Here’s an old but still good app note from Unitrode (Texas Instruments) on Adjustable Electronics Load.
Testing power supply regulation over a specified output current range is greatly simplified with the use of an adjustable electronic load. Load current can be varied from zero to the full rated value with the twist of a panel mounted potentiometer or by a digital interface. This design note will feature the design of a self biased, constant current adjustable electronic load which is fully operational with input voltages down to 2.5V.