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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Refrigerator LED lamp. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Lighting Today, many bright LEDs of various types have appeared on sale, which make it possible to make lighting lamps from them. For example, in articles [1] and [2], their authors share their experience in manufacturing the simplest lamps for a landing, consisting of two LEDs. The undoubted advantages of these lamps are efficiency, durability, low cost and the ability to make them in just a couple of hours. If a more advanced lamp is required, its manufacture is described in article [3]. I would like to share my experience in the field of manufacturing LED lamps. A very good lamp for the refrigerator can be made in the evening. By the way, its service life will be longer than that of the refrigerator itself, because the LEDs are not afraid of frequent switching on at low temperatures. Such lamps can be used not only in refrigerators, but also in sewing machines, microwave ovens, and various lamps. In order to avoid problems with installing the manufactured LED lamp in the refrigerator, its dimensions should not exceed the dimensions of the 230 V 15 W incandescent lamp, which it will replace. It was decided to use LEDs ASS28-WW120B21 size 3528 (3,5x2,8 mm) for surface mounting. The dimensions of an incandescent lamp can accommodate 60 of these LEDs. Their operating voltage is 3,2 ... 3,4 V at a current of 20 mA. This means that a circuit of series-connected LEDs will need to be supplied with a voltage of about 180 V. Only about 40 ... 50 V will have to be extinguished by the resistor, and the power dissipated on it will not exceed 1 W. Naturally, instead of the above LEDs, you can use any available for surface mounting, and it is not necessary to know exactly their rated current and operating voltage. To calculate the resistance and power of the quenching resistor, it is enough to roughly determine the voltage at which a current of 8 ... 10 mA flows through the LED and it glows with sufficient brightness using an adjustable power supply. If you use ordinary LEDs with pins for soldering into a hole, then only a few pieces will fit in the allowable dimensions. Almost all the mains voltage will have to be extinguished with a resistor. This will significantly increase the power dissipated by the resistor, therefore, you will have to increase the size of this resistor and the lamp itself. In this case, the lamp may not fit in the place allotted to it, and the "stove" in the refrigerator is not entirely appropriate. The lamp circuit is shown in fig. 1. The measured current through the LEDs when turned on turned out to be 6,5 mA, rising to 8 mA after a few minutes of operation, which is more than half the maximum allowable operating current. But even with this current, the brightness of the resulting lamp is visually much greater than that of 15 W incandescent lamps. The color of the glow of the LED lamp with the indicated LEDs is bluish. In my subjective perception, it is much more suitable for a refrigerator than the dull yellowish light of an ordinary incandescent lamp.
Now I will describe in detail the technology by which the LED lamp was made. We take a faulty incandescent lamp 230 V 15 W, wrap it with paper and break the glass bulb. We clean the inner side surface of the base from the remnants of glass and glue, with which the flask was glued to it. At the same time, we try not to change the shape of the base - it should remain round. It is necessary to work very carefully so as not to cut yourself with glass fragments and preferably in safety glasses so as not to injure your eyes with fragments. Then we glue the simplest fixture. From any solid sheet material 2 ... 3 mm thick (getinax, textolite or other plastic), we cut out three parts: a square 50x50 mm in size and two rectangles 5 ... 10 mm wide and 50 mm long. The square plate will serve as the base. We glue rectangular plates on it in parallel with a gap of about 2,8 mm between them. These are the guides between which we will lay the LEDs. The gap must be maintained so that the LEDs placed in it can be moved with little effort. It is most convenient to use hot melt adhesive to assemble the fixture, since while it cools down, the position of the guides can be adjusted. We put ten LEDs between the guides with the anode output of the next one tightly to the previous cathode output. For LEDs in the 3528 package, the cathode terminal is located at the beveled corner of the package. Then we apply a drop of neutral flux to each pair of contacting leads and solder with a low-power soldering iron. You need to solder quickly so as not to overheat the LEDs. It is advisable to check the finished strip by applying a constant voltage of 30 ... 