This example gave a good baseline specification for the ballast resistor, but it is useful to question some of the chosen values. For example, why was a 24V supply chosen when a 20V supply would reduce the power dissipation in the resistor to 0.7W? The main reason lies with the component tolerances. A typical PSU may have an output voltage tolerance of 5%, and while the current versus voltage characteristic of the LEDs is still a factor, most of the output variation will affect the voltage across the resistor. Hence, in the previous example with a 24V PSU, an increase of 5%, which is 1.2V, will result in a current increase to around 400mA, which is still close to nominal for the LEDs. However with a 20V PSU a 5% increase of 1V takes the forward current to around 450mA which is disproportionally higher than the target 350mA.