This power supply circuit module is a compact, easy to build. It uses negative variable power supply module. This power module is ideal for powering any application requiring a DC supply at current levels up to 1.5A.Diodes D1-4 form a bridge rectifier which converts the AC input voltage into a DC level. They also allow a DC input voltage to be connected either way around. Capacitor C1 smooths the DC output of the bridge whilst C2 provides high frequency decoupling. The LM337T is an adjustable regulator IC providing the desired output
voltage.
Diode D5 is reversed biased during normal operation and is used to protect the regulator if the output is connected to a voltage of the same polarity (eg battery). Diode D6 protects the regulator if a reverse polarity voltage is connected to the output.
Source:http://powersupplycircuit.blogspot.com
http://powersupplycircuit.blogspot.com/2009/02/negative-adjustable-power-supply-module.html
