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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Electronic refrigerator defroster. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Home, household, hobby Millions of our compatriots use refrigerators made in the Soviet era in everyday life. Economical, durable, unpretentious to mains fluctuations, these devices faithfully keep their watch in the kitchen for several decades, sometimes serving several generations of the family clan. Indeed, there is something to be proud of: the power consumption of the Kodry refrigerator is only 120 W, and the maximum power of the Pamir-5 refrigerator, which is 195 W, while the temperature in the evaporator is stably maintained at -12 ° C. The total usable volume is within 160...300 dm3, and the volume of the low-temperature compartment ranges from 1 6 ... 45 dm3. It would seem that everything is fine, but one parameter overshadows the operation of this device, since it is necessary to defrost the freezer for several hours regularly once every 1 months, which is upsetting, since modern refrigerators do not require this procedure. True, this advantage is given to the user by a high price: the power consumption from a 2 V network is on average in the range of 220 ... 1200 W, and the cost of the device itself is several times more expensive than domestic ones, which is doubly expensive for the user. Below is a description of a simple electronic device for the refrigerator "Dnepr", which will allow domestic refrigerators to get rid of the procedure for defrosting the freezer while maintaining its other technical advantages, i.e. will save the user from the situation when, in general, for a year the refrigerator works for 10 months, and 2 months in the defrosting mode. Circuit operation On fig. 1 shows a circuit diagram, in fig. 2 - printed circuit board.
The mains voltage of 220 V is supplied to the control circuit from the socket, which is located in the refrigerator itself. When the switch S1 is closed, the voltage enters the primary winding of the transformer T1, from the secondary - to the bridge rectifier VD1, where it is rectified, smoothed by the capacitor C2 and stabilized at 12 V by the zener diode VD3. Further, this voltage is supplied to a voltage generator assembled on a transistor VT1 with strapping. Through the frost sensor C8, which is installed in the freezer, voltage is applied to the amplifier on the transistor VT2 with strapping. The idea is that, depending on the layer of frost in the freezer, transfer a voltage level to the amplifier that can lead to the overturning of the DD1 timer circuit and the operation of relay K1, which, with its contacts K1.1, will open a powerful key transistor VT3, and he, in turn , turns on the fan motor Ml and energizes the resistors Rl2-R14, which act as heaters. When the refrigerator is turned on in a cold state, icing in the form of frost gradually appears on the walls of the freezer, which eventually turns into a thick layer of snow, popularly called a "fur coat". The purpose of this circuit is to control the level of a certain thickness of the initial layer of frost on the walls of the freezer, and in case of its excess, turn on the heater, the heated air from which is distributed throughout the freezer by a microfan. The excess frost dissolves and the controlled layer of frost remains at the same level. Since in this case the "fur coat" is not formed, then its defrosting is not required. The important points are: the correct installation of the capacitive frost sensor C8, and the selection of the heater current, taking into account the fact that the dimensions of the freezer are different for different refrigerators, and therefore, different heating intensity is required. Freezers are made of duralumin, which has good thermal conductivity, therefore, it is first necessary to correctly install the fan, which drives heated air through a plastic bell to the metal of the freezer. Under the influence of heated air, the excess frost dissolves, then the electric motor turns off, and the system goes into "standby mode" in anticipation of the growth of the frost layer. It should be noted that in order to maintain a layer of frost at the required thickness, a blowing temperature of +10 ... +20 ° C is sufficient, since the temperature inside the freezer is at -12 ° C, therefore, power costs for the control system are insignificant. Diodes VD4 and VD5 are used to protect the circuit from overvoltages. The on state of the circuit is indicated by the green LED VD2.
Design When creating such structures, one should decide on the convenience of operating this structure with the main product. In this case, the whole circuit is located in a plastic box together with a fan motor, on which a tapering plastic socket for supplying heated air to the evaporator body is installed. In the neck of the socket there are heating elements (resistors), the power of which depends on the area of the evaporator of a particular refrigerator; in addition, the socket must be able to move in a horizontal plane, which regulates the flow of heated air to the place of heating of the evaporator pointwise or at an angle. This measure changes the heating time of the entire area of the evaporator and, as a result, the overall effect of regulating the thickness of the frost on the evaporator. It should be emphasized that the correct installation of the heater socket (the distance of the mouth of the socket from the surface of the evaporator, as well as the correct angle of attack with respect to the plane of the evaporator) are decisive, since their incorrect installation can lead to the fact that, if the evaporator is overheated excessively, the frost phase will turn into the frost phase. dew, the evaporator will completely defrost, and the refrigerator compressor will work continuously, trying to reach the desired temperature in the evaporator, which is unacceptable. Therefore, without exaggeration, the setting of this system can be called jewelry. In refrigerators of the old design, the inner case is made of galvanized iron, so it is convenient to attach the device to the refrigerator body using powerful magnets. In this case, drilling of the refrigerator