1. Constant current, voltage limiting and charging circuit. This part consists of 02, R6, R8, ZD2, R9, R 10 and R 13. When the power supply is turned on, the secondary of the switching transformer T 1 induces an AC voltage. After rectification and filtering by D4 and C4, the DC voltage of about12.5V is provided. One path forms a loop through R6, R 1l, R 14, LED3 (full saturation indicator) and R 15, and LED3 is on, indicating the state to be charged: the voltage of the other path is limited by R8, stabilized by ZD2(5V 1), and then regulated by R9 and R9. The constant current source mechanism consists of Q2, its base voltage divider and ZD2. When the charged battery is loaded, the voltage of 12.5V is limited by R6, and the battery is constantly charged through the C-E pole of Q2. At this time, because Ul(Ul is a flexible packaging IC with unknown model) is connected in parallel with R6. The voltage drop at both ends of R6 makes the potential at the foot ① higher than that at the foot ③, and the foot ② outputs about two negative pulses per second.
Make LED2(CH charging indicator light) flash frequently, indicating normal charging. With the gradual increase of the terminal voltage of the charged battery, that is, when the potential of the Q2 electrode reaches the set voltage limit value (4.25V), the potential of the B electrode of Q2 remains unchanged, Q2 is turned off, and the charging is finished. At this time, the Q2c pole is suspended, the ③ pin of Ul is at a high potential, the ② pin of U 1 outputs a high level, and the LED2 goes out. At this time, the current will trickle charge the battery through R6, R 1 1 and R 14, and light the LED3. LED3 is used for triple indication of charging, saturation and trickle charging.
2. Polarity identification circuit. This component consists of R 12 and LEDl (red polarity indicator for testing). The protection circuit consists of Q3 and R7. Suppose the polarity of the rechargeable battery is reversed.
The LED 1 lights in the forward direction, and the warning switch k should be switched to charge normally. If the battery is connected reversely, the I) pole of Q3 is positively biased through R7, Q3 is turned on, and the potential of the B pole of Q2 is pulled down for short circuit, which blocks the current output (otherwise the battery will be charged and scrapped), thus protecting the safety of the battery and the charger.