A adaptor circuit for a power supply includes a first comparison circuit, a timing circuit; and a second comparison circuit. An input of the first comparison circuit is electrically connected to a PS_ON terminal. The first comparison circuit includes a diode. An input of the second comparison circuit is electrically connected to the diode of the first comparison circuit via the timing circuit. When the PS_ON signal is powered on, the diode is off, and the timing circuit charges up in a predetermined time, and the second comparison circuit outputs a PWR_GOOD signal after the predetermined time. When the PS_ON signal is powered off, the diode turns on, and the timing circuit discharges, so the second comparison circuit stops outputting a PWR_GOOD signal.