A power supply apparatus includes an inverter controller, a power transforming part and a connecting part. The power transforming part converts a direct current supplied from the inverter controller into an alternating current to output first and second polarity currents. The connecting part outputs the first and second polarity currents to first and second end portions of a load through first and second terminals, respectively and has a third terminal to receive a sensed signal in response to the first or second polarity currents so as to output the sensed signal. The first terminal is spaced apart from the second terminal by a first insulating distance, and the third terminal is spaced apart from the first or second terminals adjacent to the third terminal by a second insulating distance. Therefore, the sensed signal is independent of the output power, thereby increasing the sensing efficiency.