Various embodiments of the present invention relate to AC-DC adapters, and more particularly, to systems, devices and methods of employing a load detector to detect a load condition of the AC-DC adapters based on transient variation of a DC output voltage; and therefore, enable the AC-DC adapters to switch from a low power mode to a normal power mode. At the low power mode, a DC output voltage is generated at a target voltage by the AC-DC adapter, and subsequently drops at a decay rate dependent on load condition. The load detector is electrically coupled to monitor the output voltage and measure the decay time between two threshold voltages. An enable instruction is provided to configure the AC-DC adapter to the normal power mode as soon as the decay time is lower than the threshold decay time or the variation of consecutive decay times reaches a certain threshold.