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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Contactless charger. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells Today there is a new way to charge mobile devices - contactless. Its essence lies in the fact that the device being charged does not have direct electrical contact with the charger. This method is used to charge mobile phones, smartphones, etc. The author offers his own version of a contactless charger for charging batteries of an LED flashlight. With frequent use of any device with replaceable batteries, such as a flashlight, there is a need for frequent replacement of galvanic batteries or periodic charging if batteries are used. To charge the batteries, you have to remove them from the flashlight body, which is not always convenient. At the same time, the so-called contactless charging technology is becoming more widespread. The principle of operation of most of these chargers (charger) is based on inductive coupling between the source and consumer of energy. The memory for a flashlight offered to the attention of readers also works on the same principle. The basis of the proposed charger is an electronic ballast from a compact fluorescent lamp (CFL). As is known, the CFL electronic ballast is a pulse generator operating at a frequency of several tens of kilohertz. Due to this frequency, all elements of the device are small in size, including transformers and ballast chokes. It is the ballast choke that is the element that limits the current through the fluorescent lamp. And in this sense, it performs the same function as the ballast capacitor in the simplest chargers - it limits (sets) the charging current. The block diagram of the memory is shown in fig. 1. From the CFL, the actual electronic ballast was used, which contains a rectifier with a smoothing capacitor, a pulse generator and a ballast choke, with which not a fluorescent lamp is connected in series, but an isolation transformer. This transformer serves as a connecting element between the charger and the flashlight battery. Since it is in series with the ballast choke, the current through it will be limited, and it works partly as a current transformer, so short circuiting its secondary winding will not lead to catastrophic consequences. The primary winding of the transformer is located in the charger housing, the secondary winding is in the lantern. A current flows through the primary winding of the transformer, which depends mainly on the inductance of the ballast choke and the mains voltage, and therefore remains relatively stable.
In the lamp, an alternating voltage arises on the secondary winding of the transformer, which is rectified and fed through the voltage limiter to the battery of the lamp. Since the current in the primary winding of the transformer is limited, it will be limited in the secondary. By changing the parameters of the current transformer, you can set the required voltage and current for charging the battery. When the battery voltage reaches the maximum value, the limiter will turn on. The voltage on the battery will stop growing, and the "extra" current will flow through the limiter. The scheme of the electronic ballast of the CFL and its refinement are shown in fig. 2. All newly introduced elements and connections are highlighted in color. A CFL with a power of 18 ... 20 W was used. After opening its case, the wire leads (4 pieces) of the fluorescent lamp are removed from the board, which are usually wound on metal pins. Then disconnect the wires connecting the board to the lamp base. The board is placed in a plastic case of a suitable size with a lid. The case must be spacious enough to accommodate additional elements in addition to the board. In the author's version, a cylindrical box with a diameter of 65 and a height of 28 mm from paper clips was used (Fig. 3). In series with the standard ballast choke L2, instead of a fluorescent lamp, they include another ballast choke L3 from a similar CFL and the primary winding T2.1 of an isolation transformer. To indicate the operation of the pulse generator, a neon indicator lamp HL10 is connected to its output through current-limiting resistors R11 and R1. The entire installation is carried out by the hinged method, a hole of the corresponding diameter is made for the indicator lamp in the housing.
An LED flashlight with a body diameter of 24 and a length of 82 mm was chosen for refinement. It uses nine LEDs and a battery of three AAA batteries. The push button power switch is located in the screw-down battery cover. LED cathodes are connected to the lamp body. The scheme of completion of the lantern is shown in fig. 4, all new elements and connections are shown in red.
The alternating voltage from the winding T2.2 of the isolation transformer rectifies the diode bridge VD1, the ripple of the rectified voltage is smoothed out by the capacitor C1. Through the diodes VD2 and VD3, the charging current enters the battery. The VD2 diode prevents the battery from discharging in standby mode, and the VD3 diode, connected in parallel-opposite to the LEDs, passes the charging current. A voltage limiter is assembled on the DA1 chip (parallel voltage regulator), the LEDs HL1, HL2 indicate the battery charging modes. At the beginning of charging, when the battery voltage is less than the nominal voltage, the voltage at the control input (pin 1) of the DA1 chip is less than the threshold. Therefore, the current through the microcircuit is small, and almost all of the rectified voltage is supplied to the circuit from the current-limiting resistor R5 and the HL2 LED (green glow), which indicates that the battery is being charged. When the battery voltage reaches the threshold value, the current through the chip will increase and the voltage drop across it will decrease to about 2 V. The charging current will flow through the resistor R3 and the DA1 chip, so the battery will gradually stop charging. In this case, the HL2 LED will go out, and HL1 (red glow) will start to shine, signaling the end of charging. The design of the device is illustrated in Fig. 5. In the cover 3 of the battery compartment there is a push-button switch 5 (SA1 in Fig. 4). One output 4 of the switch 5 is mechanically connected to the metal case of the cover 3, the second - to the spring contact 6. The switch is mechanically fixed in the cover using an insulating plastic gasket 7. On the other hand, a rubber gasket 8 is put on the switch to protect against external climatic influences.
