The following diagram is a simple mobile phone battery charger circuit. The design is simple, easy to build and inexpensive. It use LM78xx regulator to make regulated and stable output voltage.
Mobile phone chargers offered in the marketplace are quite expensive. The circuit shown right here shows up as a low-cost option to charge cell phones or battery packs having a rating of 7.2 volts, for example Nokia 6110/6150.
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 configuration 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 (for example, from a automobile battery) may also be applied to energise 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 a electrical power indicator for the external DC source.
After building the circuit on a veroboard, enclose it inside a appropriate cabinet. A little heatsink is highly recommended for IC1.