Charger with timer

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

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To protect the batteries from overcharging, a conventional charger (charger) can be powered from the mains through a timer or equipped with such a node. A variant of the memory with a timer is also offered to the attention of readers. It ensures that the battery is charged for a predetermined time, after which the charging stops.

Schematic diagram of the memory is shown in fig. one.

On capacitors C1, C2, diode bridge VD1 and zener diodes VD2, VD3, a power unit is assembled. The timer is made on a specialized ("clock") chip DD1.

The memory works as follows. After connecting it with the battery installed in place (hereinafter referred to as the battery for short) to the network and pressing the "Start" button, the counters of the DD1 chip are reset and the charging time begins. At the same time, a low logic level is set at pin 5 of DD1, transistors VT1, VT2 close and charging current flows through the battery. The HL2 LED serves as an indicator of this mode (in the absence of a battery or a broken contact in it or in connector X1, it will not light). The value of the charging current is determined by the capacitance of the capacitor C1 and in this case is 13 ... 14 mA. Zener diode VD2 limits the voltage on the transistor VT1 and the battery, and in this mode no current flows through it.

The charging time depends on the oscillation frequency of the DD1 chip generator, which, in turn, is determined by the resistance of the resistor R3 and the capacitance of the capacitor C3. With the ratings indicated on the diagram, this time is approximately 15 hours. After it expires, a voltage with a high logic level appears at pin 5 of the DD1 microcircuit and transistors VT1, VT2 open. As a result, current begins to flow through the VT1HL1 circuit, the voltage at the anode of the VD5 diode decreases (due to an increase in the voltage drop across the capacitor C1) and it disconnects the battery from the power source. A burning LED HL1 signals the end of charging. At the same time, the voltage from pin 5 through the diode VD4 is supplied to the generator and stops its operation.

If during the charging process the mains voltage disappears for some time (up to several tens of minutes), the countdown will continue (the microcircuit will be powered by the energy accumulated by capacitor C2). After the mains voltage is restored, charging will resume, but as a result, the charging time will decrease (the actual charging duration will be less than required for this time interval). If there is no mains voltage for a longer time, the timer will turn off, so to continue charging after the appearance of voltage, you will need to press the SB1 button. In this case, the process will have to be completed before the timer runs out (taking into account the battery charging time until the mains voltage fails). If the actual charging time is unknown, then in order to avoid overcharging, it is better to disconnect the battery early, discharge it (in the device powered by it or in a special discharge device) and put it on charge again.

The ratings of resistors, capacitors and types of diodes and transistors are indicated on the diagram for charging batteries 7D-0,125, "Nika" and similar foreign production. It can be adapted for charging batteries and other capacities with a voltage of 6 to 12 V. The charging current is changed by selecting the capacitance of the capacitor C1, but the elements VD1-VD3, VT1, HL1, HL2 must be designed for the flow of this current. To increase the charging current, the resistance of the resistor R2 must be proportionally reduced.

The charging time tcharge can also be varied over a wide range by selecting the capacitor C3 and the resistor R3. Its value can be found from the ratio tzap = 32 768/2F, where F is the pulse repetition rate of the generator (in our case, about 0,3 Hz).

Most of the memory parts are placed on a printed circuit board made of one-sided foil-coated fiberglass (Fig. 2). The holes in it are drilled only for the button, LEDs and mounting screws. The conclusions of all parts are soldered to the printed conductors from the side of the foil. The board is placed in a plastic case measuring 17x55x80 mm, from which two cords are output: one with a power plug at the end, the other with a mating connector for connecting the battery. For the button and LEDs, holes are drilled in the case. The connector for connecting the battery must be provided with a small protective casing made of insulating material, which excludes contact with current-carrying contacts.

In addition to those indicated in the diagram, transistors KT208A-KT208M, KT209G-KT209M (VT1), KT315 with indices G-E, I, KT312B and similar (VT2) can be used in the memory. Instead of KTs407A, it is permissible to use (with a corresponding change in the configuration of printed conductors) a diode bridge from the KTs402, KTs405, KTs412 series (or a rectifier from diodes KD102B, KD105B and similar), instead of D814B - zener diodes KS191A, D818A-D818E (VD3). LEDs - any of the series AL307, AL341 or similar foreign production with a working current of up to 20 mA. Capacitors C1, C3 - K73-17, C2 - K52-1, button SB1 - any small-sized without fixing in the pressed position, but always in a plastic case.

Establishing a memory is reduced to setting the required generation frequency by selecting elements R3, C3. You can control it with a DC voltmeter with a measurement limit of 15 ... 20 V connected to terminal 12 of the DD1 microcircuit and the negative terminal of the capacitor C2: at an oscillation frequency of 0,3 Hz, the number of pulses at this microcircuit terminal for 1 min should be equal to 18 (time charging - about 15 hours). With a smaller number of them, R3 is replaced with a resistor of proportionally lower resistance, with a larger one, with a larger one. Since the charger has a transformerless power supply, each resistor replacement should be done only after the device is disconnected from the mains.

Author: I. Nechaev, Kursk

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