A direct current power supply device which generates an alarm signal when a power failure occurs in an alternating current power supply connected as an input to the device. A power failure detector circuit is connected at the input of the device and signals the failure of the AC power supply. In order to compensate for different time delays in the fall of the DC output due to varying loads at the output of the device, a reset signal generating circuit is used which is controllable in accordance with the size of the load. The circuit receives as an input, a signal identifying a power failure from the power failure detector circuit and a signal identifying the load current value from a load current value detector circuit, connected between the load and the AC/DC converter. The reset signal generating circuit, using analog or digital components, will generate a reset signal at a timing determined by the size of the load.