Build an automatic constant current charging circuit based on discrete devices

　　This article introduces a battery automatic constant current charging circuit based on discrete components, focusing on the circuit composition, structural characteristics, working principle and circuit adjustment. With the explosive growth of the digital industry, more and more high-tech digital products have appeared on the market, all of which are inseparable from rechargeable batteries, especially nickel-hydrogen rechargeable batteries, which are the main variety of large-capacity batteries at present and have been widely used in communications, transportation, electricity and other departments. At the same time, it is also the most commonly used backup battery in other intelligent instruments, and the battery is inseparable from the charger.

　　1 Overall circuit design idea

　　The overall block diagram of the simple battery automatic constant current charging circuit is shown in Figure 1. It consists of five parts: transformer rectifier circuit, constant current generating circuit, automatic power-off circuit, display circuit and power supply circuit.

　　The function of the transformer rectifier circuit is to convert the 220V AC power in the public power grid into appropriate current and voltage signals, thereby providing signals for subsequent circuits. The function of the constant current generating circuit is to use the transistor current source to generate a constant charging current for the circuit. The function of the automatic power-off circuit is to use the voltage characteristics of the transistor when it is saturated and turned on, so that the circuit can automatically cut off the power supply when the battery is fully charged. The function of the display circuit is to use the light-emitting diode to display the state of the circuit starting and ending charging. The function of the voltage-stabilized power supply circuit is to provide DC voltage for all the above circuits.

　　1.1 Design of transformer rectifier circuit and power supply circuit

　　The transformer rectifier circuit and the voltage-stabilized power supply circuit (as shown on the left side of the dotted line in Figure 2) are mainly composed of a transformer, a diode bridge circuit, and a capacitor. The transformer uses a conventional iron core transformer and converts the 220 V AC in the public power grid into 12 V AC, which is then rectified by a diode bridge circuit and filtered by capacitor C1. The rectified signal is led out by VC1. On this basis, a three-terminal voltage regulator CW7812 and capacitors C3 and C4 (as shown on the right side of the dotted line in Figure 2) are connected, so that the entire circuit constitutes a voltage-stabilized power supply circuit. A +12 V DC voltage is provided by point B.

　　1.2 Design of constant current circuit

　　As shown in Figure 3, the transistor current source composed of the voltage regulator VZ1, transistor VT1, resistor R1, and capacitor C2 provides a constant current. The voltage regulator voltage is 5 V, R1 is 51 Ω, and IC≈1OO mA, which is the charging current of the circuit.

　　1.3 Design of automatic charging detection circuit and indication circuit

　　As shown in the circuit of Figure 4, the automatic power-off circuit is composed of transistor VT2, voltage follower A1, voltage comparator A2, resistors R4, R5, R6, R7, R8, R11 and variable resistor RP1. When charging starts, the voltage comparator outputs a high level, VT2 is turned on, VT1 is also turned on, the indicator light emitting diode is on, and the battery is charged. You can first set the conversion switch to 1 to charge one battery, and the conversion switch to 2 to charge two batteries, and so on, to achieve charging of 1-4 batteries. When the battery is fully charged, the voltage comparator outputs a low level, VT2 is turned off, VT1 is not turned on, the light emitting diode is off, and charging is completed.

　　　　2 Simple charger overall circuit schematic and component list

　　2.1 Schematic diagram of the overall circuit of a simple charger

　　The overall circuit diagram of the simple battery automatic constant current charging circuit is shown in Figure 5. It consists of five parts: transformer rectifier circuit, constant current generating circuit, charging detection circuit, display circuit and power supply circuit. It should be noted in the overall circuit diagram that the connection between each unit circuit must be accurate, and the layout of each part must be reasonable.

　　2.2 Components List

　　The list of components used in this design is shown in Table 1.

　　3 Conclusion

　　The simple battery automatic constant current charging circuit introduced in this article has the characteristics of simple circuit, complete functions, and convenient adjustment. It is suitable for radio enthusiasts to make it themselves and for electronic training of higher vocational students.