Charger for digital camera. Encyclopedia of radio electronics and electrical engineering

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

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Not all commercially available chargers (chargers) have an automatic charging stop function. This can lead to overcharging of the batteries and, as a result, to failure or reduction of their service life. In the charger developed by the author, charging stops when the battery voltage reaches a predetermined value.

The memory circuit is shown in fig. 1, it is designed to charge two Ni-Mh or Ni-Cd batteries - a common power source for a digital camera. The principle of operation of the charger is based on charging batteries with a gradually decreasing current and controlling the voltage on them. When the voltage reaches a predetermined value, the charging process stops. The initial charging current is approximately equal to 0,1CA, where CA is the nominal capacity of the battery, and by the end of charging it decreases by 25 ... 35%. Although some types of batteries allow accelerated charging with a current of up to 0,5CA or more, using a current of about 0,1CA allows you to implement a gentle charging mode, but it requires more time. The service life of the batteries in this case, as a rule, increases.

Fig. 1

On the stabistor VD1, the field effect transistor VT1, which is included as a current stabilizer, and the op-amp DA1.1, a reference voltage source for the comparator on the op-amp DA1.2 is assembled. This voltage can be adjusted by resistor R3 in the range from 2,8 to 3,4 V. An RS flip-flop is assembled on the elements DD1.1 and DD1.2, an inverter is assembled on the elements DD1.3 and DD1.4, and an electronic key.

After connecting the batteries to the memory, press the SB1 "Start" button, and the RS-trigger will switch to a state in which the outputs of the elements DD1.3, DD1.4 will be set to a low level, the transistor VT2 will open and the batteries will start charging, and the HL1 LED will light , signaling this. The voltage at the inverting input of the op-amp DA1.2 exceeds the voltage at its non-inverting input, so the output will be a voltage corresponding to a low logic level. The charging current (Izar) depends on the supply voltage (UPIT), the saturation voltage of the transistor VT2 (UVt2). voltage drop across the diode VD2 (UVD2), battery voltage (UGb1) and resistance of the resistor R10: Izar = (Upit - UVT2 - UVD2 - UGB1) / R10.

With discharged batteries (Ugb1 = 2 V) and Upit = 6 V, UvtТ2 = 0,8 V, UVD2 = 0,4 V, R10 = 27 Ohm, Icharge will be about 100 mA. As the batteries charge, the voltage across them increases and the charging current decreases. For example, with UGb1 = 3 V, Icharge = 66 mA. Knowing the nominal capacity of the rechargeable batteries, based on the above ratios, the required resistance of the resistor R10 is selected.

Charging the batteries will continue until the voltage at the inputs of the op-amp DA1.2 are equal. In this case, even a slight increase in voltage at the non-inverting input will lead to a high level at the output, the RS flip-flop will switch and transistor VT2 will close. The HL1 LED will turn off and charging will stop. Diode VD2 prevents the battery from discharging through the LED HL1.

Fig. 2

Most parts of the device are mounted on a printed circuit board made of double-sided foil fiberglass, the drawing of which is shown in Fig. 2. The foil on the first side, where the parts are installed, is used as a common wire. Connections to it of the outputs of elements (microcircuits, resistors, etc.) are shown with large black dots. The terminals of the capacitor C1 are inserted into the hole of the board, separated in different directions and soldered to the pads of the second side. One of them, connected to the "negative" terminal of this capacitor, is connected through a hole in the board by a wire jumper to the foil of the first side. In the foil around the holes into which the leads of the elements are inserted, "protective" circles with a diameter of 2 ... 2,5 mm are etched (countersinking is less desirable). Transistor VT2 is attached to the board with an MXNUMX screw, it is not necessary to use a heat sink.

The device uses MLT fixed resistors, a multi-turn tuning resistor - BOURNS 3296, an oxide capacitor - K50-35, C2 - K10-17.

The VD2 diode must be germanium or Schottky, for example 1N5819, the HL1 LED can be of any glow color, for example, AL307BM, AL307VM or similar imported ones. Button SB1 - any small-sized with self-return, for example, PKn125, PKn129, PKn129M. If the resistor R10 is replaced by two resistors connected in series - a constant 8,2 Ohm and a variable 33 Ohm (PPZ-11), then the desired battery charging current can be set. To do this, an ammeter of 0,5 ... 1 A is included in the same circuit or the scale of a variable resistor is calibrated in mA or mAh. To power the device, a cell phone network charger with an output voltage of 6 V was used.

The board is fixed with three M2 screws in a case of a suitable size, on the walls of which the SB1 button, the HL1 LED, and, if desired, a socket for connecting the power supply are installed.

Setting up the device comes down to setting the battery voltage at which charging stops. To do this, each of the batteries is pre-discharged to 1 V, the maximum reference voltage is set (the slider of the resistor R3 is in the left position according to the diagram) and charging is turned on. After 17 ... 20 hours (full charging of the batteries is carried out with a decreasing current and will require more than 15 hours), slowly rotate the slider of the resistor R3 until the LED goes out.

Author: Yu. Vinogradov, Moscow; Publication: radioradar.net

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