The following circuit Diagram of (DUAL VOLTAGE POWER
SUPPLY ) can be used for Misc.. application.
It requires a few components to built. The most
important components of this circuit are REGULATORS.
1 : (AN) 7812 and 2 : (AN) 7912 AN7812 is the
Positive Voltage Regulator. It regulates the voltage
from (almost) 24vDC to 12vDC (accurate). AN7912 is
the Negative Voltage Regulator. It regulates the
voltage from (almost) -24vDC to -12vDC. A
transformer output must be between 12vAC to 24vAC @
500mA. Input of transformer (Primary)
should be about 110vAc-220vAC. It also include some
capacitors to filter the current.
this circuit in PDF