This is Adjustable Constant Current Regulator circuit. This circuit can be used in a bench power supply to prevent the circuit that are tested from being damaged. This circuit can be adjusted to any value from a few milliamp to about 500mA. The max current is 500mA because it is the limit of the BC337 transistor in the current-limiting part of the circuit. Here is the schematic diagram of the circuit:

Adjustable Constant Current Regulator circuit schematicThe input voltage (supply) has to be 5.25v above the required output voltage, because of 1.25v across the current-limiting section and Approximately 4v is dropped across the regulator.  The LM 317 3-terminal regulator will need to be heatsinked. the LM series of regulator is suitable for this circuit because they have a voltage differential of 1.25v between “adj” and “out” terminals. the LM317 can be replaced by 7805, however the losses in the BC337 will be 4 times greater as the voltage across it will be 5v.

For example, to charge 4 Ni-Cad cells, just connect them to the output and adjust the 500R pot until the required charge-current is obtained. The charger will charge 4 Ni-Cad cells at the same current. But, don’t forget to turn off the charger before the cells are fully charged or the battery will be over-charged. [Source: talkingelectronics.com]