Nickel-cadmium battery automatic charger circuit (simple)

This nickel-cadmium battery automatic charger has status indication function. The LED emits green light when charging; when fully charged, the protection circuit operates and the LED emits red light, indicating that the battery is fully charged. When the battery is fully charged, the protection circuit automatically cuts off the charging current to prevent overcharging. Therefore, this charger can charge ordinary zinc-manganese batteries. Working Principle  　　The circuit principle is shown in Figure 1

Capacitor C1 and diodes VD1-VD4 form a step-down (current limiting) and rectifier circuit. Since the internal resistance of the capacitor is very large, the output is approximately constant current, which charges the battery through the diodes VD5-VD7, and generates a voltage drop of about 2.1V on VD5-VD7 to cause the light-emitting diode to glow (green) as a charging indication. Transistor VT and potentiometer RP form an automatic protection circuit. When the battery is fully charged, VT is saturated and turned on, automatically cutting off the charging current. At the same time, the potential of point A drops to about 0.5V. At this time, VB>VA causes the red light-emitting diode to light up, indicating the end of charging. The component selection and production component list is shown in the table below. No. Name Model Quantity R1 Resistor 510K/1W 1 R2 Resistor 100Ω 1 RP Adjustable resistor 10K 1 C1 Oil-immersed paper capacitor 1.5u/400V 1 VD1-VD4 Rectifier diode IN4004 4 VD5-VD7 Crystal diode 2CP type 3 LED Light-emitting diode red , green and 1 each 2 VT transistor 9013 1 The charger debugging is very simple. The nominal voltage of a single nickel-cadmium battery is 1.2V. When it is discharged to 1V, it should be charged. When charged to 1.35V, it is basically full. Therefore, if two AA nickel-cadmium batteries are charged at the same time, after full charging, the voltage across the battery pack should reach 1.35×2=2.7 (V). At this time, adjust RP so that the transistor VT is saturated and turned on, and VT is usually cut off. If you want to charge 4 batteries at the same time, RP should be readjusted to change the operating voltage of the protection circuit. Since this circuit is directly connected to the mains power, special attention should be paid to safety during debugging and use.