Small battery charger

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

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On the power of small equipment from galvanic cells and batteries at today's prices, you can literally go broke. It is more profitable, having spent one time, to switch to the use of batteries. In order for them to serve for a long time, they must be properly operated: do not discharge below the permissible voltage, charge with a stable current, and stop charging in time. But if the user himself has to monitor the fulfillment of the first of these conditions, then it is advisable to entrust the fulfillment of the other two to the charger. It is such a device that is described in the article.

During development, the task was to design a device with the following characteristics:

• wide intervals of change of the charging current and voltage of the automatic termination of charging (APZ). providing charging of both individual batteries used to power small-sized equipment, and batteries made up of them with a minimum number of mechanical switches;
• close to uniform scales of regulators, allowing to set the charging current and APL voltage with acceptable accuracy without any measuring instruments;
• high stability of the charging current when the load resistance changes;
• relative simplicity and good repeatability.

The described device fully meets these requirements. It is designed to charge batteries D-0,03, D-0,06. D-0,125, D-0,26, D-0,55. TsNK-0,45, NKGTS-1,8, their imported counterparts and batteries made up of them. Up to the set threshold for turning on the APL system, the battery is charged with a stabilized current, independent of the type and number of cells, while the voltage on it gradually increases as it charges. After the system is triggered, the previously set constant voltage is stably maintained on the battery, and the charging current decreases. In other words, there is no overcharging and discharging of the battery, and it can remain connected to the device for a long time.

The device can be used as a power supply unit for small equipment with adjustable voltage from 1,5 to 13 V and protection against overload and short circuit in the load.

The main technical characteristics of the device are as follows:

• charging current at the "40 mA" limit - 0...40, at the "200 mA" limit - 40...200 mA;
• instability of the charging current when the load resistance changes from 0 to 40 Ohm - 2.5%;
• APZ actuation voltage regulation limits - 1,45 ... 13 V.

The schematic diagram of the device is shown in fig. 1.

A current source on a transistor \L "4 is used as a charging current stabilizer. Depending on the position of the switch SA2, the current in the load In is determined by the relations: IН \u10d (UB - UBE) / R9 and IН \u10d (UB - UBE) / (R4 + R9 ), where UB is the voltage at the base of the transistor VT10 relative to the positive bus, V; UBE is the voltage drop at its emitter junction, V; RXNUMX, RXNUMX are the resistances of the corresponding resistors, Ohm.

It follows from these expressions that by changing the voltage at the base of the transistor VT4 with a variable resistor R8. load current can be adjusted over a wide range. The voltage across this resistor is maintained by a constant zener diode VD6, the current through which, in turn, is stabilized by a field-effect transistor VT2. All this ensures the instability of the charging current specified in the technical specifications. The use of a stable current source controlled by voltage made it possible to change the charging current down to very small values, to have a current regulator scale close to uniform (R8) and simply switch the limits of its regulation.

APS system. triggered after reaching the maximum allowable voltage on the battery or battery, includes a comparator on the op-amp DA1, an electronic key on the transistor VT3, a zener diode VD5. current stabilizer on the transistor VT1 and resistors R1 - R4. The HL1 LED serves as an indicator of charging and its completion.

When a discharged battery is connected to the device, the voltage on it and the non-inverting input of the op-amp DA1 is less than the exemplary one on the inverting one, which is set by the variable resistor R3. For this reason, the voltage at the output of the op-amp is close to the voltage of the common wire, the transistor VT3 is open, a stable current flows through the battery, the value of which is determined by the positions of the variable resistor R8 and switch SA2.

As the battery charges, the voltage at the inverting input of the op-amp DA1 increases. The voltage at its output also increases, so the transistor VT2 exits the current stabilization mode, VT3 gradually closes and its collector current decreases. The process continues until then. until the zener diode VD6 ceases to stabilize the voltage across resistors R7, R8. With a decrease in this voltage, the transistor VT4 begins to close and the charging current decreases rapidly. Its final value is determined by the sum of the battery self-discharge current and the current flowing through the resistor R11. In other words, from that moment on, the voltage set by the resistor R3 is maintained on the charged battery, and the current necessary to maintain this voltage flows through the battery.

