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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Power supply for LED lamp. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies LED light sources are gradually replacing not only the usual incandescent lamps, but also the so-called energy-saving or CFLs. Therefore, when it was required to make a small table lamp, the choice fell, of course, on LEDs. It turned out that the easiest way is to buy an LED strip with a supply voltage of 12 V, with a number of LEDs of 30 pcs/m and a power of 4,7 W/m. Unfortunately, you cannot connect LEDs directly to the network, as they will fail. Requires a power supply that provides 12 VDC. However, the price of such a power supply in the store is quite high, so this option was not considered. I had to make my own power supply. It turned out that 18 LEDs are enough for comfortable lighting, but the power supply is designed with a small power margin. Main Specifications
Since the LED is a device with a pronounced non-linear CVC, the LED lamp is sensitive to even a small change in the supply voltage, so the voltage of the power supply must be stabilized. It should be noted that the LED lamp is not so sensitive to the pulsation amplitude, since the pulsation frequency is very high. Of course, the power supply must be short-circuit protected, built on common parts and have a high efficiency. In addition, it was still required to have a small height (no more than 15 mm). The most suitable for building such a power supply is a self-oscillating flyback converter (OHP). Its main advantage is simplicity and the fact that it is protected from short circuits at the output. Compared to the set, the push-pull converter-voltage stabilizer OHP has a higher efficiency. It is also important that in the event of a block failure, replacing a transistor is much easier than looking for a microcircuit. The power supply circuit is shown in fig. 1. Resistor R1 limits the charging current of the filter capacitor C1, and is also used as a fuse. Resistor R2 sets the initial base current of the switching transistor VT2. Zener diode VD9, optocoupler U1, transistor VT1, as well as resistors R3 and R8 form an output voltage stabilization circuit. The work of the OHP is described in detail in [1], so we will not dwell on it. You should pay attention to the diode VD5 in the base circuit of the switching transistor VT2, which many developers do not install. Without this diode, a breakdown of the transistor with a negative voltage at the base is possible. Oscilloscope measurements have shown that spikes in this voltage can exceed 5 V.
All parts are mounted on a printed circuit board, the drawing of which is shown in Fig. 2. To reduce the dimensions of the block, some of the elements (R2, R3, R5-R8, C3) are used for surface mounting of size 1206. Resistors R1, R4 - MLT, C2-23, oxide capacitors - imported. Since a rectified mains voltage is applied to the resistor R3, to prevent breakdown, it is composed of three 1 MΩ resistors connected in series. The MJE13003 transistor can be replaced by the ST13003 transistor. Instead of the BC847 transistor, you can use a low-power surface-mount transistor with a permissible collector current of at least 50 mA and a current transfer ratio of more than 50.
Diodes 1N4007 can be replaced by diodes KD243 with letter indices D, E, Zh or KD247 with indices G and D. Diode KD247G can be replaced by diodes KD257G KD257D, diode 1N4148 - by diodes KD510, KD521, KD522. Instead of the KD226D diode, you can use the KD226 diode with any letter index. Zener diode - with a stabilization voltage of about 11 V. If there is a zener diode for a lower stabilization voltage, a diode or a zener diode can be installed in series with it. For him, the board provides a seat on which a wire jumper is installed. The heat sink for the VT2 transistor is cut out from the heat sink of the computer power supply. For the transformer, a low-profile frame from an "electronic ballast" (CFL) was used, the brand of ferrite is unknown, its standard size is EE19/8/5. The magnetic circuit is assembled with a gap in the central core of 0,3 mm. Winding I is wound first, containing 148 turns of wire PEV-2 0,18, then winding II - 18 turns of the same wire, the last winding III, containing 28 turns of wire PEV-2 0,28. Each winding layer I is separated from the rest by one layer of capacitor paper 0,1 mm thick. Two layers of paper are laid between windings I and II, and three layers of paper between windings II and III. After checking, the transformer is impregnated with varnish. Inductor L1 - from CFL, inductance - 0,2 ... 1 mH, it can be made independently on a dumbbell-type ferrite magnetic circuit with a diameter of 6 mm. Winding - wire PEV-2 0,18 until filled, then varnished. To establish the unit, you will need a multimeter, an oscilloscope, an isolation transformer with an output voltage of about 150 V (for example, TAN-17-22050) and LATR. First, it is advisable to assemble the block on a breadboard, and after adjusting, mount the parts on a printed circuit board. The first connection of the unit to the transformer must be made through a 40 W incandescent lamp. A standard load must be connected to the output of the unit. Immediately, the oscilloscope checks the shape of the voltage on the current sensor - resistor R7, it should be approximately the same as shown in Fig. 3. They control the voltage at the output of the power supply, and if it differs from 12 V, you will have to select a zener diode (or zener diodes) with the required stabilization voltage. After 5 ... 10 minutes, they check how the power supply heats up. If it works normally, increase the voltage at its input to 250 V. The output voltage should remain stable. After a while, the unit is again checked for heating - during prolonged operation, the heat sink of the transistor, the transformer and the VD8 diode should not heat up above 50 оC. Then you should check the resistance of the unit to a short circuit of the output and disconnection of the load. In the event of a short circuit, a characteristic squeak with a frequency of 10 ... 15 kHz may appear. When the load is disconnected, the voltage may increase by 0,5 ... 1 V.
It is advisable to check the operation of the unit without a stabilization circuit - for this, terminals 1 and 2 of the U1 optocoupler are temporarily closed, and this is necessary with a connected load or its equivalent. The fact is that during the operation of the voltage stabilization circuit, the collector current of the transistor VT2 usually does not reach its maximum value, at which the magnetic circuit of the transformer can enter saturation. It can enter this mode when the mains voltage drops to 150 V or less. In all operating modes, the voltage shape across the resistor R7 should be the same as in Fig. 3. But it is best to check the transformer with a device, the description of which is presented in [2]. After checking the performance of all elements of the block, except for the heat sink, it is desirable to varnish. The appearance of the power supply installed in the body of the table lamp is shown in fig. 4.
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