Replacing the battery in a small LED flashlight. Scheme, description

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Replacing the battery in a small LED flashlight. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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LED rechargeable flashlights are very popular due to their cost-effectiveness and the availability of rechargeable power sources. Unfortunately, the cheapest flashlights often use insufficiently reliable current sources. For example, there are often rechargeable batteries consisting of two lead batteries combined in one case with a paste electrolyte. stopped shining after falling from a small height. It turned out that due to the internal shorting of the plates upon impact, the battery failed. It was not possible to buy a new one, and I thought: is it worth buying a new battery of the same type, which can fail in similar circumstances? Wouldn't it be better to replace it with a more reliable current source?

A lithium battery from a cell phone or a battery of three Ni-Cd or Ni-MH batteries of AA or AAA size (depending on the size of the flashlight) may well be suitable for replacement. In the latter case, it is advisable to use a cassette (or, as it is also called, a battery compartment) calculated for the corresponding number of elements. Such cassettes have a different design and you can choose the right one for a particular lamp.

Since the parameters of super-bright LEDs have a noticeable spread, when using a battery of AA or AAA batteries, you should first determine how many of them will be enough to operate the flashlight (it is possible that you will have to use not three, but four batteries). Since the built-in network charger of the flashlight is made according to the scheme with a ballast capacitor, it is also necessary to measure the charging current and, knowing the capacity of the battery, calculate the duration of its charging. To prevent overcharging the battery, it is recommended to install a charge limiter in the flashlight.

Replacing the battery in a small-sized LED lamp
Fig. 1

A diagram of one of the options for a rechargeable LED flashlight with such a limiter is shown in fig. 1 (the numbering of the parts of the latter continues what was started on the lantern diagram). The device is assembled on the basis of a parallel voltage regulator TL431CLP (DA1) and is included in the gap (shown in the diagram with a cross) of the wire connecting the output of the bridge rectifier (VD1-VD4) to the GB1 battery. In charging mode, capacitor C2 smooths out possible surges in voltage and charging current in case of unreliable contact of the mains plug in the socket (for more information about this method of protecting LEDs, see the article by the author "Protecting the LEDs of a battery lantern", published in Radio, 2009, No. 8, p. 47). The charging current enters the battery through the diode VD5. After charging the capacitor C3 through the resistor R4 (this happens in a fraction of a second after connecting the flashlight to the network), the field-effect transistor VT1 opens and the input of the DA1 microcircuit is connected through the tuning resistor R7 to the battery - its voltage control begins. If it does not exceed a predetermined value, a current of no more than a few microamperes flows through the DA1 chip and it does not affect the charging process.

As the battery charges, the voltage on it rises and at a certain moment the current through the DA1 chip begins to increase rapidly, and the charging current of the battery GB1 decreases. When the current through DA1 reaches 70 ... 80 ° about the output current of the charger, the HL2 LED starts to shine Further the battery charging current decreases, and the current through the stabilizer DA1 and the brightness of the HL2 LED grow, which allows you to stop the process in time and thereby exclude the possibility of recharging the battery. a small current equal to the sum of the reverse current of the VD1 diode and the current through the closed transistor VT5.

Replacing the battery in a small-sized LED lamp
Fig. 2

The details of the charge limiter are mounted on a printed circuit board (Fig. 2) from one-sided foiled fiberglass with a thickness of 1 ... 1,5 mm. The board is designed for the use of fixed resistors MLT, S2-23, R1-4, trimmer SDR-19a and imported oxide capacitors (for example, the TK series from Jamicon) LED HL2 - any (but not blinking) in a case with a diameter of 3 ... 5 mm and necessarily a red glow (such LEDs have a minimum forward voltage). The appearance of the mounted board is shown in fig. 3.

Replacing the battery in a small-sized LED lamp

Set up the device in the following order. After soldering the output of the cathode of the VD5 diode from the board, a multimeter switched to the current measurement mode is included in the open circuit. Then set the engine of the tuning resistor R7 to the lower (according to the diagram) position, connect the flashlight to the network and measure the charging current Izar1. Knowing the capacity of the SAB battery, the charging time (in hours) is calculated using the formula tzap = (1,1 ... 1,3) SAB / Iear1. After this time, the slider of the resistor R7 is transferred to a position in which the charging current decreases to the value Izar2 = (0,1 ... 0,15) Izar1 and by selecting the resistor R6, the HL2 LED is lit.

In conclusion, the resistor R7 reduces the charging current to a value of Icharge3 = 3 ... 5 mA (the brightness of the HL2 LED should increase noticeably), disconnect the lamp from the mains and solder the output of the cathode of the VD5 diode into place

Author: I. Nechaev, Moscow; Publication: radioradar.net

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Comments on the article:

Jury
A very useful article, especially in light of the fact that, according to the scheme given here, the Chinese began to rivet their lamps at an inconceivable speed. Which just as quickly stop working, like mine, for example.

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