32 V to it, observing the polarity. All LEDs must be lit. In total, we make six strips, each of ten LEDs connected in series. Then we put the strips in parallel so that next to the positive terminal of the first of them there is a negative terminal of the second, and next to the plus of the second - the minus of the third, and so on and connect them by soldering. We get a module with dimensions of 35x18 mm from 60 LEDs connected in series. We solder segments of leads from old transistors MP25, MP26, MP38-MP42 to the free terminals of the first and last (sixtieth) LED. The terminals of these transistors are made of an alloy that is a good conductor of electricity but a poor conductor of heat. Of course, you can use a regular single-core mounting wire, but there is a possibility that at the time of soldering the output to the board, it is soldered from the LED. Next, from the textolite foiled on one side, we cut out a board 20 mm wide and 45 mm long. At the same time, we narrow one of the narrow edges of the board to a width of about 17 mm by a length of 5 mm - with this edge, the board will be inserted into the base from the incandescent lamp. We adjust this size, gradually grinding the board with a needle file and constantly trying it on to the base. It is necessary to ensure that the board is inserted into the base with noticeable effort and firmly held in it. It should not be glued, because after screwing the lamp into the refrigerator socket, the position of the board will have to be corrected by turning it relative to the base in order to direct the light into the refrigerator compartment. After the board is fitted to the base, we put the manufactured LED module on it from the side where there is no foil, mark the holes for its outputs and drill them. Then we cut out the printed conductors in the foil that connect the LED module, the diode bridge and the quenching resistors in accordance with the circuit diagram of the lamp. Holes for the conclusions of the bridge VD1 and resistors R1, R2 are not drilled, but soldered to the foil "overlay". You can install a smoothing oxide capacitor with a capacity of 10 ... 20 μF at 400 V in parallel with the LED module, but this does not give a noticeable increase in the brightness of the LEDs (I checked), and their flickering at a frequency of 100 Hz in the absence of a capacitor is imperceptible to the eyes. Instead of the KTs407A bridge, any four diodes with a permissible reverse voltage of at least 300 ... 400 V and a rectified current of at least 50 mA are suitable. With a stranded insulated wire, we connect the free terminal of the diode bridge to the screw part of the base, and the free terminals of the resistors R1 and R2 to its central contact. The wires leading to the base should have a small margin in length so that it is possible to rotate the board relative to the base to adjust the lamp after installation in the refrigerator. The assembled lamp is shown in fig. 2.
Before screwing into the refrigerator cartridge, we check the lamp on the table. With error-free installation, it lights up immediately after connecting to the network. If the lamp does not light up, we are looking for an error. Usually this is the wrong polarity of turning on one or more LEDs or connecting a diode bridge to an LED module. In conclusion, screwing the lamp into the refrigerator cartridge, we correct the direction of the light flux by turning the board in the base. In this case, care should be taken, since touching the current-carrying parts of the lamp, which are under mains voltage, is unsafe. To protect against accidental electric shock during lamp operation, it is necessary to make a casing for its board from a polyester sheet, widely used for blister packaging of various goods, or other similar transparent plastic. Let's take an even piece of a sheet of material selected for the casing with a thickness of 0,3 ... 1 mm and dimensions of at least 80x60 mm. Let's draw on it a parallelepiped with a width of 21, a thickness of 14 and a height of 40 mm with a marker for writing on CDs. Do not forget to provide valves for gluing in the right places. To make the folds even, we push their lines with the back of the knife. If the material is thick (about a millimeter), it is better to cut the fold points to a depth of about a third of the thickness. Having cut out the development, we will bend the parallelepiped from it and glue it. It is better to use a thermal gun for this, then the gluing process will take a minimum of time, the gluing will be transparent and look neat. Putting the resulting casing on the board, fix it with two drops of hot melt adhesive. Manufacture time of the casing by this method is 15...20 min. The second version of the casing, shown in the photograph of Fig. 3, made from a box of "tic-tac" candies, which are very popular and sold in all shops, pavilions and stalls. Its dimensions are ideal for making a casing. The box needs to be cut to a length of 40 mm, then make only two cuts, one fold and one gluing - and the casing is ready. The manufacturing time of this version of the casing is even less - 5...10 min.
The quenching resistors are chosen so that the current through the LEDs is almost half the allowable one, so the LEDs are not afraid of fluctuations in the mains voltage in the direction of increasing. And a slight decrease in brightness with a decrease in mains voltage does not play any role in illuminating the refrigerator chamber. However, with an incandescent lamp, when the supply voltage is reduced, the brightness also decreases. The brightness of the manufactured lamp can easily be almost doubled by reducing the resistance of the quenching resistors (it is better to select them empirically). But it is not worth increasing the current through the LEDs to more than 15 mA, otherwise, with an increased mains voltage, it can exceed 20 mA. The lamp, of course, will not burn out, since the refrigerator door is not kept open for a long time, but each overload will gradually reduce the life of the LEDs. Literature
Author: A. Karpachev
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