Charger with automatic switch-off for the rechargeable flashlight. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Charger with automatic switch-off for the rechargeable 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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Most simple nickel-cadmium battery chargers, such as those used in flashlights, do not automatically stop charging. The LED signaling its progress often continues to glow (sometimes with reduced brightness) even after the battery has been fully charged. So, there is a danger of failure of some elements of the charger included in the network if the contact in the circuit of the battery being charged is broken.

Charger with automatic switch-off for the battery torch

The proposed device, the scheme of which is shown in the figure, due to a slight complication, is devoid of these disadvantages. Charging automatically stops when the battery voltage reaches the set value. The charging current depends on the capacitance of the "extinguishing" capacitor C1. The use of a full-wave rectifier (diode bridge VD1-VD4) made it possible to halve the capacitance of this capacitor compared to that required for a half-wave rectifier. This makes it possible to use a smaller capacitor. While the trinistor VS1 is closed, the rectified current flows through the LED HL1 and charges the battery GB1. The glow of the LED indicates that charging is in progress.

The opening voltage of the trinistor VS1 depends on the values of the resistors R4 and R5. As soon as it is reached, the trinistor opens, the voltage drop across it becomes less than the battery voltage. The HL1 LED will turn on in reverse polarity. All rectified current will now flow through the trinistor, and not through the LED and battery. Charging will stop and the LED will turn off.

Thanks to the capacitor C2, the current through the trinistor does not drop to zero at the end of each half-cycle of the mains voltage, which could lead to the closing of the trinistor. It remains open until the device is disconnected from the network. The trinistor will also open if the battery is accidentally or deliberately disconnected, preventing the voltage on the capacitor C2 from exceeding the permissible value and thereby protecting it and the diodes VD1 -VD4 from breakdown.

To establish the device, a variable resistor with a resistance of 4 kOhm is temporarily installed in it instead of a constant resistor R100 and a partially charged battery of three nickel-cadmium batteries is connected in series with which a variable resistor with a resistance of 100 ... 200 Ohm is connected. The battery is switched on for charging, and the total voltage on it and the series variable resistor is set by its engine to 4,3 ... Radio", 4,4, No. 2001, p. 7, 36.

By slowly reducing the resistance of the variable resistor that replaced R4, the HL1 LED is turned off. A variable resistor is soldered, its resistance is measured and replaced with a constant of the nearest value. Next, set to a minimum the engine of a variable resistor connected in series with the battery, and start charging again. Gradually increasing the resistance of this resistor, make sure that the LED goes out and charging stops at the same voltage on the battery and resistor as in the first case. Now you can, eliminating the variable resistor, connect the battery directly to the charger.

Capacitor C1 must be designed to operate at an alternating voltage with a frequency of 50 Hz, not less than 250 V. Please note that the permissible direct voltage is usually indicated on the capacitors. It must be at least 630 V. The capacitance of the capacitor is selected at the rate of 0,1 μF for every 6 mA of charging current (at a mains voltage of 220 V). Diodes and trinistor can be any that can withstand with some margin the charging current of the battery and the voltage of a fully charged battery, preferably small. Trinistor KU103A can be replaced with a more modern one and having a lower control current, for example KU112A. If its false inclusions are observed under the influence of interference, it is recommended to connect a ceramic or film capacitor with a capacity of 0,01 ... 0,1 μF between the terminals of the cathode and anode of the trinistor.

Author: A. Staroverov

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