The circuit powers the load via the regulator`s input instead of its output. Because the regulator`s output sees constant dummy load R1, it tries to consume a constant amount of current, no matter what the voltage across the actual load really is. Hence, the regulator`s input serves as a constant-current source for the actual load.
Power the circuit with any one of the commonly available Â± 15 or Â±12 V supplies. The voltage dropped across the regulator and dummy load decreased the total compliance voltage of the~circuit. You set the load"s current with Rl. The current equals 1.25 A/"J x Rl.