The proposed thermal stabilizer allows you to maintain the set temperature in 0...+25°C. the Accuracy of temperature - 1...2 degrees. Maximum load capacity - 600 watts.
Diagram of the device shown in Fig. 1.
The transformer T1 reduces the voltage up to 12...15 V. Elements R1 and VD2 form of parametric stabilizer voltage. The temperature sensor is a thermistor R2. Incandescent lamps EL1 and EL2 (300 W) perform the function of the heater. The Capacitor C2 designed to extinguish the sparks on the contacts of the relay.
The desired temperature set by the variable resistor R3. In the initial state, when the temperature has not reached the predetermined value, the resistance placed inside the greenhouses of the thermistor R2 is large. As a result, the transistor VT1 is closed, relay K1 is de-energized and its contacts are shown in the diagram position. Lamp EL1 and EL2 are included and provide the heating. As heat greenhouses the resistance of the thermistor R2 is reduced, resulting increases the voltage on the base of the transistor VT1.
When the temperature inside the greenhouse rises to a predetermined value, the VT1 transistor opens, relay K1 and contacts break the chain the power light. During the cooling of the air resistance of the thermistor R2 again increases, the transistor VT1 is closed and the heating process is resumed.
Diode Assembly VD1 can be series KTS405, KTS402 index-I.e., the Transistor VT1 series CT or CT with any letter index. Zener diode VD2 can replace XA or KS147A. Relay K1 - RES-6, passport EFR 452.104-01.
All parts of the device (except for the thermistor and lamp) it is best to mount on Board and place it in a plastic box. If the length of the cable, which is connected the thermistor exceeds 20 cm, then it is advisable to use shielded wire.
If desired, you can use the electric power up to 2 kW (Fig. 2).
The relay coil K1 is desirable shall be shorted by the diode. Its cathode you have to connect to the resistor R1 and the anode to the collector of the transistor VT1. In conjunction with relay RES-6 is better to use tubular heating element(heater). When use of lamps is desirable to use a more powerful relay.
Author: E. Natenco