Mobile phone chargers available in the
market are quite expensive. The circuit presented here comes as a
low-cost alternative to charge mobile telephones/battery packs with a
rating of 7.2 volts, such as Nokia 6110/6150.
The 220-240V AC mains supply is down
converted to 9V AC by transformer X1. The transformer output is
rectified by diodes D1 through D4 wired in bridge SANI THEO
configuration and the positive DC supply is directly connected to the
charger’s output contact, while the negative terminal is connected
through current limiting resistor R2.
LED2 works as a power indicator with
resistor R1 serving as the current limiter and LED3 indicates the
charging status. During the charging period, about 3 volts drop
occurs across resistor R2, which turns on LED3 through resistor R3.
An external DC supply source (for
instance, from a vehicle battery) can also be used to energise the
charger, where resistor R4, after polarity protection diode D5,
limits the input current to a safe value.
The 3-terminal positive voltage
regulator LM7806 (IC1) provides a constant voltage output of 7.8V DC
since LED1 connected between the common terminal (pin 2) and ground
rail of IC1 raises the output voltage to 7.8V DC. LED1 also serves as
a power indicator for the external DC supply.
After constructing the circuit on a
veroboard, enclose it in a suitable cabinet. A small heat sink is
recommended for IC1.
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