature of the setpoint scale. Intermediate divisions of the scale are applied according to the desired readings of a mercury thermometer at that point on the scale near the setpoint indicator, in which a slight movement of the setpoint handle causes the lamp to switch.

The setpoint scale has a wider temperature range at a higher R2 rating and vice versa. At the values ​​shown in Fig. 2, the scale range is about 6°C.

The circuit uses: as a temperature sensor R1 a thermistor type MMT-4 or KMT-1, MMT-1 from 2 to 10 kOhm; VT1 can be KT315, KT3102 with any letter; VT2 - type KT361, KT3107, KT209, KT313 with any letter; thyristor VS1 - type KU201, KU202 K-N; bridge diodes must be with a reverse voltage of more than 300 V and a forward current sufficient to power the heater; LEDs HL1 AL307G, HL2 - AL307B.

When the heater power is more than 100 W, the thyristor and rectifier diodes should be installed on radiators. The regulator can also be used as a temperature meter at the sensor installation site. To do this, turn the knob of the temperature controller to make one of the LEDs go out and the other lights up and vice versa. In this situation, the set pointer is directed on its scale to the measured temperature.

Structurally, it is advisable to protect the temperature sensor from mechanical influences. To do this, the thermistor is placed in a plastic tube. The thermistor type MMT-4 must first be removed from the metal case. Fill the tube with transformer oil and close tightly on both sides with rubber stoppers made of dense rubber. In one of the plugs, pierce two holes with a needle, into which two thin conductors in fluoroplastic insulation are inserted.

A similar design has a heating element for the aquarium. A chain of series-connected fixed resistors is placed in a tube of sufficient length. So, a heater with a power of 50 W consists of 23 resistors of 43 ohms, 5 W in a tube 50 cm long. Being in an oily environment (and the entire heater is in water), the resistors do not overheat. The wall thickness of the tube should be small.

When working with the circuit, it is necessary to follow the safety rules, since the mains voltage is present on the elements of the circuit.

Author: A. N. Romanenko

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

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