Thermal cabinet. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers
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The proposed, far from new and very simple device, will help keep your supplies from frost. This is not a refrigerator, but rather a "hothouse" that provides the necessary temperature for storing vegetables and pickles during the winter cold. You can install it on the balcony, loggia or outside the window.
The electrical circuit contains an electric heater, a convection fan, an electronic control unit and a power supply.
(click to enlarge)
In this particular case, wire-wound ceramic resistors R4, R5 were used as a heater, providing the necessary storage temperature in the range of +2...+5°C in a thermal box with a volume of 0,8 m3. The power of the heater depends on the volume of the thermal box, the material from which it is made, and a number of other factors. Approximately, we can say that the heater in the form of a 60-watt light bulb provided the necessary temperature for storing potatoes in a 1,5 m3 thermal box.
It should be noted that one cannot rely on natural air circulation - cold air stagnation zones may form, therefore the use of a fan, which is turned on together with the heater, is very necessary. For the fan, you can use low-power electric motors of both direct and alternating current with a power of 5 to 15 W, structurally designed for long-term operation. Fans used in computer technology are not bad for this purpose. In this case, an electric motor DSD60 (220 V, 15 W) without a gearbox was used.
The electronic control unit is made on a single CMOS chip. On two elements DD1.1, DD1.2, a temperature control circuit is assembled in a given range. At a temperature of +2°C, the heater is turned on, at +5°C and above, it is turned off.
The temperature sensor is the thermal resistance R2 with a change range from 100 kOhm at +22°C to 285 kOhm at 0°C. Since the input of the DD1 microcircuit is very sensitive to interference from extraneous electromagnetic fields, it is protected by a blocking capacitor C1. With this power supply scheme, the use of a shielded wire for the sensor is not recommended.
On the other two elements (DD1.3, DD1.4), a fan shutdown delay relay is assembled. In this case, it is approximately one minute. The switch-off delay allows better use and distribution of the accumulated heat of the heater. Depending on the design of the heater, the delay time can be changed over a wide range by the values of R8, Sat. The electronic circuit is powered by a transformerless power supply. A quenching capacitor C2 is installed in the heater circuit.
All calculations related to the manufacture of a power supply unit, a heater, the use of quenching capacitors are quite fully covered in the literature [1, 2]. In the case of using an electric lighting lamp as a heater (not the best option), a circuit for monitoring its condition should be provided. Switching relays should be selected according to the switching current, permissible values of current and voltage on the contact groups. You can use the relay RES 9 with passports RS4.524.200 or RS4.524.201 (lslab = 30 mA, lpab up to 0,3 A at 250 V).
It is better to make the case of a drawer/cabinet from polystyrene foam 40...60 mm thick, grade PVC 2, 3, followed by gluing with fiberglass with PVC glue. Granulated foam is of little use, although much cheaper.
Do not forget that in the case of using this circuit, all elements are under mains voltage relative to the "ground", and the common wire cannot be grounded.
Literature
- Bannikov V. Simplified calculation of a transformerless power supply. - Radio amateur, 1998, N1,2.
- Bastanov V. 300 practical advice.- M.R. 1989, S.35...40.
Author: I. Semenov, Moscow region, Dubna
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