what is the purpose of lm317 ic in the auto turn off mobile charger

lm317 12v battery charger

Automatic Battery Charger Circuit Design:

This automatic battery charger circuit design mainly involves two sections – power supply section and load comparison section.

The main supply voltage 230V, 50Hz is connected to the primary winding of the center tapped transformer to step down the voltage to 15-0-15V.

The output of the transformer is connected to the Diodes D1, D2. Here diodes D1, D2 are used to convert low AC voltage to pulsating DC voltage. This process is also called as rectification. The pulsating DC voltage is applied to the 470uF capacitor to remove the AC ripples. Thus the output of the capacitor unregulated Dc voltage. This unregulated DC voltage is now applied to the LM317 variable voltage regulator to provide regulated DC voltage. The output voltage of this voltage regulator is variable from 1.2V to 37V and the maximum output current from this IC is 1.5A. The output voltage of this voltage regulator is varied by varying the pot 10k which is connected to the adjust pin of LM317.

Lm317 voltage regulator output is applied to the battery through the diode D5 and resistor R5. Here diode D5 is used to avoid the discharge of battery when main supply fails. When battery charges fully, the zener diode D6 which connected in reverse bias conducts. Now base of BD139 NPN transistor gets the current through the zener so that the total current is grounded. In this circuit green LED is used for indicating the charge of the battery. Resistor R3 is used to protect the green LED from high voltages.

Charger settings:

The output voltage of the battery charger should be less than 1.5 times of the battery and the current of the charger should be 10% of the battery current. Battery charger should have over voltage protection, short circuit protection and reversed polarity protection.