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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING LED rechargeable flashlight. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Lighting LEDs are far superior to incandescent lamps in their energy consumption. They have become so popular that it is no longer possible to find flashlights with incandescent lamps on the market. The 2,5V, 3,5V, 6,3V and 8V incandescent bulbs used in the flashlights will require high energy power supplies. Most of them use galvanic cells of size 373 (D) - with a diameter of 34,2 and a height of 61,5 mm. The number of elements depends on the power of the flashlight. Often these are two, three, four and six elements. The most massive are manganese-zinc cells with a salt electrolyte or alkaline, they are also called alkaline - a derivative of the English word alkaline - "alkali". The electric capacity of an alkaline battery is about 1700 - 3000 mA·h. In terms of capacity, alkaline batteries are leading compared to salt batteries, the electrical capacity of which is less and amounts to 550 - 1100 mA·h. By the end of the voltage safety line and the capacity of current sources, due to self-discharge, is reduced by 15 - 30% for salt and 10% for alkaline. The capacity of manganese-zinc elements also decreases markedly with a decrease in temperature. At a temperature of -40˚С, the duration of the elements is about 5 - 10% of the duration of operation at a temperature of +20˚С. Alkaline cells have significantly higher capacitance characteristics when operating in the region of negative temperatures. In salt cells in the last stages of the discharge and at the end of it, an electrolyte leak can be observed, which leads to damage to the product. But the higher the performance of the batteries, the higher their cost. However, everyday practice shows that the price may not always correspond to the declared characteristics and quality [1,2]. A galvanic cell is classified as a primary current source that converts the chemical energy of active substances directly into electrical energy. Unfortunately, primary current sources allow only one-time use of active materials. You can extend their service lines of galvanic cells if you use an LED (LEDs) instead of a light bulb - fig. 1. To do this, it must be soldered into the E10 base from an incandescent bulb - fig. 2. But to save much more on galvanic cells, they will be replaced by a so-called secondary current source - a battery. A distinctive quality of batteries is that they can be charged and discharged many times.
The base of the light bulb consists of a sleeve - a threaded contact, an insulator and a bottom - the central contact. In flashlights, as a rule, the threaded contact of the bulb is connected to the negative pole of the power source, and the central contact is connected to the positive (although polarity is not important for an incandescent electric bulb, it works fine with alternating voltage). Another thing is the LED. It has a positive terminal - the anode, and a negative one - the cathode (Fig. 3). Therefore, they mount it in the base with the anode to the bottom, and the cathode to the sleeve - fig. 4. In this case, it will be connected to the batteries according to the polarity. The power of the LED and their number are selected depending on the capacity of the power source and the necessary operational needs (brightness level, duration of operation). It should be noted that when chemical current sources are connected in series, their capacitances do not add up.
The flashlight reflector has the shape of a truncated paraboloid. To form a uniform light flux, it is necessary that the light emitting element is in the focus of the paraboloid. To do this, experimentally find the position of the LED relative to the base. When making a light bulb on three or four LEDs, the lenses near the anode output must be ground off with a file. Along the output line, a face is formed with sides at an angle of 120˚ or 90˚, respectively. The anode leg on one diode is left. On the rest, they are shortened to 5 mm. After that, they are glued together with dichloroethane or Secunda 505 glue. Then the anodes are soldered and insulated with PVC or heat shrink tubing. Next, the anode lead is threaded into the contact of the bottom of the base and soldered. The cathode leads are soldered to the threaded contact of the base - fig. 5.
It is known that the LED is not able to control the current consumed. As a result, for its normal operation, it is necessary to connect a limiting resistor in series. For a white LED, the supply voltage is 3,2 volts (the simplest and best option - a flashlight with two galvanic cells will provide the corresponding power to the white LED, without any additional devices). But as the power source is discharged, the current flowing through the diode will decrease, and, accordingly, its brightness will decrease. You can get around this negative effect by including a voltage regulator in the circuit, which is necessary for the normal operation of the LED, but more on that later. The most common and relatively cheap are sealed lead-acid batteries. The battery is selected based on the size of the compartment reserved for the power source in the flashlight body. For a flashlight on six galvanic cells 373, you can use lead-acid, with a voltage of 6 V and a capacity of 1,3 A·h, overall dimensions 97 x 54 x 51,5 mm - fig. 6. A full battery discharge is defined as a discharge to 1.95 - 2.03 V per cell at room temperature, i.e. up to 5.85 - 6,09 V for a 6 V battery. The final charge voltage at a temperature of 20 C˚ is 2.05 - 2.15 volts per battery cell, 6.15 - 6.45 V for a six-volt battery [3]. When discharged below the permissible voltages, irreversible premature aging of the battery begins. Therefore, it will be useful to supplement the circuit with a battery discharge indicator.
