Thermostat for greenhouses. Scheme, description

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Thermostat for greenhouses. 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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The temperature in greenhouses should vary depending on the light (during the day the temperature is higher, at night - lower). The temperature controller, operating from two sensors (light and temperature), meets all the requirements of a greenhouse temperature controller.

(click to enlarge)

The main parameters:

Adjustable temperature range, °С.......15...50
Adjustment accuracy, ° С ....... 0,4
Setting the threshold of illumination within, lux ....... 500-2600
Temperature difference at the transition of the threshold of illumination, °С.......±12
Permissible deviations of supply voltage, %......±20

The device consists of a temperature control unit (RT) assembled on transistors V6, V8, V10, and a temperature correction unit (CT) depending on the level of illumination (transistors V2, V4). The blocks are connected by a matching device made on the transistor V5. Depending on the position of the switch S1, the set temperature value will shift in one direction or another when the lighting conditions change. The output relay K1, which is the load of the power amplifier V10, controls the operation of the heating device with its contacts (not shown in the diagram).

Sensors - photoresistor R1 and thermistor R14 - respond to changes in illumination and temperature, respectively. The environment parameters supported by the combined controller are set according to illumination with a variable resistor R2, and according to temperature - with a variable resistor R15 and the temperature bias controller - with a variable resistor R12. RT and CT blocks are based on Schmitt triggers. To reduce the dead zone of triggers (hysteresis), diodes V3 and V7 are included in their emitter circuits.

Output relay K1, which controls a powerful contactor for turning on the heater RPU-2 with a response voltage of 24 V. You can also use a reed relay of the RPG series for the same voltage. If the switched power is relatively small (tens of watts), you can use the RES-32 relay (passport RF4.500.163 or RF4.500.131).

The power transformer is made on the magnetic circuit ShL20 x 16. The primary winding contains 3300 turns of wire PEV-2 - 0,1, winding II - 350 turns of wire PEV-2 - 0,47, winding III - 100 turns of wire PEV-2 - 0,21 . Switches S1 and S2 - P2K with fixation in the pressed position.

The adjustment of the device begins with the graduation of the scale of the resistor R15 of the RT block. The engine of the resistor R12 is set to the lower (according to the diagram) position. The temperature sensor and the reference thermometer are placed in a vessel with water and heated. The scale is calibrated according to a reference thermometer, monitoring the operation of the relay K1 at various consecutive positions of the resistor R15.

Then the scale of the variable resistor R2 of the CT block is calibrated. In parallel with the resistor R7, a voltmeter is turned on, and the switch S1 is temporarily disconnected from this resistor. If the arrow of the device does not deviate, this indicates that the trigger has already switched, i.e., the illumination is higher than the specified one. Illumination is controlled by a luxmeter (for example, 10-16). It should be borne in mind that photoresistors have a pronounced spectral dependence of resistance, so the device should be calibrated with those light sources with which it will be operated.

The scale of the resistor R12 for adjusting the temperature bias is calibrated on the scale of the resistor R15 (or on the scale of a reference thermometer). Set the switch S1 to the "-" position and, by rotating the slider of the resistor R15, come as close as possible to the position at which the V6V8 trigger is triggered. By setting a certain position of the slider of the resistor R12 and increasing the illumination of the photoresistor R1 (for example, by slightly opening the closed window of the photoresistor), rotate the slider of the resistor R15 in the direction of decreasing temperature until relay K1 is activated. The difference in readings on the scale of resistor R15 is the desired bias temperature at this position of the resistor R12 slider. Similarly, other scale marks of the resistor R12 are determined.

Set the switch S1 to the "+" position, select the resistor R11 * (within 200 ... 300 Ohms), achieving the most accurate possible coincidence of the temperature offset marks with the already graduated scale.

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

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