body and other undesirable locksmith manipulations inside the refrigerator body, which can lead to leakage of cooled air from the refrigerator, are excluded. For the same reason, the connection of the power supply minus of the circuit to the evaporator of the freezer is made using a crocodile clip. The dimensions, shape, location of the freezer in each particular refrigerator have their own characteristics, so the user determines the location of the de-icer individually. It is most convenient to place it outside the freezer and under the evaporator. As a C8 sensor, it is convenient to take a contact pair from a relay of the RES-48 type or similar, clean the attachment point on the freezer from dirt with alcohol, glue the relay contact insulator to the evaporator body with Supercement or Moment glue. The second contact of the C8 sensor will be the evaporator body itself. The height of the contact above the evaporator is determined experimentally, it is approximately equal to 1,0 ... 1,5 mm. In other words, this height allows a layer of frost on the freezer. As the frost layer grows further, the tracking system will turn on the heater with a fan and dissolve this growth, keeping its layer of constant thickness. It is convenient to use ready-made resistors of the type OMLT-1, OMLT-2 as heaters, and for high powers - resistors of the C5-35 type. It is important to remember that for them the power load factor is 0,5, i.e. it is allowed to load these resistors by half of their nameplate power. The installation of the circuit can be carried out using a printed circuit board or surface mounting using a MGShV-0,2 mm wire. For safety reasons, the C8 sensor should be covered with a protective cover. Setting The following equipment is required for tuning: LATR, adjustable power supply, oscilloscope, tube voltmeter, multimeter, resistors for selection. Using LATR, apply a voltage of 220 V to the circuit, check the value of the constant voltage on the capacitor C2 with a multimeter, it should be about 15 V; LED VD2 is on. On the zener diode VD3, the tube voltmeter shows 12 V. Then connect the oscilloscope in parallel with the inductor L1, and put the potentiometer R5 in the middle position; while on the screen of the oscilloscope there should be harmonic oscillations with a frequency of approximately 10 MHz. Such a sufficiently high frequency was chosen from the considerations that the frost layer, which plays the role of a capacitive sensor here, has a small capacitance, therefore, to increase the sensitivity of the circuit, it is necessary to increase the generator frequency. Adjusting R5 should align the shape of the generator curve. The next step is to check the operation of the capacitive sensor C8. To do this, you need to set the potentiometer R8 engine up according to the scheme to the VT2 base. Connect an oscilloscope and a tube voltmeter in parallel with the L1 choke, and fill the space between the relay contact and the evaporator housing with a light fraction of snow, which should be scraped off from the freezer of another working refrigerator - this will be the equivalent of a capacitive C8 sensor with frost, with which a rough adjustment of the circuit should be made. A sinusoid should be visible on the oscilloscope screen, and a tube voltmeter (with a sensor contact gap of 1,5 mm) will show a voltage of about 100 mV (depending on the snow layer). Using a match, loosen the snow under the contact and check the voltmeter readings - they should change. This is an important point, since in a real circuit the growth of frost will go smoothly, and the circuit must respond quickly. At this voltage level, relay K1 should work; the electric motor Ml will turn on, and the heating of the resistors R12-R14 will begin. The electric motor can be turned off for the time being, and with a multimeter it is necessary to check the load current through the resistors R12-R14. Under optimal conditions, the load resistors will warm up to about +40°C within half an hour. To test the effect of this temperature on the freezer, fix a narrow fan nozzle at a distance of 10 mm from the freezer. Scrape snow from another working refrigerator into the chamber itself and cover the bottom of the freezer to be checked with it. Now turn on the heating and the fan, note the time by the clock. The light layer of snow on the bottom of the freezer should dissolve in about 30 minutes. Otherwise, the heating of the heaters should be corrected by increasing or decreasing the value of their resistance, or by changing the angle of attack of the fan nozzle in relation to the freezer body. After the rough setting, you can proceed to the finishing. To do this, it is necessary to fully assemble the entire circuit and turn on the experimental refrigerator in the network, wait until frost appears on the freezer of the desired thickness. When, in your opinion, its thickness is sufficient for testing, you can slightly press the sensor contact to the bottom of the chamber; at the same time, the heated fan should turn on, and the layer of frost should gradually dissolve within half an hour, and the fan motor and heating will turn off. If necessary, the scheme is reconfigured according to the above method. It should also be borne in mind that the sensitivity of the entire circuit is regulated by resistor R8. Details Capacitors: C1 - K73-1 1 with a capacity of 0,82 uFx400 V; C2 - K50-35 with a capacity of 1000 uFx25 V; the rest - type KM: C3 - 0,01 uF; C4 - 22 pF; C5 - 82 pF; C6 - 4,7 pF; C7 - 8,2 pF; C9 - 100 pF; SU - 0,1 uF; C11 -510pF. Resistors: constant type OMLT-0,25; R1 - 1 MΩ; R2, R4, R7 - 510 Ohm; R3 - 1 kOhm; R10 - 10 kOhm; R9 - 5,6 kOhm; R11 * - 22 kOhm; R12*-R14* - 720 Ohm; R5, R8 - V25P at 10 kOhm.
Transformer T1 type RM4LS; inductor L1 type SM-L15B; relay K1 type FSMR-12; switch S1 type VT2; fuse F1 type VP1-1 0,2 A; electric motor Ml - a computer "cooler" from Intel for a voltage of 12 V, a current consumption of 0,44 A. Correspondence of numbers of contacts to the printed circuit board is shown in fig. 2, and the connected external elements are given in the table:
According to the materials of the journal Radioamator Publication: cxem.net
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