This is a reverse engineered circuit diagram from an expensive commercial variable lab power supply. The unit is designed around an ordinary op-amp and easy to obtain parts. Although the unit uses a single transformer with two output windings the circuit works equally well using a big transformer and a little transformer. The test point at pin 8 of the op-amp goes high when the variable power supply is operating in current limited mode and it could be used to drive an LED.
Source:http://members.shaw.ca/novotill/LabSupply50V1A/index.htm