555 time base automatic fast charging circuit diagram

Photocoupler

Circuit Principle: The automatic charger circuit is illustrated in the figure below. The design consists of an RS flip-flop, a charging voltage upper and lower limit setting circuit, and a power supply section. The RS flip-flop is implemented using a 555 timer IC (A). The internal comparators of the 555 receive reference voltages from an external voltage regulator VS connected to pin 5. This means that the reset level of the circuit is set to the output voltage of VS, which is 3V in this case. The upper and lower limit voltage settings are achieved through potentiometers RP2/RP3 and resistors R3/R4 respectively. The power supply is derived from a transformer T that steps down the voltage, followed by a full-wave bridge rectifier (VD1–VD4) and a filter capacitor C1. During the charging process, adjust RP3 to set the lower limit voltage based on the battery type and number of cells, while RP2 sets the upper limit. When the battery voltage is low, the voltage at pin 2 of the 555 is below V5/2 (V5 being 3V), causing the 555 to be set, resulting in a high output at pin 3. This activates the charging process via RP1 and VD5, lighting up the indicator LED VL. Once the battery is fully charged, the voltage at pin 6 exceeds V5, resetting the 555 and stopping the charge. Adjusting RP1 allows control over the charging current, typically around 50 mA for standard Ni-Cd batteries. VD5 prevents reverse current flow from the battery back into the 555 after charging stops.

The automatic charger can charge four No. 5 Ni-Cd batteries simultaneously. Once the battery is fully charged, the circuit automatically stops the charging process. The circuit includes a power supply, a voltage comparator, and an indicator section. The power supply uses a transformer T to step down the voltage, followed by a full-wave rectifier (VD1–VD4), a 9V voltage regulator (A1), and filtering capacitors C1 and C2. After power-up, the circuit provides a stable 9V DC output. The voltage comparator is built using a second 555 timer (A2), with a Zener diode VS (5.6V) connected to its control pin (pin 5), setting the reset threshold at 5.6V. The LED VL acts as a charging indicator. A single Ni-Cd cell operates at 1.2V, with a termination voltage of about 1.4V. For a 4-cell battery pack, the total termination voltage is 5.6V. When the battery is placed in the charger and the switch S is turned on, the capacitor C3 initially holds a voltage, causing pin 2 of A2 to go low and triggering it. This results in a high output at pin 3, allowing current to flow through RP and VD5 to charge the battery. As the battery charges, its voltage rises until it reaches 5.6V, triggering A2 to reset and stopping the charge. The LED turns off, indicating the end of the charging cycle.

Review and Analysis: In the first circuit, components such as IN4001 silicon diodes are used for VD1–VD5. The Zener diode VS is a 3V, 0.5W unit. The LED VL is a standard red LED. RP is a 2W wirewound potentiometer, while RP2 and RP3 are small carbon film pots like WH5. Resistors R1–R4 are 1/8W carbon film types. Capacitor C1 is a CD11-25V aluminum electrolytic. The transformer T is a 220V/15V, 5VA unit. Four No. 5 Ni-Cd batteries are charged. In the second circuit, A1 is a LM7809 three-terminal voltage regulator, requiring a heat sink. VD1–VD5 are again IN4001 diodes. VS is a 5.6V Zener, such as UZ-5.6B or IN5232. RP remains a 2W wirewound potentiometer. Resistors R1–R4 are 1/8W carbon film. C1 is CD11-25V, while C2 and C3 are CD11-16V. Switch S is a standard 1×1 power switch. The transformer T is 220V/12V, 5VA. This setup ensures reliable and safe charging of Ni-Cd batteries.