Offer, not a new and very simple device that will help to save from the frost your supplies. It's not a refrigerator, but rather "Tallink", providing the desired temperature storing vegetables and pickles during the winter cold. You can install it on the balcony, loggia or outside the window.
The electrical circuit includes a heater, a convection fan, the electronic control unit and the power supply.
(click to enlarge)
As a heater in this particular case, the used wire ceramic resistors R4, R5, providing the desired storage temperature in the interval +2...+5°C in the box has a volume of 0.8 m3. Heating power depends on the volume of the box has, the material from which it is made, and a number of other factors. Approximately we can say that the heater in the form of 60-watt bulbs provided the necessary temperature storage of potatoes in the box has a volume of 1.5 m3.
Note that you cannot rely on natural air circulation - can form zones of stagnation of cold air, so the use the fan included with the heater, it is highly necessary. For fan you can use a low power electric motors both DC and AC power from 5 to 15 watts, structurally designed for long-term work. Not bad for this purpose, suitable fans used in the computer technology. In this case, the applied motor DSD (220 V, 15 W) without reducer.
The electronic control unit implemented on a single chip CMOS structures. Two the elements DD1.1, DD1.2 assembled circuit control temperature within a predetermined range. At a temperature of +2°C, the heater is turned on, at +5°C and above - off.
The temperature sensor is a resistance R2 range changes from 100 kω +22°C to 285 ohms at 0°C. Since the input circuit DD1 very sensitive to interference from extraneous electromagnetic fields, it is protected a blocking capacitor C1. In this scheme of power application shielded wires for the sensor, it is not recommended.
On the other two elements (DD1.3, DD1.4) assembled relay turn-off delay. In this case it approximately one minute. Sleep helps fully utilize and to distribute the stored heat of the heater. Depending on the design heater, the delay time can be varied within wide limits by the values of R8, Sat. Electronic circuit powered by a transformerless power supply. In the circuit heater installed quenching capacitor C2.
All calculations associated with the manufacture of power supply, heater, application the quenching capacitors, are adequately covered in the literature [1, 2]. In case the use of electric light bulb as a heater (not the best option) should provide circuit control its condition. Switching relay should to pick up current enable, valid values of current and voltage on contact groups. You can apply relay RES 9 with passports RS4.524.200 or RS4.524.201 (lсраб=30 mA, lpaб to 0.3 A At 250 V).
The body of the drawer/Cabinet better to produce foam of a thickness of 40...60 mm, grade PVC 2, 3, followed pasting fiberglass PVC glue. Granulated foam is not useful, though considerably cheaper.
Don't forget that in case of application of this scheme all components are at mains voltage relative to the ground, and a common wire ground is impossible.
Literature
Author: I. Semenov, Moscow region, Dubna