Power source, power failure detection apparatus, and image forming apparatus

A power source includes rectifying means configured to rectify an alternating current (AC) voltage to be input, a first and second converter configured to convert the voltage rectified by the rectifying means, zero cross detection means configured to detect a zero cross of the AC voltage, voltage detection means configured to detect the AC voltage, a first capacitance element connected across a potential after being subjected to rectification by the rectifying means and a ground, a first discharging resistor configured to discharge electric charges charged in the first capacitance element and first switch means configured to cut off a current flowing to the first discharging resistor and stopping means configured, in a case where the AC voltage is detected by the voltage detection means and the detected voltage is smaller than a threshold value, to stop the operation of the second converter.