Economical thermostat. Encyclopedia of radio electronics and electrical engineering

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

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For the preservation of vegetables in the autumn-winter period of the year, we use a container (a box with a double bottom), heated inside by incandescent lamps. The set temperature in it is maintained by a heat stabilizer made according to the scheme shown here. The operational amplifier K153UD2 (DA1) used in it, which works as a comparator, ensures the constancy of the set air temperature in the "vegetable store" with an accuracy of 1°C.

The principle of operation of the device is based on the imbalance of the measuring bridge, consisting of resistors R1 - R3 and a temperature sensor - thermistor R4. The bridge is powered by DC voltage stabilized by the Zener diode VD1.

The variable resistor R2 sets the response threshold of the electronic part, corresponding to the temperature of storing vegetables - from 0 to +10 ° C (it is assumed that the external air temperature is lower than inside the container). In this case, the voltage at the inverse input of the DA1 chip becomes less than at its direct input. The operational amplifier converts this voltage difference into a control current that opens the transistor VT1. When opening, the transistor turns on the trinistor VS1, and it, in turn, turns on the heater Rheating in the anode circuit. At the same time, the HL2 LED lights up, signaling that the heater is turned on.

Two or three incandescent lamps with a total power of about 200 W provide heating of the air in the vegetable store to the temperature set by the resistor R2. Its increase leads to a decrease in the resistance of the thermistor R4, balancing the measuring bridge and, as a result, disconnecting the heater from the network. When the air cools to a given temperature, the resistance of the thermistor increases to the nominal value and the described process is periodically repeated.

A distinctive feature of the proposed thermal stabilizer is the supply of its electronic part with half-wave mains voltage. So, with a positive half-wave of alternating voltage on the upper (according to the circuit) wire of the network, the current flows through the circuit: fuse FU1 - limiting resistor R7 - capacitor C3 - diode VD2 and charges the filter capacitor C1 to a voltage of 27 V, corresponding to the stabilization voltage of the zener diode VD1, and then through the HL1 LED ("Network" indicator) to power the DA1 chip and the measuring bridge to the FU2 fuse. With a negative half-wave of alternating voltage on the same mains wire, the current passes through fuse FU2, diode VD3, capacitor C3, resistor R7 and fuse FU1. It is easy to see that at this time, the transistor VT1, turned on by the emitter follower, is closed, there is no, therefore, the control current of the trinistor VS1 (the requirement for technical specifications is met). When using half-wave power and pulsed control of the trinistor, the incandescent lamps burn in full, which increases the efficiency and reliability of the device during operation.

Briefly about the details of the heat stabilizer. The temperature sensor (R4) is a thermistor MMT - 4 with a nominal resistance of 22 kOhm. Transistor P701 (VT1) can be replaced by any of the KT801, KT807, KT815, KT817, KT819, KT603, KT608 series. We will replace the K153UD2 (DA1) operational amplifier with any similar one, including those from other series. Variable (or tuning) resistor R2 can be of any type. The parameters of other elements of the device are indicated on its diagram.

Author: V. Velichkov, Perm

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