Here Battery charger circuit diagram designed by implementing adjustable voltage regulator LM317 with auto cut off feature. This circuit will give adjustable DC supply output and charges battery ranges from 6 volt to 12 Volt.
The LM317 is a monolithic Integrated IC comes with three different packages and it is a positive adjustable voltage regulator delivers 1.5A of load current, output voltage can be adjusted from 1.2 to 37 V.
Construction and Working
Step down (230V to 15V) transformer is used to step down the AC power supply and bridge rectifier (1N4007 X 4) converts AC supply into DC supply, C1 and C2 capacitor performs filter operation then IC1 LM317 regulates the DC power supply, VR1 variable resistor changes the supply to Adjust pin of voltage regulator and it changes the output voltage range.
Here Green LED represents the Charging condition and Red LED indicates the fully charged battery when the battery becomes fully charged then reverse voltage through Zener diode (12V) flows to transistor BD139 base and makes it turn ON due to the conduction in transistor Adjust pin of Voltage regulator connected to ground and cut the output voltage from regulator.
Connect heat sink with voltage regulator LM317 to avoid thermal runaway.
LM317 Vo Calculation
LM317 regulator gives variable output voltage and it can be varied by using Adjust pin, We can calculate Vo from IC LM317 as,
VO = VREF (1 + R2/R1) + IADJ R2
It can be modified depends on this circuit design as,
VO = VREF (1 + VR1 / R1) + I ADJ VR1
Here VO represents output voltage from regulator IC.