When viewing the literature, I was interested in “Automatic Battery Charger” described by N. Skrindevsky in Radio, 1991, No. 12, p. 28-30. I liked the idea embedded in this design to charge the battery cyclically, alternating charging intervals with intervals measuring the emf of the battery. As a result of prototyping and debugging, the proposed charger was obtained.
Key Specifications
Supply voltage, V ............. 5
Charging current, mA …………… ..150
The threshold for switching off the charging current, V ………………… 1.38
The threshold for switching on the charging current, V …………………… .. 1
The duration of the charging cycle, s .............................. 40
Duration of measurement, s ……… 1
A diagram of this device is shown in Fig. 1. A current source is assembled on the transistor VT2, resistors R9-R12 and LED HL1. It is controlled by transistor VT1. The HL1 LED has two functions: it serves as a source of stable voltage supplied to the base of transistor VT2 through resistor R10, and at the same time as a battery indicator. Resistors R11 and R12 set the charging current, the value of which in milliamperes is chosen numerically equal to the nominal battery capacity G1 in milliampere-hours. Resistor R9 limits the current through the LED HL1. The VD2 diode prevents the discharge of the G1 battery through the charger in the event of a power failure or a power outage.
On a voltage comparator DA1.1, resistors R1-R6, capacitor C1 and diode VD1, a pulse sequence generator with a duration of 40 s with a pause of 1 s is assembled. In the pauses between pulses, the EMF of the battery is measured.
During the measurement, the current source is disconnected from the rechargeable battery. At this time, the voltage on the battery is compared with the model voltage - the one to which the battery needs to be charged. The diode VD4 prevents the blocking voltage from reaching the engine trimmer resistor R14.
On the voltage comparator DA1.2 and resistors R13-R17, a Schmitt trigger is assembled, which controls the voltage on the rechargeable battery. For the trigger to work correctly, a blocking voltage is supplied to the inverting input of the comparator DA1.2 from the output of the comparator DA1.1 during charging through the VD3 diode.
Upon reaching the voltage on the battery set by the tuning resistor R14 and applied to the inverting input of the comparator DA1.2, a high level voltage appears at the output of the last one, which is fed through the diode VD5 to the inverting input of the comparator DA1. 1, blocking the operation of the generator. At the output of the comparator DA1.1, a low voltage level is set, the transistor VT1 closes, the LED HL1 goes out.
At the same time, a high level voltage from the output of the comparator DA1.2 is also supplied to the base of the transistor VT3, opening it, the HL2 LED turns on, signaling the completion of battery charging. The reference voltage at the inverting input of the comparator DA1.2 is chosen equal to 1.38 V - the same as that of an available industrial charger.
The LM393N chip can be replaced with K1401CA3A or another of its many analogues, and KT312V transistors can be replaced with similar ones with other letter indices or with KT315 series transistors. Replacement of the KT816V transistor can be the KT814V. Instead of D223 diodes, D220 or the KD522 series are suitable, and instead of KD226A, any rectifier diode with a permissible forward current of at least 200 mA is suitable. When replacing the AL307 series LEDs with more modern ones, it is recommended to increase the values of the resistors R9 and R20 in order to reduce the brightness of their glow to an acceptable level.
Oxide capacitors C1, C2 - imported or domestic series K50-16, K50-35. Capacitors C3 and C4 - any ceramic or film. Trimmer resistor R14 - imported. Fixed resistors - MLT-0.125 or similar.The charger is assembled in a small housing from the dental handpiece. With the lid open, it is shown in fig. 2. Initially, it was planned to place the battery holder (contacts X1 and X2) directly on the printed circuit board, and the board was designed specifically for this location. Subsequently, the holder was glued into the housing cover.
The drawing of the circuit board of the charger is shown in Fig. 3. For the LM393N chip, a panel is installed on it. Permanent resistors are installed both parallel and perpendicular to the surface of the board. One of the pins of resistor R2 and pinthe cathode of the diode VD1 is soldered into the board, and the remaining free conclusions of these elements are connected above it. The battery holder and LEDs glued into the housing cover are connected to the board by flexible insulated mounting wires.
In a properly assembled device, you only need to adjust the battery charging current and charging disconnect voltage. Before setting the charging current, the DA1 chip must be removed from the panel, and instead of a battery, connect a 33 Ohm resistor or a miniature incandescent lamp MN 6.3-0.3 to the terminals X1 and X2 through a multimeter in the direct current measurement mode with a limit of at least 200 mA. A selection of resistors R11, R12 should set the multimeter to 150 mA. But you can set another charging current, depending on the capacity of the battery.
Adjusting the voltage to turn off the battery charging is reduced to setting the trimmer resistor R14 voltage of 1.38 V between sockets 2 and 4 of the comparator panel. After that, you need to disconnect the device from the power source and insert the chip into the panel. The charger is ready to use.
The width of the trigger hysteresis loop on the comparator DA1.2 depends on the ratio of the resistance of the resistors R15 and R16. Decreasing the resistance of resistor R15 increases the trigger voltage.
Author: G. Kosolapov, Kirovo-Chepetsk, Kirov Region