An apparatus for detecting over voltage and/or under voltage can include: a reference voltage receiving unit configured to receive a reference voltage; a gradient measuring unit configured to measure a gradient value of a voltage waveform of the reference voltage; and a pulse conversion unit configured to convert the gradient value into a first pulse width and store it. Further, the apparatus can include: an electrical power receiving unit configured to receive an input voltage from an external power source; and a voltage detection unit configured to compare the first pulse width with a second pulse width to detect an over voltage condition or an under voltage condition.