Simple thermostat. Encyclopedia of radio electronics and electrical engineering

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

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About 18 years ago, the problem arose to assemble a simple thermostat for a vegetable store. Temperature controller circuits from available sources were reviewed, selected and tested. However, simple circuits had a number of drawbacks, and there was no desire to use microcircuits. Therefore, a simple thermostat circuit was developed from the available elements.

The thermostat has the simplest and most reliable transformerless power supply (see Figure 1) resistors - R12, R13 and voltage regulator - VD1, VD2. Resistors are selected with a power of 2 W to reduce their heating. A filter capacitor is not needed.

An incandescent lamp is used as a heater, and a standard and repeatedly tested pulse-phase regulator was chosen to control it. It does not have any changes, it has been repeatedly described, so we indicate that the value of the resistor R9 is chosen from the need to reduce the voltage on the lamp to increase its service life. Resistor R9 can be changed within 1..2 kOhm to 100 kOhm.

The main measuring circuit is assembled on transistors (the simplest differential amplifier). A thermistor (RK1) is used as a temperature sensor. The thermostat must necessarily have a hysteresis (difference) between the on and off temperatures, otherwise the lamp life will be drastically reduced and the likelihood of its burnout will increase. The hysteresis value is set by resistor R7. The temperature in the vegetable store should be maintained at the level of 2..4C0.

Now about the absence of a filter capacitor in the power circuit. The pulse-phase regulator for normal operation requires just a rectified voltage. On the other hand, the change in voltage at the base of the transistor VT1 will be commensurate with the voltage ripple in the power circuit in the presence of a filter capacitor, which negatively affects the clarity of turning the thermostat on and off. Therefore, capacitors of 1 microfarads must be included in the bases of transistors VT2 and VT10. To reduce the number of capacitors in parallel with the b-e junction of the transistor VT3, a capacitor C1 is included (optimally 50 μX3V, although it can be reduced to 33 - 22 μX3V), which replaces the capacitor filtering the power supply and eliminates additional voltage filtering in the bases of transistors VT1 and VT2.

To adjust, it is necessary to measure the resistance value of the thermistor at a temperature of 2C0 and install resistors R3 and R4 (trimming resistors can be used) of the same value. Next, check the operation of the thermostat.

Parts are non-critical and allow for wide replacement.

Author: Alexander Suchinsky, villy59@mail.ru; Publication: radioradar.net

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