Automatic temperature control in the greenhouse. Encyclopedia of radio electronics and electrical engineering

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

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For comfortable growth of plants in a greenhouse, a certain ambient temperature is required. To maintain it within the specified limits, the proposed machine was developed.

Fig. 1

Fig. 2

The basis of the device is a specialized integrated temperature sensor LM56 [1, 2], designed for use in thermostats. The functional diagram and graphs explaining the features of its work are presented respectively in fig. 1 and 2. The microcircuit contains two comparators (A1, A2), a reference voltage source Uref = 1,25 V (A3), a temperature sensor A4 and two output stages on transistors VT1, VT2 with an open collector. With the help of external resistors R1-R3 and the built-in reference voltage source A3 on terminals 3 and 2, the threshold values ​​of the switching voltage of the comparators UT1 and UT2 are set, which correspond to the specified temperature values. As a result, a low-level voltage appears at the OUT1 output (pin 7) if the temperature exceeds the T1 value, and, accordingly, a high-level voltage appears if it falls below the T1 -Thyst value (temperature hysteresis equal to approximately 5 ° C). Similarly, in relation to the temperature T2, a signal is generated at the output OUT2 (pin 6). The voltage UTEMP at the output of the microcircuit (pin 5) is proportional to the temperature in degrees Celsius with the coefficient k = 6,2 mV/оС and is offset by +395 mV.

Temperature measurement error in the range of -40...+125 °С does not exceed ±3 °С for modification LM56BIM and ±4 °С for LM56CIM.

The total resistance R of the voltage divider resistors R1-R3 recommended by the developer is 27 kOhm. The resistance of each of them separately is calculated based on the following ratios:

UT1 = Uref R3/(R1+R2+R3) = Uref R3/R;

UT2 = Uref (R3+R2)/(R1 +R2+R3) = = Uref (R3+R2)/R.

At the same time, UT1(T2) = kT + 395 mV, where k = 6,2 mV/°C, and T is the temperature value corresponding to the lower (T1) or upper (T2) limit of the specified interval. Equating the right parts of the expressions for UT1 and UT2, we obtain

R3 = RUT1 /Uref = R(kT1 + 395)/Uref;

R2 = RUT2/Uref - R3 = R(kT2 + 395)/Uref-- R3;

R1 \u2d R - (R3 + RXNUMX).

Rice. 3 (click to enlarge)

A schematic diagram of a device for maintaining a given temperature in a greenhouse is shown in fig. 3. In addition to the integrated temperature sensor DA1, it contains three electronic keys on field-effect transistors VT1 - VT3, loaded with optotriacs U1, U2, two powerful triacs (VS1, VS2) that control the heating and ventilation systems of the greenhouse, and a power supply on the DA2 PPM5- A-05ELF [3], which is a converter of alternating mains voltage to a stabilized constant 5 V. The use of field-effect transistors as keys is due to the low load capacity of the outputs of the DA1 microcircuit (the maximum collector current of its output transistors is only 50 μA), which requires a sufficiently high-resistance load .

The resistor values ​​of the voltage divider R1-R3 set the thresholds for the operation of the comparators of the DA1 chip, corresponding to temperatures of approximately 18 (T1) and 26 ° C (T2). The device operation algorithm is as follows. If the temperature in the greenhouse is below 18 °C, then after the power is turned on, a high logic level appears on both outputs of the integral sensor DA1. This opens the transistors VT1 and VT2. The first of them shunts the gate-source section of the transistor VT3 and it closes, and the second, through the current-limiting resistor R7, connects the emitting diode of the optocoupler U1 to the power source. As a result, the triac of the optocoupler opens and a voltage drop is created across the resistor R9, sufficient to open a powerful triac VS1, the load of which is the heaters of the greenhouse heating system.

When the temperature in the greenhouse rises above 18 ° C, the high level at the output OUT1 (pin 7) will change to low, the transistor VT2 will close and the heating system will turn off. However, as a rule, the heating elements are inertial, that is, after being disconnected from the network, they keep heat for some time. Therefore, the air in the greenhouse will continue to heat up, and if the temperature exceeds 26 ° C, a low logic level will appear at the OUT2 output (pin 6 of DA1), the transistor VT1 will close, and VT3 will open, turning on the optotriac DA4 and a powerful triac VS2, through which the ventilation system will turn on greenhouses.

The fans will run until the temperature in the greenhouse drops to 21°C (taking into account a hysteresis of approximately 5°C). When this happens, OUT2 will go high again and ventilation will turn off. When the temperature drops to 13 °C (taking into account the hysteresis), the heaters will turn on again. The temperature interval may be different, it depends on the type of plants that you are going to grow in the greenhouse. You can also use multiple switchable dividers or use variable resistors to set different temperature ranges in the greenhouse.

With serviceable parts and no errors in the installation, the device in question does not require adjustment. It is enough to use resistors R1-R3 with a permissible resistance deviation from the nominal value of ±1%. However, compliance with this requirement is not necessary, since the normal temperature range in the greenhouse for most grown plants is from 15 to 30 ° C, which makes it possible not to set the comparator thresholds so accurately.

The device can use any low-power field-effect transistors with an insulated gate and an n-channel, in which the maximum drain current is more than 20 mA. Optotriacs MOC3063M (U1, U2) are replaceable by other similar ones with an operating voltage of at least 400 V. Powerful triacs BTA12-600 (VS1, VS2) are replaced based on the total power of the actuators they turn on - heaters, intake and exhaust fans and fanlight openers.

Fig. 4

In the absence of the LM56 (DA1) microcircuit, it is possible to assemble its analogue based on widely used microcircuits - the LM35 analog temperature sensor and the LM393 dual comparator (Fig. 4). Divider resistors R1-R3, which determine the comparator thresholds, are calculated using the above formulas, but for LM35 the conversion factor k = 10 mV / ° C, and the offset is 0. The supply voltage +5 V can be used as a reference (Uref).

We can replace the PPM5-A-05ELF voltage converter with any power source based on discrete elements that provides a stabilized output voltage of +5 V at a load current of 50...100 mA.

Literature

1. Ivanov E. National Semiconductor integrated temperature sensors. - News of electronics, 2007, No. 10, p. 13.
2. LM56 - Dual Output Low Power Thermostat - National Semiconductor. - alldatasheet.com/datasheet-pdf/pdf/8981/NSC/LM56.html.
3. PPMxx-x-xxELF. 5-25W AC/DC CONVERTER SINGLE OUTPUT. - peak-electronics.de/DATA/acdc/PPMxx-x-ELF-Sep09.pdf.

Author: A. Kornev

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