Constant current supplies provide a fixed output current over a range of output voltages. Assume that the LED forward voltage is 3.5V, and that it nominally requires 350mA for optimum brightness. In the example shown, twenty-four 3.5V LEDs are in series requiring 84V. The series string must be powered by a constant current supply capable of supplying 84V at 350mA, totaling 29.4W. A 30W constant current supply will suffice, however, a 50W supply should probably be chosen for a safety margin. The table shows some of the constant current outputs available for a 100W supply, part family LXC100. LXC100-0350SW provides a constant 350mA of output current, driving a load between 172 and 286V_DC. A 286V load represents eighty-one LEDs with a forward voltage drop of 3.5V each.