Simple and easy-to-make multifunctional charging circuit

　　Author: Yu Rongsheng

　　This charger can charge 3.6V 1300mAh lithium battery, AA and AA Ni-MH batteries. In addition, this circuit can also be used as a regulated power supply for 1.5V or 3V radios and receivers. The charger can automatically switch from fast charging, slow charging, constant voltage charging, and finally trickle charging. The circuit eliminates the control components necessary for high- current charging, reduces costs, and simplifies the circuit, making it easy to make and debug. The circuit is shown in the figure above, and the figure below is the printed circuit board diagram.

　　Working principle: This charger circuit is essentially a low-voltage regulated power supply. The upper half of the circuit charges lithium batteries, and the lower half charges 1.2V nickel-metal hydride batteries. The charging principles of the two are the same. Transistor VT1 is a regulating tube. VT2 and VT3 form an error amplifier. LED 1 (two diodes are used in the lower half of the circuit) serves as the reference voltage and power indicator. When the charger output voltage is 4V and the lithium battery is at 2.8V, due to the current limit of R5 and the maximum charging current of 300mA, the voltage drop at both ends of VTlec is only 0.15V, and no radiator is needed. The charging current decreases as the battery voltage increases, and finally enters a trickle charging state. When charging imported lithium batteries, foreign circuits charge lithium batteries to about 3.8V, so the output voltage is adjusted to 4V; if it is a domestic lithium-ion battery, it should be adjusted to 4.2V. This charger is not afraid of short circuit. When the output terminal is short-circuited, the maximum current is 500mA and the VTlec voltage drop is 0.1V, so it is still safe for VT1.

　　The lower part of the circuit charges 1 to 3 No. 5 and No. 7 Ni-MH batteries, with a maximum charging current of about 220mA. Set the switch Sw to 1, 2, and 3 to charge 1-3 batteries respectively, and the corresponding output voltage should be adjusted to 1.3V, 2.6V, and 3.9V. LED2 lights up during charging, and the brightness of LED2 gradually dims as the battery voltage rises. When LED2 goes out, the battery enters trickle charging.

　　Use this circuit as the power supply of the radio and amplifier, and adjust the output voltage of the upper half circuit to the required voltage of 1.5V or 3V. If it cannot be adjusted to 1.5V, replace LED 1 or use 2 diodes. Circuit performance: When the output voltage is 1.5V and 300mA, the voltage drops by about 0.1V; when the output voltage is 3V and 300mA, the voltage drops by about 0.2V.