Used for limiting current output of positive voltage PSU. Very simple, two diodes in series provide fixed voltage difference for base-emitter diode and adjusting resistor. By lowering output load, current through adjusting resistor increases and when voltage on adjusting resistor and base-emitter equals voltage on diodes in series, current via adjusting resistor (÷output current) cannot increase anymore. With max. output current, voltage drop from PSU to output maybe asi high as 1 V, probably not suitable for low voltage PSU.
