The following diagram is a simple mobile phone battery charger circuit. The design is simple, easy to build and inexpensive. It used LM7806 regulator to make regulated and stable output voltage.
230volt AC 50Hz 9v transformer
D1-D6 = 1N4007
R1 = 470ohm, R2 = 47ohm, R3 = 330ohm
R4 = 47ohm
C1 = 1000uF 25V
12V DC External Battery
IC1 = 7806 (Voltage Regulator)
Note- Use a suitable value of resistor for protection of LED1
The 220-240V AC mains source is stepped down to 9V AC by transformer X1. The transformer output is rectified by diodes D1 through D4 connected in bridge type and the positive DC source is straightly wired to the charger’s output contact, while the negative terminal is connected through current limiting resistor R2.
LED2 operates as being a power indicator with resistor R1 serving as the current limiter and LED3 signifies the charging status. While in the charging period, about 3 volts drop happens across resistor R2, which switches on LED3 through resistor R3.
An external DC supply source may also be applied to energized the charger, in which resistor R4, after polarity protection diode D5, limits the input current to a secure value. The 3-terminal positive voltage regulator LM7806 (IC1) delivers a fixed voltage output of 7.8V DC because LED1 interconnected in between the common terminal (pin 2) and ground rail of IC1 increases the output voltage to 7.8V DC. LED1 also acts as being an electrical power indicator for the external DC source.
A heatsink is highly recommended for protection of IC1.
Attention! – Be carefully, when you use any transformer.