An adjustable current sink is useful for testing power supplies and figuring out how much current an LED might need. I built up this circuit to test a new power supply I was working on and revived it after seeing a couple of related post at Keith’s Electronics Blog. He wants to build a handy LED tester that can determine what resistor you need to light the LED by allowing the user to vary the LED current for desired brightness. This application could make good use of an adjustable current sink to set the LED current.
Adjustable Current Sink - [Link]