Car battery charger circuit

This battery charger has a simple structure and has corresponding protection functions. The circuit is shown in the figure.

Circuit working principle: The circuit is composed of rectifier, voltage stabilizing and current limiting circuits, and is charged by float charging.

Rectifier circuit: mainly composed of step-down transformer T and rectifier bridge BR. Convert mains power into suitable pulsating direct current.

Voltage stabilizing circuit: mainly composed of V1, VT1, R1, R2 and RB. R1 and R2 detect the output voltage of the charger and provide feedback voltage to the R terminal of V1. The output voltage of the charger cannot exceed the limit value during charging. The limit value of a 12V battery is 14.4V. When the output voltage reaches 14.4V, the divided voltage of R1 and R2 is exactly 2.5V. When the output voltage rises again, V1 turns on immediately. The shunting effect reduces the base current of VT1 and increases the partial voltage of the regulating tube VT1, thus reducing the output voltage.

Current limiting circuit: mainly composed of VT1, V2 and Re1. Re1 is used to detect charging current. In the early stages of charging (or when the polarity of the battery is mistakenly connected), the charging current is large, endangering the safety of VT1 and the battery. The maximum charging current of a 12V/44A·h battery is 11A, and the average current is 8A. When the charging current reaches the maximum value, the voltage drop on Re1 is exactly 2.5V. When overcurrent occurs, V2 is immediately turned on, causing the base of VT1 to As the current decreases, the collector current (charging current) also decreases; when the charging current does not flow, V2 does not conduct, which has no effect on VT1. In order to ensure reliable operation, the maximum charging current is generally less than the allowed maximum value, so the resistance value of Re1 can be selected to be slightly larger. In this example, Rel is 0.24Ω, and the maximum charging current is 10A.

Component selection: In order to ensure that the circuit operates reliably, the current flowing through R1 and R2 is required to be at least 1mA. In order to facilitate adjustment, R1 uses a 20kΩ multi-turn potentiometer, and seals it with paint after adjustment. It is difficult to determine Rb. The general principle is: during normal charging, the resistance of Rb should be smaller to ensure that the base of VT1 has sufficient current; when the output overcurrent occurs, Rb should be larger to ensure that V2 As soon as it is turned on, the base current of VT1 immediately decreases. Based on this, the resistance of Rb is selected as 200Ω. Both BR and VT1 generate varying degrees of heat during charging. It is recommended to install a suitable heat sink. If conditions permit, a fan can be installed for better cooling.