Yasir Qureshi posts this project on Sunday this is first post for Pakistan science club website he made before posting. He is trying to explain how people can make.
Variable DC power supply:
It can produce a range of 0-30 v direct current by variable and convert into alternating current AC to DC converters usually include a rectifier bridge to rectify the AC current of the input line and a regulating device supplying on output of one or more regulated DC voltages. Just the simple AC current is applied and through potentiometer we obtain the desired regulated DC power supply.
variable DC power supply is very common and useful project for electronics hobbyist and professionals variable power supply also available Pakistan's local markets, Karachi , Lahore, Rawalpindi, Peshawar but its too much costly, that project can be make within 500 rupees
The main components requires for building of DC Power supply are diodes, IC, Capacitors, heat sink, transformer 220 v, 0 to 30 VAC
I hope you will like that project
Please visit paksc.org for complete detail. http://paksc.org/pk/science-experiments/...upply.html

DC (Direct current) : Direct current (DC) is the unidirectional flow of electric charge. Direct current is produced by sources such as batteries, thermocouples, solar cells
AC (alternating current) the movement of electric charge periodically reverses direction.