The work is reduced to the following. A plastic casing 3 is glued to the cover 1. A hole is made in the center of the casing, in which the frame 10 is fixed with glue. The secondary winding 2 (T2.2) of the isolation transformer is wound on it. The function of the switch pusher is performed by a cylindrical magnetic circuit 11. To prevent it from falling out of the frame 10, a plastic washer 9 is glued to it. A plastic frame 12 is glued into the hole in the center of the top cover 14 of the electronic ballast housing, on which the winding 13 (T2.1) of the transformer is wound. The inner diameter of the frame for winding the coils of the transformer is chosen so that it includes a magnetic circuit 11 with a small backlash. In the author's version, a magnetic circuit with a diameter of 6 and a length of 15 mm is used from the computer power supply choke. The height of the frame is 14 - 8 ... 9 mm, the frame 10 - 6 ... 7 mm, their thickness is 0,5 ... 0,7 mm. Winding T2.1 contains 350 turns of wire PEV-2 0,18, winding T2.2 - 180 turns of wire PEV-2 0,1. Washer diameter 9 - 10 ... 12 mm, thickness - 0,5 ... 1,5 mm, the latter is selected so that the magnetic circuit 11 "does not hang out". The diameter of the casing (plastic container from the medicine) is 21 mm, its height is 11 mm. The modified lantern is shown in Fig. 6.
When using a flashlight, the magnetic circuit acts as a switch pusher. But if the lamp is turned off, the electronic ballast is connected to the network and the magnetic circuit is inserted into the frame 14 (see Fig. 5), an inductive coupling will occur between the windings T2.1 and T2.2, voltage will appear on the winding T2.2 and the battery will start charging ( Fig. 7).
The device uses small-sized fixed output resistors P1-4 or imported, LEDs - any with a case diameter of 3 mm in red and green colors of the glow. Capacitor C1 - K10-17v, it is installed on the terminals of the diode bridge VD1. The adjustment begins with the selection of the number of turns of the winding T2.2. To do this, the specified number of turns of this winding is wound and a diode bridge with a filter capacitor is connected to it. Insert the magnetic core into the frame of the winding T2.1 and put the winding T2.2 on it. A variable resistor with a resistance of 4 ohms is connected to the output of the diode bridge (see Fig. 470). By changing its resistance, the current through it and the voltage across it are controlled. It is necessary that, at the required charging current, the voltage be 4,8 ... 5 V (the voltage of a charged battery is 4,3 ... 4,4 V plus the voltage drop across the diodes VD2 and VD3). Higher voltage will increase the charging current. Since it was planned to use three batteries with a capacity of 300 ... 600 mAh in the flashlight, a charging current of about 40 mA was chosen. Based on the measurement results, a decision is made on the need to add or remove turns of the T2.2 winding. After selecting the number of turns, the winding must be protected by covering it with a layer of varnish or glue. It should be noted that their number may differ markedly from that indicated above, since this depends on the dimensions and properties of the magnetic circuit. To increase the charging current, it is necessary either to increase the number of turns of the primary winding of the current transformer, or to increase the current through it by reducing the inductance of the inductors L2 and L3 in the electronic ballast. Then, all other elements of the device are mounted on the breadboard, freshly charged batteries are installed in the battery compartment, pins 1 and 2 of the DA1 chip are temporarily closed. Insert the magnetic core into the frame of the winding T2.1, put the winding T2.2 on it and measure the voltage (vpr) at the output of the rectifier (see Fig. 4). Then, instead of the battery, a variable resistor with a resistance of 470 ohms is connected and, by changing its resistance, the same voltage is set at the output of the rectifier (vpr). Resistor R1 (see Fig. 4) is selected so that when this voltage increases (it is changed by a variable resistor) by several tens of millivolts, the HL2 LED turns off and HL1 turns on. If necessary, select the resistor R3. Its resistance should be such that when the variable resistor is turned off, the voltage at the rectifier output does not exceed and the HL1 LED lights up. It should be noted that the maximum allowable current of the TL431CLP chip is 100 mA, so the charging current should not exceed 60...70 mA. The finalization of the lamp begins with the installation of the diode VD3. To do this, remove the battery compartment, carefully remove the protective glass and squeeze out the board with LEDs from the inside. A VD3 diode is installed on the board between the outputs of the LEDs. After checking the correctness of the installation, the assembly is carried out in the reverse order and the operability of the lamp is checked. All other elements will be placed in a casing on the battery cover. Two holes are pierced in the rubber gasket 8 (see Fig. 5), into which wires are inserted in reliable insulation, for example MGTF, and soldered to the switch terminals. In this case, it may be necessary to remove the switch from cover 3 (see Fig. 5). Then the elements are placed and fixed with hot glue in the casing 1 and connected by wires. To install the LEDs in the casing, two holes with a diameter of 3 mm are made. The proposed charger can be used to charge batteries built into a variety of devices or rechargeable batteries. Depending on the design of such a device, the magnetic circuit can be installed in the frame of the T2.1 winding, and a T2.2 coil can be put on it, as well as a more radical change in the design of the transformer. Author: I. Nechaev
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