Thermostabilizer for a mini-incubator. Scheme, description

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Thermostabilizer for a mini-incubator. 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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In Ukraine, the company "Ost-Invest" (Cherkassy) produces a household mini-incubator "Kvochka". The temperature in it is maintained by a mechanical thermostat, which is used as microswitches MP9, MP11, MP24. Their reliability leaves much to be desired. An electronic temperature stabilizer is designed to replace the mechanical temperature maintenance system (Fig. 1).

(click to enlarge)

The accuracy of maintaining the temperature in the Kvochka incubator is 0,2°C. The temperature can be set within 37...38,5°С. The temperature stabilizer contains a thermistor bridge RK1, R1 ... R8, two comparators on operational amplifiers DA1, DA2, a temperature indication unit "normal", "overheat", a sound indication unit for exceeding the upper temperature threshold on a piezo-call BQ1 and a triac control circuit VS1. The temperature stabilizer uses a power supply with a quenching capacitor C7, a half-wave rectifier based on diodes VD4, VD5. The supply voltage of the circuit is stabilized by the Zener diode VD6, smoothed and filtered by capacitors C5 and C6. Since the triac VS1 can be turned on at any polarity between the anodes A1 and A2 by a negative voltage pulse on the control electrode with respect to the anode A1, then the circuit is powered by a negative voltage.

A threshold element is assembled on the comparator DA2, which turns on the heating of the incubator. When the air temperature inside the incubator is lower than that set by resistor R2, the resistance of the thermistor RK1 is large, the voltage at pin 2 of DA2 is higher than at pin 3 of DA2, set by the divider R7R8, then a low potential is set at pin 6 of DA2, allowing the pulse generator to work on DD1.3, DD1.4 .3. LED HL60 induces the "heating" mode. Since the heating element in the Kvochka incubator is four XNUMX-watt incandescent lamps connected in series, there is no need to indicate the current flow through the load.

The generator on DD1.3, DD1.4 generates high duty cycle pulses with a repetition period of 0,7 ms. The current-amplified transistor VT4 pulses of negative polarity are fed through the limiting resistor R24 to the control electrode of the triac VS1, and it turns on. As soon as the temperature in the incubator reaches the set temperature, the resistance of the thermistor RK1 decreases so much that the voltage at pin 2 of DA2 becomes lower than at pin 3 of DA2. At this time, at pin 6 of DA2, the low-level voltage changes to high. The pulse generator is turned off, therefore, heating stops. The HL3 LED goes out, and the HL2 "normal" LED lights up. The hysteresis between the "heating" and "normal" modes is 0,2°C.

For eggs of all types of poultry during all periods of incubation, the most favorable air temperature near the eggs is in the range of 37,7 ... 38 ° C. Overheating above 39,4°C is dangerous for the development of the embryo. Overheating in the last days of incubation causes mass death of embryos [1].

A node on DA1 is designed to prevent overheating of the incubation material. When the air temperature inside the incubator exceeds the threshold value set by resistor R5, a high level voltage will appear at pin 6 of DA1, the HL1 "overheat" LED will light up.

The voltage inverted by the transistor VT1 allows the operation of the low-frequency generator on DD1.1, DD1.2. This generator modulates the amplitude of the tone generator on VT2 and BQ1. An intermittent acoustic signal notifies that the temperature has gone beyond the upper permissible limit and it is necessary to additionally open the ventilation openings or turn off the incubator.

The thermostat circuit is located on a printed circuit board measuring 115 mm x 45 mm from one-sided foil fiberglass 1,5 mm thick. The location of the conductive tracks and radio elements is shown in Fig.2.

(click to enlarge)

The board is designed for the installation of fixed resistors of the MLT type. Resistors R1 ... R8 of the bridge must be used stable with a small TCS type C2-29 with a tolerance of at least 5%. Thermistor RK1 type MMT-1. Trimmer wire resistors type SP5-16, VA-0,25W.

Capacitors C1-C4, C6 type K10-17, capacitors C7 type K73-17, electrolytic type K50-35.

Operational amplifiers DA1, DA2 are recommended to be replaced with K140UD6, the DD1 chip with K561LA7. Transistors VT1-VT4 can be replaced with other appropriate structures.

It is not possible to replace the Phillips VS1 triac with a suitable one. Zener diode VD6 can be used with a stabilization voltage of 8 ... 10 V.

Setting up the thermostat is as follows. The tone generator is pre-assembled on VT2 and BQ1 on the breadboard and the resistances of the resistors R21, R23 are specified for reliable generation, then these elements are soldered into the board. The thermistor is mounted in a dielectric tube at a distance of 125 mm from the upper edge of the mini-incubator cover in place of the mechanical temperature control unit. The tube must be able to flow air from bottom to top and up to 8 holes 0,2 mm on the side surface at the bottom where the thermistor is located.

By connecting the load to the thermostat board, turn on the incubator in the network. By controlling the air temperature inside the incubator with a thermometer, for example, TL-4 (GOST 215-73) with a division value of 0,1 ° C, at a distance of 125 mm from the upper edge of the lid, the threshold for turning on the heater with resistor R2 is set at a temperature of 37,7 ... 38 ° WITH. After half an hour of operation of the incubator, the switching threshold is specified. Then, by closing the conclusions of the triac A1 and A2, an increase in temperature is observed. At a temperature of 39 ° C, by adjusting the resistor R5, a light and sound indication of "overheating" is turned on.

On this, the establishment of a heat stabilizer can be considered complete.

Trial operation of the developed temperature controller when hatching several layings of chicken, goose, duck eggs showed complete superiority over a mechanical temperature controller. This thermostat can be used in other homemade incubators with a heater power of up to 200 watts.

References:

What you need to know about poultry farming. Recommendations to owners of household incubator "Kvochka".

Author: O.V. Belousov

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

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