A dimmer reduces power by increasing the amount of time the power is “off”
Forward Phase – Leading Edge or Triac dimmers cut off the first part of the cycle. Typically used with incandescent bulbs, and produces a rush of voltage every half cycle, resulting in a rush of current to the light source. Incandescents are a resistive load. Need high initial voltage then taper off. Current and voltage are in unison
Reverse Phase – Trailing Edge (electronic dimming) utilizes a current that is turned off as the AC waveform ends, right before it crosses zero. LED power supplies are capacitive load. The current leads the voltage. Results are in a very high peak to average voltage ratio.