A power supply detection system and method for determining the AC mains voltage range when a device (e.g., LED based bulbs) is indirectly connected to the AC mains and the device does not receive the complete sinusoidal AC mains signal. The power supply detection system and method receive an input signal having a first portion that is a sinusoidal signal that generally corresponds to a first portion of the voltage source signal and a second portion that is not a sinusoidal signal that generally corresponds to a second portion of the voltage source signal. A selected reference signal is received, wherein the selected reference signal is synchronized with the input signal. The first portion of the input signal is compared with a corresponding portion of the selected reference signal, and the voltage range of the voltage source is determined based on the comparison.