The HL1 LED indicates the inclusion of the device in the network and two phases of the charging process. In the absence of a battery, a voltage is set on the resistor R11, determined by the position of the variable resistor R3 slider. Very little current is required to maintain this voltage, so HL1 glows very faintly. At the moment the battery is connected, the brightness of its glow increases to the maximum, and after the APL system is activated after charging is completed, it abruptly decreases to the average between those mentioned above. If desired, you can limit yourself to two levels of glow (weak, strong), for which it is enough to choose a resistor R6.

The details of the device are mounted on a printed circuit board, the drawing of which is shown in fig. 2. It is made by cutting through the foil and is designed for the installation of fixed resistors MLT, tuning (wire) PPZ-43. capacitors K52-1B (C1) and KM (C2). The VT4 transistor is mounted on a heat sink with an effective heat dissipation area of ​​100 cm2. Variable resistors R3 and R8 (PPZ-11 of group A) are fixed on the front panel of the device and are provided with scales with corresponding marks.

(click to enlarge)

Switches SA1 and SA2 - any type, it is desirable, however, that the contacts used as SA2 are designed for switching current of at least 200 mA.

The mains transformer T1 must provide an alternating voltage of 20 V on the secondary winding at a load current of 250 mA.

Field-effect transistors KP303V can be replaced with KP303G - KP303I, bipolar KT361V - with transistors of the KT361 series. KT3107, KT502 with any letter index (except A), and KT814B - on KT814V, KT814G, KT816V, KT816G. Zener diode D813 (VD5) must be selected with a stabilization voltage of at least 12,5 V. Instead, it is permissible to use D814D or any two low-power zener diodes connected in series with a total stabilization voltage of 12,5 ... 13,5 V. It is possible to replace PPP-11 ( R3, R8) variable resistors of any type of group A, and PPP-43 (R10) - a tuned resistor of any type with a dissipation power of at least 3 W.

Setting up the device begins with the selection of the brightness of the HL1 LED. To do this, switch the switches SA1 and SA2, respectively, to the positions "13 V" and "40 mA". and the engine of the variable resistor R8 - on average, connect a resistor with a resistance of 1 ... 2 Ohms to the sockets XS50 and XS100 and find this position of the engine of the resistor R3. which changes the brightness of the glow HL1. An increase in the difference in the brightness of the glow is achieved by selecting the resistor R6.

Then, the boundaries of the intervals for regulating the charging current and the APL voltage are set. By connecting a milliammeter with a measurement limit of 200 ... 300 mA to the output of the device. move the slider of the resistor R8 to the lower (according to the scheme) position, and the SA2 switch to the "200 mA" position. By changing the resistance of the trimmer resistor R10, the device arrow is deflected to the mark of 200 mA. Then the slider R8 is moved to the upper position and by selecting the resistor R7 they achieve readings of 36 ... 38 mA. Finally, switch SA2 to the "40 mA" position. return the slider of the variable resistor R8 to the lower position and by selecting R9 set the output current in the range of 43 ... 45 mA.

To adjust the limits of the APL voltage regulation interval, switch SA1 is set to the "13 V" position, and a DC voltmeter with a measurement limit of 15 ... 20 V is connected to the device output. positions of the slider of the resistor R1. After that, by moving SA4 to the "4,5 V" position, in the same positions of the R13 slider, set the instrument arrow to 3 and 1 V by selecting the resistor R4,5.

Next, reconnecting a milliammeter to the output, calibrate the scale of the charging current regulator (R8). and with the help of a voltmeter - the scale of the voltage regulator APZ (R3).

During operation, the APL voltage is set at the rate of 1,4 ... 1,45 V per one rechargeable battery.

If the device is not supposed to be used to power radio equipment, the indication of the end of charging by extinguishing the LED can be replaced by its flashing, for which it is enough to enter the hysteresis into the comparator - add the device with resistors R12, R13 (Fig. 3), and remove the resistor R6.

After such refinement, when the set APL voltage value is reached, the HL1 LED will turn off, and the charging current through the battery will completely stop. As a result, the voltage on it will begin to drop, so the current stabilizer will turn on again and the HL1 LED will light up. In other words, when the set voltage is reached, HL1 will start flashing, which is sometimes more obvious than a certain average brightness of the glow. The nature of the battery charging process in both cases remains unchanged.

Author: N. Herzen, Berezniki, Perm region

See other articles Section Chargers, batteries, galvanic cells.

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