The constant current source has a maximum output voltage that it can deliver, called the compliance voltage. It is the voltage that would appear across the sources output terminals if there were no load present. In this example, individual LED voltages can be estimated to be around 3V. In reality though, there is actually quite a range of voltages for individual LEDs. The voltage across any particular LED will depend on how much power is being delivered to the LED at that time. Also, the temperature of the LED will affect the voltage across it. This will change with LED brightness, heat sinking, and the ambient temperature. Also, different color LEDs will have different nominal voltages. The compliance voltage must be greater than the sum of all of the LED voltages in the string. Note that higher power LEDs do not necessarily have higher voltages. Notice in this example, how the wattage increases while the LED voltage does not change.