Answer:
To design a cell phone charger that converts AC voltage of 220V and 50Hz to a DC voltage of 3.5V, you would need the following components:
Transformer: A transformer is used to convert high voltage AC to low voltage AC. For this charger, you would need a step-down transformer that can convert 220V AC to a lower voltage, say 6V AC.
Rectifier: A rectifier is used to convert AC voltage to DC voltage. A bridge rectifier can be used to convert the 6V AC to DC voltage.
Filter: A filter is used to smooth out the DC voltage, removing any ripples in the voltage waveform. A capacitor can be used as a filter.
Voltage regulator: A voltage regulator is used to maintain a constant DC voltage output. For this charger, a 3.5V voltage regulator can be used.
USB port: A USB port can be used to connect the cell phone to the charger.
The circuit diagram for the charger would look like this:
[220V AC] --> [Step-down transformer] --> [6V AC] --> [Bridge rectifier] --> [Capacitor filter] --> [3.5V voltage regulator] --> [USB port]
The charger can be enclosed in a plastic case with a power switch and LED indicator. The charger should be designed to provide adequate protection against overvoltage, overcurrent, and short circuits to prevent damage to the cell phone or the charger itself.