Charger for a small Li-ion battery. Encyclopedia of radio electronics and electrical engineering

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

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As you know, integrated voltage regulators (ISN) with adjustable output voltage K142EN3 and K142EN4 have a built-in overheating and overcurrent protection system and allow switching on and off by an external signal (detailed information about them can be found in the article by Yu. Ignatiev "K142EN3 and K142EN4 microcircuits", published in Radio, 1986, No. The high stability of the output voltage makes it possible to use these ISNs, for example, in devices for charging small-sized Li-ion batteries.

Fig. 1

A diagram of a possible variant of such a device is shown in Fig. 1. Chip K142EN3 (DA1) is included according to the standard scheme. Resistor R4, designed to limit the maximum load current by the built-in ISN protection unit, sets the charging current to 125 mA until the battery voltage reaches the value of 6 V set by the R8-R4,2 divider. Then the current begins to decrease, and when it becomes equal to 12,5 mA, charging stops.

To turn off the charging, the input (pin 6) of the on / off ISN DA1 is used. Its state is controlled by the node on the elements VT1, VD1, R1-R3. At the beginning of charging, the voltage across the resistor R2 is in the range of 0,75 ... 0,85 V (depending on the type of diode VD1) and the transistor VT1 is open. At pin 6, relative to pin 8, there is a low voltage level (about -0,7 V), so the DA1 chip is turned on and the battery G1 is charging. LED HL1 shines brightly. At the end of charging, as the current decreases, the diode VD1 closes and the voltage at the emitter junction of the transistor VT1 is determined by the charging current flowing through the resistor R2. When it decreases, as mentioned above, to 12,5 mA, the voltage drop across this resistor becomes insufficient to keep the transistor VT1 open and it closes. The input voltage supplied to pin 6 through resistor R1 turns off the DA1 chip and stops charging, as evidenced by a sharp decrease in the brightness of the HL1 LED, up to extinction.

Diode VD1 limits the voltage drop across resistor R2 during charging, thereby providing a negative voltage level (with respect to pin 6) safe for the microcircuit at pin 8, and VD2 turns off the LED from the charged battery when charging is complete. Capacitor C2 ensures that the device is turned on when power is applied.

The author's version of the device is designed to charge a small-sized lithium-polymer battery LP052030 (EMB) with a nominal voltage of 3,7 V and a capacity of 0,25 Ah. Due to the low charging current, the heat sink for the K142EN3 chip was not required. All parts are placed on a printed circuit board made of one-sided foil fiberglass, the drawing of which is shown in fig. 2.

Fig. 2

Resistors - any small-sized dissipation power indicated on the diagram, capacitor C3 - ceramic KM, the rest - oxide imported, transistor VT1 - any low-power npn structure with a static base current transfer coefficient h_21э not less than 200. The DA1 chip is installed on the side of the printed conductors, the rest of the parts are on the opposite side. The appearance of the mounted board is shown in fig. 3. In order not to damage the printed conductors by repeated soldering, when adjusting, I recommend soldering the selection resistors R2, R4 and R8 not to the printed conductors, but to the racks of tinned wire with a diameter of 0,5 ... 0,8 mm temporarily soldered to them.

Fig. 3

The device uses K142EN3 and K142EN4 microcircuits (in a ceramic-metal package) with any letter index. You can also use KR142EN3, KR142EN4 (in a plastic case), changing the trace of printed conductors on the board accordingly.

The charging current can be increased to 1 A. To do this, of course, you will have to reduce the resistance of the resistors R2, R4, replace the more powerful diode VD1, and install the microcircuit on a heat sink. To reduce the effect on the stability of the turn-off threshold, the diode should be selected with a maximum allowable current close to the initial charging current.

As a power source, any one that provides the required charging current at an output voltage of 9 ... 11 V is suitable (a larger value corresponds to a charging current of 1 A). The need to increase the input voltage is due to an increase in the voltage drop across the regulating element of the K142EN3, K142EN4 stabilizer. When using a source based on a step-down transformer and a rectifier bridge, it is necessary to install a smoothing capacitor with a capacity of 1000 ... 10000 μF at its output at a charging current of 0,1.1 A, respectively.

Set up the device in the following order. Without connecting the battery, connect the terminals 6 and 8 of the DA1 microcircuit with a wire jumper and, having applied the supply voltage to the input, by selecting the resistor R8, a voltage of 4 V is set on the capacitor C4,2 (tolerance is no more than ± 25 mV). To facilitate this operation, you can temporarily replace the resistor R8 with an included trimmer rheostat (resistance 22 ... 33 kOhm). Having achieved with its help a voltage close to the required one, they measure the resistance of the part of the resistor introduced into the circuit, select constant resistors of close resistance from the available ones and install one on the board, when connected, the output voltage does not go beyond the above limits. I do not recommend leaving a trimming resistor in the device instead of a selected constant due to insufficient stability of the resistance between the engine and the resistive element of most types of trimmers available.

Next, a discharged battery is connected with an ammeter connected in series with it (wires of the minimum possible length!) And a selection of resistor R4 sets the charging current equal to 0,5 C (C is the battery capacity, in our case - 0,25 Ah). After that, remove the wire jumper between the pins of the microcircuit and put the battery on charge. At its end, when the charging current drops to 0,05 C, by selecting the resistor R2 (according to the sharp, almost complete extinction of the HL1 LED), the microcircuit is turned off.

Author: S. Glibin

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