A power source for applying a voltage to a pixel circuit, the power source including a sensing unit and a controlling unit. The sensing unit senses a current flowing through a power line coupled to the pixel circuit, the controlling unit increases a voltage applied to the power line by a first amount when the sensed current is lower than a reference, and the controlling unit decreases the voltage applied to the power line by a second amount when the sensed current is higher than the reference, the second amount being different from the first amount.