Lithium battery universal charger design analysis

The design has a battery polarity automatic identification circuit, which eliminates the need for manual polarity switching and is more convenient to use.

First, the circuit principle

The circuit is shown in the figure: Q1 and peripheral components constitute a self-excited oscillator. After the transformer B is stepped down and D5 is rectified, a DC voltage of 9.4 volts is obtained on C4. Current limit through R6. Q10 (TL431) after voltage regulation. The 4.23 voltage is output at the emitter of Q3 for charging the lithium battery. The polarity automatic identification circuit is composed of Q5~Q8 and R12~R15. When point A is connected to the positive pole of the battery and point B is connected to the negative pole, Q8 is turned on because R12 provides a positive bias voltage, and Q5 is biased to be turned on by R14. The charging current is formed by the charging circuit from Q5â†’A pointâ†’Battery Eâ†’B pointâ†’Q8 C pole. When point A is connected to the battery negative pole, and B is connected to the battery positive pole, the charging current is formed by Q7â†’B pointâ†’ battery Eâ†’A pointâ†’Q6 C pole to form a charging circuit. This completes the function of automatically identifying the polarity of the battery.

When a charging current flows through R6, Q4 turns on, and LED1 flashes (LED1 is a three-color LED with its own oscillator, which can randomly change the colorful flash). After charging is completed, Q4 is turned off because there is not enough conduction voltage, and LED1 is turned off. LED2 is the power indicator.

Circuit schematic

Circuit schematic

Second, common troubleshooting

Fault 1: The power indicator is off.

Overhaul: The Q1 tube is tested normally, but the b pole has no bias voltage; the R2 is measured as an open circuit, and the fault is eliminated after replacing R2.

Fault 2: The power indicator is off. Overhaul: Test Q1 has been broken, and then R1 and R3 have been opened. The other components are tested normally, and the fault is eliminated after replacing the above components.

2 times

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