The electrical circuit diagram of the converted lamp is shown in fig. 7. On transistors VT1 - 2, resistors R1 - 5, capacitor C1, LED1, a battery discharge indicator is made. Resistor R2 regulates the threshold for the LED. The value of the resistance R4 depends on the power of the LED and the power source. This indicator will inform you in time that the battery is low. The main advantage of the circuit is the clarity of operation, i.e. the signal LED lights up immediately without a smooth increase in brightness. The device quite accurately monitors the specified response threshold [4].
The integral stabilizer LM317, resistors R6, R7, capacitors C2 - C4 consists of a voltage regulator for the supply of the LED (LEDs). The selection of resistors regulates the voltage stabilization mode. To determine their value, use the program "LM317 - calculator v1.1" or "Regulator design v1.2". The load is a light bulb on LED2-4 LEDs connected in parallel, the current consumed is 35 - 70 mA each, with a lens diameter of 8 and a height of 7 mm. At 3,2 volts, their total current consumption is 180 mA (the 8-volt incandescent bulb of this flashlight consumes 600 mA!). The details of the circuit are mounted on a printed circuit board - fig. 8. The integral stabilizer LM317 is mounted on a small radiator. KT315 transistors can be replaced with KT3102, BC546, 2N5551 and others. When connecting a 12 volt power supply, it is necessary to change the resistor values: R1 - 20 k, R2 - 1,5 k, R4 - 2,2 k.
For good contact between the batteries and the light bulb, a socket with springs is provided on the back of the flashlight. It must be dismantled, but only if the back wall is used to mount the board with the battery charge indicator and the socket for connecting the charger - fig. 9. The panel with springs is transferred to another place. For example, between the board and the battery. To do this, it is fixed with self-tapping screws to the radiator - fig. 10. A socket for connecting the charger, a control unit (Fig. 11) are inserted into the flashlight body and installed on the back wall and fastened with screws and threaded couplings.
The battery is connected and inserted into the housing - fig. 12.
Connect and install contact board. Lightly press it and fix it with a bracket - fig. 13. Install the reflector with LED(s) fig. - 14.
To recharge the battery, you need a charger, which is easy to make with your own hands, while saving quite substantial funds without purchasing an industrial one. The simplest and cheapest equipment charges at a constant voltage (pociostatic mode). But more often, a combined mode is used, in which the initial current is limited. And when the specified voltage is reached, the charge is carried out when it stabilizes. It is usually called the I - U charge mode. The charge is carried out at a constant current of 0,1C (nominal battery capacity in ampere-hours) in the first stage and at a constant current source voltage in the second. Most manufacturers recommend charging cycled batteries at a constant voltage of 2,4 - 2,45 V per battery (7,2 - 7,35 V for a 6 volt battery) [3]. The charger is assembled according to the circuit shown in Figure 15. It consists of a step-down transformer Tr1, a rectifier with diodes VD1-4 and a smoothing capacitor C1, a current regulator on the integral stabilizer DA1, a resistor R1, a capacitor C2, a battery charge indicator on a transistor VT1, resistors R2-4, diode VD5 and LED1 LED, voltage stabilizer - on the integral stabilizer DA2, resistors R5-6, capacitor C3. The Bu1 plug is provided for connecting the charger to the flashlight.
Integral stabilizers are mounted on a metal case for heat dissipation. All resistors, except those indicated in the diagram, use a power of 0,125 W. For charging a 1,3A battery·h at the first stage of charging, an optimal current of 130 mA is required. To ensure the flow of current of the specified value, the resistor R1 is selected using the above programs. As the battery charges, the current decreases and the voltage rises. It is necessary to limit the final voltage value for a 6 volt battery to 7,2 V. The specified voltage is achieved by selecting the ratio of resistors R5 - 6. The glow of LED1 indicates that the battery is being charged. When the battery is fully charged, the LED turns off. For batteries with a capacity of 4,5 A·h and 7,5 A·h resistor R1 is used with a nominal value of 2,7 ohms and 1 ohms, respectively, with a power of at least 8 watt. To charge a 1 V battery, resistor R12 is used with a resistance of 5 Ohms, R470 - 6 kOhm. Diodes KD226A can be replaced with any rectifier provided for a current of at least 2 A, and VD1-4 with a diode assembly. Integrated stabilizers LM317 can be replaced by 7805. In this case, it is necessary to change the resistor values: R1 - 39 Ohm 1 W for a battery with a capacity of 1,3 A·h, 12 ohm 3 W for 4,5 A battery·h and 6,8 ohm 5 W - 7,5 A·h; R6 - 91 ohms for a 6 volt battery and R5 - 330 ohms and R6 - 510 ohms for a 12 volt battery. The KT3107 transistor can be replaced with readily available KT361, BC556, 2N5401. Literature
Author: V. Marchenko
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