In this circuit, the 7815 regulatates the positive supply, and the 7915 regulates the negative supply. The transformer should have          a primary rating of 240/220 volts for europe, or 120 volts for North America. The centre tapped secondary coil should be rated about 18 volts at 1 amp or higher, allowing for losses in the regulator. An application for this type of circuit would be for a small regulated bench power supply.